final reports - analysis & testing without image - joints -
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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.
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