purushottam(07d10028) ddp first stage report

8/12/2019 Purushottam(07d10028) DDP First Stage Report

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Dual Degree Thes

Department of Mechanical Engineering

is

Title:Composite Testing

Name:Purushottam Meena

Roll Number:07D10028

Guide:Prof. Anirban Guha

Co-Guide: Prof. Ramesh Singh


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Declaration Form

I Purushottam Meena Roll No. 07d10028 understand that plagiarism is defined as any one or

the combination of the following:

1. Uncredited verbatim copying of individual sentences, paragraphs or illustrations (such as

graphs, diagrams, etc.) from any source, published or unpublished, including the internet.

2. Uncredited improper paraphrasing of pages or paragraphs (changing a few words or

phrases, or rearranging the original sentence order)

3. Credited verbatim copying of a major portion of a paper (or thesis chapter) without clear

delineation of who did or wrote what. (Source: IEEE, The Institute, Dec. 2004)

I have made sure that all the ideas, expressions, graphs, diagrams, etc., that are not a

result of my work, are properly credited. Long phrases or sentences that had to be used

verbatim from published literature have been clearly identified using quotation marks. I

affirm that no portion of my work can be considered as plagiarism and I take full

responsibility if such a complaint occurs. I understand fully well that the guide of the project

report may not be in a position to check for the possibility of such incidences of plagiarism in

this body of work.

Signature:

Name: Purushottam Meena

Roll No.: 07D10028

Date:


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Acknowledgements

I would like to express my sincere gratitude towards my project guide Prof. Anirban Guha

and Co-guide Prof. Ramesh Singh for invaluable support and guidance throughout my stage 1

of Dual Degree Project.

I would like to thank Mr. Ganesh Soni(PhD Student-IITB-Monash) for his support and

valuable suggestions.

Purushottam Meena

(Roll no. 07d10028)


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Abstract

Fiber-reinforced composite structures have seen an increased application in aeronautics and

in other industries such as automotive, marine transportation, civil engineering, sporting

goods, medical equipment and prosthetic devices. With the increased use of composite

materials, there is a need to develop methods to predict the material properties and behavior

of composite materials and structures made of these materials under a variety of loading and

environmental conditions.

In this report chapter 1 incorporates introductory part of composite which includes properties

of composites, classification and application. Detailed study of CFRP is done. In chapter 2,

types of mechanical tests for composite are studied. Later literature review of tensile,compression, buckling and in-plane shear test has been carried out. In last chapter work done

and future proposed specimen manufacturing and experimental work is described.


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Content

Chapter 1. Introduction: composites.8

1.1 classifications of composites..8

1.1.1. Particle reinforced composites..8

1.1.2. Fiber reinforced composites..9

1.1.3. Structural composites9

1.2. Properties of composites..10

1.3. Application of composites...11

1.4. Fibre-reinforced plastic (FRP) ....13

1.4.1. CFRP (Carbon fiber reinforced polymer)14

1.5. Testing requirements in composites

1.6. Problem statement/specific objectives

1.7. Outline of report

Chapter 2: Mechanical testing of Composites and literature review..16

2.1. Mechanical testing of composites...16

2.1.1. Properties to be evaluated in testing16

2.2. Testing and literature review...17

2.2.1. Tensile testing..17

2.2.2. Literature review on tensile test..18

2.3. Compressive test..23

2.3.1. Types of Compression tests for Composites23

2.3.2. Literature review on compression test.26

2.3.3. Literature review on Buckling.33

2.4. In plane shear ..35

2.4.1. Shear Tests...35

2.4.2. Literature review for in-plane shear test.38

Chapter 3: Work done and Future work...42


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3.1. Work done...42

3.2.Future work..44


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List of figures

Figure:1.1 Classification of Composites 8

Figure:1.2 Types of composites 9

Figure:1.3 properties a) isotropic b)orthotropic c)anisotropic 11

Figure:1.4 Use of composite in military aircraft 12

Figure:1.5 Engine component made of composites 14

Figure:1.6 Use of Composites in sports 14

Figure:1.7 a) Basic structure Laminae b) Unbounded view of laminate construction 15

Figure:2.1 CFRP Specimen Shape 18

Figure:2.2 Schematic illustrations of laminates with various lay-up sequences 19

Figure:2.3 Configurations of (a) unnotched tensile, (b) notched tensile 19

Figure:2.4 a) Unnotched tensile fracture stress. b) Notched tensile fracture stress for

laminates

20

Figure:2.5 Coupons for testing the properties of CFRP 20

Figure:2.6 a) Stress-strain curves of CFRP b) The effect of strain rate on the tensile stress of

CFRP

21

Figure:2.7 a) Effect of strain rate on the modulus of elasticity of CFRP b) Effect of strain

rate on the strain to failure of CFRP

22

Figure:2.8 a) Experimental and predicted load-displacement curves b) Damage initiation

time of damaged laminae and interfaces

23

Figure:2.9 Progression of damage initiation in L1 23

Figure:2.10 Interface damage initiation and propagation in I1 24

Figure:2.11 a)Celanese test fixture b)Modified grips for IITRI

c)Northrop compression test specimen and fixture)

d)NBS compression test specimen and Fixture

26

Figure:2.12 a) SWRI compression test fixture b) Lockheed compression test fixture 26

Figure:2.13 a) Sandwich beam edgewise test configuration b) Four-point bend sandwich 27Figure:2.14 a) Longitudinal compression sandwich beam test specimen b) Stress distribution

in the thickness-wise direction

28

Figure:2.15 a) Four-point flexural test apparatus of sandwich specimen b) stressstrain curves

of compression test

29

Figure:2.16 (a) ICSTM compression test fixture and (b) clamping blocks specimen. 32

Figure:2.17 (a)Post-failure mode of unidirectional specimens b) loaddisplacement curve 33

Figure:2.18 Typical stressstrain curves of the unidirectional specimens 33

Figure:2.19 a) Sub laminate-level([45/90/-45/0]ns) b) Ply-level scaled ([45n/90n/-45n/0n]s) 34


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Figure:2.20 a) Plate geometry with parametric dimensions b) Top and bottom supports 35

Figure:2.21 a) Comparison critical buckling loads of unperforated composite plates.

b) Comparison of critical buckling ratio for various U-shaped cutouts

35

Figure:2.22 a) Two-rail shear apparatus and specimen.

b) Three-rail shear apparatus and specimen

c)Specimen geometry for two- and three-rail shear

36

Figure:2.23 a) Off-axis test specimen

b) Effects of end constraints on off-axis tensile specimens

37

Figure:2.24 a)Schematic of Iosipescu test fixture and specimen

b)Specimen configurations for determination of shear properties from the

Iosipescu test procedure

38

Figure:2.25 a) Specimen configuration of the off-axis tensile test.

b) Specimen configuration of the off-axis flexure test

40

Figure:2.26 Points of maximum normal and shear stresses in off-axis tensile specimen 40

Figure:2.27 off-axis tensile stresses as a function of the fiber orientation angle at failure load 41

Figure:2.28 Geometric loci of maximum normal and shear stresses 42

Figure:2.29 Experimental values of off-axis flexure stresses 42

Figure:3.1 machine design for testing 44

Figure:3.2 Specimen with different -2 cutouts 46

Figure:3.3 Specimen with varying angle of fibers 46

Figure:3.4 Specimen combining with different-2 angle of prepregs 47

Figure:3.5 Specimen with load axial and tranverse direction 47

Figure:3.6 Flow chart for implementation of the proposed proje 51

Figure 3.7 Timelines for proposed project 51


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CHAPTER 1

Introduction:

1. Composites Composites are combinations of two materials in which one of the

materials, called the reinforcing phase, is in the form of fibers, sheets, or particles, and is

embedded in the other materials called the matrix phase. The reinforcing material and the

matrix material can be metal, ceramic, or polymer. Typically, reinforcing materials are strong

with low densities while the matrix is usually a ductile, or tough, material. Examples of some

current application of composites include the diesel piston, brake-shoes, pads aircraft etc.

1.1 Classification of Composites On the basis of reinforced we can divided

composites in three parts which is shown in Figure 1.1.

Fig.1.1: Classification of Composites

1.1.1. Particle Reinforced Composites- These are the cheapest and most widely used. They

can be divided in two categories depending on the size of the particles: large-particle

composites, which act by restraining the movement of the matrix, if well bonded.

Dispersion-strengthened composites contain 10-100 nm particles. The matrix bears the

major portion of the applied load and the small particles hinder dislocation motion, limiting

plastic deformation. Particulate phase is harder and stiffer than the matrix. Examples:

Concrete (Matrix : Cement, Particulates : sand and gravel) Cermets- Hard Carbide Ceramic

embedded in Metal Matrix, used as cutting tools for hardened steel (Matrix : Cobalt,

Composites

Particle-reinforced

Large particleDispersion-

strengthened

Fiber-reinforced

Continuous

(aligned)

Discontinuous

(short)

AlignedRandomlyoriented

Structural

Laminatedsandwich

panels


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Particulates : Tungsten Carbide), TDNi (Thoria dispersed Nickel), SAP (Sintered Al.

powder)

Fig1.2:Types of composites a) Particle Reinforced Composite b ) Fiber Reinforced composites c) Structural

Composites [13]

1.1.2. Fiber Reinforced Composites: A fiber-reinforced composite (FRC) consists of three

components: (i) the fibers as the discontinuous or dispersed phase, (ii) the matrix as the

continuous phase, and (iii) the fine inter phase region, also known as the interface.

Reinforcing fibers can be made of metals, ceramics, glasses, or polymers that have been

turned into graphite and known as carbon fibers. Fibers increase the modulus of the matrix

material.

1.1.3. Structural Composites: Common structural composite types are two types:

1.1.3.1 Laminar: It Is composed of two-dimensional sheets or panels that have a

preferred high strength direction such as is found in wood and continuous and aligned fiber-

(a) (b)

(c)


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reinforced plastics. The layers are stacked and cemented together such that the orientation of

the high-strength direction varies with each successive layer. Example: Ski and plywood.

1.1.3.2. Sandwich Panels: They consist of two strong outer sheets which are called face

sheets and may be made of aluminum alloys, fiber reinforced plastics, titanium alloys, steel.

Face sheets carry most of the loading and stresses. Core may be a honeycomb structure which

has less density than the face sheets and resists perpendicular stresses and provides shear

rigidity. Sandwich panels can be used in variety of applications which include roofs, floors,

walls of buildings and in aircraft, for wings, fuselage and tail plane skins.

1.2. Properties of composites

Composite materials have many mechanical behavior characteristics that are different from

normal engineering materials. Some characteristics are merely modifications of conventional

behavior; others are totally new and require new analytical and experimental procedure

procedures. Composite material is often both inhomogeneous (or non homogeneous or

heterogeneous and nonisotropic (orthotropic or anisotropic):

Aninhomogeneousbody has nonuniform properties over the body, i.e., the properties

depend on position in the body. The orthotropic body has material properties that are

different in three mutually perpendicular planes of material property symmetry. Thus, the

properties depend on orientation of a point in the body. An anisotropicbody has material

properties that are different in all direction at a point in the body. No depends on material

property symmetry exist. Again, the properties depends on orientation at a point of body .

Fig 1.3: a)isotropic b)orthotropic c)anisotropic[29]

Because of the different properties from normal materials of composites materials,

they are conveniently studied from two points of view: micromechanics and macromechanics:


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Micromechanics is the study of composite material behavior wherein interaction of

the constituent materials is examined on a microscopic scale to determine their effects on the

properties of the composite material.

Macro mechanicsis the study of composite material behavior wherein the material is

presumed homogeneous and the effects of the constituent materials are detected only as

averaged apparent macroscopic properties of the composites material.

1.3. Application of composites: There are many reasons for the growth in

composite applications, but the primary impetus is that the products fabricated by composites

are stronger and lighter. Today, it is difficult to find any industry that does not utilize the

benefits of composite materials. The largest user of composite materials today is the

transportation industry. The composites application can be divided into the following industry

categories: aerospace, automotive, construction, marine, corrosion resistant equipment,

consumer products, appliance/business equipment, and others.

1.3.1. Aerospace Industry- The aerospace industry was among the first to use the benefits of

composite materials. Airplanes, rockets, and missiles all fly higher, faster, and farther with

the help of composites. Glass, carbon, and Kevlar fiber composites have been routinely

designed and manufactured for aerospace parts. The aerospace industry primarily uses carbon

fiber composites because of their high-performance characteristics. The composite

components used in fighter planes are horizontal and vertical stabilizers, wing skins, fin

boxes, flaps, and various other structural components. The use of composite materials are

used in spacecraft applications to include weight savings as well as dimensional stability.

1.3.2. Automotive Industry - Composite materials are used in some applications of theautomotive industry to deliver high-quality surface finish, styling details, and processing

options. Composite body panels have a successful track record in all categories from

exotic sports cars to passenger cars to small, medium, and heavy truck. Because the

automotive market is very cost-sensitive, carbon fiber composites are not yet accepted due to

their higher material costs. Automotive composites utilize glass fibers as main

reinforcements.


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Fig.1.4:Use of composite in military aircraft[18]

Fig: 1.5: Engine component made of composites [18]

1.3.3. Sporting Goods Industry -Sports and recreation equipment suppliers are becoming

major users of composite materials. The growth in structural composite usage has been

greatest in high-performance sporting goods and racing boats. We can see products such as


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golf shafts, tennis rackets, snow skis, fishing rods, etc. made of composite materials. These

products are light in weight and provide higher performance, which helps the user in easy

handling and increased comfort.

Fig.1.6: Use of Composites in sports [15]

1.3.4. Marine Applications - Composite materials are used in a variety of marine

applications such as passenger ferries, power boats, buoys, etc. because of their corrosion

resistance and light weight, which gets translated into fuel efficiency, higher cruising speed,

and portability. The majority of components are made of glass-reinforced plastics (GRP) with

foam and honeycomb as core materials.

1.3.5. Construction and Civil Structures- The construction and civil structure industries are

the second major users of composite materials. The driving force for the use of glass- and

carbon-reinforced plastics for bridge applications is reduced installation, handling, repair, and

life-cycle costs as well as improved corrosion and durability. It also saves a significant

amount of time for repair and installation and thus minimizes the blockage of traffic.

1.3.6. Industrial Applications - The use of composite materials in various industrial

applications is growing. Composites are being used in making industrial rollers and shafts for

the printing industry and industrial drive shafts for cooling-tower applications. Filamentwinding shows good potential for the above applications. Injection molded, short fiber

composites are used in bushings, pump and roller bearings, and pistons. Composites are also

used for making robot arms and provide improved stiffness, damping, and response time.

1.4. Fibre-reinforced plastic (FRP)

It is a composite material made of a polymer matrix reinforced with fibers. The fibers

are usually fiberglass, carbon, or aramid, while the polymer is usually an epoxy, vinylester or

polyester thermosetting plastic. FRPs are commonly used in the aerospace, automotive,


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marine, and construction industries. A polymer is generally manufactured by

polycondensation, polymerization or polyaddition. Fibre reinforced plastics are a category of

composite plastics that specifically use fibrous materials to mechanically enhance the

strength and elasticity of plastics. The original plastic material without fibre reinforcement is

known as the matrix. The matrix is a tough but relatively weak plastic that is reinforced by

stronger stiffer reinforcing filaments or fibers. The extent that strength and elasticity are

enhanced in a fibre reinforced plastic depends on the mechanical properties of the fibre and

matrix, their volume relative to one another, and the fibre length and orientation within the

matrix.

1.4.1. CFRP (Carbon fiber reinforced polymer)

It is a very strong and light fiber-reinforced polymer which contains carbon fibers.

The polymer is most often epoxy, but other polymers, such as polyester, vinyl ester or nylon,

are sometimes used. Although it can be relatively expensive, it has many applications in

aerospace and automotive fields, as well as in sailboats, and notably finds use in modern

bicycles and motorcycles, where its high strength-to-weight ratio and good rigidity is of

importance. Improved manufacturing techniques are reducing the costs and time to

manufacture, making it increasingly common in small consumer goods as well, such as

laptops, tripods, fishing rods, paintball equipment, archery equipment, racquet frames,

stringed instrument bodies, drum shells, golf clubs, and pool/billiards/snooker cues.

The physical properties of CFRP are anisotropic (different depending on the

direction of the applied force or load). The properties depends on of a CFRP panel will often

depend upon the orientation of the applied forces and/or moments and orientation of CFRP

Laminates.

To study mechanical behavior of CFRP we need to basic terminology of CFRP.

Laminae and laminates: Laminae is the basic building block of laminate which is a flat

(sometime curved as in a shell) arrangement of unidirectional fibers or woven fibers in a

matrix.


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(a) (b)

Fig:1.7 a) Basic structure Laminae b) Unbounded view of laminate construction[29]

A laminate is a bonded stack of laminae with various orientations of principal

material directions in laminae as shown in figure. The layers of a laminates are usually

bonded together by the same matrix material that is used in the individual laminae.

1.6. Requirement of composites testing

The use of composites in commercial aircraft, transportation, machinery, marine, and the

public works industries is increased too much. To use composite material in these fields, it is

necessary to determine quality or acceptability of specific components and to determine

intrinsic material properties such as modulus and strength for use in design and analysis. So

there is a need to develop methods to predict the material properties and behavior of

composite materials and structures made of these materials under a variety of loading and


1.5. Problem statement

As we know that use of composite material is increasing in automotive, marine

transportation, civil engineering, sporting goods, medical equipment and prosthetic devices,

so there is a need to develop methods to predict the material properties and behavior of

composite materials and structures made of these materials under a variety of loading and


In this research project, Experimental testing of will be carried out on CFRP composite

material varying specimen parameters like thickness of specimen, placing laminate in

different-2 angle, with cutout at different-2 place and varying depth and radius of cutout and


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varying strain rate. Then the results will be compared with numerical data which will be

obtained in ABAQUSsoftware to verify experimental results.

1.6. Outline of the report

The report has been divided into 3 chapters. Chapter 1 includes introductory part of

composite which includes properties of composites, classification, applications, CFRP

introduction, problem statement and the outline of the report. In chapter 2, types of

mechanical tests for composite are studied in detail. Later literature review of tensile,

compression, buckling and in-plane shear test has been carried out. Next chapter describe the

work done and future proposed work which includes methodology and flowchart of work.


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Chapter 2 : Mechanical Testing of Composites and Literature

Review

2.1. Mechanical testing of composites The use of composites in commercial

aircraft, transportation, machinery, marine, and the public works industries is increased too

much. So to use composite material in these fields, it is necessary to determine quality or

acceptability of specific components and to determine intrinsic material properties such as

modulus and strength for use in design and analysis.

2.1.1. Properties to be evaluated in Testing

We do experiments to find out to find out the strength and elastic propertiesof a

material. But as we know that by experiment tests we can only measure loads and

displacements. To find out the desired properties of composites from these parameters we use

the theory of elasticity for an anisotropic body. Fibrous composites with unidirectional,

laminated or spatial fiber lay-ups are inhomogeneous, are anisotropic materials. So to specify

the direction of load and its relationship with axes of elastic symmetry of the material, two

systems of coordinatesare introduced: the axes of elastic symmetry in the material (1, 2, 3)

and axes of loadingx, y,z . It is preferable to use methods in which the x, y,z axes coincidewith the 1,2,3 axes.

The majority of laminated and fibrous composites exhibit low interlaminar shear and

transverse tension strengths. Shear strength is characterized by the relations between

/x xz

E G (shear stiffness) and /u ux xz ; (shear strength). Transverse tension and compression

strengths perpendicular to the fibers are determined by the relations E x / Ez , /tu tu

x z ,

/tu cu

x z ; where Ex and Ez are the moduli of elasticity in the x and z directions; Gxz is

interlaminar shear modulus ux

and uz

are strengths in the x and z directions; uxz

is shear

strength in xz plane. Thex and y axes are located in the fiber lay-up (reinforcement) plane,

the z-axis is perpendicular to this plane; the (t) and (c) designate tension and compression,

respectively.


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2.2. Testing and Literature review

2.2.1. Tensile testing: A uniaxial tension test specimen has several functionally parts:

two loading sections, a gage section, and two transition sections. The loading sections

provide a means of fastening the specimen in the testing machine. They receive and transmit

the external loads to the gage section of the specimen. In the gage section, deformations are

measured and stresses are calculated according to the geometrical dimensions and external

load. The transition sections serve to attenuate stress-strain perturbations in the loading

section to isolate them from the gage section.

The greatest technical challenge in tension testing of composites, especially

unidirectional composites, is the reliabletransmission of tensile forces from the grips to the

specimen. This is generally performed through the use of friction forces. Tabsbonded to the

specimen improve the efficiency of load transmission considerably. For better grip we choose

the tabs made of a material that has a much lower modulus of elasticity and a higher total

elongation than the respective characteristics of the specimen material. Tabs have been made

of fiberglass reinforced composites, aluminum and wood veneers. The thickness of tabs

should be between 1.5t and 4t, where t is the specimen thickness. The tabs must have a large

enough area that the ultimate shear load capacity of the bond between the tabs and thespecimen is greater than the breaking load of the specimen gage section.

The mode of failure in tension depends on the relationship between the external load

and the reinforcing fibers and on the type of reinforcement lay-up. When unidirectional

composites are loaded in the reinforcement direction, they fail by breakage of the reinforcing

fibers. This is accompanied by transverse cracks, longitudinal shear cracks and

delamination of the polymer matrix. Increasing the angle between the load and the

reinforcing fibers causes the mode of failure to change gradually from shear and splitting of

the polymer matrix parallel to the fiber direction to pure transverse tensile crackingof the

polymer matrix. The failure mode of composites with balanced angle-ply reinforcement

depends on the angle of the fiber lay-up.


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Fig.2.1:CFRP Specimen Shape [?]

2.2.2. Literature review on tensile test

1) The effect of the lay-up sequences on the mechanical properties and fracture

behavior of the advanced carbon fiber-reinforced plastics (CFRP) composite was determined

in [1]. Specimens with angles of 0, 45 and 90between the fibers of the 0 layers and the

longitudinal direction of the specimen ((0), (45) and (90) specimens) were used. The

mechanical properties were evaluated by tensile test. he found that the tensile fracture stress

of the 0/90 ply-(0) and (90) specimens were about one-half that of the 0/0 ply-(0)

specimen.

Specimen used

Used specimen was 3.5-mm-thick carbon/epoxy laminates whose schematic illustrations are

shown in Fig 2.2(b) ; unidirectional (designated as 0/0 plies) orthotropic (designated as

0/90 plies) and quasi- isotropic (designated as 0/90/. 45 plies) laminates (20 plies) of

0.205 mm prepregs consisted of long carbon fibers (6 7 m in diameter)

Types of specimen used 1) unidirectional (designated as 0/0 plies)

2) orthotropic (designated as 0/90 plies)

3) quasi-isotropic (designated as 0/90/ 45 plies)

Fig2.2: Schematic illustrations of laminates with various lay-up sequences. Fig 2.3: Configurations of (a)

unnotched tensile, (b) notched tensile,[1]


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Configuration of specimens

1) unnotched- width 7 mm thickness- 3.5mm Tab length- 60 mm gage length- 20

2) notched -width 11 mm thickness- 3.5mm Tab length- 60 mm gage length- 20

notch 2mm with 45 angle

Result

So he found out as he increase angle from 00 to 900Tensile fracture stress decreased. As it

was above 1000MPa for 00and at 900it decreased to less than 100mpa.Tensile fracture stress

of the 0/0 ply-(0) specimen at and above 1000 MPa was independent of specimen type, but

that of the 0/0 plies-(45) and (90) specimens was less than 100MPa.The anisotropy of the

properties for the 0/90 plies was improved compared to that of the 0/0 plies. However,

the tensile fracture stress of the 0/90 ply- (0) and (90) specimens was only about one-half

that of the 0/0 ply-(0) specimen. The tensile fracture stress of the 0/90 ply-(45)

specimen was less than 200 MPa. The 0/90/ 45 ply specimen exhibited an isotropy of the

properties, but its fracture stress was about 40% of that of the 0/0 ply-(0) specimen.

Fig 2.4: a) Unnotched tensile fracture stress for laminates with various lay-up sequences. b) Notched tensile

fracture stress for laminates with various lay-up sequences[1]

2) The mechanical properties of unidirectional normal modulus carbon fibrereinforced polymer (CFRP) sheet under quasi-static and medium impact tensile loads with

different strain rates was experimentally investigated in [2]. He found that mechanical

properties are strain rate dependent. The tensile stress, modulus of elasticity and strain at

failure obtained at 3 strain rates (54.2, 67.2 and 87.4 s-1) were compared with those obtained

from quasi-static tests to demonstrate the influence of strain rate on mechanical properties

Test specimenMaterial- fibre reinforced polymer sheet (CF130).


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Process and shape size- wet-lay-up process was used to manufacture a test panel (thin

laminate carbon/epoxy) which was later cut into several coupons. Steel tabswere bonded on

both sides at the ends of specimens to avoid damage on gripping CFRP sheets. Length for test

specimen is 138 mm.

Fig.2.5: Coupons for testing the properties of CFRP [ 2 ]

Equipments and test procedure

The drop mass rig which was modified to carry out the impact tensile tests was utilized for

testing CF130. The strain rate for the quasi-static tests was 2.42 x 10-4 s-1. The strain rates

for the impact tests were 54.2, 67.2 and 87.4 s-1. The different strain rates were generated by

dropping a mass of 156kg from various heights (0.375m, 0.575m and 0.975m).

10 static tests were conducted in an Instron testing machine to serve as reference tests. The

measured properties include tensile stress, modulus of elasticity and strain at failure.

The influence of axial strain rate on the stress-strain curve of carbon/epoxy specimens isillustrated in Fig.2.6 (a). The effect of strain rate on tensile stress,the tensile stress against the

strain rate is presented in Fig.2.6 (b) for all the aforementioned strain rates. It is clear that

there is a general trend of increase in tensile stress as the strain rate increases. The increase in

tensile stress varies from 20% to 43% when compared with that under quasi-static load.

Increase in modulus of elasticity was about 20% when the strain rate was above 52.4 s-1.

Fig 2.7(b) illustrates the influence of strain rate on the strain at failure for CF130.

Again there is a general trend of increasing strain at failure as the strain rate increases. For

strain rate of 54.2 and 67.2 s-1 the increase is about 9%. A greater increase (24%) was found

when the strain rate reached 87.4 s-1.


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Fig 2.6: a) Stress-strain curves of CFRP for various strain rates b) The effect of strain rate on the tensile stress

of CFRP [2]

Fig 2.7: a) The effect of strain rate on the modulus of elasticity of CFRP b) The effect of strain rate on the strain

to failure of CFRP[2]

3) Tensile behavior of the 17-ply anti-symmetric carbon/epoxy composite which

were arranged in the ply sequence of [-602/0/605/0/-605/0/602] subjected to tensile load was

examined, subjected to uniaxial tensile loading both experimentally and numerically in [4].

The author described the damage behavior of the plies, whereas damage initiation and

progression of the interfaces. Force displacement curves obtained numerically and

experimentally. His Results shows that all laminae and interfaces experienced the damage

except laminae with 0o fibre. In addition, damage was concentrated at the tab and central

regions of the tensile specimen. Edge delamination was observed in all interfaces. He

observed that damage was first initiated at the tab region, which slowly-2 propagated and

accompanied by damage at the central region. He observed for interfaces delamination also

occurs along the edges.


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Results: Load-displacement curves: The measured and predicted load-displacement

responses of the composite specimen under tensile loading was compared which is shown in

Fig.2.8 (a). Gauge length of 3 mm specimen was used. The linear plot suggests that no

apparent damage is observed during loading until the final catastrophic fracture occurs. Thepeak load at fracture was compared well to predicted load magnitude. The difference could

be due to inability of the numerical computation to account for the load carrying ability of the

fibre after complete failure of the matrix.

Fig 2.8: a) Experimental and predicted load-displacement curves b) Damage initiation time of damaged laminae

and interfaces[4]

Damage behavior of laminae and interfaces: Evolution of internal states and

damage in the laminae and interfaces throughout the loading was predicted by the finite

element model. Fig.2.8(b) describes the corresponding time at damage initiation of each

lamina and interface. Damage is initiated in all laminae and interfaces except laminae with 0 o

fibre. It is predicted that only tensile matrix failure occurs in the lamina during the tension

test.

Next, the progression of damage in L1is described. Fig.2.9 illustrates the sequence

of matrix damage propagation with continuously increasing tensile loading of the specimen.

Results show damage is initiated at near tab region (Fig.2.9 (a)). Then, the stresses and thus

matrix damage variable are increased uniformly across the whole lamina (Fig.2.9 (b)). This is

followed by matrix damage initiation at isolated region nearby (Fig. 5. (c)) that propagated to

the edges (Fig.2.9 (d). Fig.2.9.(e) illustrates the damage contour at peak load, where matrix

cracking at the central region is observed. Besides, anti-symmetric feature of the lamina is

obvious as reflected in anti-symmetric distribution of matrix cracking damage.


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Fig.2.9: Progression of damage initiation in L1[4]

Fig.2.10: Interface damage initiation and propagation in I1 [ 4 ]

Fig.2.10 describes the initiation and progression of delamination damage in I1. Since damage

is initiated at the edges near the tabs (circled in red in Fig.2.10.(a)), delamination propagates

across the width and connects after that (Fig.2.10. (b)). It is accompanied by edge

delamination along the edges. Then, delamination is predicted to propagate slightly further

from the tabs (Fig.2.10. (c)). When peak load is attained, delamination is found at the central

edges of the specimen (Fig.2.10. (d)).

He observed that damage is first initiated at the tab region, which slowly-2 propagated and

accompanied by damage at the central region. He observed for interfaces delamination also

occurs along the edges.

2.3. Compressive test

2.3.1. Types of Compression tests for CompositesIn compression testing of unidirectional composites in the fiber direction, three basic modes

of failure are observed: buckling of the reinforcing fibers, transverse cracking of the matrix,

and shearing of reinforcing fibers at a 450angle without local buckling of the reinforcement.

Transverse cracking is caused by differences in the Poisson's ratios of the material

components and by a non-uniform transverse strain distribution along the specimen length.

Materials reinforced at an angle to the specimen's longitudinal axis fail in shear without

crumpling at the end faces because the entire shear load is taken up by the matrix. Theaforementioned basic modes of failure can be accompanied by a series of other phenomena:


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inelastic and non-linear deformation of the reinforcing fibers and matrix, delamination,

surface peeling, overall buckling and crushing of the end faces. Failures with different

combinations of these phenomena can make the determination of the failure mode very

difficult. In compression testing, great care must be taken to ensure stability of the specimen,

especially in the gage section. Buckling of the specimen side face is not always detectable

and will cause erroneous strain measurements. Special test fixtures are used to prevent overall

buckling of the specimen. There arethree accepted test methods. Each is briefly outlined and

schematic diagrams of grip arrangements are presented. Strain gages are generally used for

each of these test methods.

Type 1

This method is characterized by having a completely unsupported specimen with a relatively

short test section length. Several types of fixtures exist for this method. The Celanese (ASTM

D-3410-75) test fixture and associated specimen geometry are shown in Figure 2.11(a). The

Illinois Institute of Technology Research Institute (IITRI) test fixture uses a test specimen

identical to the Celanese fixture and is shown in Figure 4.11(b).

Fig.2.11: a)Celanese test fixture and specimen (ASTM D 3410-75) b)Modified grips for IITRI compression test

c)Northrop compression test specimen and fixtured) d)NBS compression test specimen and

Fixture[19][20][21][22]

(a) (b)

(c) (d)


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Strain gages are mounted on the specimen, which is loaded through serrated wedges

constrained by solid steel bases. The Northrop test fixture is simpler than the Celanese or

IITRI fixtures and is shown in Figure 2.11(c) The final example of typeI compression testing

is the NBS (National Bureau of Standards) test fixture.This fixture combines aspects of the

Celanese and IITRI fixtures and adds features that allow for tensile tests. The NBS fixture is

shown in Figure 2.11(d). All four of the Type I test methods yield acceptable results, but are

difficult to conduct because of load line eccentricity.

Type 2

In this class of tests the specimens are characterized as having a relatively long test

section that is fully supported. The SWRT (Southwest Research Institute) and the Lockheed

type fixtures are schematically shown in Figures2.12 (a) and (b), respectively. Results from

experiments using these grips are comparable to data from Type I tests. The SWRI grip has a

cut in one support to allow for a transverse gage to measure Poissons ratio in compression.

Longitudinal strain is measured by an extensometer or strain gage placed on the edge of the

specimen. The specimen is a modified tensile specimen in which the overall length is reduced

while the end tab lengths are increased. The entire specimen length is supported by the

fixture. The Lockheed fixture uses side supports only over the gage section of the specimen,

which is the primary difference between it and the SWRI fixture.

Fig 2.12: a) SWRI compression test fixture b) Lockheed compression test fixture [22][23]

Type 3

The final class of compression test methods involves two sandwich beam specimen

configurations. In each case straight-sided coupons are bonded to a honeycomb core, which

supplies lateral support. The elastic moduli and Poissons ratio are determined from

(a) (b)


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relationships between applied loads and strain gage readings taken from the specimen

.Results of failure strengths from this method are usually higher than those from the other

methods. The sandwich beam method can also be used to determine tensile properties. The

two specimen configurations are shown schematically in Figure 2.13. The specimen in

Figure2.13.(a) is referred to as the edgewise compression test specimen and is used to

determiney

E , and yx from the initial linear portion of the load-displacement curves

generated during testing.

Fig.2.13: a) Sandwich beam edgewise compression test configuration b) Four-point bend sandwich beam

compression test[26]

The applied load is assumed to be distributed equally between the top and bottom specimens.

The core is assumed to carry no in-plane load and is intended to supply lateral stability so that

the potential for buckling is reduced. The elastic modulus and Poissons ratio are determined

from strain gage readings to be

x

yx

y

= .2.1

2 2

y

y

y y y

P PE

A bh

= = =

.2.2

The specimen in Figure 2.13(b) is somewhat different because it is loaded in four

point bending. The specimen is the top sheet, which experiences compression. The bottom

face sheet is in tension and is metal. Since the sandwich beam is subjected to flexure, various

parameters (metal face sheet strength, core cell size, etc.) can be changed to achieve the

desired compression failure of the specimen.Poissons ratio for this specimen is determined

from direct strain gage readings to be /xy y x = . The elastic modulus Ex, is somewhat

harder to establish since it requires an assumption of uniform deformation in each face sheet

while bending stresses in the core are neglected.

(a) (b)


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2.3.2. Literature review

1) Nonlinear mechanical behaviors of unidirectional composites under

compressive loading using a sandwich beam specimen with the emphasis on the elastic

material nonlinearity in the fiber direction was studied in [6]. Measured compressive

behaviors were compared with those from coupon-type compression tests of unidirectional

and quasi-isotropic laminates. The compressive strength of unidirectional laminates

evaluated, using the present sandwich method, was much higher than that of coupon

specimens, whereas the failure strain of the former was almost identical to that of coupon

type quasi-isotropic laminates.

Test specimen

A sandwich beam flexure specimen consisting of unidirectional CFRP and honeycomb core

was utilized to measure compressive response and strength of 00 laminates. In order to

prevent unwanted failure modes (e.g., core failure, skin/core debonding) Sandwich

construction was applied only to the gauge section of the specimen used here. In addition,

sandwich specimen with a relatively thin core wad used in his[6] study to make the ratio of

span length to specimen thickness large so that applied load in four-point bending is small

enough not to induce failure near load pins (even when flexible cushion is placed between

load pins and specimens).

Fig2.14: a) longitudinal compression sandwich beam test specimen b) Assumed stress distribution in the

thickness-wise direction.[6]

In figure parameters are hf(mm)- 1.1 hc(mm) = 5.0 ho(mm)= 2.2 lc(mm)= 20 lp(mm) =

25 lm(mm)= 105.

This study used a T800H/3633 carbon/epoxy. An aluminum honeycomb (1/8-5052-002,

Showa Aircraft Industry Co. Ltd.) is used as the core of the sandwich beam in the gaugesection. Sandwich beams were manufactured in the manner that the longitudinal direction of

(a) (b)


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the honeycomb core coincides with the [0]8 direction of CFRP. In order to prevent failure in

the tension side, [0]8 laminates and [0]16 laminates were used in the compression side and

tension side, respectively. The rest of the core consists of [0]32 laminates. The fiber volume

fractions are almost identical between the three unidirectional laminates (Vf=57%). CFRP

laminates and aluminum core were bonded using structural epoxy film adhesives (AF163-2K,

3M) in order to prepare sandwich specimens with 280mm length, 20mm wide, and 8mm

thickness.

Experimental Procedure - The manufactured specimen and test apparatus is shown in

Figure 2.15(a). Flexible rubber sheets were placed between the load pins and specimens in

order to avoid unwanted failures near contact regions. Strain gauges were attached onto both

surfaces in the central gauge section. The four-point flexure tests of the sandwich specimens

were performed using a mechanically driven machine (4482, Instron) until failure. The

crosshead speed was set to be 2 mm.

Fig.2.15: a) Four-point flexural test apparatus of sandwich specimen b)Experimental stressstrain curves of

compression test.[6]

Test result - The measured compressive stressstrain relations of the three specimens are

shown in Figure2.15 (b). Clear nonlinear response can be observed in high strain ranges.

Compressive failure occurred in the gauge section. No damage was observed in the

honeycomb core until skin compressive failure. The tensile strain in the opposite skin was

less than 1% at specimen failure, which was low enough compared to the tensile failure strain

to be considered as no tensile damage in the opposite skin. By this results he concluded that

this is the fairly adequate compressive testing of unidirectional laminates could be performed.

The average compressive strength and failure strain were 1860MPa and 1.67%, respectively.

The coefficient of variation estimated from three specimens is relatively small (5.4%).


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The results obtained from this experiment were compared with coupon type test

methods. Which has same volume fraction with unidirectional [0]20laminates. The stress

strain relation of coupon specimen coincides well with that of sandwich specimen, which

shows the validity of the present test method.

However, compressive strength of the sandwich specimen was found much higher

than that of the coupon specimen, at 1860MPa for the sandwich and 1420MPa for the

coupon.

The effect of specimen size on the axial compressive strength of IM7/8552 carbon

fibre/epoxy unidirectional laminates (UD) was experimentally studied evaluated in [5].

Laminate gauge length, width and thickness were increased by a scaling factor of 2 and 4

from the baseline specimen size of 10 mm * 10 mm * 2 mm. In all cases, strength decreased

as specimen size increased, with a maximum reduction of 45%; no significant changes were

observed for the axial modulus. Optical micrographs show that the failure mechanism is fibre

micro buckling accompanied by matrix cracking and splitting. The location of failure in most

specimens, and especially the thicker ones, was where the tabs terminate and the gauge

section begins suggesting that the high local stresses developed due to geometric

discontinuity contribute to premature failure and, hence, reduced compressive strength. Two

generic quasi-isotropic multi-directional (MD) lay-ups were also tested in compression,

one with blocked plies [45n/90n/-45n/0n]s and the other with distributed plies [45/90/ /45/0]ns

with n = 2, 4 and 8.Strength results showed no evidence of a size effect when the specimens

are scaled up using distributed plies and compared to the 2 mm thick specimens. All blocked

specimens had similar compressive strengths to the sub laminate ones apart of the 8 mm

specimens that showed a 30% reduction due to extensive matrix cracking introduced during

the specimens cutting process. The calculated unidirectional failure stress (of the 00 ply

within the multidirectional laminate) of about 1710 MPa was slightly higher than the average

measured value of 1570 MPa of the 2 mm thick baseline unidirectional specimen, suggesting

that the reduced unidirectional strength observed for the thicker specimens was a testing

artefact. It appears that the unidirectional compressive strength in thicker specimens (>2 mm)

is found to be limited by the stress concentration developed at the end-tabs and

manufacturing induced defects such as fibre misalignment, ply waviness and voids.


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Experimental procedure

Specimen:

Material: The specimens were fabricated from carbon/epoxy pre impregnated tapes 0.125

mm thick. The tapes were made of continuous intermediate modulus IM7 carbon fibers pre-

impregnated with Hexcel 8552 epoxy resin (34 vol% resin content).

Specimen type: 1.Unidirectional plates [04]nswith n = 2, 3, 4, and 8.(thickness)

2. Quasi-isotropic

1. Blocked plies [45n/90n/-45n/0n]s

2. Distributed plies [45/90/-45/0]nswith n = 2, 4 and 8

Specimen geometry-

Design of a compressive test specimen certain constraints considered: (i) end-tabs were

required both to effectively transfer load from the test machine grips to the specimen and to

provide an adequate restraint against specimen buckling; (ii) the overall specimen stability

needs to be sufficient to ensure that compressive failure occurs significantly before potential

buckling, implying correct combination of specimen length and flexural stiffness.

The maximum allowable specimen gauge section length was determined on the

basis of an Euler column buckling analysis assuming a pinned end strut with a rectangular

cross-section and corrected to account for the influence of shear deflection due to transverse

shear forces developed in anisotropic materials.

Critical gauge lengths for plain specimens

Thickness (mm) 2 4 6 8 10

Lmax(mm) 13.9 27.6 45.9 55.9 69.9

Minimum gauge length for waisted specimens

t0(mm) 1.34 2.67 4 5.34 6.67

Lmin (mm) 8.2 16.6 24.9 33.1 41.5

In order to avoid failure occurring at the junction of end-tab and the gauge section

or within the tabs, then a specimen with waisted gauge section could be considered. Waisting

the specimen through its thickness, which is in fact an optimized form of tabbing, has to be

carried out carefully in order to maintain a symmetric specimen, otherwise one would be

likely to introduce bending and cause premature failure.

Test fixture and mechanical tests


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Generating a uniform one-dimensional stress state was the main objective of a compression

test fixture. Stress concentrations due to load introduction present the most serious problems

for unidirectional lay-ups and may cause premature failure of the specimen. Currently, there

are no universally accepted test standards for testing specimens thicker than 2 mm. In[5]

compression tests on the unidirectional specimens were performed using the Imperial College

ICSTM test jig at a constant compression rate of 1 mm/min on a servo-hydraulic machine

with a load capacity of 1000 kN, which is shown in Figure 2.16.

By this fixtures specimen was loaded purely on the ends. However, a small amount

(in the region of 10%) is applied by shear loading via the end-tabs, thus lowering the average

stresses at the end of the test piece. The fixture consists of two grip blocks, Fig. 2.16 that

accommodates the specimen and prevent debonding of the tabs from the specimen end, shear

failure of the end or compression failure under the tab. The clamping blocks rest on hardened

and ground steel plates, a measure that was necessary to avoid indentations of the loading

surfaces. The high precision die set shown in Fig. 2.16(a) was designed to eliminate specimen

misalignments. The lower grip was not attached to the lower plate of the die set in order to

minimize additional constraints during testing, like bending of the specimen due to

misalignment between the upper and lower grips. The bolt torque applied in the clamping

blocks for the 2 mm thick specimen was in the region of 810 N m and increased slightly for

the thicker specimens. An advantage of the fixture was that by adjusting the size of the

clamping blocks a variety of specimen sizes can be accommodated; it was also used to test

multidirectional laminates.

Fig.2.16: (a) ICSTM compression test fixture and (b) clamping blocks for a 40 mm 40 mm 8 mm

specimen.[5]


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Foil strain gaugeswere used on both faces to tested degree of Euler bending and measure

axial strain and, hence, axial modulus. The location and nature of damage in the UD

laminates was obtained by optical and scanning electron microscopy (SEM).

Compressive test results: The experimental results consist of stressstrain plots,fracture stresses and strains for the unidirectional and multidirectional laminates, scanning

electron micrographs of some of the fracture surfaces, and photographs showing the overall

failure mode of selected specimens.

Unidirectional laminates: Initial compression tests on unidirectional specimens with

relatively thin end-tabs showed that failure occurred within the tabbed region, Fig. 2.17(a),

resulting in relatively lower compressive strengths (2030% lower than expected). It appears

that damage initiated on the end of the specimen (top corner) at the load introduction point

and propagated down the length and across the width of the specimen. Due to the clamping

constraining effect failure was progressive in nature, resulting in a more ductile load

displacement curve, Fig. 2.17(b), rather than the relatively brittle catastrophic failure that is

usually observed when the specimen breaks within the gauge section or near the end-tab.

Fig 2.17:(a)Post-failure mode of unidirectional specimens b) loaddisplacement curve of a 4 mm thick

specimen that failed within the tabbed region[5]

Representative stressstrain curves of 2 mm (plot A), 4 mm (plot B) and 8 mm (plot C) thick

unidirectional specimens obtained at the centre of each specimen from back-to-back strain

gauges are shown in Fig.2.18.


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Fig.2.18:Typical stressstrain curves of the unidirectional specimens obtained from back-to-back strain gauges

(A: 10 mm *10 mm * 2 mm, B: 20 mm *20 mm * 4 mm and C: 40 mm *40 mm* 8 mm.).[5]

Multidirectional laminates -stressstrain curves of the 2 mm, 4 mm and 8 mm thick

multidirectional specimens laminated with distributed plies [45/ 90/45/0]ns (sub laminate-

level scaling)and blocked plies [45n/90n/45n/0n]s (ply-level scaling)stacking sequences are

shown in Fig. 2.19:a and b, respectively. Plots B for the 4 mm thick specimen and C for the 8

mm thick specimen are offset by 0.5% and 1.0% strain, respectively.

Fig.2.19: a) Sub laminate-level([45/90/-45/0]ns) laminate b) Ply-level scaled ([45n/90n/-45n/0n]s) laminate[5]

2.3.3. Buckling

The influence of cutout shape upon the buckling stability of multilayered

rectangular epoxy plates reinforced by glass fiber, with different orientation angles was

studied in [17].U-shaped cutouts were made on the long sides symmetrically. The

investigated plates were simply supported on the loaded edges (i.e. short sides) and free on

the unloaded edges. The plates without cutouts were examined theoretically to confirmexperimental and Finite Element (FE) results. The FE and experimental results were found


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out for different U-shaped cutouts sizes. U-notch shape effects are examined depending on

notch depth and notch root radius. It was found that the effect of notch depth was stronger

than that of the notch root radius on buckling loads of plates. But, in some cases, although

plates containing U-notch, no reduction was obtained in buckling loads.

Fig 2.20: a) Plate geometry with parametric dimensions b)Top and bottom supports[17]

Specimen-

Material- LY5082 epoxy resin matrix (Ciba Geigy) and E-Glass fiber.

glass fiber volume fractions- 30%.

Process used lay- up

Specimen size- 200mm long*100mm wide *2.25 mm thickness

unperforated (r=0, a=0), and three type notched panels with r=1 a=0, r=1 a=1, and r=1 a=2

cm .The panels consists three plies which have 0, 30, 45, 60 and 900fiber orientation angles.

Test fixture and testing speed- INSTRON 4301 universal uniaxial tensioncompression

testing machine was used. Fixtures were designed to apply compression load with simply

supported edges. The unloaded edges of the panel were free. The panels were loaded slowly

at 1 mm/min rate and critical buckling load were obtained. To determine the initial buckling

load two methods were used, firstly the point of inflection in the plot of load against in-plane

displacement and secondly the point of reversal in the membrane strain.

Results-


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Fig. 2.21: a) Comparison of FE, experimental, and theoretical critical buckling loads of unperforated composite

plates. b) Comparison of critical buckling ratio for various U-shaped cutouts, based on fiber orientation angle

of ply.[17]

2.4.In Plane Shear

2.4.1. Shear Tests The material properties in the plane of lamination (1 -2) are

commonly termed in plane, while those in the 1-3 and 2-3 planes are known as inter laminar

properties. There are some commonly accepted methods for in-plane shear testing and out

plane testing which are presented here.

1) Shear Rail Test-

There are two acceptable configurations for the shear rail test: two-rail and three-rail. A

schematic of the load fixture for each is shown in Figures 2.22(a) and (b), respectively. For

both tests the shear stress in the strain gaged region of each specimen is defined in terms of

the applied load P and the specimen thickness (h), as well as the distance between each

vertical rail (b). The shear stress for each configuration is approximated by

xy

p

bh = (Two rail ) ..2.3

2xy

p

bh = (Three rail).2.4


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Fig.2.22: a) Two-rail shear apparatus and specimen.(ASTM D-30) b) Three-rail shear apparatus and specimen

c)Specimen geometry for two- and three-rail shear tests[24][25]

Free surfaces at the top and bottom of each specimen experience large normal stresses

concentrated at the corners.A length-to-width ratio of 10:l has been shown to approximate a

state of pure shear stress, provided the edges are perfectly clamped. The requirement of

perfect clamping can be met if the bolts in the rails each apply the same clamping pressure to

the edges. Since a state of pure shear is only approximated with the two- and three-rail

configurations, a single element strain gage oriented at 450 to the load axis may not

adequately define the true state of strain.

2) 100Off-Axis Test: An off-axis test is generally performed in order to establish stress-

strain responses in directions other than the principal material directions. The off-axis test is

a tension test and no special fixtures or specimen preparation is required. Consider the

unidirectional test coupon loaded as shown in Figure 2.23(a). The rectangular rosette in this

figure is not required for establishing G12. Its presence is solely for the purpose of indicating

that an off-axis test can be used for defining more than one parameter. The strains indicated

by each gage in the rosette are related to Cartesian (x-y) strains by the strain transformation.

The relations between gage strain and the Cartesian strains are A = x , y = C , and

2xy B A C = , where A , B , and C are the strains indicated by gages A, B, and C,

respectively. The normal stressx

, and strainx

, (from strain gage measurements) are related

by /x x xE = .


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variety of material properties than other procedures. Analysis of the procedure has led to the

evolution of several specimen and fixture geometries.

Fig:2.24: a) Schematic of Iosipescu test fixture and specimen b) Specimen configurations for determination of

shear properties from the Iosipescu test procedure.[26]

4) [ 4 5 ]2s Coupon Test: This procedure involves a uniaxial tension test of a [ 45]2s

laminate, with strain gages. Although a biaxial rosette is sufficient, a three-element rosette

provides additional information that can be used to verify the state of stress in the specimen.

Specimen preparation and testing are identical to a conventional tension test. Results from the

[

45]2stest are in good agreement with those from other procedures, and it is considered tobe a reliable test configuration

2.4.2. Literature review for in-plane shear test

In-plane Shear Strength of Unidirectional Composite Materials Using the Off-axis Three-

point Flexure and Off-axis Tensile Tests was determined and compared in [9]. In the case of

the off-axis three-point flexure test, the condition of small displacements and the condition of

lift-off between the specimen and the fixture supports were taken into account. Some

considerations regarding stress and displacement fields were presented. The in-plane shear

characterization were performed on a carbon fiber reinforced unidirectional laminate with

several fiber orientation angles: 100, 200, 300, and 450.Test conditions for both off-axis

experimental methods, in order to ensure their applicability, are presented. By investigation

results the author found out Off-axis flexure test more suitable than off-axis tensile test for

the determination of in-plane shear strength.

Material and Apparatus


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Material - Toughened epoxy matrix-based carbon fiber reinforced. All specimens had the

same Cross section- width b =12.5mm and thickness h=1 mm. with off-axis fiber orientation

angles: 100, 200, 300, and 450, with different length-to-width ratios. A universal testing

machine Instron- 4206 with two load cell of 100 and 1 kN, and standard test fixtures, were

used.

Geometric parameters in off-axis tests:

Off-axis tensile test Off-axis flexure test

h L (mm) L (mm) c L0(mm) L (mm) c cLO

100 275 175 14 90 80 6.5 6.34

200 275 175 14 70 60 4.6 3.19

300 250 150 12 60 50 3.8 2.25

450 200 100 8 50 40 3.1 2.20

Off-axis Tensile Test

A cross-head speed of 1 mm/min was considered for tensile tests. Both the nominal gage

length L and the nominal overall-length L of the tensile specimens are consigned in Table

(Figure 2.25) as well as the nominal length-to-width ratio c. All authors suggest a high

length-to-width ratio, in order to reduce the end-constraint effect: cmax 9. cmax=10 , cmax

16, cmax=18 and cmax=28. The gripping system consisted of sandpaper placed between the grip

and the specimen along the gripping length H presented in Figure 2.25(a). For all off-axis

tensile tests H=50 mm.

Fig.2.25: a) Specimen configuration of the off-axis tensile test. b) Specimen configuration of the off-axis flexure

test.[9]

Off-axis Flexure Test

The flexure tests were performed at a nominal strain rate of 1%/min. The nominal span L, the

nominal overall-length L, and span-to-width ratio c of the flexure specimens showed in fig.


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In order to increase the shear stresses at the failure critical point the span-to width ratio must

be as high as possible, bearing in mind the small displacement condition, ensuring that the

maximum deflection during the test do not exceed the 10% of the span.

Result

Off-axis Tensile Test

there are three critical points in the off-axis tensile test: point F near the gripping zone, point

M located along the longitudinal middle line of the specimen, and point F located along the

longitudinal edge of the specimen where it is assumed that failure starts.

Fig.2.26:Points of maximum normal and shear stresses in off-axis tensile specimen[9]

The longitudinal normal stress, transverse normal stress, and the in-plane shear stress

as function of the fiber orientation angle, at points F, M, and F, are presented in Figure 2.26.

Results show that all stress components are higher at F than at F and at M. In spite of the fact

that all in-plane stress components are a little greater at point F than at point F, the failure

starts at point F due to the greater probability of containing machining defects through the

longitudinal edge of the specimen.

Fig:2.27: Experimental values of off-axis tensile stresses as a function of the fiber orientation angle at failure

load: (a) longitudinal normal stresses, (b) transverse normal stresses, (c) in-plane shear stresses.[9]


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Figure 2.27(c)According to [9] results it was revealed that in-plane shear stress

decreases as the fiber orientation angle increases, due to increasing influence of Tas seen in

Figure 2.27(b), and that determination of (in plane shear strength)XLTby means of the off-

axis tensile test provides a conservative value. It is recommended to consider low fiber

orientation angles (e.g., =100). Comparing in-plane shear stress at critical points with the

nominal interfacial shear stress it can be affirmed that, on the one hand, 0 is quite similar to

( LT)Mspecially for high off-axis angles. On the other hand, ( LT)Fand ( LT)Fare higher

than 0in all cases. Regarding the in-plane shear strength experimentally obtained by the off-

axis tensile test, it can be inferred from the results that the present approach, considering

XLT=( LT)F, gives values up to 24% higher than values obtained with the conventional

approach, based on 0 .

Off-axis Flexure Test

In-plane shear strength by the off-axis flexure test was determined at critical point K and K.

Figure 2.28 shows the longitudinal normal stress, the transverse normal stress, and the in-

plane shear stress as a function of the fiber orientation angle at both K and K points.

Fig.2.28: Geometric loci of maximum normal and shear stresses in the off-axis flexure specimen[9]

Figure 2.29(a) shows that longitudinal normal stress decreases as the fiber orientation angleincreases, and Figure 2.29(b) indicates that transverse normal stress has the opposite trend; it

increases as increases. These results lead to conclude that for fiber orientation angles

between 100and 450, for the span-to-width ratios considered, failure is mainly due to shear

stresses. In-plane shear stresses at points K and K for all analyzed orientations are depicted

in Figure 2.29(c), and compared to the reference value 114 MPa.


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Fig.2.29: Experimental values of off-axis flexure stresses as a function of the fiber orientation angle at failure

load: (a) longitudinal normal stresses, (b) transverse normal stresses, (c) in-plane shear stresses.[9]


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Store the parameter ,non-loss electric, simple operating process Lead mechanism of

translation .radium of rotation and motion parameter is set up , can memory, avoid the

reset parameter when open and close PC, avoiding the error from artificial.

According to the needs we can choose pulses, mm, microns for motion unit. pluses: The basic unit of controller.

Mm/microns: Show the Linear displacement.

Any position can be set to be the user working origin position. The convenience of setting to

zero position is to simplify the experiment and reduce time.

Real-time read the control system logical position, real position, drive speed and the

acceleration during the movement, which will help us to find out the instantaneous speed

and position of fixtures.

Parameters can be stored and with memory function in case of power-down.

Fig 3.1: Set-up for testing


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3.2. Future work: It is divided in four parts

1) Specimen

2) Fixture

3) Testing parameters

4) Experiment

1) Specimen

1) Material:Raw Materials for type of CFRP prepregs has to be choose according to

availability of material and our process.

2) Process: Prepreg lay-up process will be chosen as it is Simple to manufacture

complex parts easily using this process.It has the advantage of low tooling

cost. Very strong and stiff parts can be fabricated using this process.

3) Cutting process: The test coupons will be cutout from stacked composite sheets

using required precision machining process.

2) Fixtures: Fixtureswill be chosen according to our experiments. We have three option

for compressive test:1) Celanese (ASTM D-3410-75) test fixture, Illinois Institute of

Technology Research Institute (IITRI) , Northrop compression test fixture and NBS

compression test specimen so on the basis of our requirement and for better result best

fixture will be chosen. For type 2. Experiment we have option of a) SWRI compression test

fixture b) Lockheed compression test fixture . For In-plane we will have to decide the fixture.

3) Testing parameters:

We will do experiment on the specimen by varying following parameters of the CFRP

specimen.

1) Specimen with cutout at different-2 place, with varying shape of cutout and depth of

cutout.


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Figure3.2: Specimen with different -2 cutouts

2) Specimen with varying thickness and placing the laminas in different -2 angles

witdifferent-2 number of lamina stakes.

Radius

L

b

Depth of cut

Thickness ofLaminate(t)

t


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Figure.3.3.:Specimen with varying angle of fibers

Figure:3.4. Specimen combining with different-2 angle of prepregs

900

600

300

Prepreg

with angle

900

Prepreg

with angle

00

Prepreg

with angle45

0


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3) Angle between load and fiber direction.

Figure:3.5. Specimen with load axial and transverse direction

4) Experiment- Experiment will be done at various parameters which is decided above to

determine the influence of fiber orientation on the CFRP mechanical properties. Each of the

specimens will be tested in a random order and the stress and strain will be calculated from

the load and displacement results.

The results will be compared with numerical data which will be obtained in abaqussoftware to verify experimental results.

Flow chartfor implementation of the proposed project in shown in figure: 3.6.

F

F

FF


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Fig.3.6: Flow chart for implementation of the proposed project

Figure 3.7 shows the start and completion of various tasks mentioned in Figure.3.6 The

entire project duration is 8 months which is divided into 8 quarters.

Task/Month Oct Nov Dec Jan Feb Mar Apr Jun

Task1 Done

Task2

Task3

Task4

Task5

Task6

Fig.3.7: Timelines for proposed project

Expected outcome from the project

After successful completion of the project in mechanical testing of CFRP in varying fiber

orientation and varying thickness.Following specific objectives are identified with the above

mentioned study:

Detailed understanding about mode of failures during testing of specimen.

Detailed understanding of mechanical properties of CFRP laminates with varying

parameters(thickness, fiber orientation, strain rate) in specimen

Validation of experimental data with modeling data .

Task1:

Literature survey

Task2:

Selection of materialand manufacturing of

specimen

Task3:

Design,Fabrication andinstrumentation ofexperimental setup

Task4:design anddevelpoment of

specimen

Task5:

Experiments onspecimen

Task6:

Experimantal dataproceesing and

analysis

Task7:

Publishing results


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References

1) Kojiro morioka,Yoshiyuki tomita, Effect of lay-up sequences on mechanical properties

and fracture behavior of CFRP laminate composites,, journal of Materials Characterization

45 (2000) 125-136, 29 February 2000.

2) Haider AL-Zubaidy et al, Mechanical Behavior of Normal Modulus Carbon Fibre

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Engineering 10 (2011) 18651870

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19521960

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purushottam(07d10028) ddp first stage report

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