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Article

Strength and Performance Enhancement of Bonded Jointsby Spatial Tailoring of Adhesive Compliance via 3D Printing

Shanmugam Kumar, Brian L. Wardle, and Muhamad F. ArifACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13038 • Publication Date (Web): 14 Dec 2016

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1

Strength and Performance Enhancement of Bonded Joints by

Spatial Tailoring of Adhesive Compliance via 3D Printing

S. Kumar *,†, Brian L. Wardle ‡, Muhamad F. Arif†

†Institute Center for Energy (iEnergy), Department of Mechanical and Materials

Engineering, Masdar Institute of Science and Technology, Abu Dhabi 54224, UAE

‡Department of Aeronautics and Astronautics, Massachusetts Institute of

Technology, Cambridge, MA 02139, USA

* Corresponding author. Fax: +971 2 8109901.

Email address: [email protected], [email protected] (S. Kumar)

Abstract

Adhesive bonding continues to emerge as a preferred route for joining materials with

broad applications including advanced structures, microelectronics, biomedical

systems, and consumer goods. Here, we study the mechanics of deformation and

failure of tensile-loaded single-lap joints with a compliance-tailored adhesive.

Tailoring of the adhesive compliance redistributes stresses and strains to reduce

both shear and peel concentrations at the ends of the adhesive that determine failure

of the joint. Utilizing 3D printing, the modulus of the adhesive is spatially varied along

the bondlength. Experimental strength testing, including optical strain mapping,

reveals that the strain redistribution results in a greater than 100% increase in

strength and toughness concomitant with a 50% increase in strain-to-break, while

maintaining joint stiffness. The tailoring demonstrated here is immediately realizable

in a broad array of 3D printing applications, and the level of performance

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enhancement suggests that compliance tailoring of the adhesive is a generalizable

route for achieving superior performance of joints in other applications, such as

advanced structural composites.

Keywords: Adhesive joints, 3D printing, Interface compliance/stiffness

tailoring, Graded materials, Polymer composites

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1. Introduction

Adhesive bonding is becoming a preferred means of joining materials and

structures in the pursuit of realizing cost-effective, lightweight and efficient

structural and/or functional systems. However, steep shear and peel

stress/strain gradients and concentrations exist at the ends of the overlap

causing concern in many applications such as bonded advanced composites,

to the point where mechanical fasteners are utilized to supplement the

adhesively bonded joint, adding cost, weight, and complexity1. The most

commonly occurring configuration to join dissimilar or similar adherends is the

single-lap joint (SLJ) due to its ease of manufacture and efficiency of load

transfer2,3, although much work has also addressed double lap joints4. In the

SLJ, stress/strain shear concentrations in the adhesive layer originate from

unequal axial straining of the adherends, and in the case of peel

stresses/strains, because of the eccentric load path, concomitantly dictating

the failure strength of these joints5–7. Classical stress analysis of SLJ by

Volkersen8 comprised of two identical elastic adherends of uniform thickness

)(t , and Young’s moduli )(E bonded over a length )(l by an adhesive of

thickness )(h with shear modulus )(G , finds that when bonded members are

in pure tension, the shear stress/strain in the adhesive layer is a maximum at

each end of the overlap governed by a concentration factor

=

Eht

Gl

Eht

Glk

22

coth . Volkersen’s analysis suggests that for a given

adherend material and geometric configuration, the lower the shear modulus of

the adhesive used in the bondline, the lower the stress concentration, leading

to higher joint strength. Therefore, in such a configuration (see Fig. 1a), an

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adhesive that is very compliant relative to the adherends is commonly utilized

to achieve a shear-dominated load transfer across the joint and to minimize the

propensity of interfacial and/or cohesive brittle failure due to high peel stresses

a relatively stiffer adhesive may experience9. While relatively compliant

adhesives are attractive, a compromise must be maintained in that joint

stiffness has the opposite trend. In many applications, such as advanced

structural composites, adhesive resistance to peel stresses is of paramount

concern.

Numerous ways of reducing the shear and peel stress concentrations such as

modifying the adherend geometry (see, e.g., 10,11), the adhesive geometry (see,

e.g.,12) and employing a spew (a bead at the lap ends of the adhesive)

geometry (see, e.g.,13,14) have been tried to minimize such stress/strain

concentrations to improve the structural response of joints. Research on joints

with material-tailored adhesives was pioneered by Hart-Smith4 and Srinivas15,

towards increasing the structural performance by redistributing the adhesive

stresses. Recently, there is growing interest in this area, e.g., it was

experimentally shown that the joint strength can be increased about 50-120%

by employing a bi-adhesive bondline.16 The extant work has as of yet

considered only a single-step variation in adhesive over the bondlength, with

the step-change in both the modulus as well as adhesion energy of the

adhesive. However, several recent theoretical studies have considered

compliance-tailoring of the adhesive along the bondlength to provide a

framework for the mechanics of such bonded systems (see e.g.,17–21). However,

manufacturing of bonded systems with spatially varying elastic properties of the

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adhesive along the bondlength is challenging, and therefore, the potential of

such compliance-tailored adhesive designs have not been explored.

3D printing is a facile approach to realize compliance tailoring of interfaces in

multi-material systems so as to produce stronger, tougher and more efficient

aerospace/automotive structures22, wearable soft-hard prosthetics23, energy

dissipating structures24, gripping mechanisms25, robotic arms26, multilayer

dental27 and orthopedic implants28,29, among others. Here, we focus on 3D

printing of SLJs with compliance-tailored adhesive layers. As shown in Fig. 1a,

a SLJ with far-field tensile stress ∞σ contains an eccentric load-path, and both

shear and peel stresses/strains are generated in the adhesive, including steep

gradients and relatively high absolute values at the ends of the adhesive. The

well-known (e.g.,9) typical distribution of adhesive stresses over the bondlength

in a SLJ with a constant modulus adhesive is shown in Fig. 1a bottom.

Cohesive and/or adhesive failure originates at the stress/strain concentration

zones at the adhesive ends leading to catastrophic failure of the joint.

Increasing compliance of the adhesive requires a larger length of the bondline

to participate in shear stress transfer and has the positive effect of reducing the

concentrations at the ends. However, as mentioned earlier, the bondline must

be stiff and therefore quick (as measured along the bondlength) load transfer

via shear is desired. To this end, we explore two different ways of spatially

tailoring the elastic modulus of the adhesive over the bondlength, with moduli

profiles shown by the green and orange lines in Fig. 1a center. The compliance

tailoring is expected to provide strain tolerance to the joint, reducing

stress/strain concentration at the ends, while maintaining load transfer near the

center of the joint. The results discussed in Section 3 reveal that SLJs with

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compliance-enhanced adhesive ends have a significant increase broadly in

performance, including load-bearing capacity.

2. Materials and Methods

2. 1 3D Printing

We utilize 3D printing of photopolymers technology that offers the possibility of

multi-material deposition26,30,31 to print SLJs. Heterogeneous composites that

combine stiff reinforcing elements with a soft organic matrix have been

successfully exploited for decades in the form of glass or carbon fiber

reinforced polymers32,33. Compared to this established technology, multi-

material printing offers the possibility of cost-effective automation of the

fabrication process and provides greater flexibility to locally design the material

architecture in three dimensions22,34–44. The emergence of such multi-material

and composites 3D printing technologies facilitates the design of functionally

graded designs at sub-millimeter scale enabling fabrication of multi-material

structures simultaneously exhibiting both strength and toughness which are

often mutually exclusive and thus difficult to reach in homogeneous

materials45,46.

CAD models of the joints were created using Solidworks (Dassault Systemes,

France) and fabricated using an Object Connex260 Polyjet 3D multi-material

printer (Statasys Ltd., USA). Object Connex family 3D printers can print two

different polymers simultaneously and could combine these two basic polymers

in different proportions to produce a range of materials called digital materials.

This printer deposits the polymeric material via an array of 8 print heads that

sprays the liquid polymer, subsequently the polymer is cured by UV light,

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before the next layer is added. The nozzles are mounted on the print head

which moves in the x and y directions. The print head only jets the liquid

polymer when it is moving across the tray in the x direction. Therefore, the

printing direction could influence the material behavior of printed specimens47,

although this is avoided in the comparisons undertaken here. A support

material (Objet Support SUP705) was used during printing to enable fabrication

of overhang portions of geometries and was subsequently removed through

water washing. The resolution of the 3D multi-material printer is 16 µm in the z

direction (thickness) and 42 µm in the x and y directions. Therefore, the

smallest geometric feature of the joints was designed to be at least an order of

magnitude greater in size than the resolution of the 3D printer to minimize any

effects of the 3D printing process on the mechanical response48.

2. 2 Adhesive Tailoring

The compliance tailoring approach adopted here takes advantage of the 3D

printing capabilities to control the relative volume fraction of polymer 26,30,49 so

as to modify the compliance of the adhesive while maintaining the same

material interfaces between the adherends and the adhesive. The adherends

were 3D printed using a rigid polymer VeroWhitePlusTM RGD835 (VW) and it’s

Young’s modulus ( aE ) is 2250 MPa. The adhesive was printed using both

TangoPlusTM FLX930 (TP), and a digital material S40 having Young’s moduli

1E =0.536 MPa and 2E =1.098 MPa, respectively.47,48 See discussion and Fig.

S1 for properties of the different polymers, principally noting that

21 5.0 EEEa ≅>> . The different configurations tested are shown in Fig. 1b,

and include both stiffness (in the center) and compliance (at the ends) addition

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to the base adhesive by printing widthwise-constant circular features into the

adhesive in a staggered pattern. Preliminary testing looking at different

distributions and shapes of features (see Figs. S3 and S4 and associated

discussion) to impart compliance change were investigated leading to the

geometries and materials used herein. A refined version of preliminary Design

1 shown in Fig. S3a with =2l 22.14mm, =L 50mm and number of circular

features =in 20 were considered for center stiffness tailored adhesive (see Fig.

S5a) with local volume fraction of circular features in the central region of 15%,

giving a local modulus change of +16% versus the 1E adhesive. Similarly, an

improved version of preliminary designs shown in Fig. S4a with == 31 ll

9.53mm, =L 50mm and number of soft circular features =in 7 at each end

zone were considered for the edge compliance tailored adhesive (see Fig S5b)

with local volume fraction of circular features in the end zone being 15%, giving

a local modulus change of -7% versus the stiffer 2E adhesive. In addition,

adhesives that have these features along the entire bondlength for both cases

were considered. The 3D printed SLJs were tested on a Zwick-Roell tensile

testing machine with a 2.5kN load cell. The load was applied at a constant

speed of 5 mm/minute. Digital image correlation (DIC) was used to map the

two-dimensional state of deformation and calculate strain in the joint as a

function of load. Random speckle patterns were formed on the specimen’s

surface prior to testing by applying a thin coat of white acrylic paint followed by

spraying random dots of black acrylic paint using an airbrush. A monochrome

5.0 MP camera was used to capture the speckle images during the test. Vic-2D

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software was used to compute the evolution of the strain field as a function of

load. Each of the SLJ designs was tested three times to ensure repeatability.

Figure 1. Spatially-tailored adhesive compliance in single-lap joint (SLJ): (a)

Conceptual representation of SLJ subjected to a far-field tensile stress ∞σ with

different adhesive modulus profiles. Bottom shows stress distributions in the adhesive

for a constant modulus adhesive (either 1E or 2E ). (b) Optical images of 3D printed

joints with adhesive compliance tailoring considering all four profiles in a), including

center stiffness- and edge compliance-tailored cases. The thickness of the adhesive in

all cases is 7mm. Note that 21 5.0 EE ≅ .

Figure 2. Performance of tailored- and constant-modulus adhesive joints: (a) Load-

displacement curves using compliant adhesive with modulus 1E , and (b) with modulus

2E . Note that 21 5.0 EE ≅ .

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3. Results and Discussion

Table 1. Summary performance of 3D printed tailored and non-tailored joints.

Changes (shown in red) are calculated vs. the constant modulus adhesive in each

case, and only statistically significant increases are shown.

Design Configuration Joint stiffness

(N/mm)

Maximum

load (N)

Deflection at break

(mm)

Toughness

(×103 N*mm)

�� adhesive 26.8 ± 0.493 231 ± 9.39 9.70 ± 0.163 1.24 ± 0.07

�� tailored with center stiffness 29.1 ± 0.295

(+8.58 %)

443 ± 7.87

(+91.8 %)

14.6 ± 0.142

(+50.5 %)

3.34 ± 0.076

(+169 %)

�� non-tailored (stiffness added) 31.6 ± 0.03

(+17.9 %)

263 ± 6.59

(+13.9 %)

9.33 ± 0.226

1.36±0.067

(+9.68 %)

�� adhesive (��~2 ∗ ��) 44.9 ± 0.375 403 ± 3.33 9.21 ± 0.07 1.95 ± 0.03

�� tailored with edge compliance 47.2 ± 0.316

(+5.12 %)

848 ± 24.3

(+110 %)

13.7 ± 0.281

(+48.8 %)

5.24 ± 0.213

(+169 %)

�� non-tailored (compliance added) 45.4 ± 0.19

686 ± 10.9

(+70.2 %)

12.7 ± 0.130

(+37.9 %)

4.10 ± 0.098

(+110 %)

Representative load-displacement responses are shown in Fig. 2 for the four

SLJ configurations in Fig. 1b. As expected, the relatively stiffer and stronger

adhesive imparts a stiffer and also stronger (~2×) SLJ response than the more

compliant adhesive, given the geometric parameters considered. It can also be

seen that both tailoring approaches have merit relative to the constant-modulus

adhesives, where joint stiffness is maintained and strength is enhanced (see

summary data in Table 1). The compliance tailored ends of the adhesive in Fig.

2b is shown to be the highest performing across all relevant metrics, achieving

a large strength (110% increase relative to the homogeneous adhesive 2E and

even greater compared to homogeneous adhesive 1E ), and toughness

increase while maintaining joint stiffness (see Table 1). Importantly, all

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configurations tested contain the same adhesive-adherend interfaces – unlike

prior work that varies both the modulus and the energy of adhesion of the

adhesive and the adherend by tailoring the adhesive in a step-wise fashion.

Thus, in the work herein, we can conclude that the increased performance is

(solely) attributable to stress/strain redistribution due to spatially tailoring

compliance. This is explored further in strain imaging for the specimens tested.

It should be noted that the case of non-spatially tailored features (where

stiffness or compliance is added over the entire bondlength) is also presented

in Table 1, with discussion to follow.

The improved joint performance in Fig. 2 and Table 1 is due to the

redistribution of stress and strain in the adhesive as a direct result of the

compliance tailoring along the bondlength. At loads where the response

remains linear elastic, the strain re-distribution is evident in Figs. 3 and 4 for the

various modulus profiles. Note that in Fig. 3, the 2E circular features add

stiffness to the 1E adhesive, whereas in Fig. 4, the 1E features add compliance

to the 2E adhesive. In Fig. 3, the peel and shear strain distributions are shown

at a load of 50 N (see Fig. 2a) and the trend is consistent with expectations: (1)

the compliant adhesive (relative to the adherend) in the top row transfers the

load in shear with the development of strain concentrations at the free edges of

the adhesive; (2) adding stiffness across the entire bondlength (see bottom row

of Fig. 3 where 2E features are added) maintains the same distributions but

reduces the magnitude. This is also similar to the 2E adhesive case in the top

row of Fig. 4, as expected although the magnitudes are again reduced. The

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distributions in Figs. 3 and 4 are noted to agree with the expected behavior of

compliant adhesive SLJs.

Even at the relatively low load and strain levels in Figs. 3 & 4, the effect of the

features (center stiffness added, and edge compliance added, respectively) are

noted to redistribute strains in the adhesive layer, and reduce magnitudes of

both at the critical end zones of the adhesive. The compliance tailoring acts to

reduce strains overall, both shear and peel, particularly in the critical high-strain

regions at the adhesive ends (compare the top and middle rows in Fig. 3), with

similar behavior in Fig. 4. The influence of spatial tailoring is revealed in higher

values of loading and strain as shown in the strain maps in Figs. 5 (at 130 N

load) and 6 (at 237 N load) and the failure images in Figs. 7 & 8, for the center

stiffness and edge compliance- tailored cases, respectively. Here, the

redistribution of strains can be seen in both tailored cases (center rows in Figs.

5 & 6), in particular reducing strain concentrations (both shear and peel) near

the adhesive edges. It is worth noting that the strain maps for the (unfailed)

compliance tailored adhesives shown in Figs 5 and 6 (middle rows) correspond

to a load level at which failure initiates in constant moduli adhesives (top rows

of Figs 5 and 6).

Joint failure (focusing on loss of load-carrying capability, as in ultimate strength)

can be observed in the strain map shown in Fig. 6 for the constant modulus E2

joint where a crack forms near the adhesive-adherend interface. Indeed, in all

cases failure is observed as the initiation and growth of a crack in the region of

strain/stress concentration at the adhesive ends. From Figs 7, 8, S7 and videos

of their failure (see Supporting Information), we observe that in all cases

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macroscopic failure near the peak load emanates as a crack close to the corner

either at the adhesive-adherend interface (adhesive failure) or within the

adhesive (cohesive failure) very close to the bondline. The homogeneous

adhesive 1E and 2E cases are exemplary, in that the emanated failure

propagates very quickly as adhesive failure over the entire bondlength. The

purpose of our study is not to consider or engineer tougher behavior, but rather

to highlight the effects of spatial compliance tailoring of the adhesive to reduce

critical strains (shear and peel) driving failure at the adhesive ends. Noting that

toughness is influenced by the relative strengths and toughness of the 1E and

2E materials, as well as by compositing effects of the features and new failure

modes such as feature debonding (all of which are uncontrolled in the current

study), we instead focus on strength with toughness reported in Table 1 simply

for completeness. In all cases, the peak load is associated with a crack

appearing and propagating at the peak load, such that comparing relative

strengths of the joints due to compliance tailoring can be made.

Figure 3. Strain field distribution in 3D printed SLJ with/without stiff center features

under tensile load by DIC at 50 N (see Fig 2a): a), schematic of the adhesive, b) shear

strain, and c) peel strain in the joint, respectively. 1E is the modulus of the adhesive

and 2E is the modulus of the stiffer circular features.

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Figure 4. Strain field distribution in 3D printed SLJ with/without compliant edge features

under tensile load by DIC at 200 N (see Fig 2b): a), schematic of the adhesive, b) shear


and 1E is the modulus of the soft circular features.

Figure 5. Strain field distribution in 3D printed SLJ with/without center stiffness features

under tensile load by DIC at 130 N (see Fig 2a): a), schematic of the adhesive, b) shear


and 2E is the modulus of the stiffer circular features.

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Figure 6. Strain field distribution in 3D printed SLJ with/without edge compliance

features under tensile load by DIC at 247 N (see Fig 2b): a), schematic of the adhesive,

b) shear strain, and c) peel strain in the joint, respectively. 2E is the modulus of the

adhesive and 1E is the modulus of the soft circular features.

Figure 7. Deformations near failure in 3D printed SLJ with/without center stiffness

features under tensile load: a), representative load-deflection response, and specimens

under loading at b) point A, c) point B, and d) point C in Fig. 7a.

Figure 8. Deformations near failure in 3D printed SLJ with/without edge compliance

features under tensile load: a), representative load-deflection response, and specimens

under loading at b) point A, c) point B, and d) point C in Fig. 8a.

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4. Conclusions

In this study, compliance-tailored adhesive designs are explored via 3D printing

to illustrate the potential of such an approach towards generating superior

multi-material adhesive interfaces. The dependence of (single-lap) joint

performance, focusing on strength while maintaining stiffness, was found

through experiments including optical strain mapping. While retaining joint

stiffness, more than a 100% increase in strength and 150% increase in

toughness were observed for both center stiffness and edge compliance

tailored adhesive designs of the joints in comparison with their non-tailored

(stiffness/compliance added) and constant moduli counterparts due to

reduction in peak peel and shear strains at the ends of the adhesive. The

approach of compliance-tailoring demonstrated here is quite general and

immediately realizable and useful in both current (e.g., multi-polymer 3D

printers as used here) and emerging (polymer, metal, ceramic) multi-material

3D printers, e.g.,28,35,50–54 . In addition to joining in multi-material additive

manufacturing, the approach may be applied to joining separately additively

manufactured structures, and other joining problems such as advanced

composites. If the correct levels of spatial tailoring in the adhesives can be

achieved in structural composite joining, perhaps performance can approach

what some have called optimal design of such interfaces by bio-mimicking55–61.

Although not explored in the current study, tailoring of the adhesive can also

impart new mechanisms of toughness to joints that allow some load-carrying

capability beyond the critical load, by imparting the usual composite toughening

mechanisms, such as localized shear banding leading to increase in tortuosity

of the crack path11,62–70. The change in failure mode/mechanism and delay in

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growth of multi-site cracks due to tailoring may provide design opportunities to

increase both strength and toughness, properties that are often mutually

exclusive.

Acknowledgements

We gratefully acknowledge financial support from the ADNOC/GASCO under

Award No: EX2016-000010. Authors would like to thank Siddarth Tampi and

Edem Dugbenoo for their help on 3D printing of prototypes.

Supporting Information

Supporting information includes material and geometric properties of the 3D

printed joints, preliminary results for different cases of compliance tailored

adhesive joints, fracture images, and synchronized videos of load-

displacement curves and deformation maps of representative 3D printed SLJs

(SV1 and SV2).

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