Laminated Composite Stiffened Panels Application and Behaviour

A

SEMINAR REPORT

ON

“Laminated Composite Stiffened Panels Application and Behaviour”

Submitted in partial fulfillment of the requirements

for the award of degree

of

MASTER OF TECHNOLOGY

in

CIVIL ENGINEERING

(With specialization in Computer Aided Design)

By

HEMENDRA KUMAR JAIN

Under the Guidance of

Dr. AKHIL UPADHYAY

Associate Professor

Department of Civil Engineering

DEPARTMENT OF CIVIL ENGINEERING

INDIAN INSTITUTE OF TECHNOLOGY ROORKEE

ROORKEE 247667 (INDIA)

AUGUST-2009

i

ACKNOWLEDGEMENT

I wish to affirm my earnest acknowledgement and indebtedness to my supervisor

Dr. Akhil Upadhyay, Associate Professor, Department of Civil Engineering, IIT-Roorkee,

for his intuitive and meticulous guidance and perpetual inspiration in completion of this

Seminar work. In spite of his busy schedule, he rendered help whenever needed, giving

useful suggestions and holding informal discussions.

I would like to express my gratitude to my family, their blessings, motivation and

inspiration have always provided me a high mental support.

I am also grateful to my friends who are actively involved in providing me vital

support and encouragement whenever I needed.

Place: Roorkee (HEMENDRA KUMAR JAIN)

Date: 31 / 8 / 2009 Enrollment no. 08518004

M. Tech – II Year

(Computer Aided Design)

ii

CONTENTS

TITLE Page no.

Acknowledgement .... i

Contents .... ii

Certificate …. iv

Abstract …. v

1. INTRODUCTION ….1

1.3 Making a Composite ….2

1.4 Choosing the Manufacturing Process ….3

1.5 Advantages of Laminated Composites Compared to Conventional Materials

….5

2.1 Classification of Composite Material ….8

2.2 Laminated Fibrous Composite ….8

2.3 Laminae ….8

2.4 Laminate ….9

2.6 Laminate Type ….10

2.7 Stiffened Panels ….11

2.8 Types of Stiffeners ….12

2.9 Role of Stiffeners ….13

2.10 Laminated Composite Stiffened Panels ….13

1.2 Characteristics of Composite ….2

1.1 Composite Material ….1

2. DESCRIPTION OF LAMINATED COMPOSITE STIFFENED PANELS ….8

2.5 Laminate Code ….9

iii

3. APPLICATION OF LAMINATED COMPOSITE STIFFENED PANELS ….15

3.1 Application of Laminated Composite Stiffened Panels in Marine

Engineering ….15

3.2 Application of Laminated Composite Stiffened Panels in Aerospace

Engineering ….20

3.3 Application of Laminated Composite stiffened Panels in Civil Engineering

....22

3.3.1 Laminated Composite stiffened Panels in Bridges ….23

3.3.2 Multicellular Bridge Decks Presently in Use ….29

3.3.3 Laminated Composite Stiffened Panels in Building ….31

3.4 Other Applications of FRP Panels ….32

4 BEHAVIOUR OF LAMINATED COMPOSITE STIFFENED PANELS ….36

5 CONCLUSION ….38

REFERENCES ….39

iv

CANDIDATE’S DECLARATION

I hereby declared that the work is being presented in the seminar entitled “Laminated

Composite Stiffened Panels Application and Behaviour” in partial fulfillment of

the requirement for the award of the degree of Master of Technology in Civil Engineering

with specialization in Computer Aided design, submitted in the Department of Civil

Engineering, Indian Institute of Technology, Roorkee is an authentic record of my own work

carried out for a period of one month i.e., July 2009 to August 2009 under the supervision of

Dr. Akhil Upadhyay, Associate Professor, Department of Civil Engineering, Indian Institute

of Technology, Roorkee.

The matter embodied in this seminar report has not been submitted by me for the award

of any other degree or diploma.

Place: Roorkee

Date: 31 / 8 / 2009 (HEMENDRA KUMAR JAIN)

CERTIFICATE

This is to certify that the above statement made by the candidate is correct to the best of

my knowledge.

Dr. Akhil Upadhyay

Associate Professor

Department of Civil Engineering

Indian Institute of Technology

Roorkee -247667 (India)

v

ABSTRACT

Composite materials are formed by the combination of two or more materials that retain their

respective characteristics when combined together to achieve desired properties (physical,

chemical, etc.) that are superior to those of individual constituents. The main components of

composites are reinforcing agents and matrix. Composites have high strength-to-weight ratio

and high stiffness-to-weight ratio as compared to conventional materials. Composite are used

in many sectors like civil, aerospace, automobiles, marine, medical and power transmission

etc., but in this paper more emphasis is given on structural applications.

This seminar presents the literature review of application and behaviour of laminated

composite material in structural members. Being a thin walled structure, their behaviour is

governed by stability criteria. Here brief review of stability behaviour of laminated composite

stiffened panels is presented along with addition the knowledge are also identified into which

more comprehensive analysis is needed.

1

1. INTRODUCTION

The quest for a light weight structures is an ongoing task for engineering community.

However, Fibre reinforced polymers (FRP) do meet some of these requirements. The

development in FRP commenced only in 1940s. After the use of FRP in defence, aircrafts,

automobiles etc, it was employed in structural applications. FRP have many excellent

structural qualities and some examples are high strength, material toughness, fatigue

endurance and light weight. Other highly desirable qualities are high resistance to elevated

temperature, abrasion, corrosion and chemical attack.

1.1 Composite Material

Since the early dawn of civilization, the strong and light material has always fascinated

mankind for typical applications. The idea of composite materials are formed by the

combination of two or more materials that retain their respective characteristics when

combined together to achieve desired properties (physical, chemical, etc.) that are superior to

those of individual constituents. The main components of composites are reinforcing agents

and matrix. The fibers, particulates & whiskers act as the reinforcement and provide most of

the stiffness & strength. The matrix binds the reinforcement together thus effecting the load

transfer from matrix to reinforcement. Other substances such as fillers are used to reduce the

cost and improve processsability & dimensional stability.

Laminated composites are a special form of FRP which belongs to the new generation of

energy efficient materials, almost dominating over the metallic materials. The potential of

laminated composites offer several possibilities but on the other hand the mechanical

characterization of a composite structure is more complex than that of metal structures.

Composites exist in nature. A piece of wood is a composite, with long fibers of cellulose (a

very complex form of starch) held together by a much weaker substance called lignin.

Cellulose is also found in cotton and linen, but it is the binding power of the lignin that makes

a piece of timber much stronger than a bundle of cotton fibers.

Fig. 1.Fibers Fig. 2.Composite material

2

1.2 Characteristics of Composites

Composites have high strength-to-weight ratio and high stiffness-to-weight ratio as compared

to conventional materials. Composites have many excellent structural qualities and some

examples are high strength, material toughness and fatigue endurance. Other highly desirable

qualities are high resistance to elevated temperature, abrasion, corrosion and chemical attack.

The real advantage of the use of laminated composites lies in the fact that they provide

flexibility to tailor different properties of the structural elements to achieve strength and

stiffness requirements. The tailoring results in large mass savings. These advantages include

lightweight, resistance to environmental damage, long life, and automated fabrication, ease of

manufacturing, handling, erection and rapid installation. In fact, with the technological leaps

in recent times, focus has been on developing the materials required to perform in stringent

conditions - high temperature & pressure, highly corrosive environment, higher strength but

without much weight implications etc. which the conventional materials failed to service.

The mechanical properties of composites depend on many variables such as fiber types,

orientations and architecture. The fiber architecture refers to the preformed textile

configurations by braiding, knitting, or weaving. Composites are anisotropic materials with

their strength being different in any direction. Their stress-strain curves are linearly elastic to

the point of failure by rupture.

Fig. 3.Light weight composite panels Fig. 4.High strength of composite

1.3 Making a Composite

Most composites are made up of just two materials. One material (the matrix or binder)

surrounds and binds together a cluster of fibers or fragments of a much stronger material (the

reinforcement). In fiberglass, the reinforcement is provided by fine threads or fibers of glass,

often woven into a sort of cloth, and the matrix is a plastic. The threads of glass in fiberglass

3

are very strong under tension but they are also brittle and will snap if bent sharply. The

matrix not only holds the fibers together, it also protects them from damage by sharing any

stress among them. The matrix is soft enough to be shaped with tools, and can be softened by

suitable solvents to allow repairs to be made. Any deformation of a sheet of fiberglass

necessarily stretches some of the glass fibers, and they are able to resist this, so even a thin

sheet is very strong. It is also quite light, which is an advantage in many applications.

Fig. 5.Alignment of fibers

1.4 Choosing the Manufacturing Process

Making an object from a composite material usually involves some form of mould. The

reinforcing material is first placed in the mould and then semi-liquid matrix material is

sprayed or pumped in to form the object. Pressure may be applied to force out any air

bubbles, and the mould is then heated to make the matrix set solid. The molding process is

often done by hand, but automatic processing by machines is becoming more common. One

of the new methods is called pultrusion (a term derived from the words 'pull' and 'extrusion').

This process is ideal for manufacturing products that are straight and have a constant cross

section, such as bridge beams.

In many thin structures with complex shapes, such as curved panels, the composite structure

is built up by applying sheets of woven fiber reinforcement, saturated with the plastic matrix

material, over an appropriately shaped base mould. When the panel has been built to an

appropriate thickness, the matrix material is then cured. In many advanced composites (such

as those used in the wing and body panels of aircraft), the structure may consist of a

honeycomb of plastic sandwiched between two skins of carbon-fiber reinforced composite

material. Such sandwich composites combine high strength, and particularly bending

stiffness, with low weight. Like everything to do with aircraft, they can be very costly.

4

5

Fig. 6.Manufacture process

1.5 Advantages of Laminated Composites Compared to Conventional Materials

Laminated composite stiffened panels have the advantage of both features structural form and

material

A few significant advantages are:

Composites can provide structures that are 25-45 % lighter than the conventional

structures designed to meet the same functional requirements.

Tensile strength of composites is four to six times greater than that of steel or

aluminium.

Unidirectional fiber composites have specific modulus (ratio of material stiffness to

density) about 3 to 5 times greater than that of steel and aluminium.

Fatigue endurance limit of composites may approach 60% of their ultimate tensile

strength. For steel and aluminium, this value is considerably lower.

Corrosion resistance of fiber composites leads to reduced life cycle cost.

Unlimited capacity to be molded (fabric impregnated in situ) and perfect adaptability

to the original shape of the structural element that needs to be reinforced, stiffened or

integrated.

Simple application methods with absence of complex preparation.

Better durability, excellent damage tolerance, non invasive application, improved

torsional stiffness and impact properties.

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Composites are less noisy while in operation and provide lower vibration transmission

than metals.

Composites are more versatile than metals and can be tailored to meet performance

needs and complex design requirements.

Excellent fatigue, impact, environmental resistance and reduced maintenance.

Composites enjoy reduced life cycle cost compared to metals.

Composites exhibit excellent corrosion resistance and fire retardancy.

Improved appearance with smooth surfaces and readily incorporable integral

decorative melamine are other characteristics of composites.

Composite parts can eliminate joints/fasteners, providing part simplification and

integrated design compared to conventional metallic parts.

In addition to above, From an economic point of view, the main factors contributing to their

competitiveness with respect to conventional materials are time saving, flexibility, low labor

costs, low tooling and machinery costs on the construction site because of the light weight

and manageability of tools, possibility of restoring a structure without interrupting its

utilization by users and durability. In spite of many advantages of laminated composites over

traditional materials, complex mechanics involved in laminated fibrous composites poses

new challenges for the construction industry.

The downside of composites is usually the cost. Although manufacturing processes are often

more efficient when composites are used, the raw materials are expensive. At present

composites can not totally replace traditional materials like steel, but in many cases they are

just what we need. And no doubt new uses will be found as the technology evolves.

7

Fig. 7.Comparison of different properties of composite materials with traditional material

8

2 DESCRIPTION OF LAMINATED COMPOSITE STIFFENED PANELS

2.1 Classification of Composite Material

There are three commonly accepted types of composite materials

1) Fibrous composite: it consists of fibre and matrix

2) Laminate composite: it consists of layers of various materials

3) Particulate composite: it is composed of particle in matrix

2.2 Laminated Fibrous Composite

Laminated fibrous composite are a hybrid class of composite involving both fibrous

composite and lamination technique. A more common name is fiber-reinforced composites.

Here, layers of fiber-reinforced material are built-up with the fiber direction of each layer

typically oriented in different direction to give different strength and stiffnesses in the various

directions. Thus, the strength and stiffness of the laminated fiber-reinforced composite can be

tailored to the specific design requirement of the structural element being built. Some of the

basic terminology of laminated fiber-reinforced composite material is as discussed below.

2.3 Laminae

A lamina is a flat arrangement of unidirectional fibers or woven fibers in a matrix. Two

typical laminae are shown if fig.8 Along with their principal material direction. The fibers

are the principal load carrying agent. They are typically strong and stiff. The function of

matrix is to support and protect the fibers and provide a means of distributing load among and

transmitting load between fibers.

Fig. 8.Typical laminate configuration

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2.4 Laminate

A laminate is the stack of laminae with various orientation of the principle material direction

in the laminae. A typical laminate construction is shown in fig.8. The layers of the laminate

are usually bound together by the same matrix material that is used in laminae. Laminate can

be composed of different material or, in the present context, layers of fiber- reinforced

laminae. The main purpose of lamination is to tailor the directional dependence of strength

and stiffness of a material to match loading environment of the structural element.

The effect of proper bond between lamina can be seen by the considering an example of two

laminas each having width ‘b’ and thickness ‘t’ as shown in fig.9.

Case1: when no bond exists between two laminas

Section modulus Z1 = 2(b x t3/12)

Case2: when a bond between the laminas

Section modulus Z1 = b x (2 t3)/12 = 4Z1

Fig. 9.Effect of proper bond between laminas

2.5 Laminate Code

An x, y, z orthogonal coordinate system is used in analyzing laminates with the z coordinate

being perpendicular the plane of the laminate. The orientations of the unidirectional laminas

are specified by the angle θ (in degree) with respect to the x-axis. The angle θ is positive in

the counter clockwise direction. The number of lamina group is specified by a numerical

subscript. For example the laminate consisting of unidirectional plies as shown in fig.10 is

designated as [453/03/902/60]

This laminate contains four lamina group

1) Three plies in the 450 direction

2) Three plies in the 00 direction

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3) Two plies in the 900 direction

4) One plies in the 600 direction

Fig.10.Fibre orientation of laminas

2.6 Laminate Type

Symmetrical laminate

When the laminate is symmetrical with respect to the mid-plane it is referred to as a

symmetrical laminate. A symmetric laminate is represented as

[-452/04/-452] = [-452/02]s

[45/-452/452/-452/45] = [45/-452/45]s

The subscript s indicates symmetry about the mid-plane.

Anti-Symmetrical laminate

When the laminate is anti-symmetrical with respect to the mid-plane it is referred to as an

anti-symmetrical laminate. A anti-symmetric laminate is represented as

[-452/04/-452]

[45/-452/452/-452/45]

Cross-ply laminate

In cross-ply laminate fibers are only in the 0 and 90 degree direction. Cross-ply laminate may

be symmetrical or unsymmetrical. Since there is no distinction between the +0 and -0 and

between +90 and -90 degree directions, cross-ply laminate are balanced. A cross-ply laminate

is represented as

[0/90/90/0] or [0/90]s

[0/90/0/90]

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Angle-ply laminate

Angle-ply laminate consists of plies in the +θ and -θ directions. Angle-ply laminate may be

symmetrical or unsymmetrical. An angle-ply laminate is represented as

[45/-45/-45/45] = [45/-45]s and

[45/-45/45/-45]

2.7 Stiffened Panels

Stiffened panel consist of plate provided with stiffeners in the longitudinal and/or transverse

direction. These plates are extensively used in many industrial structures, aircrafts and ship

hulls etc. The stiffened elements representing a relatively small part of the total weight of the

structure substantially influence their strength, stiffness and stability. Fig 11 and 12 shows a

schematic representation of different type of stiffeners. Each geometry has its merit and

demerits over other types of stiffeners. Each type of stiffeners is used to serve an intended

purpose. These are broadly classified as open type close type or box type. Open type are

torsionally week while close type or box type are torsionally stiff. Blade stiffener, T stiffener,

J stiffener, I stiffener fall under the category of open type of stiffener and hat stiffener, U

stiffener, V stiffener, Y stiffener fall under the category of close or box type.

Some various shaped stiffening members commonly used for panel structural concepts are

“T”, “Z”, “I”, “C”, “J”, and hat. The stiffening member provides the benefit of added load-

carrying capability with a relatively small additional weight penalty. Most stiffened panel

designs provide high bending stiffness in only one direction. However, unidirectional

designed panels cost less and are easier to inspect and most applications do not require high

bending stiffness in both directions. One very common type of stiffener is the so-called blade

stiffener which actually is a plate perpendicularly attached to the laminated composite plate.

A typical arrangement of this class can be found in Fig. 13.

Fig. 11.Example for stiffeners cross section

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Fig. 12.Panel cross-section, dimensions, and support locations for repeating element of: (a, b)

metal J- and blade-stiffened panels; and (c, d) composite blade- and hat-stiffened panels.

Fig. 13.Laminated composite blade stiffened panel

2.8 Types of Stiffeners

There are three arrangements of stiffeners commonly used to reinforce plates.

1) The longitudinal stiffeners placed to the in-plane load to carry a portion of the applied

load.

2) The transverse stiffeners used merely to subdivide the plate into smaller panels, since

the portion of the load carried by them is relatively small.

3) A combination of longitudinal and transverse stiffeners resulting in an orthogonally

stiffened plate.

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2.9 Role of Stiffeners

1) In a stiffened plate subject to large deflection, lateral load is resisted by simultaneous

bending and member action. Sufficient weight reduction can be achieved by the use of

stiffened plates.

2) The stability of a plate depends on the thickness of the plate. It increases with increase

in the plate thickness. By using stiffened plates, stability can be increase keeping the

plate thickness as small as possible.

2.10 Laminated Composite Stiffened Panels

In all cases of buckling of plates, critical loads are proportional to the flexural rigidity of the

plates. The stiffeners are of very less weight but at the same time enhance the in-plane critical

load carrying capacity when compared with unstiffened panels. Stability of the plate

increases with the increase in the thickness of the plate but a more economical solution is

obtained by keeping the thickness of the plate as small as possible and increasing the stability

by introducing stiffeners. Being a thin walled structure the design of stiffened plates is

governed both by stability and strength criterion.

Stiffened panels are generic structural elements in weight sensitive structural, aerospace and

marine applications. These panels are becoming increasingly used in structural applications

because of their high specific stiffness (stiffness per unit weight) and specific strength

(strength per unit weight). The stiffened elements representing a relatively small part of the

total weight of the structure substantially influence their stiffness and stability. When these

stiffeners are arranged in a regular orthogonal grid, and the spacing is small enough to smear

the stiffeners to a continuum in the analysis, such a stiffened plate is called orthogonal

anisotropic plate or in short, an orthotropic plate. The lateral load is resisted by a stiffened

panel by simultaneous bending and membrane action. Sufficient weight reduction can be

achieved by the use of stiffened panels. Laminated composite stiffened panels, which are

anisotropic and orthotropic are gaining popularity in structural applications such as long span

bridge decks, ship deck hulls and superstructure of offshore oil platforms etc. The use of

laminated composite provides flexibility to tailor different properties of the structural

elements to achieve strength and stiffness requirements.

It is well known that stacking sequence optimizations are indispensable for laminated

composite structures. Stiffened composite panels usually have more than two stacking

sequences because they consist of a panel skin laminate and stiffener laminates. This means

that the stacking sequences need to be jointly optimized to achieve structural optimization of

the stiffened composite panel.

both the stiffeners and laminated composite material such weight reduction, better durability

and excellent damage tolerance. Because of the use of such panels in bridge, they can be

subjected to patch load in the form of vehicle and

patch load and their effects on stiffened panels are important aspect of research for efficient

use of composite in bridges. There is also lack of database for laminated composite

orthotropic panels subject to pa

Fig.14.

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the stiffened composite panel. Laminated composite stiffened panels have the advantage of



subjected to patch load in the form of vehicle and pedestrian traffic. Therefore, such type of



orthotropic panels subject to patch load.

14. Laminated composite stiffened panels



have the advantage of



pedestrian traffic. Therefore, such type of



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3 APPLICATION OF LAMINATED COMPOSITE STIFFENED PANELS

Laminated composite stiffened panels are widely employed in many engineering application.

Such structures are usually thin-walled and therefore susceptible to buckling. Stiffened panels

are generic structural elements in weight sensitive structural, civil, aerospace and marine

applications. These panels are becoming increasingly used in structural applications because

they have high specific stiffness and specific strength. A laminated composite stiffened panel,

which is anisotropic and orthotropic, are gaining popularity in structural applications such as

long span bridge decks, buildings, ship deck hulls, wing panels and superstructure of offshore

oil platforms etc.

3.1 Application of Laminated Composite Stiffened Panels in Marine Engineering

With composites exhibiting excellent resistance to the marine environment, their applications

have made good inroads in the marine sector worldwide. Complex configurations & the

advantages of seamless hulls were the main driving factors in the development of FRP boats.

Racing power boats employ advanced & hybrid composites for a higher performance craft

and driver safety. Major structural elements viz. deckhouses, hatch covers, kings posts & bow

modules appears to be very well suited for FRP construction. Deck and ship bottom

structures are modelled as assemblies of stiffened composite panels. In India, composite

applications in marine segment has made some beginning in the last decade in high speed

boats, naval vessels, sail boats, fishing boats, high capacity trawlers, barges & other ship

components.

The technological advances in materials and new understanding of how composite materials

behave under load have been crucial in the developments for the marine industry. It is

essential to fully understand how these composites behave, because only that we can build

stronger and lighter marine application. Advanced composites materials on vessels have a

potential to reduce fabrication & maintenance cost, enhance styling, reduce outfit weight and

increase reliability. The consumption of composites by this industry is mainly glass fibre

reinforced polyesters.

The performance of future navy ships requires novel and innovative material and structural

systems to meet ever-increasing design requirements. The application of composite materials

for the primary structure of shipbuilding surface combatants offers the potential to meet these

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performance goals in the areas of increased payload fraction reduced life cycle costs and

improved survivability.

Fig.15.Constitution of laminate

Fiber/matrix composite laminates in both military as well as industrial applications have

weight savings up to 70% compared to traditional metallic structures. These weight savings

can be used to maintain the necessary stability criteria as a ship accommodates additional

payload or weapons systems and increases in tonnage throughout its service life. The reduced

weight may also be used to increase ship speed or mission range. The layered configuration

of laminated structures allows opportunities to embed and integrate specialized materials into

the composite lay-up which provide improved electromagnetic performance.

A further challenge is that even as current composite applications become more accepted and

standardized in design and manufacturing, the underlying technology and threats continue to

move forward. Next generation composite ship structures will need to provide better

performance to meet more severe blast, ballistic, and electromagnetic requirements than ever

before. Achieving this in an affordable system will require improved resin systems, better

core materials and a total integration of technologies into a true multi-layered, multi-

functional structural system. Incorporation of these technologies and many others will forever

change the appearance, design, and performance of the navy’s future surface ships, as seen

projected in Figure 16.

17

e

Fig. 16.Evolution of the surface ships

A new composite perform framing technology shows promise in the reduction of fabrication

costs for large ship construction. Larger ships (over 150 ft in length), however, have been

constructed traditionally from steel and other conventional materials because higher cost of

composite panels compared to conventional materials. Therefore, new higher quality

materials with lower costs and new fabrication methods need to be developed before

composite materials will be fully accepted for the construction of large ships. The U.S.

shipbuilding industry recently has started incorporating composite materials in the

construction of both military and commercial ships due to the advantages of composite

construction. Composite construction has many advantages compared to steel construction.

Weight savings lead to larger cargo capacity, fuel savings, and increased stability of the ship.

Composite materials are corrosion resistant requiring less maintenance and can be tailored to

meet certain performance requirements allowing more design flexibility. Composite materials

provide improved stealth characteristics and better protection against shrapnel for military

applications.

One major disadvantage of composites is the higher construction costs compared to steel and

other traditional materials. This is partially due to the labor-intensive nature of composite

construction. Conventional hand lay-up, open mold fabrication is cost effective for small

boats; however, hand lay-up typically is not used for larger vessels due to the high

construction costs. One exception to this is minehunters. As in the case of minehunters,

composites have been fully embraced only for larger vessels with very specific functions

where wood construction has been preferred over steel construction. Before composites are

completely accepted by the shipbuilding industry for the large ship construction, new lower

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cost/higher quality materials need to be developed and current fabrication methods need to be

improved.

This technology replaces more labor-intensive, traditional framing methods. These traditional

methods involve forming trapezoidal foam blocks which are bonded onto the laminate, then

fabricating frame laminates which are placed onto the foam blocks. The new technology uses

preform frames which are manufactured by casting a dry GRP fabric into shape in a closed

mold with a two-part, self-rising urethane foam core. Radius edges to reduce stress

concentrations and tabbing material to provide primary bonding are built into the preform

frames. Preform frames can be used in resin infusion processes, but are typically installed in

an open mold. Hand lay-up installation is accomplished by first using resin to wet the bottom

of the frame tabs, then placing the frame onto the laminate panel, and finally applying resin to

the frame laminate and rolling out the tabs.

New framing technology has been used in the construction of recreational boats and yachts

producing significant costs savings (Walsh, 1996). It also shows promise in the reduction of

fabrication costs in the large composite ship construction for both military and commercial

applications. Before this preform frame technology can be accepted by the shipbuilding

industry, more technical information is needed. Sarah E Mouring (1998) focuses on different

fiber orientations for the frames and summarizes preliminary results of the testing of

composite stiffened panels with preform frames under in-plane uniaxial compressive loads.

Such loads are representative of the forces caused by wave bending moments which develop

from alternative hogging and sagging wave conditions. Biaxial (0, 90), quadaxial (0,

90,+45,−45), and triaxial (+45,−45,0) laminates were used in the frames.

Fig. 17.profile of AMT validation modal Fig. 18.Midships FEM of AMT validation modal

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Fig. 19.Lay-up configuration for advanced material transporter (AMT) validation model

[Nguyen, 93 Sml Boat]

Fig. 20.Laminated hat stiffened deck house panels test element [Scoot Bartlett, NSWC]

Fig.21. MV85 BIGLIANI Class 45-Knot Fast Patrol Craft From Crestitalia SpA, Italy [Jane’s

High-Speed Craft]

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Fig. 22.The mid-ship cross section

During the 1960s, the navy conduct a series of studies to consider the feasibility of using an

FRP hull for minesweepers. In 1969, Peterson builders, Inc. Of Sturgeon Bay, WI completed

a 34 foot midship test section.

Fig. 23.Ship made by composite material

3.2 Application of Laminated Composite Stiffened Panels in Aerospace Engineering

The Air Force Research Laboratory Space Vehicles Directorate (AFRL/VS) USA is

exploring new structural configurations and corresponding methods for fabricating launch

vehicle fairings. The goal of this research is to reduce the cost of these components while also

enabling large structures to be fabricated. This Process for fabricating Advanced Grid

Stiffened panel (Fig.24) composite structures have been developed that show promise to help

achieve these goals. These procedures were successfully demonstrated in an Advanced Grid

Stiffened panel fairing for the Ballistic Missile Defense Organization’s (BMDO) Combined

Experiments Program. One of the most common structural element in aerospace structures

stiffened panels. The aerospace industry began to use composites in pressure vessels,

21

containers, and non -structural aircraft components. Composite panels are used some stressed

panels in aircraft.

Fig. 24.Advanced Grid Stiffened Structure Configuration

A structurally efficient hat-stiffened panel concept that utilizes structural foam as a stiffener

core material has been designed and developed for aircraft primary structural applications.

This hat-stiffener concept is structurally more efficient than most other prismatically stiffened

panel configurations in a load range that is typical for both fuselage and wing structures. The

panel design is based on woven/stitched and braided graphite-fiber textile preforms, an epoxy

resin system, and Rohacell foam core. The structural response of this panel design was

evaluated for its buckling behavior with and without low-speed impact damage. The results

from single-stiffener and multi-stiffener specimen tests suggest that this structural concept

responds to loading as anticipated and has excellent damage tolerance characteristics

compared to a similar panel design made from preimpregnated graphite-epoxy tape material.

A fire retardant, low smoke-emitting, thermally stable, light weight sandwich paneling,

suitable for high traffic flooring on a passenger aircraft, was evaluated. The material is of

sandwich panel construction, with graphite face sheets (a phenolic-vinyl resin) and

honeycomb core.

York et al. (1993) are design the some laminated composite stiffened panels for aircraft wing

panels. They are presented for the most heavily and lightly loaded of eight benchmark

stiffened laminated wing panels defined from a Dornier wing by a Group for Aeronautical

Research and Technology in Europe working party. These benchmark panels had three

identical and equally spaced blade stiffeners. Fig.25.shows an isometric view of the

22

components of the Dornier wing and also shows a plan view of its top skin and stacking

sequence.

Fig.25. Isometric view and a plan view of the Dornier benchmark problem showing with

staking sequence (a) Isometric view of the components of the Dornier wing (b) is a plan view

of the top skin (c) Staking sequence of benchmark panels

Fig.26.The entire fuselage of the boeing’s 787 Dreomliner jet built with carbon fiber

3.3 Application of Laminated Composite Stiffened Panels in Civil Engineering

Laminated composite stiffened panels have long been used in the construction industry.

Applications range from non-structural gratings and claddings to full structural systems for

industrial supports, buildings, long span roof structures, tanks, bridge components and

complete bridge systems.

Composite panel’s benefits of corrosion resistance and low weight have proven attractive in

many low stress applications. Laminated composites present immense opportunities to play

23

increasing role as an alternate material to replace timber, steel, aluminium and concrete in

buildings.

3.3.1 Laminated Composite stiffened Panels in Bridges

The existing inventories of deployable bridges in the Indian Armed Forces are quite large and

do generally meet the user aspirations. However, India being a vast country and having sub

continental weather and terrain conditions, there is a huge scope for light weight, modular

FRP composite panels deployable bridges or components like FRP bridge decks for specific

areas like glaciated areas, states like Himachal Pradesh, Uttrakhand, Jammu and Kashmir,

North Eastern states where landslides, avalanches, etc do occur at regular intervals and all

flood affected states. They can be located at important places so that they can be rushed to the

affected areas at a very short notice. Besides, India has a vast number of road bridges which

do need repairs and replacement. FRP composites panels as deployable bridges can ensure a

smooth transition from old to new.

Civil engineers are beginning to gain confidence and experience in applying this technology

to civil structures. In October 1996, the Federal Highway Administration organized and

sponsored a scanning tour on advanced composites in bridges to three selected European

countries and Japan. The purpose of the tour was to assess the state-of-the-art in the use of

composites stiffened panels in bridge construction. The technical findings from the scanning

team are basically categorized into new bridge construction, and strengthening of existing

bridges. The scanning report states that the U.S. composite materials bridge technology has

developed concurrently with those of the world and is neither behind nor significantly ahead

of the countries visited.

With the ageing of (more than 50 years) many bridges in the US, alternative options are being

explored to retrofit the bridges or replace the old decks with lighter decks which meet the

service criteria of the existing decks and even replace the bridges itself. Considering, the

sheer number (almost 23% 0f 600,000 bridges) of bridges which are structurally deficient and

functionally obsolete (Hwang-Chu Wu et al. 2006 and Halvard E. Nystrom et al. 2003).

Structurally deficient are those which are closed or restricted to light vehicles only and

functionally obsolete are those which are cannot safely service the volume or type of traffic

using them. According to a data base recently compiled by the Composites Institute, there are

more than 80 bridge projects worldwide using FRP composites panels. The U.S. has a modest

24

beginning with 30 projects, 26 of which were built within the last 4 years. The remaining

discussion in this will focus on some of the initial successful bridge applications in the U.S.

using FRP composites panels. The discussion will also include the advantages,

characteristics, concerns, and future needs to advance the composite technology into the civil

infrastructure. In new bridge construction, the FRP composite panels may be used in the

entire structure, or they could be used as structural members or components (Fig. 27).

Fig. 27.FRP Bridge Decks: Panels being placed into position

The first civil application in composites was a dome structure built in Benghazi in 1968, and

other structures followed slowly. At the beginning of new millennium, deterioration of

concrete structures has become an important issue in the civil engineering community. Nearly

15% of the 600,000 bridges in USA suffer from loss of material properties due to

environmental degradation and age, and additional 14% are experiencing more traffic than

originally intended (FHWA/USDOT 2005). The annual direct cost of corrosion for highway

bridges is $8.3 billion and life-cycle analysis estimates indirect costs (due to traffic delays

and lost productivity) to be 10 times the direct cost (FHWA). There is an urgent need for

cost-effective and durable technologies for repair, or retrofit the aging structures to meet the

increased traffic demands. In the last 20 years, a construction component manufacturing

industry has emerged with several commercial companies offering FRP composite deck

systems as drop-in replacements for steel or concrete deck systems. Fig.28 shows the FRP

composite panels bridges.

25

Fig. 28.Some of the FRP Composite panels bridges

Cross section (typical) of Friedberg bridge (Knippers Helbig Consulting Engineers,

Stuttgart)

Composite bridge in Bath Twp., near Fairborn in Greene County, US. Installation of deck panels took less than three hours.

World's Largest Carbon Composite Bridge Span at JEC-Fair in Paris (24.5 X5 M2 : 12

MT)

Composite bridge in Dronten, Germany(Cycle & pedestrian bridge 10.5 m long)

An artists redition of futuristic composite bridge

All-composite footbridge in Kolding, Denmark. Erected in 1997

26

The main cause is deterioration of the materials used in construction and repair of these

facilities. A key factor behind bridge deterioration is the corrosion of reinforcing steel in

concrete decks, exacerbated by road salt used to combat winter ice and snow. FRP panels are

expected to be much more durable than ordinary concrete and steel. A rapid growth of

interest in FRP composites panels for bridge construction has resulted in a multitude of recent

research and development initiatives and demonstration projects. The high strength, high

fatigue resistance, low density, and excellent corrosion resistance of composites are desirable

characteristics for bridge application, especially for decks. Bridge decks are subjected to

severe environmental conditions and heavy traffic loads. They sometimes account for a major

percentage of a bridge structure’s dead load. An FRP deck weighs approximately 80% less

than a concrete deck. Reducing the dead load will increase the allowable live load capacity of

the bridge without significant repair to the existing superstructure, thus lengthening its

service life. Hence, lightweight and durable FRPs can be an excellent candidate for replacing

concrete decks.

Composite bridge decks are typically made of glass fiber and a polyester resin or vinyl ester

resin matrix. Each of these deck systems is factory assembled into deck panels that are sized

appropriately for shipping to the work sites. The panels are then erected and bonded together

at the site using high performance adhesives, mechanical connections, or both. A great deal

of work has been carried out in developing FRP bridge decks. It is estimated that over $30

million has been spent by the US government in developing composites for infrastructure

usage. Over half of that has been spent on bridges. In analyzing the structural behaviour of

many of these decks, the FRP deck is isolated from its support system. The bridges’ super- or

substructures are typically not included in the analysis. There are some concerns that such a

procedure may lead to underestimated deck deflections, especially for flexible superstructure

truss bridges. Currently there are no proven analysis procedures or design standards available

for FRP bridge decks. Thus, FRP deck behaviour under load and long term durability is not

well understood by civil engineers. An as-installed field evaluation through load testing and

further analytical investigations is considered essential to ensure the safe and cost-effective

use of FRP decks (Bin Mu et al. 2006).

Literature review indicates that extensive research is being carried out both in laboratory as

well as on ground to design the FRP decks and study the response to various external effects.

Lot of on ground applications in the form of pedestrian bridges, hybrid bridges (with RC), all

27

composite bridges and bridge decks as replacement in old bridges are underway in US,

Europe, Australia, Korea etc. Two major types of FRP composite bridge decks are currently

in use: sandwich type construction and cellular or stiffened structure.

Multicellular FRP bridge decks with an advantage of box structure have a promising role to

play. Some of the additional benefits of these decks are reduction in dead load and

subsequent increase in live load rating, rehabilitation of historic structures, widening of

bridges without imposing additional dead load, faster installation reducing cost and traffic

congestion, and enhanced service life even under harsh environment. However, since it is a

new material no design codes, guidelines or specifications are available. There is a need to

create a database for better understanding and analysis of multicellular FRP bridge decks.

The present numerical study in an attempt in this direction. Among the refined methods for

analysis of complex composite structures, the finite-element method is probably the most

involved and time consuming. However, it is still the most general and comprehensive

technique for static and dynamic analysis, capturing all aspects affecting the structural

response. Theoretically, the torsional rigidity of multicellular decks reduces slightly with

increase in number of cells. However, due to increase in number of webs, which also share

the load, the deformation reduces with increase in number of cells. On the basis of FEM

studies a large data base on the pattern of stress and deformations have been generated, which

will be helpful in understanding the multicellular FRP bridge deck better.

Fig. 29.Remote installation of FRP pedestrian bridges

The first US advanced composite vehicular bridge superstructure was dedicated into service

on December 4, 1996 in Russell, Kansas. Besides US, composite bridge projects are being

developed the world over especially in Europe, Korea and Australia. The number of such

bridges is continuously growing as bridge engineers become comfortable with the material

28

and its performance. However, most of these bridge and decks have been built using

proprietary technology. Many pedestrian bridge projects have been constructed throughout

the United States using pultruded composite structural shapes.

The range and scope for use of multicellular FRP bridge decks is limitless. An imaginative

designer with knowledge of additional applications of the structural mechanics can make the

design of FRP decks efficient, cost effective vis-à-vis reinforced or pre-stressed concrete or

steel structures and thus make them a viable option for the bridge industry. An effort has been

made to model multicellular FRP bridge decks and do parametric study of Stress and

Deformation patterns.

Since the 1990s, numerous FRP composite in Deployable bridges have been built in the

United States, and the number of such bridges is continuously growing as bridge engineers

become comfortable with the material and its performance. Rapid construction being the

major criteria during disasters FRP has become a promising material for Deployable bridges.

FRP with box sections of single or multi cells further help reduce the weight of the bridge

simultaneously increasing the load carrying capacity or making leaner sections as required by

the user. The future of Deployable bridges is certainly with FRP Composites.

FRPs are developing to be a promising alternative construction material for bridge decks

today. The favorable characteristics of FRP bridge decks are a high strength combined with a

small dead load and a large tolerance for frost and de-icing salts. The small dead load of

approximately 20% of a concrete deck enables short installation times of the prefabricated

panels with minimum traffic interference as well as a possible increase in the allowable live

loads of existing bridges via replacement of the heavy concrete decks. Furthermore,

construction details can be designed much simpler compared to concrete decks. Different

deck systems have already been developed, two most commonly used construction forms are:

multi-cellular deck panels from adhesively bonded pultruded shapes and sandwich panels

with different core structures. Stiffened foams or thin walled cellular materials are most

commonly used for the cores. Most of the bridges constructed to date, however, use multi-

cellular pultruded deck systems. The panels consist of differently-shaped cross-sections:

hexagonal plus half-depth trapezoidal sections, triangular single or dual-cell sections, box

sections as well as trapezoidal dual-cell sections. Fig.29 shows the general cross section of

multicellular FRP bridge deck.

29

Fig. 30.FRP bridge deck cross sections

Stiffened panels are used in long span box girder; a box girder is built using deck plate,

vertical or inclined webs and bottom plates. The deck plates carries heavy traffic load so need

stiff stringers and transverse beans to transfer the load to the box web by bending.

Fig. 31.Box girder

3.3.2 Multicellular Bridge Decks Presently in Use

The deck (see Fig.32) is made up of quadratic tubes of 152-mm depth and two 9.53-mm thick

face panels. The tubes and plates are bonded with an epoxy adhesive before being

mechanically fastened to the steel girders. A vinyl ester resin of 6.4-mm thickness and

various sizes of angular quartz formed the wearing surface. Before installing the bridge deck

at Troutville weight station (Virginia, USA) several tests on the section were conducted at

Virginia Polytechnic Institute

Fig. 32.End view of Virginia Tech Deck

30

Smart Composite Bridge (see Fig.33), which is located on the University of Missouri-Rolla

(UMR) campus. The bridge is approximately 9.1 m (30 ft) long by 2.8 m (9 ft) wide (25.5

m2 or 274 ft2) and consists of a modular assembly of pultruded 76-mm-square FRP tubes.

The bonded assembly consists of alternating layers of tubes running longitudinally and

transverse to the span length. The top two layers and bottom two layers are continuous and

the intermediate layers are limited to four webs for material savings. Carbon/vinyl-ester tubes

are used in the top and bottom layers. Lower-cost, lower-stiffness glass/ vinyl-ester tubes are

used elsewhere for economy.

Fig. 33.Smart Composite Bridge on UMR campus

St. James Bridge (see Fig.34), located on St. Francis Street in a residential area of St. James,

Missouri. The St. James Bridge is approximately 8.00 m (26.25 ft) long by 8.33 m (27.33 ft)

wide (66.64 m2 or 717 ft2) and consists of four self-supporting FRP honeycomb sandwich

panels, which are 600 mm thick. The panels have glass FRP faces and a corrugated glass FRP

core with a 9.5 mm polymer concrete wear surface.

Fig. 34.Composite bridge in city of St. James, Mossouri

31

A large number of numerical studies are planned to study the stresses and deformations in

multicellular FRP bridge decks. The Finite Element Analysis (FEA) method has been

accepted as the most powerful tool in structural analysis problems involving complex

geometries, arbitrary loadings and general material properties (Bonet and Wood, 1996). Due

to versatility of the FEA technique, many general purpose programs developed in the past,

may be directly employed to analyse a broad range of the problems. Such a program not only

reduces the development cost which is quite high but also eliminates the duplication of effort.

Among the most prevalent programs are the ANSYS and the NASTRAN (NASA Structural

Analysis) are probably the largest and most widely used. Therefore FEA study is used to

analyse the multi cellular FRP bridge decks for the stress calculations.

Lot of research is underway to develop FRP bridge decks into a viable product. Finite

element analysis is a potent method to analyse the complex FRP composites. Numbers of

FRP decks have been modeled using ANSYS software. Variation in parameters like numbers

of cells, angles of fibre orientation, boundary conditions and types of loadings have been

studied. High initial cost of FRP is a limiting factor in extensive use of it in structural

applications. An extensive database needs to be generated before we embark on laboratory or

field trials. Wide ranging numerical studies to analyse these decks for different parameters is

the need of the hour.

3.3.3 Laminated Composite Stiffened Panels in Building

The composite panel is an ideal material for the manufacture of prefabricated, portable and

modular buildings as well as for exterior cladding panels which can simulate masonry or

stone. The all too familiar translucent roof sheeting is now supplied in a variety of colours

and profiles to suit both commercial and domestic building needs. Stiffened panels are used

in wall, ceiling, door, building blocks and primary load bearing structure etc. With the

growing population pressure and increasing labour & material costs, composite usage in

construction may provide cheaper solutions to a large extent. This area holds priority for the

induction of composites in place of conventional materials being used as in doors &

windows, paneling, furniture and other interiors. Components made of composite panels find

extensive applications in shuttering supports, special architectural structures imparting

aesthetic appearance etc. with the advantages like longer life, low maintenance, ease in

workability, fire retardancy etc.

32

Fig.35.Site office/Visitor’s Centre at Severn Crossing is made by composite stiffened panels

High strength, lightweight shoring panel and method of preparing the same has been

described by M/s Comcore Utilities Products, Falls Church, Virginia in April 1994. Methods

of preparing the panels and methods of assembling the panels into excavation support

systems are also mentioned. The translucent roof sheeting is now supplied in a variety of

colours and profiles to suit both commercial and domestic building needs.

The key restricting factors in the application of composites are initial costs due to raw

materials and also inefficient conventional moulding processes. Industry & design experts are

view that with the adoption of advanced technologies and some extent of standardization.

3.4 Other Applications of FRP Panels (Ref 8)

Commercial Canopy

Custom color translucent blue panels gave the architect of this stage canopy the look and

function he was designing for.

33

Day Lighting Metal Buildings

Translucent Panels offer architecturally pleasing options to building exteriors, while

providing efficient natural day lighting to building interiors

Metal Process - Aluminum - Primary & Recovery

Aluminum operations have extremely challenging

environments. This secondary aluminum processor

chose translucent Tred-Safe panels to replace their

original metal roof that had failed in less than 4 years.

This owner was able to benefit by retrofitting with

Tred-Safe compared to traditional metal products.

These benefits included: lower installation costs,

longer service life, better working conditions with the added day lighting of the translucent

panels.

Water & Waste Water – Sludge Bed Covers

Translucent Tred-Safe panels were used for roofing

and siding of sludge beds at this wastewater treatment

plant. The FRP panels shield the beds from

precipitation, yet transmit sunlight and heat to speed

up the sludge drying process.

34

Water & Waste Water - Trickling Filters

FRP panels offer an economical alternative for siding of trickling filters in this hostile

environment of a wastewater treatment plant trickling filter

Water & Waste Water -Composting Facility

The design of this west coast composting facility

called for an insulated roof system along with

translucent wall panels.

Salt – Storage

Opaque and translucent roofing and siding panels were specified by the owner of this salt

storage facility to combat the corrosive salt environment and also provide natural day lighting

into the building.

Industrial - Power Plants

Coal dust in a damp environment becomes extremely corrosive. Translucent panels were

selected by the owner of this power plant for roofing and siding of this coal conveyor and

35

transfer station. The natural day lighting the panels provided was a much appreciated

maintenance and safety benefit.

Commercial - Walkway Covers

Translucent White FRP panels offer a functional yet architecturally pleasing cover for this

hotel walkway.

Cooling Tower

In the cooling tower industry, has long known the benefits of incorporating FRP panels into

their cooling tower design. The FRP panels are a cost effective solution for tower casing

(siding) and louvers, offering long term service in a continuously wet environment

Marley Cooling Tower,

36

4 BEHAVIOUR OF LAMINATED COMPOSITE STIFFENED PANELS

The laminated composite panels are high strength material so thickness requirement is less as

compare to conventional material. There are two major categories leading failure of a

component: material failure and structural instability, which is often called buckling. The

load at which buckling occurs depends on the stiffness of the components, not upon strength

of its material. Buckling refers to the loss of stability of components and independent of

material strength. This loss of stability usually occurs in elastic rang of material. Buckling

failure is primarily characterized by loss of structural stiffness and is not modeled by the

usual linear finite element analysis, but by eigenvalue solution. Slender or thin walled

component under compressive stress are susceptible to buckling. Laminated composited

stiffened panels are thin walled structure so buckling is a major problem in its design.

Stiffened panels in real structures can be loaded under a combination of in-plane and out-

plane loading. In plane loads include in-plane compression, in-plane shear and a combination

of these loadings. Out-of-plane loading would include lateral pressure or bending about the

transverse and longitudinal axes of the stiffened panel. Generally these plates are flexible

when the loads are applied normal to the plane of the plate but it is very stiff when the loads

are applied in the plane. It is this in-plane high stiffness which is utilized in thin-walled

structures. The relatively high out of plate flexibility of the plate element make these

elements more susceptible to the buckling under in-plane compression and in-plane shear.

The buckling behaviour of stiffened panels is more complex when compared to unstiffened

panels.

Stiffened panels when subjected to these in-plane loading may buckle in local mode or global

(general instability) mode. Stiffened panels are said to buckle locally when one of the

following takes place

The plate between the stiffeners buckle. This termed as plate buckling. This mode of

failure illustrated in Fig.36. Researchers reported that this failure result in more

significant drop in load-carrying capacity after buckling then the overall buckling

failure modes.

The stiffeners itself buckles. This is termed as torsional buckling of the stiffener or

stiffener tripping. Researchers reported that this failure by tripping of the stiffener is

the least desirable since it is characterized by a sudden drop in load capacity just after

the peck load is reached. Because of the sudden drop in load-carrying capacity

37

accompanying stiffener tripping this mode of failure is considered a more critical

failure mode then the other stiffened plate failure modes.

Fig. 36.Local mode of buckling

On the other hand, stiffened panel is said to buckle globally when the stiffener and the plate

buckle simultaneously in such way that the half sign wave extends over more than one plate

element between the stiffeners (Fig.37). Generally this failure is the most favourable mode of

failure since it behaves in a more stable manner after buckling.

Fig. 37.Global mode of buckling

38

5 CONCLUSION

Laminated composite stiffened panels are used throughout the civil, marine and aerospace

industry for numerous structure applications. Light weight composite stiffened panels may be

utilized with great advantage in bridge decks as well as structural component of buildings.

The technological advances in composite stiffened panels and new understanding of how

panels behave under load have been crucial in the development for the structural application.

It is essential to fully understand how these panels behave, because only that way we can

built stronger and lighter structural components. Stiffened panel are relatively new material

so no design codes, guidelines or specifications are available for the uses in structural

application. There is need to develop data base which will be helpful to new designers.

Despite extensive research and on ground applications in US, Europe, Australia, Korea etc it

is infancy in India. Presently, it is limited to academic research. In the absence of

standardised codes and practices there is no alternative to recourse to the practices of the

above mentioned countries.

39

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3. Laxman K. G. 2007. Parametric study on laminated stiffened panels subjected to

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Laminated Composite Stiffened Panels Application and Behaviour