METAL AND POLYMER FOAM HYBRID MATERIALS: DESIGN, … · 1 Materials to Fill the High-Strength, Low-Density Void 1 1.1 Materials Selection Charts: Looking at Materials Space .

METAL AND POLYMER FOAM HYBRID MATERIALS: DESIGN,FABRICATION AND ANALYSIS

by

Julianna E. Campbell

A thesis submitted in conformity with the requirementsfor the degree of Master of Applied Science

Graduate Department of Materials Science and EngineeringUniversity of Toronto

Copyright © 2009 by Julianna E. Campbell

Abstract

METAL AND POLYMER FOAM HYBRID MATERIALS: DESIGN,

FABRICATION AND ANALYSIS

Julianna E. Campbell

Master of Applied Science

Graduate Department of Materials Science and Engineering

University of Toronto

2009

Two novel hybrid materials for use in sandwich cores of structural materials are designed,

manufactured and mechanically tested. Each material is a hybrid of metal and polymer

foam. One set of hybrids is fabricated using an aluminium micro-truss filled with varying

densities of polyurethane foam. Increases up to 120% in stiffness, 372% in strength, 740%

in resilience and 106% in impact energy over the aluminium micro-truss are obtained

from compression and impact testing. Furthermore, the stiffness of these hybrids can be

tailored according to the density of the polyurethane foam. Another set of hybrids is

fabricated using a rapid prototyped ABS polymer truss that is foamed and electroplated

with nanocrystalline nickel. Increases up to 1525% in stiffness, 1165% in strength and

650% in energy absorption over the foamed ABS truss are obtained. Furthermore, the

gain in strength, stiffness and energy absorption outweigh the gain in density in these

hybrid materials.

ii

Acknowledgements

This work could not have been completed without the help and support of many col-

leagues and friends.

First and foremost I would like to thank my supervisors, Dr. Hani Naguib and Dr.

Glenn Hibbard. Their guidance and support was invaluable throughout the course of

this research.

Further thanks goes to the Hybrid Materials group, especially to Marc Suralvo for

helping with the electroplating of the ABS trusses, to Ian Stewart for helping to fabricate

the aluminium PCMs and to Brandon Bouwhuis and Eral Bele for their help with the

inelastic buckling models.

A special thanks also to those in the SAPL group: Linus Leung, Christine Chan,

Aaron Price, Reza Rizvi, Eunji In, Choonghee Jo, Joe McRae, Jack Chang, Dina Badawy

and all of the summer students for their support, help and advice, and most importantly

for making this experience enjoyable.

To my parents, Ian and Linda, sisters, Katie and Laura, brothers-in-law, Roland and

Bryan and many friends who have been waiting for me to finish school for many years

now - I think this is it - thanks for your support through all of the years!

Most of all I would like to thank my husband, Scott. I am forever grateful for his

endless patience and support throughout this process and for all of his help with my

research and latex. Without him, this thesis would never have been completed.

iii

Contents

1 Materials to Fill the High-Strength, Low-Density Void 1

1.1 Materials Selection Charts: Looking at Materials Space . . . . . . . . . . 2

1.2 Hybrid Materials that Fill the Empty Space in Materials Selection Charts 4

1.3 Objective of Thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

1.4 Overview of Thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

1.5 Conclusion: Developing Hybrid Materials to Fill Materials Space . . . . . 10

2 Structural Materials: Sandwich Structures 11

2.1 Sandwich Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.2 Lattice Sandwich Core Materials . . . . . . . . . . . . . . . . . . . . . . 13

2.2.1 Polymer Foams: Bending-dominated cellular materials . . . . . . 14

2.2.2 Periodic Cellular Metal Micro-Trusses: Stretch-dominated lattice

materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

2.3 Hybrid Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

2.3.1 Polymer Foam Matrix Hybrid Materials . . . . . . . . . . . . . . 19

2.3.2 Plated Hybrid Materials . . . . . . . . . . . . . . . . . . . . . . . 20

2.4 Conclusion: Current Hybrid Materials Missing the Low-Density Advan-

tage of Foam . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

3 Pyramidal PCM and Polyurethane Hybrid Materials 23

3.1 Materials and Sample Manufacture . . . . . . . . . . . . . . . . . . . . . 23

iv

Contents

3.2 Experimental Method and Mechanical Testing . . . . . . . . . . . . . . . 26

3.2.1 Compression Testing of PCM, PU Foam and Hybrid Materials . . 27

3.2.2 Impact Testing of PCM, PU Foam and Hybrid Materials . . . . . 27

3.3 Results of Mechanical Testing . . . . . . . . . . . . . . . . . . . . . . . . 29

3.3.1 Stiffness of PCM, PU Foam and Hybrid Materials . . . . . . . . . 31

3.3.2 Strength of PCM, PU Foam and Hybrid Materials . . . . . . . . . 36

3.3.3 Resilience of PCM, PU Foam and Hybrid Materials . . . . . . . . 40

3.3.4 Impact Resistance of PCM, PU Foam and Hybrid Materials . . . 41

3.4 Conclusion: PCM/PU Foam Hybrid Materials Offer Advantages Over

Constituent Parts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

4 Rapid prototyped ABS Truss Cores Plated with Nanocrystalline Nickel 50

4.1 Sample Development and Manufacture . . . . . . . . . . . . . . . . . . . 50

4.1.1 Rapid Prototyping the ABS trusses . . . . . . . . . . . . . . . . . 51

4.1.2 Batch Foaming of the ABS Trusses . . . . . . . . . . . . . . . . . 51

4.1.3 Electroplating of ABS Trusses . . . . . . . . . . . . . . . . . . . . 59

4.1.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

4.2 Experimental Method and Mechanical Testing . . . . . . . . . . . . . . . 60

4.3 Results of Mechanical Testing . . . . . . . . . . . . . . . . . . . . . . . . 62

4.3.1 Mechanical Properties of Foamed and Plated ABS Trusses . . . . 67

4.3.2 Effects of Foaming and Plating . . . . . . . . . . . . . . . . . . . 71

4.3.3 Buckling Analysis of Plated ABS Trusses . . . . . . . . . . . . . . 80

4.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

5 Conclusions and Future Work 85

References 89

v

List of Figures

1.1 Typical material selection chart . . . . . . . . . . . . . . . . . . . . . . . 3

1.2 Hybrid materials are a combination of two or more existing materials . . 4

1.3 Four main types of hybrid materials . . . . . . . . . . . . . . . . . . . . . 6

1.4 Materials selection chart of Young’s modulus versus density . . . . . . . . 8

2.1 Examples of sandwich structures . . . . . . . . . . . . . . . . . . . . . . 12

2.2 Example of honeycomb . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.3 Examples of periodic cellular metal (PCM) micro-trusses . . . . . . . . . 17

3.1 Manufacturing the pyramidal PCMs . . . . . . . . . . . . . . . . . . . . . 24

3.2 Schematic of the mold used to create the hybrid materials . . . . . . . . 25

3.3 Pyramidal PCM, PU foam and hybrid samples . . . . . . . . . . . . . . . 27

3.4 Gardner impact tester . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

3.5 Representative stress-strain curves - high density . . . . . . . . . . . . . . 30

3.6 Representative stress-strain curves - low density . . . . . . . . . . . . . . 30

3.7 Comparison of stiffness for pyramidal PCM, polyurethane foam and hybrids 32

3.8 Comparison of stiffness and density for the PCM, foams and hybrids . . . 34

3.9 Comparison of hybrid stiffness and foam stiffness . . . . . . . . . . . . . 35

3.10 Comparison of strength for pyramidal PCM, polyurethane foam and hybrids 36

3.11 Comparison of strength and density for the PCM, foams and hybrids . . 37

3.12 Comparison of the strength of the polyurethane foam samples found ex-

perimentally and using Menges model . . . . . . . . . . . . . . . . . . . . 39

vi

List of Figures

3.13 Comparison of the strength of the hybrid samples found experimentally

and using Menges model . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

3.14 Comparison of resilience for pyramidal PCM, polyurethane foam and hybrids 40

3.15 Comparison of resilience and density for the PCM, foams and hybrids . . 41

3.16 Damage profile for the pyramidal PCM . . . . . . . . . . . . . . . . . . . 42

3.17 Damage profile for the PU foams . . . . . . . . . . . . . . . . . . . . . . 43

3.18 Damage profile for the PCM/PU foam hybrids . . . . . . . . . . . . . . . 43

3.19 Comparison of impact failure modes for the PCM, foams and hybrids . . 45

3.20 Comparison of impact energy for crack formation in the PU foam samples 46

3.21 Comparison of impact energy for pyramidal PCM and hybrids . . . . . . 47

3.22 Comparison of impact energy of the hybrid versus the sum of its parts (the

PCM and PU foam) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

3.23 Comparison of impact energy and density . . . . . . . . . . . . . . . . . 48

4.1 Schematic diagram of fused deposition modeling (FDM) process . . . . . 52

4.2 CAD drawing of polymer truss . . . . . . . . . . . . . . . . . . . . . . . . 53

4.3 Rapid prototyped polymer truss sample . . . . . . . . . . . . . . . . . . . 54

4.4 SEM micrograph of the cross-section of the ABS truss . . . . . . . . . . . 54

4.5 Photo of rapid prototyped ABS trusses . . . . . . . . . . . . . . . . . . . 56

4.6 Percentage of volume expansion of rapid prototyped ABS trusses versus

foaming temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

4.7 Micrographs of the foamed structure of the rapid prototyped ABS trusses 57

4.8 Nanocrystalline nickel plated ABS truss . . . . . . . . . . . . . . . . . . . 59

4.9 Failure of ABS trusses due to edge effects . . . . . . . . . . . . . . . . . . 61

4.10 Restriction plate used during compression testing to eliminate edge effects 61

4.11 Representative stress-strain curves where strain is calculated using both

the total truss height and the core height . . . . . . . . . . . . . . . . . . 62

4.12 Representative stress-strain curves for the unplated ABS trusses . . . . . 63

4.13 Representative stress-strain curves for the plated ABS trusses . . . . . . 63

vii

List of Figures

4.14 Comparison of representative stress/strain plot and derivative/strain plot

for plated samples foamed at 85 ◦C . . . . . . . . . . . . . . . . . . . . . 65

4.15 Comparison of representative stress/strain plot and derivative/strain plot

for unplated samples foamed at 85 ◦C . . . . . . . . . . . . . . . . . . . . 66

4.16 Fracture at the node joint of the plated ABS truss at peak strength . . . 67

4.17 Representative stress-strain curves of the plated and unplated rapid pro-

totyped ABS trusses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

4.18 Mechanical properties of the nano-Ni plated and unplated ABS trusses . 70

4.19 Material selection charts for mechanical properties of the nano-Ni plated

and unplated ABS trusses . . . . . . . . . . . . . . . . . . . . . . . . . . 72

4.20 Decreasing trends in specific stiffness, specific strength and specific energy

absorption of the foamed ABS trusses . . . . . . . . . . . . . . . . . . . . 73

4.21 Comparison of the strength of the ABS foam trusses found experimentally

and using the Gibson/Ashby model . . . . . . . . . . . . . . . . . . . . . 75

4.22 Relative ratios for mechanical properties of the foamed ABS trusses . . . 77

4.23 Relative ratios for mechanical properties of the nano-Ni plated and un-

plated ABS trusses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

4.24 Comparison of the theoretical and experimental force per strut versus the

cross-sectional area of the core for pinned (k=1) end conditions . . . . . 82

4.25 Comparison of the theoretical and experimental strength versus the cross-

sectional area of the core for pinned (k=1) end conditions . . . . . . . . . 82

4.26 Comparison of hybrid strength with previous studies . . . . . . . . . . . 83

5.1 Materials selection chart with PCM/PU foam and ABS/nanoNi hybrid

materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

viii

List of Tables

3.1 Nine different sample types . . . . . . . . . . . . . . . . . . . . . . . . . . 26

3.2 Average results of strength, stiffness and resilience from compression tests 31

3.3 Percentage increase of density, strength, stiffness and resilience in the hy-

brid samples compared to the PCM . . . . . . . . . . . . . . . . . . . . . 32

3.4 Average results for the impact energy for given failure modes of the PCM 44

3.5 Average results for the impact energy for given failure modes of the PU

foams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

3.6 Average results for the impact energy for given failure modes of the PCM/PU

foam hybrids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

4.1 ABS truss dimensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

4.2 Foaming parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

4.3 Summary of truss dimensions after foaming . . . . . . . . . . . . . . . . 58

4.4 Thickness of nano-Ni coating on ABS trusses . . . . . . . . . . . . . . . . 60

4.5 Average results of strength, stiffness and energy absorption . . . . . . . . 69

4.6 Percentage increase of strength, stiffness and energy absorption of the

nanocrystalline nickel plated trusses over the ABS foamed trusses . . . . 71

ix

Chapter 1

Materials to Fill the High-Strength,

Low-Density Void

There is a large demand for lightweight structural materials in the aerospace, automotive

and consumer goods industries. In the aerospace industry, a reduction in the overall mass

of an aircraft directly relates to an increase in the amount of payload or reduction in fuel

consumption. The structural weight of an aircraft contributes 23 - 29% of its total take-

off weight [1]. In an industry that relies on the transportation of goods and passengers

for its profits, a decrease in the structural weight, and thus an increase in payload, would

be highly beneficial. By reducing the number of total flights in order to move a specific

total payload, aerospace companies would have lowered fuel costs, fewer crew hours and

fewer flight hours. The aerospace industry requires materials that are high in strength

and stiffness, but low in density for the fuselage and wings of aircraft. A reduction in

density of the aircraft’s structural materials could greatly reduce the overall mass of the

aircraft.

In the automotive sector, a reduction in the overall mass of the body of an automobile

would improve fuel economy. Similar to the aerospace industry, the automotive industry

requires high strength materials, but here, an emphasis on impact resistance and energy

absorption is also paramount for safety reasons.

1

Chapter 1 Materials to Fill the High-Strength, Low-Density Void 2

Moving away from vehicular applications, there are many consumer goods that also

make use of similar high-strength, low-density structural materials such as skis and snow-

boards. For these applications high strength is important, as well as the ability to tailor

the stiffness of the final product in order to create a superior piece of equipment for a

given user.

These three industries are just a sampling of those that require high-strength materials

with low density that also have some additional requirement such as the ability to tailor

the stiffness of the material, or increased impact resistance. The materials used for these

applications have changed over time as new materials are developed with improvements

over their predecessors. Ultimately, engineers and materials scientists are continually

trying to improve upon or develop new materials that offer better mechanical properties

for a given application, or are cheaper or faster to manufacture. A recent trend to develop

multi-functional materials, materials that have desirable properties beyond strength, has

also pushed the industry to develop new materials. Currently, there are many deficiencies

in materials selection; to understand this we look to materials selection charts.

1.1 Materials Selection Charts: Looking at Materials

Space

Materials selection charts are used to aid engineers in choosing the optimal material for a

specific task. Based on a certain set of design criteria, these charts help to narrow down

the choices of available materials for a given application. Modulus, strength, density and

cost are some of the primary properties that materials selection charts map out, however,

many other properties are charted where a given application requires them, such as impact

resistance. An example of a materials selection chart for strength versus density is given

in Figure 1.1 [2]. In this figure, it is easy to pick out the high strength, high density

metals in the upper right-hand corner of the chart, and the lower density, lower strength

polymer foams in the lower left-hand corner of the chart. However, there are many areas


Figure 1.1: Typical material selection chart [2].


Figure 1.2: Hybrid materials are a combination of two or more existing materials chosento provide the final hybrid material with properties that are not inherent to any of itsconstituent materials [3].

in the chart which are void of any materials including the high strength, low density

region. Although some of these areas can never be accessed due to restraints on atomic

size and forces, other areas could be filled with new materials that are developed either

by new alloys, polymers or the like, or by combinations of two or more existing materials.

The former option can be prohibitively costly and uncertain, so the latter option is the

method chosen by many researchers [3].

1.2 Hybrid Materials that Fill the Empty Space in

Materials Selection Charts

Hybrid materials are a combination of two or more existing materials as shown in Fig-

ure 1.2. The materials are chosen in such a way as to design a final hybrid material

that has properties that are not inherent to any of its constituent parts. There are four


main types of hybrid materials outlined in Figure 1.3 including composites, sandwich

structures, lattices and segmented structures [3].

This study examines hybrid materials made of lattices from the third group to be used

as cores for the sandwich structures of the second group. The third group of materials,

lattices, is considered to be a hybrid of solid (typically a metal or polymer) and gas,

wherein the properties of the gas become a relevant consideration in terms of thermal

conductivity, compressibility and other properties. The two main types of lattice mate-

rials are bending-dominated and stretch-dominated lattices. Bending-dominated lattices

are typically foams that fail due to the yielding, buckling or fracturing of their cell walls,

whereas stretch-dominated lattices are typically triangulated lattice structures that are

designed to ensure the struts of the lattice stretch rather than bend. By stretching, the

stretch-dominated latices have a higher structural efficiency over the bending-dominated

lattices [3].

The hybrids that will be developed in this study make use of both types of lat-

tice structures in order to make use of the lower density properties of polymer foams

(bending-dominated lattices) and the higher strength properties of metal trusses (stretch-

dominated lattices). By using these two types of materials, the newly developed hybrids

become multi-functional by offering additional advantages over their individual counter-

parts, such as greater impact resistance, along with improved mechanical properties such

as strength and the ability to tailor the stiffness of the final material.

An emerging trend in materials science focuses on multi-functional materials, or rather

materials that offer something in addition to load carrying ability such as enhanced

vibrational or acoustical damping, or heat transfer capabilities [4–9]. Periodic cellular

metals (PCMs) are a stretch dominated lattice structure that have been identified as being

potential core materials for sandwich structures since they offer this multi-functionality.

PCMs have superior load carrying capabilities and offer additional properties such as

heat transfer and energy absorption [4, 5]. The hybrids developed in this study make

use of the PCM architecture in order to take advantage of the stretch-dominated lattice


Figure 1.3: Four main types of hybrid materials: composites, sandwich structures, latticesand segmented structures [3].


system and the potential for multi-functionality in terms of impact resistance and the

ability to tailor the stiffness of the hybrid material.

1.3 Objective of Thesis

The overall objective of this thesis is to develop low density, high strength materials to be

used as structural materials in the aerospace, automotive and consumer goods industries.

These hybrid materials are also multi-functional in terms of greater impact resistance and

the ability to tailor their stiffness. In so doing, an empty area of materials space will

be filled with new hybrid materials as suggested by Ashby [3]. In particular an attempt

will be made to fill the area found between existing bending lattice polymer foams and

stretch lattice PCMs as shown in Figure 1.4, giving engineers a new material option for

structural applications. In addition, a multi-functional hybrid material is developed that

offers improved impact resistance as well as increased strength, stiffness and resilience.

Two types of novel hybrid materials are designed, fabricated and tested. Each of the

hybrid materials makes use of both metal and polymer foam using a stretch-dominated

lattice structure in hopes of capitalizing on the strength of the metal truss while reducing

the overall density of the final material by including polymer foam. The overall objective

is considered in two ways: first, a hybrid material is created using a periodic cellular

metal (PCM) with metal struts which are surrounded by polymer foam, and second, a

hybrid material is created using a PCM where a foam truss core is coated in metal. By

examining these two distinct cases, this work expands materials space and provides new

options for structural materials for use in the aerospace, automotive and consumer goods

industries.


Figure 1.4: Materials selection chart of Young’s modulus versus density. Stretch domi-nated lattices have been found to fill an existing hole in this chart [3].


1.4 Overview of Thesis

The hybrid materials in this work are specifically designed using a periodic cellular archi-

tecture with both polymer foam and metallic components. Chapter 2 gives an overview

of these periodic structures and how they have been have been used in hybrid materials

in the past. Further insight is given into the use of these structures as cores in sand-

wich structures, and how sandwich structures have evolved and become invaluable in

structural applications.

The main work of this thesis follows in Chapters 3 and 4, split over two chapters in

order to discuss the two distinct hybrid materials that are developed and tested. Chapter

3 discusses the first set of hybrid materials which make use of an aluminium metal truss

system surrounded by polyurethane foam. An overview of the design, manufacture and

testing of these hybrid materials is given followed by a discussion of the results of the

mechanical testing. Overall, these hybrids exhibit up to 372% higher strength, 740%

higher resilience and 106% greater impact energy than their PCM truss counterparts. It

is also found that the stiffness can be tailored based on the density of the foam.

Chapter 4 examines the second set of hybrid materials. These hybrids make use of a

foamed ABS polymer truss system which is then electroplated with nanocrystalline nickel.

Similarly, an overview of their design, manufacture and testing is given, followed by a

discussion of the results and the effects of foaming and plating. Overall, the plating of

the trusses greatly increases their mechanical properties including up to 1165% greater

strength, 1525% greater stiffness, 650% greater energy absorption compared to thier

foamed ABS truss counterparts. It is also found that plating, and foaming the ABS

trusses is advantageous in terms of strength, stiffness and energy absorption, despite the

gain in density due to the addition of the nanocrystalline nickel.

Chapter 5 summarizes the key contributions of this work and how they relate back

to the objectives of creating new structural materials for use in the aerospace, automo-

tive and consumer goods industries, thus filling some of the holes in materials space.

Recommendations are made for future work on this topic.


1.5 Conclusion: Developing Hybrid Materials to Fill

Materials Space

The need for high strength, low density materials is apparent for structural applications in

the aerospace, automotive and consumer goods industries. A trend towards finding multi-

functional materials has pushed the need for new material development. Hybrid materials

created using two or more existing materials in various designs are a relatively inexpensive

approach to this problem. This work explores two such materials and determines their

feasibility as materials options for the given applications.

Chapter 2

Structural Materials: Sandwich

Structures

Sandwich structures are a commonly used structural material used for aircraft, auto-

motives and consumer goods such as skis and snowboards. There are many different

materials used for the core of sandwich structures, including foams and micro-trusses.

Much research has been performed on improving sandwich structures including devel-

oping new core materials by examining different truss architectures, filling trusses with

various materials, and plating trusses and metallic foams. This chapter will elucidate

the background and literature summary of sandwich structures and their core materials

including various truss architectures and hybrid materials.

2.1 Sandwich Structures

There is an ongoing need for lightweight structural materials in the aerospace and au-

tomotive industries. Past trends led to the use of sandwich structures in which two

high-strength skin layers are separated by a central core which provides stiffness against

bending and buckling [3,10]. Examples of sandwich structures with various core materi-

als are given in Figure 2.1. The face sheet materials are selected based on their strength

11

Chapter 2 Structural Materials: Sandwich Structures 12

Homogeneous core materials:

Structured core materials:

Wood Cores Foam Cores

Honeycomb Cores Corrugated Cores Textile Cores

Figure 2.1: Examples of sandwich structures [11].

and stiffness as they carry most of the load [3]. Lightweight, stiff materials are chosen for

the core as they must have the shear strength, shear modulus and compressive strength

needed to withstand the shear stresses that the core undergoes [3, 12]. Less material

is used in a sandwich structure than in its monolithic counterpart, which can allow for

significant savings due to reduced material costs [13].

Sandwich structures are commonly manufactured with a lattice core made of hon-

eycomb, metallic foam or polymer foam, and composite or metal face sheets [10, 14].

Honeycomb cores can be made in varying shapes and with various materials such as alu-

minium, glass reinforced plastics, aramid, carbon fibre and kevlar [16]. Hexagonal hon-

eycomb cores, as shown in Figure 2.2, made with aluminium 5052 alloy have compressive

strengths of up to 15 MPa depending on their wall and cell thickness [16]. Common

polymer foams used for sandwich cores include ABS, epoxies, phenolics, polypropylene,

polyurethane and polyvinyl chloride. The hybrid materials developed in this study will

use ABS and polyurethane (PU) foam. In sandwich core structures, ABS foam typically

has a density in the range of 641 - 897 kg/m3 and a compressive strength ranging from

15.8 to 25.5 MPa, while rigid polyurethane foam has densities varying from 21 to 400

kg/m3 and compressive strengths varying from 0.10 to 13.8 MPa [16,17].


Figure 2.2: Example of honeycomb [15].

Much research has been focused on sandwich structures and how to optimize their

design based on a given set of loading conditions [13, 18–20]; however there have since

been advances in the materials available for sandwich construction. Periodic cellular

metals (PCMs) make use of a truss-like geometry which can be used as a core material

since they offer superior load carrying capacity and can provide multi-functionality such

as heat transfer capabilities and energy absorption [4, 5].

2.2 Lattice Sandwich Core Materials

Most sandwich structures make use of lattice materials as their core material. Lattice

materials are lightweight due to their inherent cellular structure. As mentioned in Chap-

ter 1, they are considered to be hybrids of a solid material, such as a polymer or metal,

and gas. The gas is an important component of these structures as it contributes to

various material properties such as thermal conductivity and compressibility [3].

There are two types of lattice materials; bending-dominated foams and stretch-

dominated micro-trusses [21]. Both have been used as sandwich core materials in the

past, and this study uses both bending- and stretch-dominated lattices to create novel

hybrid materials which capitalize on the best properties of both types of materials.


2.2.1 Polymer Foams: Bending-dominated cellular materials

Polymer foams are bending-dominated cellular materials that fail along their cell edges

due to either plastic bending, elastic buckling, or fracture for plastic, elastomeric, or

brittle foams, respectively [3]. Sandwich cores are commonly made using rigid polymer

foams [22]. Among the most frequently used polymers for this application is polyurethane.

Polyurethane (PU) foam is available commercially as a spray-foam product for insu-

lation, or for other applications in a two-phase system in which the two components are

mixed thoroughly before curing. For sandwich structures, the foam is injection molded

into a cold-cavity die [16]. PU offers superior thermal insulation and the ability to bond

well with the sandwich face sheets [10]. It is also dimensionally stable and maintains

high mechanical properties at high and low temperatures [23]. Studies into the effect

of the density of polyurethane foam show that at lower densities, the damping capacity

increases [24,25]. The failure modes of rigid polyurethane foam change depending on the

porosity of the foam [26, 27]. Theocaris tested various porosities of rigid polyurethane

foams in compression and tension and found that with an increase in porosity, the failure

mode would change from being stronger in compression to being stronger in tension [26].

Various models have been developed to predict the mechanical properties of polymer

foams based on their relative density and the properties of the parent polymer [28, 29].

Menges et al. developed the following model to predict the compressive strength [29],

βD = αEpja0.0425χ2 (2.1)

where α is a clamping factor, Ep is the elastic modulus of the polymer, j is a determination

factor (0.53 for PU), a is a reduction factor (a = βD−measured/βD−calculated) and χ is the

relative density. In particular, for rigid polyurethane foam, this reduces to [29]:

βD = 1250χ2 [kp/cm2] (2.2)


in units of kilopond per centimeter squared (1 kp=9.80665 N). This model was derived

by theoretically determining the buckling behaviour of the cell bars during compression,

as the cell walls contribute little to the structural strength of the foam, and comparing

the model with experimental data [29]. Rigid polyurethane foam is used in the design

and manufacture of the first set of hybrid materials in the present work.

Another model that is commonly used to predict the strength of polymer foams is the

Gibson/Ashby model [28]. For elastic-plastic foams in compression, the model states [28]:

σ

σY S= C1

(φρ

ρs

)3/2

CDCF + C2(1− φ)

(ρ

ρs

)(2.3)

where σY S is the yield strength of the parent polymer, φ is a constant between 0 and 1

based on the number of open and closed cells in the foam (φ = 0 for closed cells, φ = 1

for open cells), ρ is the density of the foam, ρs is the density of the solid polymer, C1

and C2 are constants (for φ = 1, C1 = 0.3 , φ = 0, C1 = 0.44 for relative density < 0.2)

and CDCF is a density correction factor (1 + (ρ/ρs)1/2) which can be included, but has

small influence.

The second set of hybrid materials in this study will be manufactured using acrylonitrile-

butadiene-styrene (ABS). Although ABS is not as commonly used in a foamed structure

compared to polyurethane, it is often used in rapid prototyping manufacturing, and has

been selected based on its ability to form a porous (foam) structure [30–35]. The second

group of hybrid materials in this study will use a rapid prototyped structure using ABS.

Due to the current methods of rapid prototyping available, the polymer making up

the sample can not be foamed as it is being manufactured. Therefore a batch foaming

method will be used to generate the porous structure [31] after the trusses have been

manufactured. In this method, a sample is placed in a pressurized chamber which is

filled with a gas (commonly CO2 or N2) at a given pressure. The sample becomes

saturated with the gas over a period of time after which the pressure is rapidly released.

The sample is then placed in a hot water bath wherein the thermodynamic instability


of the rapid pressure drop and increase in temperature causes the cells to nucleate and

grow. Finally, the sample is then quenched in a cool water bath in order to control the

cell growth and left to air dry to allow the remaining gas to escape [36].

Although rapid prototyped ABS parts which has been foamed has not been considered

in past studies, solid ABS rapid prototyped parts have been researched. For example,

methods have been developed to optimize the design criteria for rapid prototyped ABS

parts [37]. Much of the research surrounding the use of ABS as a core material is related

to its use in hybrid materials which will be discussed in Section 2.3.2.

2.2.2 Periodic Cellular Metal Micro-Trusses: Stretch-dominated lat-

tice materials

Stretch-dominated lattices are open-cell systems that make use of a truss-like geometry

in order to reduce the overall amount of material used and thus reduce the structure’s

mass [6,15,38]. The term ’periodic cellular metals (PCMs)’ refers to tubes, beams or wires

arranged in a three-dimensional repeating architecture that are used to make up their

lattice structure [3]. Figure 2.3 shows some examples of periodic cellular metal micro-

trusses developed using various geometries including pyramidal, tetrahedral, kagome and

others [7].

Previous studies show that PCMs offer the same lightweight advantages as honeycomb

cores, but with additional multi-functionality such as cooling and vibration control [5].

Compared to metallic foams, PCMs have higher specific strength and stiffness [6, 7, 39].

Additional properties such as thermal management, dynamic load protection, acoustic

damping and better crush strength make PCMs an attractive alternative to honeycomb

or metallic foam cores [7–9].

Much research has centered around the processing of these micro-trusses [40–43].

Some studies examined the effects of heat treatments on these structures, especially

when adding faceplates [40, 41]. Other studies looked at the effects of using different

manufacturing techniques [42,43].


(a) tetrahedral (b) pyramidal (c) 3-D Kagomé

(d) Diamond weave (e) hollow truss (f) egg-box

Figure 2.3: Examples of periodic cellular metal (PCM) micro-trusses [7].

Mostly, PCM research has focused on the mechanical testing, failure analysis and

modeling of these structures [41, 44–49]. Mechanical properties of PCMs vary greatly

depending on geometry of the truss, geometry of the strut and the material used. Pyra-

midal PCMs manufactured using aluminium alloy 3003 that have been resistance brazed

to face plates of the same material have a compressive strength of up to 0.95 MPa [41].

In terms of failure, it has been found that PCMs typically fail by Euler buckling, shear

buckling or face wrinkling and that this failure depends on the properties of the bulk

material and the geometry of the PCM [6, 45]. PCMs that are not restricted with face

sheets fail by plastic hinging collapse [47]. McShane et al. examined the energy absorp-

tion and shock resistance of these lattices [50]. They found that the sandwich plates have

a higher shock resistance than similar monolithic plates which was verified with finite

element method (FEM) simulations [50].

A model for the strength of an ideal PCM in compression was developed by Deshpande

et al. [51]:

σPCM = σFρRsin2ω (2.4)

where σF is the failure strength of the strut, ρR is the relative density and ω is the strut

angle. The failure strength of the strut, σF , is dependent on the slenderness ratio L/r


(where L is the length of the strut and r is the radius of gyration). For small slenderness

ratios, corresponding to short, stocky struts which fail by yielding, σF ≡ σY S, the yield

strength. However, for larger slenderness ratios, the struts of the PCM fail by buckling

and σF ≡ σCR, the critical buckling stress.

The critical buckling stress was developed by Shanley and is given by [52]:

σCR =k2π2EtI

AL2=k2π2Et(L/r)2

(2.5)

where k accounts for the rotational stiffness of the strut (k=1 corresponds to pinned ends,

k=2 corresponds to fixed ends), Et is the tangent modulus, I is the moment of inertia,

A is the cross-sectional area and L is the length of the strut. For very high slenderness

ratios, elastic buckling will occur and Et ≡ E, Young’s modulus. However, in the elas-

tic to plastic region of the stress strain curve, various models have been developed to

model the strain behaviour [53,54]. The Ramberg-Osgood model is one such constitutive

relationship [53]:

ε =σ

E+ ε0

(σ

σY S

)N(2.6)

where ε is the strain, σ is the stress, E is the Young’s modulus, ε0 is the plastic strain

corresponding to the yield strength, σY S (0.002) and N is a strain hardening exponent.

By finding the derivative to equation 2.6,

Et =

(∂ε

∂σ

)−1

=

(1

E+N

ε0σY S

(σ

σY S

)N−1)−1

(2.7)

the critical stress can be calculated. By solving equations 2.5 and 2.7 together, the

slenderness ratio required for σ = σCR is:

(L

r

)= π

√Etσ. (2.8)

In the Deshpande model described above, the PCM is assumed to have perfectly aligned

struts with perfectly uniform cross-sections, which is rarely the case. Therefore it often


over-predicts the actual strength of the PCM [40,42,55–57]. A knockdown factor has been

included in many studies to account for the slight abnormalities in strut cross-section and

alignment [58–60].

Overall the research trends for PCMs found that they offer greater strength-to-weight

and stiffness-to-weight ratios over metallic foams. PCMs have been manufactured using

various techniques, materials and geometries. They outperformed metallic foams in terms

of strength and stiffness and are comparable to honeycombs, yet at reduced cost and in-

creased multi-functionality [61]. The multi-functional benefits include properties such as

heat transfer and impact resistance. Although considered to be a hybrid material with gas

themselves, these structures were also used with other materials to create further hybrids.

Their open porosity allows ample opportunity to fill them with various materials in order

to further enhance their mechanical properties, or add additional multi-functionality [62].

2.3 Hybrid Materials

Hybrid materials can be developed by combining existing materials in order to access

new regions of materials property space [63]. This section will explore the development

of previous hybrid materials used for sandwich cores including polymer foam matrix

hybrids and plated trusses.

2.3.1 Polymer Foam Matrix Hybrid Materials

Many studies have examined using polymer foams as a matrix material for various hy-

brid structures. For example, by adding fibers or fabrics to polyurethane (PU) foam it

has been found that there is an optimum fiber content that increases tensile strength,

hardness and impact strength of the PU foam [64–67]. Other groups had success im-

proving the impact resistance of honeycomb cores by fully or partially filling the cells of

the honeycomb with polymer foam [68–71]. Similarly, the cells of PCMs have been filled

with polymers and hard ceramics in order to increase impact resistance [56,62,72].


Very little research has considered the addition of polymer foams to PCMs, however

similar studies of filling other lattice structures with polymer foams exist [73, 74]. One

such study examined the effects of adding a phenolic polymer foam to the empty spaces

in a corrugated lattice made from fiber reinforced plastic [73]. Other similar studies found

that energy absorption could be improved by adding polyurethane foam to egg-box type

lattices manufactured using fabric composites [74].

The first set of hybrid materials in this study extends this previous work by examining

PCMs filled with polyurethane foam. In these hybrids, the foamed polymer further

decreases the density of the overall hybrid material compared to using a solid polymer,

and improvements are made in strength, stiffness and impact resistance over the PCM

alone.

2.3.2 Plated Hybrid Materials

Some of the hybrid materials examined in this study are electrolytically plated with

nanocrystalline nickel. Nanomaterials are named such due to their small grain size.

They received recent acclaim due to their desirable properties such as increased strength,

hardness and toughness [75–77]. Electrodeposition has become a common method to

produce nanostructured materials as it is simple, inexpensive and versatile [78, 79]. The

second set of hybrid materials in this study use electrolytically deposited nanocrystalline

nickel plating due to its high yield strength [79]. Although this is a fairly recent branch of

research, there have been some studies that follow similar trends. Some of this research

has focused on plating PCMs, while other research has explored plating polymer lattices.

Bouwhuis et al. examined the effects of plating nanocrystalline nickel on plain car-

bon steel PCMs and found that a thin coating of approximately 50 µm would double

the inelastic buckling resistance of the struts of the micro-truss [60]. This same group

examined the effects of plating nanocrystalline nickel on metal foams and found that

their samples had a non-uniform coating thickness. Though they observed an increase

in the overall strength, there was not a significant increase in the specific strength of the


coated foams [59]. They concluded that a uniform coating thickness would have increased

the specific strength. In a similar study, Boonyongmaneerat et al. examined the effects

of electrodepositing Ni-W on reticulated aluminium foams and found that the plated

foams had greater absolute and specific strength and absolute energy absorption [80].

They were able to obtain uniform coating thicknesses on their samples by adjusting the

deposition time, the bath chemistry and the applied current.

Looking at polymer lattices, Gordon et al. explored the idea of plating nanocrystalline

nickel on pyramidal periodic cellular structures made of a rapid prototyped acrylic based

polymer [58]. They found that at a coating thickness of approximately 15 µm (on a poly-

mer strut cross-section of 0.18 mm by 0.39 mm), a 350% increase in elastic modulus and a

500% increase in peak strength could be obtained over that of the polymer core. Further

increases were found by increasing the thickness of nanocrystalline nickel. Markkula et

al. used rapid-prototyped ABS and plated pyramidal, tetrahedreal and strut-reinforced

tetrahedral lattices with copper and nickel [81]. They found that their plated ABS hy-

brids had increased stiffness, yield strength, ultimate strength, and strain-to-failure over

the pure ABS lattices.

Electroplating of other rapid prototyping materials has also been examined. Saleh

et al. looked at the mechanical properties of electroplated sterolithographed parts [82].

They used rapid prototyped parts made from an epoxy based resin which were then

coated with varying thicknesses of copper/nickel. Testing of tensile coupons resulted in

higher Young’s modulus, tensile strength and impact strength. Liu et al. studied the

bending properties of nickel coated photo-polymers used in stereolithography [83]. They

found that a thin layer of nickel could improve the strength and stiffness of the rapid

prototyped parts.

The second set of the hybrid materials in this study follow a similar trend to Gordon

and Markkula [58,81]. Rapid prototyped ABS pyramidal lattice structures are foamed to

obtain different densities. These lattices are then electrodeposited with nanocrystalline

nickel. Although Markkula has looked at the effects of plating rapid prototyped ABS


lattice structures with nickel, the effect of foaming these trusses, and thus further reducing

their density will add value to the current research trends.

2.4 Conclusion: Current Hybrid Materials Missing the

Low-Density Advantage of Foam

Much research has gone into the area of hybrid materials in hopes to fill in the empty

areas of materials space with the goal of developing materials with superior mechanical

properties. In terms of structural materials, there is a great need for low density, high

strength materials in the aerospace, automotive and consumer goods industries. Many

of those exploring hybrid materials to date had success increasing strength, stiffness and

impact resistance, while maintaining a low density; however, the materials studied to

date lack the additional low density advantage of the cellular structure of foam. PCMs

have been filled with many materials including polymers and ceramics, but foam offers

the opportunity to further reduce the density.

The first set of hybrid materials in this study extends the range of PCMs by filling

them with polyurethane foam. This set of hybrids constitutes a metal PCM strut sur-

rounded by a polymer foam. Conversely, the second set of hybrid materials in this study

examines materials made of a polymer foam strut which is plated with metal. These

hybrid materials extend the work done by Markkula et al. by foaming the ABS lattices

and plating them with nanocrystalline nickel rather than a copper/nickel combination.

The present work will demonstrate that the small grain size in the nanocrystalline nickel

provides increased strength to the ABS trusses.

These novel hybrid materials will help to expand materials space and give engineers

further options when looking for structural materials which are low in density and high

in strength, stiffness and impact resistance.

Chapter 3

Pyramidal PCM and Polyurethane

Hybrid Materials

This chapter will focus on hybrid materials developed using aluminium periodic cellular

metals (PCMs) with a pyramidal architecture that are filled with polyurethane foam. In

this case, the hybrid materials have a metal strut which is reinforced by the surrounding

polymer foam. Various densities of polyurethane (PU) foam were combined with pyrami-

dal PCMs to create the hybrid materials. These materials were then mechanically tested

in compression and impact in order to compare their stiffness, strength, resilience and

impact energy with that of the PU foam and PCM.

3.1 Materials and Sample Manufacture

To manufacture the hybrid samples, two separate materials: the PCM and the polymer

foam, were required.

The pyramidal PCMs were manufactured using a perforation stretching method [15].

A sheet of aluminium 3003 with square perforated holes was trimmed to fit the PCM

press as shown in Figure 3.1(a). The trimmed sheet was annealed at 600 ◦C for one hour

in order to increase its formability while being shaped into the pyramidal truss form of

23

Chapter 3 Pyramidal PCM and Polyurethane Hybrid Materials 24

(a) (b)

(c) (d)

Figure 3.1: Manufacturing the pyramidal PCMs requires (a) a sheet of Al 3003 withsquare perforations which is placed in (b) a press with pins on alternating nodes bothabove and below the Al sheet. The press is compressed and the (c) resultant PCM isremoved. Faceplates are adhered to the top and bottom of the PCM (d).

the PCM. It was then quenched in water and placed in the press. The PCM press had

pins on alternating nodes both below and above the sheet as shown in Figure 3.1(b). The

press was then compressed at a constant displacement of 5 mm/min. The pins applied a

force to the nodes to stretch them to an overall displacement of 6.5 mm which resulted

in the desired pyramidal geometry as shown in Figure 3.1(c).

In the final step, faceplates of the same perforated aluminium 3003 were adhesively

bonded to the PCM by surface roughing the nodes and faceplates before applying a

small amount of acrylic adhesive. The final pyramidal PCM which was used to create

the hybrid materials is shown in Figure 3.1(d). These samples were approximately 56 mm

by 56 mm with a thickness of 10 mm and a density of 337 ± 1 kg/m3.

Two types of commercially available polyurethane foam were used to create the hybrid

materials for this study. The first was a single-phase rigid polyurethane insulating foam

by Dow Chemical, while the second was a two-phase rigid polyurethane foam produced


Wooden FramePolypropylene

PCM Foam

Figure 3.2: Schematic of the mold used to create the hybrid materials. Two woodenframes coated with either a sheet of polypropylene or cotton fabric, sandwiched thePCM and uncured PU foam. The clamped wooden frames restricted the direction offoam expansion and provided a level surface on the final hybrid materials.

by Smooth-On. This two-phase foam consisted of two liquid subcomponents that were

mixed in equal amounts by volume before being applied.

The hybrid PCMs were fabricated using an upper and lower wood and plexiglass

frame. Layers of either cotton fabric or polypropylene, for the one-phase and two-phase

PU foams respectively, were used between the PCM and the frame to allow for easy

release after the foam was fully cured. To produce the hybrids using the one-phase foam,

some of the foam was layered onto the bottom plate, then the PCM was placed on top

of the uncured foam, and more foam was layered on top of the PCM. To produce the

hybrids using the two-phase foam, the PCM was placed directly on the lower plate and

the uncured foam mixture was poured over top of the PCM. Finally, the top plate was

placed over the PCM and uncured foam mixture. A schematic of this setup is given in

Figure 3.2. The two plates were then clamped together to ensure that the pressure of

the foam expansion during the curing process did not cause the plates to separate. This

restricted the direction of foaming and allowed for the sample to have a level and uniform

top and bottom face.

The hybrids were then left for at least 2 or 24 hours for the two- and one-phase foams,

respectively, to cure. Once the curing process was complete, the clamps were released

and the hybrid was removed from the mold. Excess foam was trimmed from the hybrids

to create the final samples.


Table 3.1: Nine different sample types.

PCM Polyurethane Foam Hybrid Materials(density, kg/m3) (PU foam density, kg/m3)

Pyramidal One-phase (83 ± 3) Pyramidal PCM/(83 ± 3)Two-phase (113 ± 2) Pyramidal PCM/(113 ± 2)Two-phase (232 ± 2) Pyramidal PCM/(232 ± 2)Two-phase (290 ± 2) Pyramidal PCM/(290 ± 2)

Reference foam samples were made in a similar manner as the hybrid materials as

described above, however, for the foam samples, the PCM was excluded from the mold.

These samples had dimensions that were comparable to the PCMs and hybrid materials

and along with mechanical testing, these samples were used to calculate the density, ρ,

of the PU foams using

ρ =m

V(3.1)

where m is the mass of the sample and V is the volume of the sample. The one-

phase Dow Chemical foam had a density of 83 ± 3 kg/m3, while three densities of the

two-phase Smooth-On foam were used to create the hybrid materials: 113 ± 2 kg/m3,

232 ± 12 kg/m3 and 290 ± 6 kg/m3 (supplier reported nominal densities of 80 kg/m3,

160 kg/m3 and 240 kg/m3, respectively).

Overall, nine different sample types were manufactured including the pyramidal PCMs,

four different polyurethane foams and four different hybrid materials. The different sam-

ples are shown in Figure 3.3 and outlined in Table 3.1.

3.2 Experimental Method and Mechanical Testing

The nine different samples underwent two types of mechanical testing: compression test-

ing to obtain the stress-strain curves of the materials; and impact testing to obtain the

impact resistance of the materials.


(a) (b)

(c)

Figure 3.3: Samples tested include: (a) pyramidal PCM with faceplates, (b) polyurethanefoam and (c) hybrid.

3.2.1 Compression Testing of PCM, PU Foam and Hybrid Materials

All compression testing was performed using a Shimadzu AG-1 load frame at a constant

displacement of 1 mm/min due to the strain rate sensitivity of the PU foams. The PCM

and hybrid samples were loaded in uniaxial compression until truss core collapse occurred

by inelastic buckling failure [6]. Foam samples were loaded in uniaxial compression until

failure due to bending and crumpling of the cell walls [84]. Nominal strains were measured

from the cross-head displacement [85–89].

3.2.2 Impact Testing of PCM, PU Foam and Hybrid Materials

Impact testing of the PCM, PU foams and PCM/2-phase PU foam hybrid materials was

performed using a Gardner Impact tester (Qualitest IG-1142) as shown in Figure 3.4a.

This impact tester comprises an aluminium tube with graduated markings that is used to

guide a cylindrical mass to the point of impact with a specimen as shown in Figure 3.4b.


(a)

Sample

Mass

Tube with graduated markings

(b)

Figure 3.4: Photo (a) and schematic diagram (b) of Gardner impact tester. A 0.227 kgmass was initially dropped from a height of 25.4 mm. The sample was inspected fordamage, then replaced. The test continued with the mass being released from increasingincrements of 25.4 mm in height to obtain the entire damage profile of the samples.

An ASTM standard was not followed due to the strict dimensions of the samples, instead,

the impact energy was measured as by placing the mass over the central node of the PCM

and hybrid samples and releasing it from increasing heights. First, the 0.227 kg (0.5 lb)

mass was released from a height of 25.4 mm (1 in.) above the sample as shown in the

schematic in Figure 3.4b. The sample was then removed and inspected for damage after

which the test was repeated with the mass being released from an increasing height in

increments of 25.4 mm until surface damage was observed. This method provided the

impact energy at which the sample was damaged. In this case the impact energy was

equivalent to the potential energy (PE) where

PE = mgh (3.2)

and m is the mass, g is the gravitational constant and h is the height at which the mass

was dropped from. Frictional effects in the tube were considered to be negligible.


3.3 Results of Mechanical Testing

Stress-strain curves for each sample were used to determine the strength, stiffness and

resilience. For these curves, the apparent stress, σ, was calculated as

σ =F

Ap(3.3)

where F is the force measured under the compression test and Ap is the projected area of

the PCM truss or hybrid sample. Representative curves for each of the various densities

of polyurethane foam, the PCM and the hybrids are given in Figures 3.5 and 3.6. For

each of the stress-strain curves there is an initial linear elastic region, a collapse plateau

and for some curves, the final densification section. The shape of the hybrid curves

changes between Figure 3.5 and Figure 3.6. For the hybrids made with the lower density

foams shown in Figure 3.6, the shape of the stress-strain curve tends to follow that of

the PCM, while for the hybrids made with the higher density foams, the shape of the

stress-strain curve tends to follow that of the foam. This suggests that either the PU

foam or the PCM may dominate in the hybrid material depending on the density of the

PU foam. This becomes more evident when examining the stiffness of these materials in

section 3.3.1.

The stiffness, strength and resilience were calculated using the stress-strain curves.

The stiffness was calculated from the maximum slope of the curve before the initial peak.

The strength was calculated using the peak stress value. However, in the cases where

there was no definitive peak, an intersection between the maximum slope before the

first inflection point, and the minimum slope after the first inflection point was used to

determine the strength. Finally, the resilience, which is the maximum energy per volume

that can be stored elastically, was calculated by integrating up to the peak (as defined

by the strength).

The results from at least three samples were used to obtain an average result for

each sample type. The error was calculated based on the standard deviation of the


0

1

2

3

4

5

6

7

0 0.1 0.2 0.3 0.4 0.5

Strain (mm/mm)

Stre

ss (M

Pa)

PU: ρ=232 kg/m³

PU: ρ=290 kg/m³

H: ρ=232 kg/m³

H: ρ=290 kg/m³

PCM

Figure 3.5: Representative stress-strain curves for the PCM, the two higher densitypolyurethane foams and their hybrid counterparts.

0

0.5

1

1.5

2

0 0.05 0.1 0.15 0.2

Strain (mm/mm)

Stre

ss (M

Pa)

PU: ρ=113 kg/m³

H: ρ=113 kg/m³

PCM

PU: ρ=83 kg/m³

H: ρ=83 kg/m³

Figure 3.6: Representative stress-strain curves for the PCM, the two lower densitypolyurethane foams and their hybrid counterparts.


Table 3.2: Average results of strength, stiffness and resilience from compression testsover at least three samples. Labels 83, 113, 232, 290 indicate density of foam in kg/m3,PU refers to the polyurethane foam samples and H refers to the hybrid samples.

Sample Density (kg/m3) Stiffness (MPa) Strength (MPa) Resilience (kJ/m3)

PCM 337 ± 1 34.4 ± 4.2 1.13 ± 0.02 37 ± 383PU 83 ± 3 1.5 ± 0.5 0.17 ± 0.04 14 ± 883H 357 ± 10 21.3 ± 1.2 1.23 ± 0.03 47 ± 2

113PU 113 ± 0 7.7 ± 0.8 0.51 ± 0.05 21 ± 4113H 395 ± 3 34.6 ± 6.2 1.80 ± 0.03 86 ± 9

232PU 232 ± 12 12.9 ± 1.1 2.56 ± 0.07 243 ± 33232H 537 ± 17 24.9 ± 3.2 3.86 ± 0.53 309 ± 42

290PU 290 ± 6 80.9 ± 6.1 4.46 ± 0.02 117 ± 8290H 650 ± 44 75.8 ± 6.9 5.33 ± 0.20 186 ± 32

three different sample results. The percentage difference for each of the properties was

calculated using

%difference =Phybrid − PPCM

PPCM∗ 100 (3.4)

where P refers to a specific property, either density (ρ), stiffness (E), strength (σ) or re-

silience (J). The average results for the apparent density and absolute strength, stiffness

and resilience for each sample type are listed in Table 3.2, while the percentage difference

for density, strength, stiffness and resilience of the hybrid samples compared to the PCM

are given in Table 3.3. The apparent density was calculated by

ρ =m

V(3.5)

where m is the mass of the sample and V is the volume of the sample.

3.3.1 Stiffness of PCM, PU Foam and Hybrid Materials

The results for the average stiffness of the materials are represented by Figure 3.7. The

foam samples, represented by the columns labeled with a ’PU’ show a trend of increasing

stiffness with increasing foam density. However, the stiffness of the hybrids shows a


Table 3.3: Percentage increase of density, strength, stiffness and resilience in the hybridsamples compared to the PCM. Labels 83, 113, 232, 290 indicate density of foam inkg/m3.

Sample Density Stiffness Strength Energy Absorption(kg/m3) (MPa) (MPa) (MJ/m3)

83H 6% -38% 9% 26%113H 17% 1% 59% 132%232H 59% -27% 241% 740%290H 93% 120% 372% 405%

PCM 83PU 83H 113PU 113H 232PU 232H 290PU 290H0

20

40

60

80

100

Stiff

ness

(MPa

)

Figure 3.7: Comparison of stiffness for pyramidal PCM, polyurethane foam (PU) andhybrids (H). Labels 83, 113, 232, 290 indicate density of foam in kg/m3.


different trend. For the hybrids made with the 113 and 290 kg/m3 PU foams the stiffness

of the overall sample is dominated by the component with the greatest stiffness. For

example, in the case of the 113 kg/m3 PU foams, the stiffness of the PCM was much

greater than that of the foam which resulted in the overall stiffness of the hybrid being

similar to that of the PCM alone. Whereas for the 290 kg/m3 PU foam, the foam had a

greater stiffness than the PCM and therefore the stiffness of the corresponding hybrid was

similar to that of the foam alone. These results suggest that the stiffness of the hybrids

can be tailored for a specific application to be greater than or equal to the stiffness of

the PCM alone, depending on the density of the polyurethane foam used to create the

hybrid.

For the hybrids made with the 83 and 232 kg/m3 PU foams, the results do not follow

the same trend. Though the stiffness of these hybrids is greater than their PU foam

counterparts, it is also below that of the PCM. This suggests that the stiffness of the

PCM did not dominate in these cases, and in fact, was somehow reduced by the presence

of the foam. In these cases it is suspected that the face plates of the PCM fully or partially

separated during the manufacturing process, possibly due to the expansion forces of these

particular foams. During the compression test, the face plates would be forced to make

contact with the PCM, so it was difficult to determine if this was the case, however,

by considering how the failure progressed, this theory is possible. If the face plates did

separate during manufacturing, the struts of the PCM trusses in the hybrid may start

to splay out slightly before buckling. The PU foam would inhibit the truss movement

to a degree, however, as the cells of the PU foam begin to buckle, they would condense

and would restrict the movement of the PCM nodes much in the same way that the face

plate did. The failure of the inner PCM would then revert back to buckling, however, a

greater overall strain would be seen for the sample as is the case for the hybrids made

with the 232 kg/m3 foam.

Although the greater strain is not evident in the hybrid samples made with the

83 kg/m3 PU foam, it is suspected that a similar effect occurred. If the face plates


0

20

40

60

80

100

0 200 400 600 800 1000Density (kg/m3)

Stiff

ness

(MPa

)

PCM83PU83H113PU113H232PU232H290PU290H

PU Foams Hybrids

Figure 3.8: Comparison of stiffness and density for the PCM, foams (PU) and hybrids(H). Labels 83, 113, 232, 290 indicate density of foam in kg/m3. Error bars are omittedfor standard deviations less than 2 MPa and 10 kg/m3.

had partially separated for these samples, the density of the foam would not be high

enough to restrict the movement of the struts to the same degree as in the case of the

samples made with the 232 kg/m3 foam, and again the trusses would start to splay out

instead of buckling. However, in the case of the 83 kg/m3 foam, the cell walls of the foam

would also collapse and condense under a smaller force than for the samples made with

the 232 kg/m3 foams. This would create the restriction around the nodes which is needed

to cause buckling in the struts. Overall, this effect would take place under a lesser overall

strain than for the samples made with the 232 kg/m3 foam due to the difference in foam

density. Ultimately, further testing and examination of the hybrid materials made with

these foams would be required in the future to determine if this were in fact the case, or

if some other effect reduced the stiffness of these materials.

The relative density and stiffness of the PCM, foams and hybrids are illustrated in

Figure 3.8. In this plot, the low density foams form a group at the left-hand side (the

outlined symbols), while the hybrids form a group towards the right-hand side (the solid


0

20

40

60

80

100

0 20 40 60 80Foam Stiffness (MPa)

Hyb

rid S

tiffn

ess

(MPa

)

100

83H113H232H290H

Figure 3.9: Comparison of hybrid stiffness and foam stiffness. Labels 83, 113, 232, 290indicate density of foam in kg/m3. Error bars are omitted for standard deviations lessthan 2 MPa.

symbols) due to their greater density caused by the addition of the PCM. When looking

for a material with a stiffness in the range of 20 - 40 MPa, both the PCM, and lower

density foam hybrids are viable options, whereas the higher stiffness options include the

290 kg/m3 foam and hybrid. The stiffness property can be examined along with other

material properties in order to optimize the material selection process.

By comparing the stiffness of the hybrid samples and the stiffness of the polyurethane

foam in Figure 3.9 the effect of the dominating PCM stiffness is more evident. The 83,

113 and 232 kg/m3 foam hybrids fall within the area bounded by the dashed lines which

correspond to the stiffness of the PCM. Outside of the dashed lines, the stiffness of the

foam will dominate, as is the case for the 290 kg/m3 foam hybrid.



1

2

3

4

5

6

Stre

ngth

(MPa

)

Figure 3.10: Comparison of strength for pyramidal PCM, polyurethane foam (PU) andhybrids (H). Labels 83, 113, 232, 290 indicate density of foam in kg/m3.

3.3.2 Strength of PCM, PU Foam and Hybrid Materials

The results for the average absolute strength of the materials are represented by Fig-

ure 3.10. Both the foam samples (represented by columns labeled ’PU’) and the hy-

brid samples (represented by the columns labeled ’H’) show an increasing trend of

strength with increasing foam density. Furthermore, the hybrid samples have a greater

strength than either of the PCM or foam components. For example, by examining

the columns for the 113 kg/m3 foam (113PU) and hybrid (113H), the increase in the

113 kg/m3 foam/PCM hybrid is evident next to the 113 kg/m3 foam.

For the most part, the strength of the hybrid is approximately equal to the sum of

its constituent parts, the PCM and the PU foam. However, in the hybrid made with

the 113 kg/m3 foam, the strength of the hybrid is greater than the sum of the PCM

and the 113 kg/m3 foam strengths. In all cases, by adding the foam to the PCM, the

struts of the PCM have been reinforced against buckling, their first failure mode [6]. The

foam supports the struts from every side and restricts their movement to provide greater


0

1

2

3

4

5

6

7

0 100 200 300 400 500 600 700Density (kg/m3)

Stre

ngth

(MPa

)

PCM83H113PU113H83PU232PU232H290PU290H

PU Foams

Hybrids

Figure 3.11: Comparison of strength and density for the PCM, foams (PU) and hybrids(H). Labels 83, 113, 232, 290 indicate density of foam in kg/m3. Error bars are omittedfor standard deviations less than 0.1 MPa and 6 kg/m3.

overall strength. In developing a hybrid material, the strength of the new hybrid has

become equal to or greater than the sum of its constituent parts.

A comparison of the strength and density of the PCM, foams and hybrids is given

in Figure 3.11. From this plot, the trend of the lower density foams on the left-hand

side, and higher density hybrids on the right-hand side is again apparent. The increasing

trend of strength with density is also apparent for both the foams and the hybrids.

Furthermore, it is evident that the hybrids have greater strength when compared to the

foams.

The strength of the PU foams and the hybrids has been successfully modeled using

the theory developed by Menges [29]. In order to model the behaviour of the strength in

rigid polyurethane foams Menges developed the following equation [29]:

βD = αEpja0.0425χ2 (3.6)


where β is the strength, D refers to the compressive mode, α is a clamping factor, Ep is the

elastic modulus of the polymer, j is a determination factor (0.53 for PU), a is a reduction

factor (a = βD−measured/βD−calculated) and χ is the relative density. In particular, for rigid

polyurethane foam, this reduces to [29]:

βD = 1250χ2 [kp/cm2] (3.7)

in units of kilopond per centimeter squared (1 kp=9.80665 N). In Figures 3.12 and 3.13

the strength of the two-phase polyurethane foams and hybrids follow the trend of the

Menges model. However, the one-phase, 83 kg/m3 foam and hybrid samples are slightly

over-predicted in the Menges model. This is likely due to the fact that although the one-

phase 83 kg/m3 foam was marketed as a rigid foam, which is what the Menges model

considers, it actually had some characteristics inherent to flexible polyurethane such as

large strain response which the two-phase foams did not exhibit [90].

In terms of the expected strength of the PCM, the Deshpande model was used to

predict the strength of the PCM [51]. The Deshpande model uses the equation [51]

σPCM = σFρRsin2ω (3.8)

where σPCM is the strength of the PCM, σF is the failure strength, ρR is the relative

density and ω is the truss angle. The failure strength, σF , is dependent on the slenderness

ratio of the struts. For small slenderness ratios, σF is equal to the yield strength (σY S).

However, for medium to high slenderness ratios, σF is equal to the critical buckling stress,

σCR, determined by [52]

σCR =k2π2EtI

AL2=k2π2Et(L/r)2

(3.9)

where k accounts for the rotational stiffness, Et is the tangent modulus, I is the moment

of inertia, A is the cross-sectional area and L is the length of the strut. Alternatively,

the critical stress can be defined in terms of the slenderness ratio (L/r) where r is the


0

1

2

3

4

5

6

7

0 50 100 150 200 250 300 350 400

Density (kg/m3)

Stre

ngth

(MPa

)

Foams Menges

Figure 3.12: Comparison of the strength of the polyurethane foam samples found exper-imentally and using Menges model. Error bars are omitted for standard deviations lessthan 0.1 MPa and 3 kg/m3.

0

1

2

3

4

5

6

7

0 100 200 300 400 500 600 700 800

Density (kg/m3)

Stre

ngth

(MPa

)

Menges Hybrids

Figure 3.13: Comparison of the strength of the hybrid samples found experimentally andusing Menges model. Error bars are omitted for standard deviations less than 0.1 MPaand 3 kg/m3.



100

200

300

400

Res

ilien

ce (k

J/m

3 )

Figure 3.14: Comparison of resilience for pyramidal PCM, polyurethane foam (PU) andhybrids (H). Labels 83, 113, 232, 290 indicate density of foam in kg/m3.

radius of gyration (r = I/A, or t/√

12 for a rectangular cross-section).

Experimentally, the PCM was found to have a strength of 1.13 ± 0.02 MPa, whereas

the Deshpande model calculated the strength of the PCM to be 50.84 MPa. It is common

for this model to over-predict the strength of the PCM as the model is taken as an ideal

case for a perfectly uniform, perfectly straight strut [40, 42,55–57,59].

3.3.3 Resilience of PCM, PU Foam and Hybrid Materials

The results for the average absolute resilience of the materials are represented in Fig-

ure 3.14. The addition of the foam to the PCM increased the resilience of the hybrids

regardless of the foam density.

In general, there was an increasing trend in the resilience of the sample with foam den-

sity for both the PU foam and hybrid samples, with the exception of the 232 kg/m3 foam

samples. The comparatively lower modulus for the 232 kg/m3 foam and hybrid, as dis-

cussed in section 3.3.1, resulted in larger elastic energy absorption.


0

100

200

300

400

0 100 200 300 400 500 600 700Density (kg/m3)

Res

ilien

ce (k

J/m

3 )

PCM83PU83H113PU113H232PU232H290PU290H

PU FoamsHybrids

Figure 3.15: Comparison of resilience and density for the PCM, foams (PU) and hybrids(H). Labels 83, 113, 232, 290 indicate density of foam in kg/m3. Error bars are omittedfor standard deviations less than 10 kJ/m3 and 7 kg/m3.

A comparison of the resilience and density of the PCM, foams and hybrids is given in

Figure 3.15. This material selection chart can be used with those in Figures 3.8 and 3.11

to determine the ideal material for a given application in terms of its density, stiffness,

strength and resilience.

3.3.4 Impact Resistance of PCM, PU Foam and Hybrid Materials

A Gardner impact test was performed on each of the PCM, two-phase PU foam and two-

phase PU foam/PCM hybrid samples. The surface damage of each sample was observed

throughout the test and it was found that the three distinct sample types (PCMs, PU

foams and PCM/PU hybrids) each had different damage profiles.

Initially, the PCMs were able to withstand any failure as shown in Figure 3.16a.

Eventually they began to fail by the inelastic buckling of their struts at the point of

impact as shown in Figure 3.16b. Next, a depression became visible on the top face sheet


(a) (b)

(c) (d)

Figure 3.16: Damage profile for the pyramidal PCM includes (a) no damage, (b) inelasticbuckling of the local struts, (c) top face sheet depression, and (d) base sheet deformation.

of the PCM as shown in Figure 3.16c. Finally, the base sheet began to deform due to

the continuous buckling of the struts as shown in Figure 3.16d.

The damage profile for each of the PU foam densities contained the same failure

modes. An initial, undamaged foam sample is shown in Figure 3.17a. Upon first impact,

the foam displayed slight surface depression as shown in Figure 3.17b. As the impact

energy increased, a crack formed in the depression as shown in Figure 3.17c. Finally,

complete penetration of the foam would occur as shown in Figure 3.17d.

The hybrid samples had fewer visible failure modes. Initially they withstood the

impact as shown in Figure 3.18a. Eventually, a depression would form on their surface as

shown in Figure 3.18b. Finally, shearing would occur at the metal/foam interface. This

final mode could not be captured in a photo so a schematic is given in Figure 3.18c.

The impact energies required to reach the various failure modes for the PCM, PU

foams and hybrid materials are summarized in Tables 3.4, 3.5 and 3.6, respectively.

Figure 3.19 compares the failure modes of the various samples. The only common


(a) (b)

(c) (d)

Figure 3.17: Damage profile for the PU foams includes (a) no damage, (b) surface de-pression, (c) crack in surface depression, and (d) complete penetration.

(a) (b)

(c)

Figure 3.18: Damage profile for the PCM/PU foam hybrids includes (a) no damage, (b)surface depression and (c) shearing at the metal/foam interface.


Table 3.4: Average results for the impact energy for given failure modes of the PCM.

Impact Energy (mJ)Inelastic Buckling Depression Base Sheet Deformation

PCM 283 ± 0 339 ± 0 640 ± 33

Table 3.5: Average results for the impact energy for given failure modes of the PU foams.Labels 113, 232, 290 indicate density of foam in kg/m3.

Impact Energy (mJ)Depression Crack Penetration

113PU 57 ± 0 337 ± 0.033 697 ± 33232PU 57 ± 0 640 ± 182 1187 ± 57290PU 57 ± 0 867 ± 131 1488 ± 33

Table 3.6: Average results for the impact energy for given failure modes of the PCM/PUfoam hybrids (H). Labels 113, 232, 290 indicate density of foam in kg/m3.

Impact Energy (mJ)Depression Shearing at Metal/Foam Interface

113H 414 ± 33 678 ± 0232H 546 ± 33 867 ± 33290H 697 ± 33 1130 ± 655


290H232H113H290PU232PU113PUPCM0.00

0.25

0.50

0.75

1.00

1.25

1.50

1.75

2.00

Impa

ct E

nerg

y (J

)

PenetrationShearingBase Deform.CrackDepressionInelast. Buckl.No Failure

Figure 3.19: Comparison of impact failure modes for the PCM, foams (PU) and hybrids(H).

failure mode was the depression of the surface face. As mentioned above, the PCMs

underwent inelastic buckling of the struts, surface depression and deformation of the

base sheet. The PU foams underwent surface depression, cracking in the depression and

complete penetration. Finally, the hybrids underwent surface depression and shearing at

the metal/foam interface.

Each of the foam samples exhibited surface damage in the form of an indentation from

the first impact test at a height of 25.4 mm. The PCM and hybrid samples continued

to resist the impact beyond the first test, and up to a greater impact energy. These

samples did not begin to show a surface depression until a height of at least 152.4 mm.

Since the PU foams all exhibited damage from the first impact, smaller increments of

impact energy are required in order to compare their initial damage. However, upon

continuing the testing, the sample would eventually crack as the impact energy was

increased. Figure 3.20 compares the impact energies of the foam for samples that have a

visible crack in the depression that formed on their surface. This figure shows that there


113PU 232PU 290PU0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Impa

ct E

nerg

y (J

)

Figure 3.20: Comparison of impact energy for crack formation in the PU foam samples.

is an increasing trend of crack resistance with increasing foam density.

In order to compare all of the different types of samples, the surface depression failure

mode was used. Figure 3.21 compares the impact energy for surface depression of the

PCM, the PU foams and the hybrids. Since the PU foams all exhibited surface damage

upon first impact at an impact energy of 57 mJ, they are represented as one entry in

Figure 3.21.

Figure 3.21 shows that the impact energies of the hybrid materials are greater than

that of the PU foam or PCM alone. There is also an increasing trend in impact energy

with the density of the foam used to create the hybrid. So by increasing the density of the

foam a hybrid material with greater impact energy is created. In this case, the addition

of the PU foam allows for energy to be transferred from the PCM to the PU, creating

a material with a greater overall impact resistance. Furthermore, with each of the foam

densities the impact energy of the hybrid is greater than the sum of the impact energy

of the PU foam and PCM. This is shown more clearly in Figure 3.22. The diagonal line

indicates the values where the impact energy of the hybrid would be equal to that of the


PU PCM 113H 232H 290H0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Impa

ct E

nerg

y (J

)

Figure 3.21: Comparison of impact energy for pyramidal PCM and hybrids (H). Labels113, 232, 290 indicate density of foam in kg/m3.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Impact Energy of PCM + PU foam

Impa

ct E

nerg

y of

Hyb

rid

Figure 3.22: Comparison of impact energy of the hybrid versus the sum of its parts (thePCM and PU foam).


0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 100 200 300 400 500 600 700 800Density (kg/m3)

Impa

ct E

nerg

y (J

)

PCM113PU113H232PU232H290PU290H

PU Foams

Hybrids

Figure 3.23: Comparison of impact energy and density. Labels 113, 232, 290 indicatedensity of foam in kg/m3. Error bars are omitted for standard deviations less than 0.04J and 2 kg/m3.

PU foam plus the PCM. Any point above this line would indicate that the impact energy

is greater than the sum of the PCM and PU foam. The data points in Figure 3.22 line

up above one another due to the fact that the impact energy of the PCM was constant

and the impact energy of the PU foams was also constant, regardless of foam density.

Therefore, the sum of the impact energy of the PCM and PU foam would be the same

regardless of PU foam density.

A comparison of the impact energy and density of the PCM, foams and hybrids is

given in Figure 3.23. The low density foams appear towards the left-hand side of the

plot, however they offer little in terms of impact resistance as can be seen by the low

impact energy at which they fail. The PCM performs relatively well, with an average

density and impact energy, however the hybrid samples offer much more in terms of the

impact energy which they can undergo before failure. Although there is a slight loss in

terms of the density of the hybrids, the gain in impact resistance is substantial.


3.4 Conclusion: PCM/PU Foam Hybrid Materials Offer

Advantages Over Constituent Parts

A novel hybrid created from a pyramidal PCM architecture and rigid polyurethane foam

has been designed, fabricated and tested in uniaxial compression and impact resistance.

The hybrid materials exhibited a number of interesting properties including the ability to

tailor the stiffness of the hybrid by using different densities of polyurethane foam. Also,

the strength and resilience of the hybrid was greater than the strength and resilience

of the PCM and the polyurethane foam components. In some instances the strength

and/or resilience of the hybrid samples even exceeded the sum of the strength and/or

resilience of the PU foam and PCM. Furthermore, the impact energy required for surface

deformation of the hybrids was greater than both the PU foam and the PCM, and by

increasing the foam density, the impact energy also increased. Finally, in general, the

strength, resilience and impact energy had an increasing trend with foam density.

In developing a new material of PU foam and pyramidal PCM architecture, a hybrid

material that offers up to 372% greater strength, 740% greater resilience and 106% greater

impact resistance over its PCM counterpart has been created. These properties increase

the options of materials available for structural applications by increasing the property

space as discussed previously.

Chapter 4

Rapid prototyped ABS Truss Cores

Plated with Nanocrystalline Nickel

This chapter will focus on hybrid materials developed with an ABS periodic cellular

polymer structure, created by rapid prototyping, which has been foamed and plated

with nanocrystalline nickel. Unlike the previous chapter, in which the hybrid materials

consisted of a metal truss in which the struts were surrounded by polymer foam, the

hybrids in this chapter have polymer foam struts surrounded by metal. The ABS trusses

were foamed using a batch foaming method in order to obtain three different densities for

which the amount of polymer was kept constant, and thus three different strut dimensions

were obtained. After plating, the hybrids were mechanically tested in compression in

order to compare their properties.

4.1 Sample Development and Manufacture

The samples for this study were created using three main steps. First, the initial samples

were manufactured using ABS in a rapid prototyping method. Next, the as-received

samples were foamed using a batch foaming method. Finally, a subset of the foamed and

unfoamed samples were plated with nanocrystalline nickel to increase their strength.

50

Chapter 4 Rapid prototyped ABS Truss Cores Plated with Nanocrystalline Nickel 51

4.1.1 Rapid Prototyping the ABS trusses

Rapid prototyping is a method that uses a CAD drawing to create a three-dimensional

model built up in layers, typically using a polymer. There are many different types of

rapid prototyping processes. Stereolithography is a method that uses liquid resin which

is polymerized using photons (light) to create the final model. Selective laser sintering

(SLS) uses layers of powder which are sintered using a carbon dioxide laser. In fused

deposition modeling (FDM) layers of extruded polymer are built up to create the model.

Laminated object manufacturing (LOM) uses a laser to cut bonded layers of paper,

plastic, metal or composite into the model shape. In ballistic particle manufacturing

(BPM) layers are built up with droplets of melted materials which are shot at previous

layers, much like an ink jet printer. Finally, three-dimensional printing uses a similar

concept in which drops of binder are shot at a layer of powder to build up the layers of

the model [91].

The method used in this chapter is a fused deposition modeling method (FDM). In

this method, layers of the model are built up using a thermoplastic filament that is

extruded through a heated nozzle as shown in Figure 4.1. The samples in this study used

acrylonitrile butadiene styrene (ABS), a common thermoplastic used in FDM which is

also known to be foamable [30–35].

The samples were manufactured according to the dimensions in Table 4.1, using the

file shown in Figure 4.2. The final sample is shown in Figure 4.3, while Figure 4.4 shows

an SEM micrograph of the inner structure of one of the individual struts. In this figure,

you can see the individual layers of ABS that were used to create the sample.

4.1.2 Batch Foaming of the ABS Trusses

In order to create the cellular, foamed sample, a batch foaming process was used [31].

In this method, a sample is placed in a pressurized chamber which is filled with a gas

(commonly CO2 or N2) at a given pressure. The sample becomes saturated with the gas


Figure 4.1: Schematic of fused deposition modeling (FDM) process [91].

Table 4.1: ABS truss dimensions

Lattice geometry PyramidalMaterial ABSplusNumber of unit cells 4 x 3/4Strut length 7.3 mmStrut cross-section SquareStrut thickness 3.10 mmStrut angle 45 ◦

Total truss length 22.8 mmTotal truss width 22.8 mmTotal truss thickness 10 mmTotal surface area 2313 mm2


c4_rpCAD

c4_rpCAD2

5 mm

Truss Width

Truss Length

Strut Width

5 mm

Truss Height

Strut Length

Strut Thickness

(a)

c4_rpCAD

c4_rpCAD2

5 mm

Truss Width

Truss Length

Strut Width

5 mm

Truss Height

Strut Length

Strut Thickness

(b)

Figure 4.2: CAD drawing of polymer truss a) oblique and b) edge on.


5 mm

Figure 4.3: Rapid prototyped polymer truss sample.

500 μm

Figure 4.4: SEM micrograph of the cross-section of the ABS truss.


Table 4.2: Foaming parameters.

Parameter Value

Gas CO2

Saturation pressure 4 MPaSaturation time 24 hrsFoaming water bath temperature 85, 90 or 95 ◦CFoaming time 240 sQuenching water bath temperature Room temp.Quenching time 60 s

(the rate of which depends on the material’s solubility and diffusion rate), over a period

of time after which the pressure is rapidly released. The sample is then immediately

placed in a hot water bath wherein the thermodynamic instability of the rapid pressure

drop and increase in temperature causes the cells to nucleate and grow. The sample is

then quenched in a cool water bath in order to control the cell growth and left to air dry

to allow the remaining gas to escape [36].

The parameters used for foaming in this study are given in Table 4.2 and are based

on some of the parameters found in a previous study [31]. CO2 gas was used to saturate

the samples at a pressure of 4 MPa over a period of 24 hours. The samples were placed

in a hot water bath at temperatures of either 85, 90 or 95 ◦C for 240 s after which they

were quenched in a room temperature water bath to control the cell growth.

The resultant foamed trusses are shown in Figure 4.5. The three different foaming

temperatures resulted in three different volume expansions. Overall, there is an increasing

trend of percentage volume expansion with foaming temperature as shown in Figure 4.6.

Volume expansion ratios of 80 ± 1 %, 137 ± 3 % and 281 ± 3 % were obtained at foaming

temperatures of 85 ◦C, 90 ◦C and 95 ◦C, respectively.

The individual foamed structures were also analyzed by scanning electron microscopy

(SEM). A strut of the foamed ABS trusses was fractured in order to reveal the inner

cross-section. These samples were then coated with platinum using a sputter coater in

order to make their surfaces conductive. The micrographs of the foamed struts shown in


Figure 4.5: Photo of rapid prototyped ABS trusses, from left: as received, 85 ◦C, 90 ◦C,95 ◦C foaming temperature.

0

50

100

150

200

250

300

84 86 88 90 92 94 96

Water Bath Temperature (°C)

% V

olum

e Ex

pans

ion

Figure 4.6: Percentage of volume expansion of rapid prototyped ABS trusses versusfoaming temperature. Errors ranging from 1.4 to 2.9 % are not shown.


(a) (b)

(c) (d)

Figure 4.7: Micrographs of the foamed structure of the rapid prototyped ABS trusses:(a) as received, (b) 85 ◦C, (c) 90 ◦C and (d) 95 ◦C foaming temperatures.

Figure 4.7 were taken using a JEOL JSM 6060 scanning electron microscope (SEM). A

reference micrograph of the solid ABS truss is given in Figure 4.7a, while micrographs

for the foamed structures at foaming temperatures of 85 ◦C, 90 ◦C and 95 ◦C are given in

Figures 4.7b, c, and d, respectively. The average cell size was calculated using ImageJ,

an image processing software package. The average cell size increased with foaming

temperature from 8 ± 2 µm to 11 ± 4 µm based on an average of at least 12 cells.

The cell density, N, is the number of cells per unit volume and was calculated using the

following equation,

N =

(n

A

) 32

∗ ρpρf

(4.1)

where n is the number of cells in a viewing area, A, ρp is the density of the solid ABS


Table 4.3: Summary of truss dimensions after foaming. Labels 80, 135 and 280 correspondto the approximate percent volume expansion of the ABS foamed trusses.

Solid 80 135 280

Truss PropertiesWidth (mm) 22.91 ± 0.03 26.80 ± 0.14 29.66 ± 0.09 35.20 ± 0.17Length (mm) 22.94 ± 0.05 26.83 ± 0.12 29.62 ± 0.16 35.26 ± 0.12Height (mm) 10.13 ± 0.03 13.30 ± 0.08 14.35 ± 0.11 16.35 ± 0.08

Strut PropertiesWidth (mm) 3.07 ± 0.02 3.33 ± 0.12 3.74 ± 0.16 4.37 ± 0.14Thickness (mm) 3.10 ± 0.03 3.82 ± 0.06 4.18 ± 0.05 4.70 ± 0.06Length (mm) 7.28 ± 0.06 9.47 ± 0.07 10.27 ± 0.48 12.11 ± 0.69Angle ( ◦) 44.67 ± 2.08 46.67 ± 2.25 47.67 ± 0.58 44.67 ± 1.53Slenderness Ratio 8.13 ± 0.10 8.60 ± 0.14 8.51 ± 0.41 8.94 ± 0.52Cross-sectional Area (mm2) 9.51 ± 0.10 12.71 ± 0.50 15.62 ± 0.69 20.54 ± 0.70Second Moment ofInertia (mm4) 7.62 ± 0.13 15.42 ± 0.69 22.74 ± 1.07 37.72 ± 1.46

truss, and ρf is the density of the foamed ABS truss. The cell density ranged from 5.7 x

109µm/cm3 to 8.1 x 109

µm/cm3 based on an average of three viewing. Both the average

cell size and the cell density are in the range for microcellular foams which have a cell

size in the range of 10 µm and a cell density in the range of 109 cells/cm3 [35].

A summary of the dimensions of the various samples is given in Table 4.3. Values for

truss width, length and thickness and strut width, length and thickness were measured

using calipers at various locations on the sample for each of the six samples. The values

reported in Table 4.3 are the average of at least three measurements per sample. The

standard deviation of these measurements is reported as the error. The slenderness ratio

is calculated using L/r where L is the length of the strut and r is the radius of gyration

which is equivalent to t/√

12 for rectangular cross-sections (t is the thickness of the strut).

Finally, the second moment of inertia is calculated using

I =wt3

12(4.2)

where t is the thickness of the strut and w is the width of the strut.


5 mm

Figure 4.8: Nanocrystalline nickel plated ABS truss.

4.1.3 Electroplating of ABS Trusses

The electroplating of the ABS trusses was performed at Integran Technologies (Toronto,

Ontario). In order to plate the ABS trusses using the method of electrodeposition, the

samples were first metallized using a proprietary process in order to create an electrically

conductive surface. Once the samples were metallized, nanocrystalline nickel was elec-

trolytically deposited on them using a procedure similar to Cheung et al. [92]. The final

nanocrystalline nickel plated ABS truss is shown in Figure 4.8.

During plating, a thickness of approximately 250 microns was desired. To determine

the actual thickness of a given sample the mass of the sample before and after plating

was compared in the following equation

t =m2 −m1

ρSA(4.3)

where t is the thickness of the metal coating, m is the mass of the sample before (m1) and

after (m2) plating, ρ is the theoretical density of Ni (8.9 g/cm3) and SA is the predicted

surface area of the sample, based on the original CAD model for the unfoamed sample,

and a scaled CAD model for the foamed samples. The calculated coating thickness are


Table 4.4: Thickness of nano-Ni coating on ABS trusses.

Sample Thickness(% volume expansion) (µm)

Solid 235 ± 1780 233 ± 19135 255 ± 13280 242 ± 7

given in Table 4.4.

4.1.4 Summary

Eight different sample types have been manufactured: four ABS trusses at varying den-

sities both plated and unplated. These samples will each undergo mechanical testing in

order to obtain their stress-strain profile and calculate their stiffness, strength and energy

absorption during strut failure.

4.2 Experimental Method and Mechanical Testing

The eight different sample types underwent uniaxial compression testing to obtain the

stress-strain curves of the materials. Past studies have found that edge effects are rela-

tively small in PCM samples with a 2 x 2 unit cell size when periodically rigid boundary

conditions were applied to each node [93]. Preliminary studies on solid ABS trusses

showed fracture of the face sheet struts before buckling of the core struts as shown in

Figure 4.9. Therefore, the restriction plate shown in Figure 4.10, similar to that used

in [93], was designed in order to eliminate the possibility of edge effects that would not be

apparent in samples with a greater number of unit cells. As in Chapter 3, all compression

testing was performed using a Shimadzu AG-1 load frame at a constant displacement of

1 mm/min. The ABS trusses, both plated and not-plated, were loaded in uniaxial com-

pression until truss core collapse occurred by inelastic buckling failure. Nominal strains

were measured from the cross-head displacement [85–89].


5 mm

Figure 4.9: Failure of ABS trusses due to edge effects.

Figure 4.10: Restriction plate used during compression testing to eliminate edge effects.


0

10

20

30

40

50

60

0 0.2 0.4 0.6 0.8 1 1.2 1.4

Strain (mm/mm)

Stress (MPa)

Truss Height

Core Height

C4_ssstraincomp

Figure 4.11: Representative stress-strain curves where strain is calculated using both thetotal truss height and the core height.

4.3 Results of Mechanical Testing

Stress-strain curves for each sample were used to determine the strength, stiffness and

energy absorption during strut failure. Typically for periodic structures the faceplate is

considered effectively rigid and is therefore not included in the overall height when calcu-

lating strain. However, for these samples, the thickness of the faceplates is a significant

fraction of the overall height of the truss. By comparing stress-strain curves where the

strain has been calculated using both the total truss height and only the core height, as

shown in Figure 4.11 it is apparent that the curve calculated with the core height returns

strains greater than one. This indicates that the faceplates in these samples cannot be

considered effectively rigid and are in fact contributing a small amount of strain to the

overall sample. Therefore, the strains for subsequent samples were calculated using the

total truss height rather than just the core height.

Representative curves for each of the various densities of ABS, unplated and plated

are given in Figures 4.12 and 4.13, respectively. The curves in Figure 4.12 for the


0

1

2

3

4

5

6

7

8

9

10

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35

Strain (mm/mm)

Stre

ss (M

Pa)

Solid80135280

Figure 4.12: Representative stress-strain curves for the unplated ABS trusses. Labels80, 135 and 280 correspond to the approximate percent volume expansion of the ABSfoamed trusses.

0

5

10

15

20

25

30

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45

Strain (mm/mm)

Stre

ss (M

Pa)

Solid80135280

Figure 4.13: Representative stress-strain curves for the plated ABS trusses. Labels 80,135 and 280 correspond to the approximate percent volume expansion of the ABS foamedtrusses.


unplated samples follow the typical curve trend for the compression of polymer foam.

There are three distinct areas: the initial linear elastic region, the collapse plateau and

the final densification section [94]. These figures show that with increasing density, the

Young’s modulus increases and the plateau stress increases. Typically, the strain at

the onset of densification would decrease with increasing density, however the trends in

Figure 4.12 show the opposite. This is likely due to the design of the restriction plates

used for the compression tests in order to control edge effects. Unfortunately, the top and

bottom plate came into contact during testing in the plateau region, so the curves include

the compression of the steel confinement plates as well as the compression of the ABS

trusses. This would skew the location of the onset of densification and inhibits the ability

to calculate energy absorption by published methods as discussed below in Section 4.3.1.

Different sets of restriction plates were manufactured for each set of samples due to the

differing faceplate thickness which was dependent on volume expansion. Therefore, the

faceplates would meet at varying strain values as shown in Figure 4.12.

It is apparent from the stress-strain curves for the plated samples, given in Figure 4.13,

that these samples have a much greater strength due to the nanocrystalline nickel coating.

These curves follow similar trends to those in Figure 4.12, however with some distinct

differences. In Figure 4.13, there is again an initial elastic region. However, before peak

strength is obtained, there are a few instantaneous load drops. These drops can be more

easily identified by plotting the slope of the tangent of the stress-strain curves versus

strain as in the example for the plated samples foamed at 85 ◦C in Figure 4.14. However,

in comparison, Figure 4.15 which compares the same data for unplated samples, has

no discernible steep troughs. These load drops relate to small cracks at the joint of

the nodes. These cracks grew until peak stress as shown in Figure 4.16. After each load

drop, the slope of the curve remains constant until peak strength. Beyond peak stress, the

load drops became greater as the nanocrystalline nickel coating began to fracture and

delaminate from the ABS truss core. Beyond the collapse strength final densification

occurred.


0

2

4

6

8

10

12

14

16

0 0.02 0.04 0.06 0.08

Strain (mm/mm)

Stress (MPa)

c4_deriva

-800

-600

-400

-200

0

200

400

600

0 0.02 0.04 0.06 0.08

Strain (mm/mm)

dσ/dε

c4_derivb

0

0.4

0.8

1.2

1.6

2

0 0.02 0.04 0.06 0.08 0.1

Strain (mm/mm)

Stress (MPa)

c4_deriv2a

(a)

0

2

4

6

8

10

12

14

16

0 0.02 0.04 0.06 0.08

Strain (mm/mm)

Stress (MPa)

c4_deriva

-800

-600

-400

-200

0

200

400

600

0 0.02 0.04 0.06 0.08

Strain (mm/mm)

dσ/dε

c4_derivb

0

0.4

0.8

1.2

1.6

2

0 0.02 0.04 0.06 0.08 0.1

Strain (mm/mm)

Stress (MPa)

c4_deriv2a

(b)

Figure 4.14: Comparison of (a) a representative stress/strain plot and (b) a deriva-tive/strain plot for plated samples foamed at 85 ◦C.


0

2

4

6

8

10

12

14

16

0 0.02 0.04 0.06 0.08

Strain (mm/mm)

Stress (MPa)

c4_deriva

-800

-600

-400

-200

0

200

400

600

0 0.02 0.04 0.06 0.08

Strain (mm/mm)

dσ/dε

c4_derivb

0

0.4

0.8

1.2

1.6

2

0 0.02 0.04 0.06 0.08 0.1

Strain (mm/mm)

Stress (MPa)

c4_deriv2a (a)

0

10

20

30

40

0 0.02 0.04 0.06 0.08 0.1

Strain (mm/mm)

dσ/dε

c4_deriv2b

0

10

20

30

40

50

60

70

80

90

100

9 11 13 15 17 19 21

Strut Cross-Section Area (mm2)

∆σ (MPa)

Experimental

Theoretical

Theoretical with knockdown

c4_buckling2

(b)

Figure 4.15: Comparison of (a) a representative stress/strain plot and (b) a deriva-tive/strain plot for unplated samples foamed at 85 ◦C.


(a)

5 mm

(b)

Figure 4.16: (a) SEM fracture at the (b) node joint of the plated ABS truss at peakstrength.

By comparing pairs of plated and unplated curves at a given density, as in Figure 4.17,

an increase in the plateau stress of the unplated samples and the collapse strength of the

plated samples can be seen.

4.3.1 Mechanical Properties of Foamed and Plated ABS Trusses

Using the stress-strain curves, the stiffness, strength and energy absorption were obtained.

The stiffness was calculated from the maximum slope of the curve before the initial peak.

The strength was calculated using the peak stress value. In the cases where there was

no definitive peak, an intersection between the maximum slope before the first inflection

point, and the minimum slope after the first inflection point was used to determine the

strength. Energy absorption during strut failure depends on the onset of densification

strain, for which there are many definitions [8, 40, 85, 95–98]. For metallic foams, it has

been defined as: the strain at twice the peak stress [95], the strain at 1.5 times the

stress at 50% strain [96], the strain where the stress-strain curve starts to rise [97], or

it can scale with relative density [85]. For PCMs the densification strain has multiple

definitions as well: the strain at which the stress returns to peak stress [40], or a range

of values between 0.5 and 0.6 [8, 98]. Due to the fact that the confinement plates made

contact during compression testing of the uncoated samples, an arbitrary value of strain


0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

0 50 100 150 200 250 300

% Volume Expansion

En

erg

y A

bso

rpti

on

(M

J/m

3)

NOT-PLATED

PLATED

c4_energy

0

5

10

15

20

25

0 0.1 0.2 0.3 0.4

Strain (mm/mm)

Str

ess (

MP

a)

0

5

10

15

20

0 0.1 0.2 0.3 0.4

Strain (mm/mm)

Str

ess (

MP

a)

c4_ss_subfig1 c4_ss_subfig2

0

5

10

15

0 0.1 0.2 0.3 0.4

Strain (mm/mm)

Str

ess (

MP

a)

0

2

4

6

8

10

0 0.1 0.2 0.3 0.4

Strain (mm/mm)

Str

ess (

MP

a)


(a)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

0 50 100 150 200 250 300

% Volume Expansion

En

erg

y A

bso

rpti

on

(M

J/m

3)

NOT-PLATED

PLATED

c4_energy

0

5

10

15

20

25

0 0.1 0.2 0.3 0.4

Strain (mm/mm)

Str

ess (

MP

a)

0

5

10

15

20

0 0.1 0.2 0.3 0.4

Strain (mm/mm)S

tress (

MP

a)


0

5

10

15

0 0.1 0.2 0.3 0.4

Strain (mm/mm)

Str

ess (

MP

a)

0

2

4

6

8

10

0 0.1 0.2 0.3 0.4

Strain (mm/mm)

Str

ess (

MP

a)


(b)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

0 50 100 150 200 250 300

% Volume Expansion

En

erg

y A

bso

rpti

on

(M

J/m

3)

NOT-PLATED

PLATED

c4_energy

0

5

10

15

20

25

0 0.1 0.2 0.3 0.4

Strain (mm/mm)

Str

ess (

MP

a)

0

5

10

15

20

0 0.1 0.2 0.3 0.4

Strain (mm/mm)S

tress (

MP

a)


0

5

10

15

0 0.1 0.2 0.3 0.4

Strain (mm/mm)

Str

ess (

MP

a)

0

2

4

6

8

10

0 0.1 0.2 0.3 0.4

Strain (mm/mm)

Str

ess (

MP

a)


(c)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

0 50 100 150 200 250 300

% Volume Expansion

En

erg

y A

bso

rpti

on

(M

J/m

3)

NOT-PLATED

PLATED

c4_energy

0

5

10

15

20

25

0 0.1 0.2 0.3 0.4

Strain (mm/mm)

Str

ess (

MP

a)

0

5

10

15

20

0 0.1 0.2 0.3 0.4

Strain (mm/mm)S

tress (

MP

a)


0

5

10

15

0 0.1 0.2 0.3 0.4

Strain (mm/mm)

Str

ess (

MP

a)

0

2

4

6

8

10

0 0.1 0.2 0.3 0.4

Strain (mm/mm)

Str

ess (

MP

a)


(d)

Figure 4.17: Representative stress-strain curves of the plated and unplated rapid proto-typed ABS trusses: (a) as received, (b) 85 ◦C, (c) 90 ◦C and (d) 95 ◦C foaming tempera-tures.


Table 4.5: Average results of strength, stiffness and energy absorption from compressiontests over three samples. Labels 80, 135, 280 indicate % volume expansion of the ABScore, S refers to the as-received unfoamed samples, ABS refers to the ABS unplatedsamples and H refers to the hybrid Ni plated samples.

Sample Density Stiffness Strength Energy Absorption(kg/m3) (MPa) (MPa) (MJ/m3)

S-ABS 341 ± 1 97.3 ± 4.9 5.38 ± 0.10 1.14 ± 0.03S-H 1031 ± 18 404 ± 9 22.30 ± 1.26 3.88 ± 0.1280ABS 193 ± 2 37.9 ± 0.5 2.08 ± 0.07 0.43 ± 0.0180H 946 ± 52 350 ± 115 14.28 ± 0.01 2.25 ± 0.04135ABS 147 ± 2 23.1 ± 2.8 1.31 ± 0.05 0.27 ± 0.01135H 875 ± 23 309 ± 53 14.04 ± 0.52 1.86 ± 0.24280ABS 92 ± 1 10.7 ± 0.8 0.58 ± 0.02 0.12 ± 0.01280H 653 ± 9 173 ± 7 7.3 ± 0.5 0.91 ± 0.08

corresponding to 25% of the original micro-truss height was chosen as an upper boundary.

The results from three samples were used to obtain an average result for each sample

type. The error was calculated based on the standard deviation of the three different

sample results. The average results for density, strength, stiffness and energy absorption

for each sample type are listed in Table 4.5.

The results of the average stiffness, strength and energy absorption of the materials

are given in Figure 4.18. There are a couple of generalized trends in Figure 4.18. First of

all, there is a general decreasing trend with decreasing density. Although not linear, this

trend occurs for both strength and energy absorption. For stiffness, however, there is a

slight increase from the stiffness of the solid plated sample, to the stiffness of the plated

sample with the 80% volume expansion ABS truss. Although there is a slight increase,

the solid sample falls within the error bounds of the foamed sample, and it is expected

that with further testing that the trend would become strictly decreasing.

The second trend in Figure 4.18 is that the plated samples have a higher absolute

stiffness, strength and energy absorption than the unplated, however, the percentage

increase in the strength and stiffness of the plated samples over the unplated was greatest

in the samples foamed at 280% volume expansion (1165% and 1525%, respectively). The


0

50

100

150

200

250

300

350

400

450

500

0 50 100 150 200 250 300

% Volume Expansion

Sti

ffn

ess (

MP

a)

NOT-PLATED

PLATED

c4_stiffness

0

5

10

15

20

25

0 50 100 150 200 250 300

% Volume Expansion

Str

en

gth

(M

Pa)

NOT-PLATED

PLATED

c4_strength

(a)

0

50

100

150

200

250

300

350

400

450

500

0 50 100 150 200 250 300

% Volume Expansion

Sti

ffn

ess (

MP

a)

NOT-PLATED

PLATED

c4_stiffness

0

5

10

15

20

25

0 50 100 150 200 250 300

% Volume Expansion

Str

en

gth

(M

Pa)

NOT-PLATED

PLATED

c4_strength (b)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

0 50 100 150 200 250 300

% Volume Expansion

En

erg

y A

bso

rpti

on

(M

J/m

3)

NOT-PLATED

PLATED

c4_energy

0

5

10

15

20

25

0 0.1 0.2 0.3 0.4

Strain (mm/mm)

Str

ess (

MP

a)

0

5

10

15

20

0 0.1 0.2 0.3 0.4

Strain (mm/mm)

Str

ess (

MP

a)


0

5

10

15

0 0.1 0.2 0.3 0.4

Strain (mm/mm)

Str

ess (

MP

a)

0

2

4

6

8

10

0 0.1 0.2 0.3 0.4

Strain (mm/mm)

Str

ess (

MP

a)


(c)

Figure 4.18: Mechanical properties of the nano-Ni plated and unplated ABS trusses: (a)stiffness, (b) strength and (c) energy absorption.


Table 4.6: Percentage increase of strength, stiffness and energy absorption of thenanocrystalline nickel plated trusses over the ABS foamed trusses. Labels 80, 135, 280indicate % volume expansion of ABS, S refers to the as-received unfoamed samples.

Sample Density Stiffness Strength Energy Absorption

S 203% 315% 315% 241%80 391% 994% 588% 425%135 495% 1239% 968% 593%280 613% 1525% 1165% 650%

percentage difference for each of the properties was calculated using

%difference =Pplated − Pnot−plated

Pnot−plated∗ 100 (4.4)

where P refers to a specific property, either density (ρ), stiffness (E), strength (σ) or

energy absorption (J). A summary of the percentage difference for density, stiffness,

strength and energy absorption is given in Table 4.6.

The materials selection charts for these mechanical properties are given in Figure 4.19.

As with the first set of hybrid materials in Chapter 3, the low density ABS foam occupies

the lower left area of each of the curves while the higher density hybrid occupies the upper

right corner of each curve. So again, for a small gain in density a large gain in strength,

stiffness and energy absorption is obtained.

4.3.2 Effects of Foaming and Plating

In order to examine the effects of foaming on the ABS trusses, the specific strength,

stiffness and energy absorption are plotted versus volume expansion in Figure 4.20. There

is a decreasing trend in each of the specific strength, stiffness and energy absorption with

increasing volume expansion. This is due to the overall cellular structure of the struts of

the trusses. The increasing cell size produces a weaker overall structure with increasing

volume expansion.

The strength of the ABS foam trusses has been modeled using the theory developed

Chapter 4 Rapid prototyped ABS Truss Cores Plated with Nanocrystalline Nickel 720.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

-50 0 50 100 150 200 250 300 350

% Volume Expansion

No

rmalized

En

erg

y A

bso

rpti

on

Rati

o

c4_energyratio

0

50

100

150

200

250

300

350

400

450

500

0 200 400 600 800 1000 1200

Density (kg/m3)

Sti

ffn

ess (

MP

a)

NOT-PLATED

PLATED

c4_stiffnessmsc (a)

0

5

10

15

20

25

0 200 400 600 800 1000 1200

Density (kg/m3)

Str

en

gth

(M

Pa)

NOT-PLATED

PLATED

c4_strengthmsc

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

0 200 400 600 800 1000 1200

Density (kg/m3)

En

erg

y A

bso

rpti

on

(M

J/m

3)

NOT-PLATED

PLATED

c4_energymsc

(b)

0

5

10

15

20

25

0 200 400 600 800 1000 1200

Density (kg/m3)

Str

en

gth

(M

Pa)

NOT-PLATED

PLATED

c4_strengthmsc

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

0 200 400 600 800 1000 1200

Density (kg/m3)

En

erg

y A

bso

rpti

on

(M

J/m

3)

NOT-PLATED

PLATED

c4_energymsc (c)

Figure 4.19: Material selection charts for mechanical properties of the nano-Ni platedand unplated ABS trusses: (a) stiffness, (b) strength and (c) energy absorption.


0

10

20

30

40

50

60

0 200 400 600 800 1000 1200

Density (kg/m3)

Str

en

gth

(M

Pa)

Rapid Prototyped ABS

Menges

strengthMengesABS

0

50

100

150

200

250

300

350

0 50 100 150 200 250 300

% Volume Expansion

Sp

ecif

ic S

tiff

ness (

MP

a / g

/cc)

c4_specstiffness

(a)

0

2

4

6

8

10

12

14

16

18

0 50 100 150 200 250 300

% Volume Expansion

Sp

ecif

ic S

tren

gth

(M

Pa / g

/cc)

c4_specstrength

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0 50 100 150 200 250 300

% Volume Expansion

Sp

ecif

ic E

nerg

y (

MJ/m

3 / g

/cc)

c4_specenergy

(b)

0

2

4

6

8

10

12

14

16

18

0 50 100 150 200 250 300

% Volume Expansion

Sp

ecif

ic S

tren

gth

(M

Pa / g

/cc)

c4_specstrength

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0 50 100 150 200 250 300

% Volume Expansion

Sp

ecif

ic E

nerg

y (

MJ/m

3 / g

/cc)

c4_specenergy (c)

Figure 4.20: Decreasing trends in (a) specific stiffness, (b) specific strength and (c)specific energy absorption of the foamed ABS trusses.


by Gibson and Ashby [28]:

σ

σY S= C1

(φρ

ρs

)3/2

CDCF + C2(1− φ)

(ρ

ρs

)(4.5)

where σY S is the yield strength of the parent polymer, φ is a constant between 0 and 1

based on the number of open and closed cells in the foam (φ = 0 for closed cells, φ = 1

for open cells), ρ is the density of the foam, ρs is the density of the solid polymer, C1

and C2 are constants (for φ = 1, C1 = 0.3, for φ = 0, C1 = 0.44 for relative density <

0.2) and CDCF is a density correction factor (1 + (ρ/ρs)1/2) which can be included, but

has small influence. To determine the strength of the ABS foam used in the trusses, the

following relationship was used:

σABS−foam =σABS−trussAtrussNsAcsin(ω)

(4.6)

where Atruss is the projected area of the ABS truss, Ns is the number of struts per

truss sample, Ac is the cross-sectional area of the strut and ω is the strut angle. In

Figure 4.21 the strength of the ABS foam trusses is plotted along with the Gibson and

Ashby model using values of 0.46, 0.67 and 0.94 for φ, C1 and C2, respectively. Although

the experimental data does not match the model exactly, it does follow the overall trend.

Variations from the projected model are due to the truss structure of the ABS foam, and

the layering effect of the rapid prototyping process which created a cellular structure of

its own not accounted for in the Gibson/Ashby model.

In order to determine whether the foaming of the ABS trusses was advantageous, the

ratio of the foamed samples and the solid samples is compared. By further normalizing

these values with the ratio of the density of the foamed sample and the density of the

solid sample it can be determined whether foaming of the ABS trusses was advantageous.

An example of this calculation used to compare the strength of the samples is outlined in

equation 4.7, which is equivalent to the ratio of specific strength (or strength-to-weight


0

5

10

15

20

25

30

35

40

0 200 400 600 800 1000 1200

Density (kg/m3)

Strength (MPa)

Rapid Prototyped ABS Trusses

Gibson Model

strengthGAABS

Press

Figure 4.21: Comparison of the strength of the ABS foam trusses found experimentallyand using Gibson/Ashby model.


ratio) of the foamed and solid samples.

rf =

σfoamed

σsolidρfoamed

ρsolid

=

σfoamed

ρfoamed

σsolid

ρsolid

(4.7)

In this equation, if rf is greater than one, then foaming alone is beneficial. In other

words, any decrease in strength is less than the decrease in density, so if one is restricted

to a certain value for density when selecting a material, a greater strength material could

be obtained by foaming a higher density material then by using a solid material. By

comparing the values of r, the effect of foaming can be determined. For example, the

greater the r value, the less the decrease in strength compared to the decrease in density.

Figure 4.22 compares the r values (the normalized ratios) for the foamed ABS trusses

for each of the mechanical properties investigated including stiffness, strength and energy

absorption. For each of the properties, the r value is below one, indicating that there

is no advantage in terms of strength, stiffness or energy absorption due to the foaming

of the ABS truss. This is due to the change in the cellular structure as mentioned

previously. However, the addition of nanocrystalline nickel shows the value of foaming

the ABS trusses.

In order to determine whether the addition of nanocrystalline nickel or the foaming

of the ABS trusses was advantageous, the ratio of the plated samples and the unplated

samples is compared similar to the method above. By normalizing these values with the

ratio of the density of the plated sample and the density of the unplated sample the value

of plating the ABS trusses with nanocrystalline nickel can be determined. An example of

this calculation used to compare the strength of the samples is outlined in equation 4.8,

which is equivalent to the ratio of specific strength (or strength-to-weight ratio) of the

plated and not-plated samples.

r =

σplated

σnot−plated

ρplated

ρnot−plated

=

σplated

ρplated

σnot−plated

ρnot−plated

(4.8)

In this equation, if r is greater than one, then the addition of the nanocrystalline nickel


0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

-50 0 50 100 150 200 250 300 350

% Volume Expansion

No

rmalized

Str

en

gth

Rati

o

c4_strengthratiof

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

-50 0 50 100 150 200 250 300 350

% Volume Expansion

No

rmalized

Sti

ffn

ess R

ati

o

c4_stiffnessratiof (a)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

-50 0 50 100 150 200 250 300 350

% Volume Expansion

No

rmalized

Str

en

gth

Rati

o

c4_strengthratiof

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

-50 0 50 100 150 200 250 300 350

% Volume Expansion

No

rmalized

Sti

ffn

ess R

ati

o

c4_stiffnessratiof

(b)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

-50 0 50 100 150 200 250 300 350

% Volume Expansion

No

rmalized

En

erg

y A

bso

rpti

on

Rati

o

c4_energyratiof (c)

Figure 4.22: Relative ratios for mechanical properties of the foamed ABS trusses: (a)stiffness, (b) strength and (c) energy absorption.


is beneficial. In other words, the gain in strength is greater than the gain in density,

so if one is restricted to a certain value for density when selecting a material, a greater

strength material could be obtained by plating a lower density material (in this case, the

ABS truss) then by using a solid material. Furthermore, by comparing the values of r,

the effect of foaming can be determined. For example, the greater the r value, the greater

the gain in strength compared to the increase in density.

Figure 4.23 compares the r values for the nanocrystalline nickel coated samples for

each of the mechanical properties investigated including stiffness, strength and energy

absorption. Starting with Figure 4.23a, the normalized ratio is always greater than one.

Therefore, the gain in stiffness obtained by plating the samples with nanocrystalline

nickel is greater than the gain in density. Furthermore, the normalized ratio increases

and then plateaus with increasing foamed volume expansion. This tells us that foaming

the ABS truss is also advantageous. A similar trend is observed in Figure 4.23b for

strength wherein the addition of the nanocrystalline nickel and the foaming of the ABS

truss core is also advantageous. The normalized ratio for energy absorption is given in

Figure 4.23c. Again, the ratio is always greater than one, indicating that the gain in

energy absorption is greater than the gain in density, however in this figure the trend

does not continually increase with increasing volume expansion. There is a peak effect

around those samples with a volume expansion of 135%. This indicates that there may

be an ideal amount of foaming for these truss materials, or that there may be an ideal

coating thickness-to-strut thickness ratio which is exemplified in the samples foamed

with a volume expansion of 135% and a coating of approximately 250 µm. This theory

is supported by the fact that the strength and stiffness plateau around the samples with

a volume expansion of 135% as well. Further investigation by examining various volume

expansions and various coating thicknesses would be needed to determine if there is in

fact an ideal amount of foaming or coating thickness-to-strut thickness ratio.

Overall, the trends for stiffness, strength and energy absorption all show that it is

advantageous to plate the ABS foamed trusses with nano-nickel. Furthermore, it is also

Chapter 4 Rapid prototyped ABS Truss Cores Plated with Nanocrystalline Nickel 790.0

0.5

1.0

1.5

2.0

2.5

-50 0 50 100 150 200 250 300 350

% Volume Expansion

No

rmalized

Str

en

gth

Rati

o

c4_strengthratio

0.0

0.5

1.0

1.5

2.0

2.5

3.0

-50 0 50 100 150 200 250 300 350

% Volume Expansion

No

rmalized

Sti

ffn

ess R

ati

o

c4_stiffnessratio

(a)

0.0

0.5

1.0

1.5

2.0

2.5

-50 0 50 100 150 200 250 300 350

% Volume Expansion

No

rmalized

Str

en

gth

Rati

o

c4_strengthratio

0.0

0.5

1.0

1.5

2.0

2.5

3.0

-50 0 50 100 150 200 250 300 350

% Volume Expansion

No

rmalized

Sti

ffn

ess R

ati

o

c4_stiffnessratio

(b)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

-50 0 50 100 150 200 250 300 350

% Volume Expansion

No

rmalized

En

erg

y A

bso

rpti

on

Rati

o

c4_energyratio

0

50

100

150

200

250

300

350

400

450

500

0 200 400 600 800 1000 1200

Density (kg/m3)

Sti

ffn

ess (

MP

a)

NOT-PLATED

PLATED

c4_stiffnessmsc

(c)

Figure 4.23: Relative ratios for mechanical properties of the nano-Ni plated and unplatedABS trusses: (a) stiffness, (b) strength and (c) energy absorption.


advantageous to foam the ABS trusses, but there is likely an ideal amount of foaming or

coating thickness-to-strut thickness ratio.

4.3.3 Buckling Analysis of Plated ABS Trusses

In order to determine the theoretical strength of the nanocrystalline nickel coated ABS

trusses, a model using hollow nanocrystalline tubes was considered. In this model, the

critical strength (σCR) of nanocrystalline tubes with a rectangular cross-section was cal-

culated using [52]

σCR =k2π2EtI

AL2=k2π2Et(L/r)2

(4.9)

where k accounts for the rotational stiffness of the strut (k=1 corresponds to pinned ends,

k=2 corresponds to fixed ends), Et is the tangent modulus, I is the moment of inertia,

A is the cross-sectional area and L is the length of the strut [52]. Alternatively, the

critical stress can be defined in terms of the slenderness ratio (L/r) where r is the radius

of gyration. The tangent modulus in this case was calculated based on the Ramberg-

Osgood model applied to nanocrystalline nickel [53],

ε =σ

E+ ε0

(σ

σY S

)N(4.10)

where ε is the strain, σ is the stress, E is the Young’s modulus, ε0 is the plastic strain

(0.002) corresponding to the yield strength, σY S and N is a strain hardening exponent.

By finding the derivative to equation 4.10,

Et =

(∂ε

∂σ

)−1

=

(1

E+N

ε0σY S

(σ

σY S

)N−1)−1

(4.11)

the critical stress can be calculated. Using the critical stress of the nanocrystalline nickel

sleeve, the force per strut can be calculated using

Fper−strut = σCRAc (4.12)


where Ac is the cross-sectional area of the nanocrystalline nickel sleeve. Using equa-

tion 4.12 the resolved critical force of the nanocrystalline nickel truss can be calculated

using

FPCM = Ns(Fper−strut)sin(ω) (4.13)

where Ns is the number of struts per truss sample and ω is the strut angle. Finally, the

overall strength of a truss made with nanocrystalline sleeves can be calculated by

σPCM = βFPCMAt

(4.14)

where At is the projected area of the truss sample and β is a knockdown factor used

to account for the slight abnormalities in strut cross-section and alignment. A previous

knockdown factor of 0.59 has been reported for a nanocrystalline nickel plated truss using

an acrylic polymer with differing truss dimensions to the present study [58].

Figure 4.24 shows the theoretical and experimental force per strut versus the cross-

sectional area of the core for pinned (k=1) end conditions. The experimental force per

strut was calculated similarly using equations 4.15 and 4.14,

Fper−strut:experimental =

(σPCMAtNssin(ω)

). (4.15)

The pinned end conditions have been found to work for nanocrystalline nickel coated

micro-trusses in previous studies [60]. Using a knockdown factor of β=0.18 a close match

between the experimental and theoretical data can be obtained. This trend continues

with the comparison of the strength of the truss as shown in Figure 4.25. Here, the

experimental strength was calculated as

∆σ = σplated − σunplated. (4.16)

Again, a knockdown factor of β=0.18 is used to obtain a correlation between the theo-

retical and experimental data.


0

1

2

3

4

5

6

7

8

9

9 11 13 15 17 19 21


Fo

rce p

er

str

ut

(kN

)

Experimental

Theoretical


c4_buckling1

0

10

20

30

40

50

60

70

80

90

100

9 11 13 15 17 19 21


∆σ

(M

Pa)

Experimental

Theoretical


c4_buckling2

Figure 4.24: Comparison of the theoretical and experimental force per strut versus thecross-sectional area of the core for pinned (k=1) end conditions.

0

1

2

3

4

5

6

7

8

9

9 11 13 15 17 19 21


Fo

rce p

er

str

ut

(kN

)Experimental

Theoretical


c4_buckling1

0

10

20

30

40

50

60

70

80

90

100

9 11 13 15 17 19 21


∆σ

(M

Pa)

Experimental

Theoretical


c4_buckling2

Figure 4.25: Comparison of the theoretical and experimental strength versus the cross-sectional area of the core for pinned (k=1) end conditions.


0

2

4

6

8

10

12

14

16

18

20

0.0 0.2 0.4 0.6 0.8 1.0

Density ∆ρ (g/cc)

Str

en

gth

∆σ

(M

Pa)

Foamed ABS/Nano-Ni Trusses

Bouwhuis et al. (2008) CellMet

Bouwhuis et al. (2009) Acta Mater.

Bouwhuis et al. (2008) Compos. Sci. Technol.

Bele et al. (2009) upcoming Acta Mater.

Gordon et al. (2009) Acta Mater.

c4_deltastrength

Micro-Truss Hybrids

Micro-Truss/Foam Hybrids

Foam Hybrids

Figure 4.26: Comparison of hybrid strength with previous studies.

To see how the results of these hybrid materials compare with other similar ones, the

change in strength and the change in density is compared in Figure 4.26. In this figure,

the plated ABS trusses fill an area between the hybrids from previous studies which

fulfills the objective of designing a hybrid material to fill an empty area of materials

space.


4.4 Summary

In this chapter novel hybrid materials were designed and manufactured using rapid proto-

typed ABS truss cores that were foamed to varying volume expansions and electroplated

with nanocrystalline nickel. The hybrid materials had up to 1165% greater strength,

1525% stiffness and 650% energy absorption compared to their unplated counterparts.

Also, there was a decreasing trend in the absolute value of these mechanical proper-

ties with decreasing core density, however in general, there was an increasing percentage

difference in the plated trusses with decreasing density.

Furthermore, by examining their normalized ratios, the plating of the trusses with

nanocrystalline nickel was advantageous in terms of the mechanical properties of stiffness,

strength and energy absorption, despite the gain in density. Similarly, by examining the

normalized ratios, the foaming of the samples is advantageous as well, and there may

be an ideal amount of volume expansion, or an ideal coating thickness-to-strut thickness

ratio.

Overall these hybrids give a new option for low density, high strength materials that

can be used in the aerospace, automotive or consumer goods industries as cores for

sandwich materials.

Chapter 5

Conclusions and Future Work

Two sets of novel metal and polymer foam hybrid materials were designed, developed,

manufactured and tested. In the first set the effect of surrounding a metal strut with a

polymer foam was examined. Aluminium periodic cellular metals made of a pyramidal

architecture were filled with varying densities of polyurethane foam. The PU foam acted

to inhibit the struts of the PCM from buckling, thus increasing the strength of the hybrid

material. Furthermore, the foam also contributed to energy transfer and absorption

during impact testing, creating a hybrid material that can withstand greater impact

energy.

These hybrid materials offered a number of advantages over their micro-truss or foam

counterparts:

� the ability to tailor the stiffness depending on the density of the polyurethane foam;

� an increase in stiffness of up to 120% over the aluminium PCM in the hybrid using

the 290 kg/m3 density polyurethane foam;

� an increase in strength between 9% and 372% over the aluminium PCM in the

hybrids using the 83 kg/m3 density polyurethane foam and the 290 kg/m3 density

polyurethane foam, respectively;

85

Chapter 5 Conclusions and Future Work 86

� an increase in resilience between 26% and 740% over the aluminium PCM in the

hybrids using the 83 kg/m3 density polyurethane foam and the 232 kg/m3 density

polyurethane foam, respectively;

� an increase in impact energy between 22% and 106% over the aluminium PCM in

the hybrids using the 113 kg/m3 density polyurethane foam and the 290 kg/m3 den-

sity polyurethane foam, respectively;

� a greater than sums effect where the impact energy of the hybrids made with the

113, 232 and 290 kg/m3 PU foams is greater than the sum of the impact energy of

the PCM and the PU foam combined; and

� an increase in mechanical properties with an increase in foam density.

In the second set of hybrid materials the reverse effect was examined by looking at

a foamed polymer strut surrounded by metal. The ABS cores for these hybrid mate-

rials were manufactured using a rapid prototyping technique. They were then foamed

and electroplated with nanocrystalline nickel. The hybrid materials have greater overall

strength, stiffness and energy absorption over the ABS trusses. More importantly, the

slight increase in density gained by plating the ABS trusses was insignificant compared

to the increase in stiffness, strength and energy absorption. Furthermore, the value of

foaming the ABS truss cores could also be seen by examining the normalized ratios.

These hybrid materials also offered a number of advantages over their ABS micro-truss

counterparts:

� an increase in stiffness between 315% to 1525%;

� an increase in strength between 315% to 1165%;

� an increase in energy absorption between 241% to 650%; and

� the increases in mechanical properties was greater than the increase in density due

to plating.


This preliminary study of these novel hybrid materials has resulted in a new set of

materials each with a unique set of properties. Both sets of hybrids saw an increase in

strength and resilience. Furthermore, in the first set of hybrids, the stiffness of the final

hybrid material can be tailored by adjusting the density of the polyurethane foam used

in the manufacture of the hybrid. Increased impact resistance could be obtained with

increasing foam density.

The value of adding nanocrystalline nickel, and foaming the ABS trusses could also be

seen in terms of increased strength, stiffness and energy absorption. In this set of hybrid

materials, although there was a gain in density due to the addition of the nanocrystalline

nickel, it was outweighed by the gain in strength, stiffness and energy absorption.

These results provide a framework for further investigation into these hybrid materi-

als. There is still some question about delamination of the face sheets of the PCM/PU

foam hybrids. This could be controlled by spot welding the face sheets to the nodes.

Further testing in terms of three-point bending would also be valuable. For the second

group of hybrids, varying the thickness of the nanocrystalline nickel in the ABS/nanoNi

trusses would help to identify if there is an ideal coating thickness to strut thickness

ratio. Further investigation into various foamed densities would also help to identify

if there is an ideal foam density for these hybrid materials. Due to geometrical con-

straints of the foaming and testing equipment used, impact testing was not performed

on the ABS/nanoNi trusses. By creating a larger sample, with more cells, an impact test

could be performed and would provide valuable insight to the impact resistance of these

materials.

In attempting to find new low density, high strength materials for the aerospace,

automotive and consumer goods industries, hybrid materials have been developed that

offer substantial gains in strength, stiffness, resilience, energy absorption and impact re-

sistance. Furthermore, these new hybrid materials offer multi-functionality in terms of

impact resistance and the ability to tailor the stiffness of the material. Figure 5.1 is a

materials selection chart that shows the hybrid materials developed in this thesis. As


0

10

20

30

40

50

60

0 200 400 600 800 1000 1200

Density (kg/m3)

Strength (MPa)

Rapid Prototyped ABS

Menges

strengthMengesABS

0

5

10

15

20

25

0 0.2 0.4 0.6 0.8 1 1.2

Density (Mg/m3)

Strength (MPa)

ABS

ABS/nano-Ni Hybrids

PU

PCM/PU Hybrids

PCM

c5_strengthmsc

c4_deltastrength Figure 5.1: Materials selection chart with PCM/PU foam and ABS/nanoNi hybrid ma-terials.

demonstrated previously in Figure 4.26, these new hybrids fill a hole in the materials

selection charts of current research trends for micro-truss hybrid materials. With con-

tinued research into the properties of these novel hybrid materials, it is expected that

these materials will quickly become valuable for use as the cores of sandwich structures

in structural applications in the aerospace, automotive and consumer goods industries.

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