ABSTRACT

ABSTRACT

The composite structural members are highly used in the applications like

aerospace, automobiles, robotic arms, architecture etc., has attracted extensive

attention in the past decades. One of the important issues in the composite

technology is the repairing of aging aircraft structures. In such applications and

also for joining various composite parts together, they are fastened together either

using adhesives or mechanical fasteners. In this project work, modeling and

analysis of 3-D models of the Bonded, Riveted and Hybrid joints were carried out

using ANSYS 11 FEA software and also Tensile Strength of these joints were

carried out using Universal Testing Machine(UTM) . A parametric study was also

conducted to compare the performance of the hybrid joint with varying adherent

thickness, adhesive thickness and overlap length. To utilize the full potential of

composite materials as structural elements, the strength and stress distribution of

these joints must be understood .ANSYS FEA tool and UTM (Experimental) is

used to study the stress distribution and Tensile Strength in the members involved

under various design conditions and various joints.

LIST OF FIGURES

S.No Name of the Figure Page No.

1 3.1 Flow Chart 14

LIST OF TABLES

S.No Name of the Table Page No.

TABLE OF CONTENTS

Chapter No. Description Page No.

1 Introduction 1

1.1 OBJECTIVE 2

1.2 METHODOLOGY 4

INTRODUCTION

CHAPTER 1

INTRODUCTION

Over the past three decades, application of composite materials are

continuously increasing from traditional application areas such as military aircraft,

commercial aircraft to various engineering fields including automobiles, robotic

arms and even architecture. Due to its superior properties, composites have been

one of the materials used for repairing the existing structures. In such applications

and also for joining various composite parts together, they are fastened together

either using adhesives or mechanical fasteners. Nowadays, a novel method called

hybrid joint is also being employed, where a combination of both adhesive and

mechanical fasteners is used.

1.1 OBJECTIVE

To investigate the stress distribution and Tensile Strength of various

configuration of single lap joint. A parametric study of hybrid joint by varying the

three dimensional parameters of the joint will be carried out.

1.2 METHODOLOGY

Modeling and analysis of 3-D models of the Bonded, Riveted and Hybrid

joints were carried out using ANSYS 11 FEA software and also Tensile Strength

of these joints were carried out using Universal Testing Machine(UTM).

1.3 IMPORTANCE OF THE WORK

Composite materials have been widely used as structural elements in aircraft

structures due to their superior properties. Aircraft structure is a huge assembly of

skins, spars, frames etc. The structure consists of an assembly of sub-structures

properly arranged and connected to form a load transmission path. Such load

transmission path is achieved using joints. Joints constitute the weakest zones in

the structure. Failure may occur due to various reasons such as stress

concentrations, excessive deflections etc. or a combination of these. Therefore, to

utilize the full potential of composite materials, the strength and stress distribution

in the joints has to be understood so that suitable configuration can be chosen for

various applications.

Analysis using FEA tool is necessary to standardize the experimental

procedures and testing sequence.

LITERATURE REVIEW

CHAPTER 2

LITERATURE REVIEW

2.1 LITERATURE SURVEY

Any number papers can easily be found for mechanically fastened

joints and adhesive bonded joints whereas for hybrid joints, there are only a

limited number of papers available. Some of them are briefly discussed below.

1. Hart-Smith conducted a theoretical investigation of combined

bonded/bolted stepped lap joints between titanium and Carbon Fiber

Reinforced Plastic (CFRP). While no significant strength benefits were found

in comparison to perfectly bonded joints, the combined bolted-bonded joint

was found to be beneficial for repairing damaged bonded joints and limiting

damage propagation. Under room temperature and ambient humidity

conditions, 98% of the applied load was predicted to be transferred by the

adhesive.

2. Chan and Vedhagiri investigated the use of bolted, bonded and

combined bonded-bolted joints used in repair. A single strap joint with CFRP

adherends was considered with two bolts used in the overlap region. The

study primarily considered stress distributions in the laminates and limited

consideration was given to load transfer through the joint.

The structural response of various configurations of single lap joint, namely,

bonded, bolted and bonded-bolted joints, was analyzed by three-dimensional

finite element method. For the case of hybrid joints, it was found that the bolts

do not take an active role in load transfer before the initiation of the failure.

However, the bolts in the hybrid joint actually reduce the in-plane axial stress

near the edge of overlap.

3. Gordon Kelly investigated the load distribution in hybrid joints

numerically through the use of finite element analysis. A three dimensional

finite element model was developed including the effects of bolt-hole and

nonlinear material behavior and large deformation. From the study, it was

found that load transferred by the bolt increases with increasing adherend

thickness and adhesive thickness and decreases with increasing overlap

length, pitch distance and adhesive modulus.

4. Fu and Mallick investigated the static and fatigue strength of hybrid

joints in a Structural Reaction Injection Moulded (SRIM) composite materials.

The authors performed an experimental investigation on a single lap joint

considering the effect of different washer designs. It was concluded that the

performance of the hybrid joints was dependent upon the washer design which

affected the distribution of the bolt clamping force. The hybrid joints were

shown to have higher static strength and longer fatigue life than adhesive

bonded joints for the studied material system.

5. Jin-Hwe Kweon, Jae-Woo Jung and others conducted tests to evaluate

the strength of carbon composite to aluminum double lap joints with two

different adhesive materials, film and paste types. The three types of joints-

bonded, bolted and hybrid- were considered. It was found that the strength of

hybrid joints with film type adhesive is dominated by the strength of the

adhesive itself. On the contrary, the strength of the joints with paste type

adhesive was mainly affected by the bolt joint. In general, it was found that

hybrid joining is effective when the mechanical fastening is stronger than the

bonding. On the contrary, when the strength of the bolted joint is lower than

the strength of the bonded joint, the bolt joining contributes little to the hybrid

joint strength.

Previous studies on hybrid joints have considered fixed joint

geometries and material systems. Limited consideration has been given to the

prediction of load distribution in the joints. This study was focused on the

analysis of stress distribution in three prominent joining methods namely,

bonded, bolted and hybrid joints. FEA is used to study the stress distribution

in the members involved under various design conditions.

2.2 COMPOSITE MATERIALS: AN OVERVIEW

Basic requirements for the better performance efficiency of an aircraft are

high strength, high stiffness and low weight. The conventional materials such as

metals and alloys could satisfy these requirements only to a certain extent. This

lead to the need for developing new materials that can whose properties were

superior to conventional metals and alloys, were developed.

A composite is a structural material which consists of two or more

constituents combined at a macroscopic level. The constituents of a composite

material are a continuous phase called matrix and a discontinuous phase called

reinforcement.Matrix gives shape and protects the reinforcement from the

environment. It also makes the individual fibers of the reinforcement act together

and provides transverse shear strength and stiffness to the laminated composites.

The matrix factors which contribute to the mechanical performance of composites

are transverse modulus and strength, shear modulus and strength, compressive

strength, inter-laminar shear strength, thermal expansion co-efficient, thermal

resistance and fatigue strength. Reinforcement provides strength and stiffness and

controls thermal expansion co-efficient. It also helps to achieve directional

properties. Reinforcements may be in the form of fibers, particles or flakes. The

fiber factors which contribute to the mechanical performance of a composite are

length, orientation, shape and material.

The factor which influences the mechanical performance of composites other than

the fiber and the matrix is the fiber-matrix interface. It predicts how well the matrix

transfers load to the fibers.

Composites are classified by

1. the geometry of the reinforcement as particulate, flakes and fibers

2. the type of matrix as polymer, metal, ceramic and carbon

The most commonly used advanced composites are polymer matrix

composites. These composites consists of a polymer such as epoxy, polyester,

urethane etc., reinforced by thin-diameter fibers such as carbon, graphite, aramids,

boron, glass etc. Low cost, high strength and simple manufacturing principles are

the reason why they are most commonly used in the repair of aircraft

structures.To measure the relative mechanical advantage of composites, two

parameters are widely used, namely, the specific modulus and the specific

strength. These two parameter ratios are high in composites.

The building block of a laminate is a single lamina. Therefore the

mechanical analysis of a lamina precedes that of a laminate. A lamina is an

anisotropic and non-homogeneous material. But for approximate macro-

mechanical analysis, a lamina is assumed to be homogeneous where the

calculation of the average properties are based on individual mechanical

properties of fiber and matrix, as well as content, packing geometry and shape of

fibers.

Fig.2.1 Aircraft Structures

The lamina is considered as orthotropic, so it can be characterized by nine

independent elastic constants: three Young’s moduli along each material axis,

three Poisson’s ratio for each plane and three shear moduli for each plane. Once

the properties for each lamina are obtained, properties of a laminate, made of

those laminae can be calculated using those individual properties.

In the highly competitive airline market, using composites is more

efficient. Though the material cost may be higher, the reduction in the number of

parts in an assembly and the savings in the fuel cost makes more profit. It also

lowers the overall mass of the aircraft without reducing the strength and stiffness

of its components.

Fig.2.2 Comparative characteristics of metals and composites

2.3 EPOXIES

Epoxies are polymer materials, which begin life as liquid and are converted

to the solid polymer by a chemical reaction. An epoxy based polymer is

mechanically strong, chemically resistant to degradation in the solid form and

highly adhesive during conversion from liquid to solid. These properties, together

with the wide range of basic epoxy chemicals from which an epoxy system can be

formulated, make them very versatile.

Epoxy systems physically comprise two essential components, a resin and a

hardener. Sometimes there is a third component, an accelerator, but this is not so

common. The resin component is the ‘epoxy’ part and the hardener is the part it

reacts with chemically and is usually a type of ‘amine’. Whereas the resin

component is usually light, sometimes almost clear coloured and near odourfree,

hardeners are usually dark and have a characteristic ‘ammonia-like’ odour. When

these two components are brought together and mixed intimately in a prescribed

way, they will react chemically and link together irreversibly, and when the full

reaction has been completed they will form a rigid plastic polymer material.

This polymer is called a ‘thermoset’ plastic because, when cured, it is irreversibly

rigid and relatively unaffected by heat. Epoxy polymers have many uses: as

industrial adhesives, or as coatings, or as matrices in which to embed

reinforcement fibres to form advanced reinforced plastics, and also as

encapsulation media.

2.3.1 USES

The uses for epoxies span many markets including aerospace, transport,

marine, civil engineering and general industry, and such is the versatility of

epoxy’s chemistry that chemists are able to fine tune the formulations for a wide

variety of specific tasks. Some epoxies, which are used as coatings, are dispersed

in solvents, but the majority used for structural applications are solvent-free, and

these are the types which require more care in their use and which are featured

here.

2.4 POLYESTER RESINS

The resin and hardener are usually supplied as two liquids in separate

containers. To bring about a reaction, not only must they be combined in exactly

the right quantities relative to one another (ratio), but the individual molecules

must be brought into contact with one another by stirring, in order that the reaction

is initiated and the ‘cure’ can proceed. A little too much of either component will

adversely affect the chemical link-up which is taking place (the cure) and the user

will soon notice that the material has not ‘gone off’, the mix remaining softer than

it should be. Epoxies must not be confused with another very common group of

thermoset polymers, the ‘polyesters’, which although they may look similar, have a

different curing mechanism involving the use of a ‘catalyst’. In a catalytic reaction,

the liquid resin component reacts with itself to form the hard plastic polymer, but

only when the catalyst has been added. Only a small amount of catalyst (usually 1-

2%) is needed to start this type of reaction.

Polyester resins characteristically have a smell of styrene which is both a 'solvent'

for the polyester and a reactive component. The other component the user adds to

initiate the cure reaction is added in very small volumes relative to the resin, which

may be varied according to speed of reaction required. As this material is only a

catalyst, the quantity added is not critical in the same way that a hardener is to

epoxy resin and the two systems must be thought of as being completely different.

2.5 UNIVERSAL TESTING MACHINE:

The most common testing machines are universal testers, which test

materials in tension, compression, or bending. Their primary function is to create

the stressstrain curve described in the following section in this chapter. Testing

machines are either electromechanical or hydraulic. The principal difference is the

method by which the load is applied. Electromechanical machines are based on a

variable-speed electric motor; a gear reduction system; and one, two, or four

screws that movethe crosshead up or down. This motion loads the specimen in

tension or compression. Crosshead speeds can be changed by changing the speed

of the motor. A microprocessor-based closed-loop servo system can be

implemented to accurately control the speed of the crosshead.

Fig.2.3 Universal Testing Machine

Hydraulic testing machines are based on either a single or dual-acting piston that

moves the crosshead up or down. However, most static hydraulic testing machines

have a single acting piston or ram. In a manually operated machine, the operator

adjusts the orifice of a pressure-compensated needle valve to control the rate of

loading. In a closed-loop hydraulic servo system, the needle valve is replaced by an

electrically operated servo valve for precise control.

2.6 INTRODUCTION TO TENSILE TESTING

TENSILE TESTS are performed for several reasons. The results of tensile

tests are used in selecting materials for engineering applications. Tensile properties

frequently are included in material specifications to ensure quality. Tensile

properties often are measured during development of new materials and processes,

so that different materials and processes can be compared. Finally, tensile

properties often are used to predict the behaviour of a material under forms of

loading other than uniaxial tension. The strength of a material often is the primary

concern. The strength of interest may be measured in terms of either the stress

necessary to cause appreciable plastic deformation or the maximum stress that the

material can withstand. These measures of strength are used, with appropriate

caution (in the form of safety factors), in engineering design. Also of interest is the

material’s ductility, which is a measure of how much it can be deformed before it

fractures.

Rarely is ductility incorporated directly in design; rather, it is included in material

specifications to ensure quality and toughness. Low ductility in a tensile test often

is accompanied by low resistance to fracture under other forms of loading. Elastic

properties also may be of interest, but special techniques must be used to measure

these properties during tensile testing, and more accurate measurements can be

made by ultrasonic techniques.

FINITE ELEMENT

ANALYSIS

CHAPTER - 3

SOFTWARE OVERVIEW

Fig. 3.1 Flow Chart

3.1 CATIA

CATIA is one of the world’s leading CAD/CAM/CAE package. Being a

solid modeling tool, it not only unites 3D parametric features with 2D tools, but

also addresses every design through manufacturing process.

CATIA- Computer Aided Dimensional Interactive Application.

CATIA, developed by Dassult systems, france, is a completely re-

engineered, next generation family of CAD/CAM/CAE software solution.

CATIA serves the basic design task by providing different workbenches,

some of the workbenches available in this package are

Part design workbench

Assembly design workbench

Drafting workbench

Wireframe and surface design workbench

Generative shape design workbench

DMU kinematics

Manufacturing

Mold design

3.1.1 PART DESIGN WORKBENCH

The part workbench is a parametric and feature-based environment, in which

we can create solid models. In the part design workbench, we are provided with

tool those convert sketches into other features are called the sketch-based features.

3.1.2 ASSEMBLY DESIGN WORKBENCH

The assembly design workbench is used to assemble the part by using

assembly constraints. There are two type of assembly design,

Bottom –up

Top- down

In bottom –up assembly, the parts are created in part workbench and assembled in

assembly workbench.

In the top-down workbench assembly, the parts are created in assembly workbench

itself.

3.1.3 WIREFRAME AND SURFACE DESIGN WORKBENCH

The wire frame and surface design workbench is also parametric and feature

based environment. The tools available in this workbench are similar to those in

the part workbench, with the only difference that the tool in this environment are

used to create basic and advance surfaces

3.1.4 DRAFTING WORKBENCH

The drafting workbench is used for the documentation of the parts or the

assemblies created in the form of drafting.

There are two types of drafting techniques:

Generative drafting

Interactive drafting

The generative drafting technique is used to automatically generate the drawing

views of parts and assemblies.In interactive drafting, we need to create the drawing

by interactive with the sketcher to generate the views.

3.1.5 DMU KINEMATICS

This workbench deals with the relative motion of the parts.DMU kinematics

simulator is an independent CAD product dedicated to simulating assembly

motions. It addresses the design review environment of digital mock-ups (DMU)

and can handle a wide range of products from customer goods to very large

automotive or aerospace projects as well as plants, ships and heavy machinery.

We created model of joints (bonded, riveted and hybrid) by using CATIA

software. The models are shown below…

Fig 3.2 Model of Bonded Joint

Fig 3.3 Model of Riveted Joint

Fig 3.4 Model of Hybrid Joint

3.2 ANSYS:

ANSYS is a complete FEA simulation software package developed by

ANSYS Inc – USA. It is used by engineers worldwide in virtually all fields of

engineering.

Structural

Thermal

Fluid (CFD, Acoustics, and other fluid analyses)

Low-and High-Frequency Electromagnetic.

PROCEDURE:

Every analysis involves three main steps:

Pre-processor

Solver

post processor

3.2.1 STRUCTURAL ANALYSIS

Structural analysis is probably the most common application of the finite

element method. The term structural (or structure) implies not only civil

engineering structures such as bridges and buildings, but also naval, aeronautical,

and mechanical structures such as ship hulls, aircraft bodies, and machine

housings, as well as mechanical components such as pistons, machine parts, and

tools.

TYPES OF STRUCTURAL ANALYSIS

The seven types of structural analyses available in the ANSYS family of

products are explained below. The primary unknowns (nodal degrees of freedom)

calculated in a structural analysis are displacements. Other quantities, such as

strains, stresses, and reaction forces, are then derived from the nodal

displacements.

Structural analyses are available in the ANSYS Multiphysics, ANSYS

Mechanical, ANSYS Structural, and ANSYS Professional programs only.

STATIC ANALYSIS--Used to determine displacements, stresses, etc. under static

loading conditions. Both linear and nonlinear static analyses. Nonlinearities can

include plasticity, stress stiffening, large deflection, large strain, hyperelasticity,

contact surfaces, and creep.

MODAL ANALYSIS--Used to calculate the natural frequencies and mode shapes

of a structure. Different mode extraction methods are available.

HARMONIC ANALYSIS--Used to determine the response of a structure to

harmonically time-varying loads.

TRANSIENT DYNAMIC ANALYSIS--Used to determine the response of a

structure to arbitrarily time-varying loads. All nonlinearities mentioned under

Static Analysis above are allowed.

SPECTRUM ANALYSIS--An extension of the modal analysis, used to calculate

stresses and strains due to a response spectrum or a PSD input (random

vibrations).

BUCKLING ANALYSIS--Used to calculate the buckling loads and determine the

buckling mode shape. Both linear (eigenvalue) buckling and nonlinear buckling

analyses are possible.

EXPLICIT DYNAMIC ANALYSIS--This type of structural analysis is only

available in the ANSYS LS-DYNA program. ANSYS LS-DYNA provides an

interface to the LS-DYNA explicit finite element program. Explicit dynamic

analysis is used to calculate fast solutions for large deformation dynamics and

complex contact problems.

In addition to the above analysis types, several special-purpose features are

available:

Fracture mechanics

Composites

Fatigue

p-Method

Beam Analyses

3.2.2 STRUCTURAL STATIC ANALYSIS

A static analysis calculates the effects of steady loading conditions on a

structure, while ignoring inertia and damping effects, such as those caused by time-

varying loads. A static analysis can, however, include steady inertia loads (such as

gravity and rotational velocity), and time-varying loads that can be approximated

as static equivalent loads (such as the static equivalent wind and seismic loads

commonly defined in many building codes).

Static analysis is used to determine the displacements, stresses, strains, and

forces in structures or components caused by loads that do not induce significant

inertia and damping effects.

Steady loading and response conditions are assumed; that is, the loads and

the structure's response are assumed to vary slowly with respect to time. The kinds

of loading that can be applied in a static analysis include:

Externally applied forces and pressures

Steady-state inertial forces (such as gravity or rotational velocity)

Imposed (nonzero) displacements

Temperatures (for thermal strain)

Fluences (for nuclear swelling)

PERFORMING A STATIC ANALYSIS

The procedure for a static analysis consists of these tasks:

Build the Model

Set Solution Controls

Set Additional Solution Options

Apply the Loads

Solve the Analysis

Review the Results

3.2.3 LOAD TYPES

All of the following load types are applicable in a static analysis.

DISPLACEMENTS (UX, UY, UZ, ROTX, ROTY, ROTZ)

These are DOF constraints usually specified at model boundaries to define

rigid support points. They can also indicate symmetry boundary conditions and

points of known motion. The directions implied by the labels are in the nodal

coordinate system.

FORCES (FX, FY, FZ) AND MOMENTS (MX, MY, MZ)

These are concentrated loads usually specified on the model exterior. The

directions implied by the labels are in the nodal coordinate system.

PRESSURES (PRES)

These are surface loads, also usually applied on the model exterior. Positive

values of pressure act towards the element face (resulting in a compressive

effect).

TEMPERATURES (TEMP)

These are applied to study the effects of thermal expansion or contraction

(that is, thermal stresses). The coefficient of thermal expansion must be defined if

thermal strains are to be calculated. We can read in temperatures from a thermal

analysis [LDREAD], or we can specify temperatures directly, using the BF

family of commands.

FLUENCES (FLUE)

These are applied to study the effects of swelling (material enlargement due

to neutron bombardment or other causes) or creep.

GRAVITY, SPINNING, ETC.

These are inertia loads that affect the entire structure. Density (or mass in

some form) must be defined if inertia effects are to be included.

APPLY LOADS TO THE MODEL

Except for inertia loads, which are independent of the model, we can

define loads either on the solid model or on the finite element model.

3.2.4 COMPOSITES IN ANSYS

Composite materials have been used in structures for a long time. In recent

times composite parts have been used extensively in aircraft structures,

automobiles, sporting goods, and many consumer products.

Composite materials are those containing more than one bonded material,

each with different structural properties.

The main advantage of composite materials is the potential for a high ratio

of stiffness to weight. Composites used for typical engineering applications are

advanced fiber or laminated composites, such as fiberglass, glass epoxy, graphite

epoxy, and boron epoxy.

ANSYS allows us to model composite materials with specialized elements

called layered elements. Once we build our model using these elements, we can

do any structural analysis (including nonlinearities such as large deflection and

stress stiffening).

LOAD TRANSFER IN HYBRID COMPOSITE SINGLE-LAP JOINTS

The load transfer in hybrid joints is complicated due to the difference in

stiffness of the alternative load paths. The load distribution in hybrid composite

single-lap joints has been predicted through use of a three-dimensional finite

element model including the effects of bolt–hole contact and non-linear material

behaviour. The effect of relevant joint design parameters on the load transferred by

the bolt have been investigated through a finite element parameter study.

Joint configurations where hybrid joining can provide improved structural

performance in comparison to adhesive bonding have been identified. Experiments

were performed to measure the distribution of load in a hybrid joint. A joint

equipped with an instrumented bolt was used to measure the load transfer in the

joint. The measured bolt load values were compared to predictions from the finite

element model and the results were found to be in good agreement.

ANSYS RESULTS

Fig 3.5 Meshed Model: Bonded Joint

Fig 3.6 Meshed Model: Rivited Joint

Fig 3.7 Meshed Model: Hybrid Joint

Von Misses Stress

Fig 3.8 Von Misses Stress: Bonded Joint

Fig 3.9 Von Misses Stress: Riveted Joint

Fig 3.10 Von Misses Stress: Hybrid Joint

Comparison Graph

Fig 3.11 Von Misses Comparison Graph

COMPOSITE JOINTS

CHAPTER 4

COMPOSITE JOINTS

Ideally, it is always preferred to make monolithic structures, that is,

structures without joints. This ideal can never be realized for many reasons like

size limitations imposed by materials or the manufacturing process, need for

disassembly of structure for transportation and access for inspection and repair etc.

Basically, there are two types of load-carrying joints available: mechanically

fastened joints and adhesively bonded joints. Nowadays, a novel method called

hybrid joint is also being used in certain applications.

A short description of the three types of joints used in the present work

namely, bonded, riveted and hybrid joints is given below.

Fig.4.1 Hybrid Joints

4.1 BONDED JOINT

Bonded joints can be made by gluing together pre-cured laminates with the

suitable adhesives or by forming joints during the manufacturing process, in which

case the joint and the laminate are cured at the same time (co-cured). Here, load

transfer between the substrates take place through a distribution of shear stresses in

the adhesive.

In general, there are numerous advantages of adhesive bonded joints over

the traditional mechanical fastened joints. These advantages include large bond

area for load transfer, low stress concentration, smooth external surfaces at the

joint, less sensitivity to cyclic loading, time and cost saving, high strength to

weight ratio, electrical and thermal insulation, conductivity, corrosion and fatigue

resistance, crack retardation, damping characteristic and so on.

Some of the disadvantages of bonded joints are

1. Disassembly is impossible without component damage.

2. They can be severely weakened by environmental effects.

3. They require surface preparation.

4. Joint integrity is difficult to confirm by inspection. Thus ensuring a quality

of bonding has been a challenging task.

4.2 RIVETED JOINT

Riveted joints can be used quite successfully on laminates up to about 3mm

thick and also where a tight fit, called interference fit, is necessary. The choice lies

between solid & hollow types and whichever is chosen, care must be taken to

minimize damage to the laminate during hole-drilling and closing of rivet. In

addition to material and configurational parameters, the behavior of riveted joints

is also influenced by rivet parameters such as rivet size, clamping force, hole size

and tolerance. Of these parameters, the clamping force, that is, the force exerted in

the through – thickness direction by the closing of the fastener, is of critical

importance.

Some advantages of riveted joints are that

1. No surface preparation of composite is required.

2. There are no abnormal inspection problems.

Though mechanically fastened joints involve simple processing and

handling, they have several disadvantages. The holes create stress concentration

and reduce the strength of the substrate laminates. Other than this, these joints

incurs large weight penalty and also create a potential corrosion problem resulting

from contact with the composites. Also disassembly is not possible in riveted

joints.

4.3 HYBRID JOINT

Hybrid joints have a combination of adhesive bonding and mechanical

fasteners. In the present case, rivet has been used as the mechanical fastener. The

advantages of using a combined bonded-riveted design apply mainly in a repair

situation. It is generally accepted that a bonded joint is stronger than a

mechanically fastened joint and a well-designed bonded joint is stronger than a

hybrid joint. However, in a repair situation, limitations force the repair design to be

less than an optimum joint design.

In this case, the inclusion of rivets can be advantageous in one of the

following two ways.

1. If a bond line is subjected to forces that induce out-of plane peel stresses, the

addition of rivets can reduce these stresses, thereby, increasing the strength of

the joint.

2. Once damage has initiated in a bonded-only repair, it can propagate quickly

and the bond line will completely unzip. The additions of rivets arrest the

propagation of the crack, thereby increasing the strength or the life of the

joint.

MANUFACTURING

CHAPTER 5

MANUFACTURING

5.1 LAMINATE PREPARATION

The laminate size is 200mm × 200mm x 3.5 mm

No. of layer is 3.

5.2 RESIN

Resin is to transfer stress between the reinforcement fibers, act as a glue to

hold the fiber together.

Commonly used resin are:

o Epoxy, polyester and vinyl ester

o Epoxy LY556 is selected.

5.3 TYPES OF HARDENER

HY951 – at room temperature.

HT927 – temperature ranging from 80˚C - 130˚C

HT974 - temperature ranging from 70˚C - 80˚C

HZ978 - temperature ranging from above 100˚C

5.4 PREPARATION OF EPOXY AND HARDENER

Epoxy LY556 and it mixed with Hardener HY951.

Ratio of mixing epoxy and hardener is 10:1

5.5 SPECIMEN PREPARATION FOR GLASS FIBER

The mould should be well cleaned and dry.

Release agent is applied.

The epoxy mixture is uniformly applied.

First woven mat is laid into the moulded.

Apply the resin on mat by brush.

Second mat is laid to first mat

Repeated the process up to 3 layers

Mould is closed.

5.6 MOULDING PREPARATION

Two rectangular steel plate having dimensions of 200mm × 200mm x 8 mm.

Chromium plated to give a smooth finished as well as to protect from

rusting.

Four beading are used to cover compress the fiber after the epoxy is applied.

Bolt and nuts are used to lock the plate.

5.7 STAGES OF THE EPOXY REACTION

5.7.1 THE CURE

The cure of epoxies is the conversion of the liquid resin and hardener components

to a solid high-performance plastic material. Cure is only initiated once the

components are metered in the correct ratio to one another and are physically

mixed together. The cure of all epoxies is an exothermic process where heat is

liberated as a natural consequence of the chemical reaction. Success in using

epoxies most efficiently is dependent upon handling the product in the correct way

in order to avoid wastage and premature cure, and this can be achieved by some

understanding of the basic chemistry and the various stages of the chemical

transformation.

5.8 THIN FILM SOLVENT-FREE EPOXY CURE STAGES

5.8.1 MIXING IN POT

Bringing together resin and hardener as a uniform liquid, to initiate the chemical

reaction, followed by transfer to a large surface area tray to produce a thin film.

The chemical reaction begins, producing heat, much of which escapes to the

environment, because of the large surface area.

5.8.2 GELATION

Point of which a significant proportion of the reaction has occurred. The cross-link

density (number of resin and hardener molecules linked together) is high enough

for the liquid to have become the consistency of a thick gel. Usually between 1 - 4

hours.

5.8.3 ‘ALMOST TACK FREE’

The last stage at which the surface is ‘active’ and can accept further layers/coats of

epoxy and still form good bond. This stage is indicated when a finger on the

surface does not indent the layer deeply, but does leave a fingerprint. Usually

between 1 hour to 6 hours.

5.8.4 HARDENING

Occurs after almost tack free - touching the surface with a finger leaves no

fingerprint. However, the epoxy film is still less than 30% cured and can be

indented with a thumbnail. Usually not before 3-6 hours.

5.9 EARLIEST SANDING TIME

Epoxy film is firm enough to withstand abrasion. At this stage sanding will

produce a fine dust rather than crumbling lumps. Usually 8-24 hours.

5.10 EARLIEST LOADING TIME

For adhesives, this is the time after which sufficient cure has occurred to enable the

polymer to be loaded without it stretching irreversibly, or breaking. However, this

is rather arbitrary and it is better to wait for full cure.

5.11 FULL CURE

At room temperature ‘full cure’ for most epoxies means 80-90% of their

theoretical maximum crosslinking, which can only be obtained by heating to

higher temperatures (40-100°C typically). ‘Room temperature cure’ epoxies are

designed to give good performance even though not cured to their theoretical

maximum. ‘Full cure’ is usually after a minimum of one week.

5.12 ESSENTIAL PRACTICAL NOTES FOR USERS OF SOLVENT-FREE

EPOXY SYSTEMS

Being sophisticated chemicals, epoxies respond to careful use. Epoxy ‘systems’

(the particular epoxy resin and hardener combination) vary considerably depending

on what particular task they are designed to perform. Therefore, one system will

display characteristics and working properties which may vary from another.

Reading the product data sheet which accompanies the product is therefore

strongly recommended in order that the user achieves the expected results.

Although variation can be expected, all systems have some common requirements

which can be considered as the ‘general rules’ of using epoxies and these are

outlined as follows:

5.12.1. STORE THE RESIN AND HARDENER IN A WARM

ENVIRONMENT BEFORE USE

If the storage temperature is too cold, the hardener and particularly the resin will

become too thick to both measure out and to mix efficiently. A resin which has the

viscosity of thick ‘syrup’ is too thick to use and should be warmed until it is the

consistency of thin ‘motor oil’. Resin and hardener are usable at any temperature

above 15°C, but 18-25°C is considered ideal. Storing the containers either indoors

at room temperature or warming the contents prior to use by a safe heater is good

practice. Some professional users, who have limited heating facilities, construct a

heated cabinet (using lightbulbs) as an economical method of maintaining their

epoxy at a suitable temperature, and this technique has proved very successful.

5.12.2 USE THE PRODUCTS IN A WARM ENVIRONMENT

It is the nature of epoxies that they cure by chemical reaction, the rate of which is

temperature dependent. In fact, for every 10°C rise in temperature, reaction rate

will double. In order that epoxies can be fitted in to a wide spectrum of uses and

temperature requirements, most epoxies usually have two or more hardeners giving

different reaction speeds. The user should acquaint himself with the expected

working characteristics given by each system before commencing the work.The

relevant information is included in the product data sheet. Ideally, no epoxy should

be left to cure at a temperature of lower than 12-15°C, even using fast hardener.

This applies particularly to coatings. In reality, even though the epoxy may appear

to be ‘hard’ and perform its task satisfactorily, the cure may not be complete,

particularly with epoxy adhesives. It may take a prolonged period to cure fully at

such ‘low’ temperatures and some epoxy reactions may only be completed by the

subsequent application of elevated temperatures. Whereas warm, thermostatically

controlled workshops are the best solution, these are expensive to run and certainly

are in the minority, at least in the marine industry.

Certain less expensive techniques however, can be very useful for raising the cure

rate. Local high temperature levels are effective for initiating the cure and, if used

sensibly in short bursts, do not have the expected result of causing the mix to

harden prematurely. An electric hot air gun is particularly invaluable when using

epoxy in cold conditions (down to 5°C), to aid application. The hot air gun (paint-

stripper type) can enable small scale work to proceed in cold cure environments.

Coating and laminating applications have benefited most but the technique can also

assist glue application or the application of epoxy fillers. But care should be used

to ensure that heat is only applied to the surfaces to be bonded or coated and in a

limited number of short bursts.

5.12.3 MEASURE ACCURATELY IN THE CORRECT RATIO

Read the data sheet supplied with the product to determine the ratio of your

particular epoxy. Although usually given on a volume basis (e.g. 5:2 parts resin to

hardener respectively), some systems also give a weight ratio, which is nearly

always different.

For instance, a system of 5:2 mix ratio by volume may be 3:1 by weight. The

following methods are available to the user for metering purposes:

5.13 GRADUATED SYRINGES

50cc and 10cc syringes are two convenient sizes that are easily obtainable. The

larger one should be reserved only for the resin and the smaller one for the

hardener. Used in this way they do not require cleaning after use. Simply allow

each to drain on to some absorbent paper. Syringes are ideal for measuring out

small volumes of less than 200cc and certainly the most economical, accurate

method for volumes of less than 20cc. However, when using syringes on the small

size packs, plastic extension tubes should be fitted. Syringes are consistently

accurate and convenient to use and recommended for packs up to 1kg size.

5.14 NON-CALIBRATED MINI-PUMPS

These, the smallest size of mini-pumps, are designed to fit on top of the containers

of small pack sizes of resin and hardener, replacing the container caps. These non-

calibrated pumps dispense approx. 7cc of liquid and when fitted should be used on

the ‘number of strokes principle’ where the number of strokes of resin (always the

greater proportion) to hardener corresponds to the system mix ratio, e.g. 5:1ratio =

5 strokes of the resin pump to 1 stroke of the hardener pump.

5.15 CALIBRATED PUMPS (MAXIPUMPS)

As the pumps are calibrated, work on the principle of one stroke resin to one stroke

hardener to give the correct metered ratio amount of resin and hardener.

Larger volumes of mixed resin/hardener are obtained by several equal strokes of

the resin pump and hardener pump. Each pack size and each system has its own

designated calibrated pump pack, and each cannot be used with any other system

or pack size.

5.16 GRADUATED RE-USEABLE MIXING POTS

These are graduated up to 500cc and are useful for all systems where volumes

required in one mix are greater than 150cc. They are commonly the preferred

metering system when the total mixed volumes required are 400-500cc. They are

made of flexible plastic, to which the epoxy will not adhere. Thus they can be re-

used by allowing the remaining epoxy to cure in the pot, which can then be

‘popped’ out by flexing the container.

5.17 ELECTRONIC SCALES

Weight measurement is always the most consistently accurate measurement for

any system for volumes over 200cc. We recommend scales with a tare facility of

2kg and 5kg capacity where the maximum mix required may be up to 1.5 litres or 4

litres respectively. Metering using scales is becoming increasingly popular and is

particularly convenient if different epoxy systems or different size packs are being

used which would otherwise require a range of specific pump dispensing systems.

4. MIX THE COMPONENTS THOROUGHLY

No reaction between resin and hardener will occur, even at the interface of the two

liquids, if they are not physically mixed together for a suitable period.

Stirring by hand with a suitable flat-sided stirring stick and scraping round the

sides of the container should be done for a period of not less than 1-2 minutes,

depending on volume. Electric stirrers are efficient but do not touch the container

sides, and therefore this method should always be accompanied by a final hand

stirring to mix material stuck to the container side walls and base.

5. MIX ONLY SUFFICIENT PRODUCT FOR CONVENIENT USE

It is relatively common to find users, who are not familiar with epoxy systems,

mixing up too great a volume and that a proportion of the mix ‘hardens’ in the pot

before being removed for use. All resin and hardener mixes generate heat so do not

be surprised if you detect a heat build-up, however small, in the base of your

mixing container. This heat build-up signals that the material will soon become

unusable and is particularly noticeable when large volumes are mixed. Using the

mixed product is somewhat of a ‘balancing act’ - the aim of which is to remove

material from the pot for use at a rate which is faster than the build-up of heat in

the container, which is tending to make the product unusable. If epoxy is simply

left in the container in large volumes, considerable heat will build up and an

uncontrollable 'exotherm' will result. This happens when the retained heat speeds

the reaction, which in turn generates more heat, further speeding the reaction, and

so on. If the user mixes only sufficient for use within the working time of the

product, none or little detectable heat will be felt. For larger volumes (over half a

litre), transferring mixed resin and hardener from the mixing vessel into a shallow

container or tray is strongly recommended.

Once the epoxy mix is in a thin film, the heat generated is readily liberated and the

material stays relatively cool, giving a suitably long working time. The shallow

tray pot-life can typically be ten times the life in the mixing pot before the material

becomes unworkable. In practice, one should always be conscious of the limited

amount of time available to both transfer the epoxy from the mixing pot to the

work and to manipulate the product on the working surface. The product data sheet

will give some guidance on this, but it is always a good idea for the sake of

economy to start with small volumes, until familiarisation will allow larger

volumes to be mixed.

TESTING

CHAPTER 6

TESTING

Fig 6.1 Universal Testing machine

Fig. 6.2 Model Specimen

Fig. 6.3 Testing Of Bonded Joint( Before Load Applied)

Fig. 6.4 Testing Of Bonded Joint( After Load Applied)

Fig. 6.5 Testing Of Riveted Joint(Before Load Applied)

Fig. 6.6 Testing Of Riveted Joint(After Load Applied)

Fig. 6.7 Testing Of Hybrid Joint(Before Load Applied)

Fig. 6.8 Testing Of Hybrid Joint(After Load Applied)

GRAPHS:

Fig 6.9 Sample Specimen1: Bonded Joint – Load Vs Displacement

Fig 6.10 Sample Specimen2: Bonded Joint – Load Vs Displacement

Fig 6.11 Sample Specimen1: Rivited Joint – Load Vs Displacement

Fig 6.12 Sample Specimen2: Rivited Joint – Load Vs Displacement

Fig 6.13 Sample Specimen1: Hybrid Joint – Load Vs Displacement

Fig 6.14 Sample Specimen2: Hybrid Joint – Load Vs Displacement

TEST RESULTS

SAMPLE ID DISPLACEMENT (mm) BREAKING LOAD(MPa)

BONDED JOINT 2.54 22.56

RIVETED JOINT 2.32 46.40

HYBRID JOINT 3.12 56.96

Table 6.1

Comparison Graph

Fig 6.15 Sample Specimen: Comparison Graph

Average Graph

Fig 6.16 Sample Specimen1: Average Graph

CONCLUSION

CONCLUSION

In the proposed work, FEA for the prediction of stress distribution in

bonded, riveted and hybrid joints has been carried out. 3-D models were created

and analyzed using ANSYS FEA software and also Tensile Strength of these joints

were carried out using Universal Testing Machine (UTM).

Thus from the present study, it was found that a well-designed hybrid

joint is very efficient when compared to bonded or riveted joints in the case of

repair situation in aircraft structures.

Through the experimental work the found that, the tensile specimens were

prepared with all above discussed joints and their test were carried out by

Universal Testing Machine with Data Aquisition System. The results says that, the

hybrid joint strentheing is more than the other two joints. Also through the hybrid

joints the application of the Aircraft and Automobile Structure is mostly acheived

tough the strenthen provided by the hybrid joints.

FUTURE SCOPE

In other ways these three joints were prepared and it could be undergoes for

further mechanical testing like Fracture, Impact and other destructive and non-

destructive tests.