fracture and failure analyses of plastics and reinforced plastics

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Fracture and Failure Analyses of Plastics and Reinforced Plastics

Dr. K. PadmanabhanProfessor & Assistant Director

School of Mechanical & Building Sciences VIT-University, Vellore-632014

Email: padmanabhan.k@vit.ac.in

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Definitions

• Fracture is the creation of at least two surfaces from a body due to the action of forces.

• Fracture results in the release of stored elastic energy ( strain energy) and creation of surfaces with a surface energy.

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Contents

• Definitions• Structure of Plastics and Reinforced Plastics• Property Correlations• Introduction to Fracture and Fracture Mechanics• Basics of Failure Theories and Analyses• Test Specific Failure Characterization• Application Specific Failure Characterization• Feedback from Failures and Correlation• Failure processes, Improved Designs and

Manufacturing• Summary

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Structure and Properties

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Chemical Groups and Bond Energy

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Polymer Chain Structure

Intermolecular and Super molecular Structures haveInfluence on structure, processing and properties

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Stereoisomerism

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Types of Copolymers

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0.51 nm

Schematic diagram of Kevlar® 49 fiber

showing the radially arranged pleated sheets

Microstructure of aramid fiber

Solvent Spun Kevlar fibres

3.5 GPa of Tensile Strength and E= 130 GPa

For reference material on Kevlar see bibliography

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Kevlar Fibre Fibrillation

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Zylon Fibrewww.toyobo.co.jp

ZYLON   consists of rigid-rod chain molecules of poly(p-phenylene-2,6-benzobisoxazole)(PBO).

Tensile Strength : 5.8 GPa

Tensile Modulus : 270 GPa

ILSS with Epoxy : ~ 40 GPa

Ref: K. Padmanabhan , Toyobo Confidentiality Report, 2002.

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Types of Foams

• Thermoset, thermoplastic, elastomeric, ceramic, rock wool, metallic….

• Rigid, semi rigid, flexible….• Filled and unfilled• Poisson’s ratio variations• Auxetic, syntactic ….. • Formed by for example Polyol, MDI

Reactions like in PUF

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Closed pore rigid foam

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Open pore rigid foam

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SEM of a PU Foam

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PUF 125 DLS Foam Fracture

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Poisson’s Ratio and Foams

Shear strain at maximum load vs foam density

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Rigid Thermoset Unfilled Foam

PUF 500 PUF 125

PUF 250 PUF 64

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Polymer Crazing

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Fracture Testing

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Fracture Modes

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Static mechanical properties

• Tensile properties• In accordance with ASTM D3039 standard• Specimen preparation, adhesive bonding at the

end grips• Recommended cross head speed of 0.02

cm/min• Strain gaging, rosette strain gaging.• 0 and 90° (fibre direction) tests for E11and E22..

Similarly for major and minor poisson’s ratios .

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Tensile Failures

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Tensile cracking

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Failure and Survival

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Micro buckling and Design • Micro buckling can be

prevented by choosing higher fibre diameter, higher fibre elastic modulus and high interfacial bond strength between fibre and matrix

• Boron fibres are the best for design against micro buckling

• High pressure Compression moulding with superior bond strength

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Mechanical Testing

Ref: K. Padmanabhan and Kishore , ` Failure behaviour of carbon/epoxy composites in pin ended buckling and bending tests’, Composites, Vol:26,No: 3, 1995, p201.

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Asymmetric Hybridization

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ILSS of Kevlar/Epoxy Composites

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Damage in Fatigue

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Damage in Fatigue

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Damage in Fatigue

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Damage in Fatigue

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Multiple Cause Fatigue

• Thermomechanical Fatigue• Fatigue –Vibration Interactions• Thermal Fatigue with Moisture Attack• Fatigue after DBT• Hygrothermal Fatigue• Creep –Fatigue Interactions.• Electro-hygrothermo-mechanical Fatigue.

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Impact Behaviour of Plastics and Reinforced Plastics

• Izod Impact• Charpy Impact• Drop Weight Impact• Low Velocity Impact Tests• Repeated Impact Tests• Medium and High Velocity Impact Tests• Crashworthiness Tests• Ballistic Impact Tests.• Single Point Bird Hits .

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Tensile Impact

The tensile impact test is a unique test. It is the extensional answer to crashworthiness. Characterized by gross and fast fibre pull out and interfacial Fracture, it is very dependent on the interfacial shear stress in UD conditions !

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Creep of Plastics and FRPs

The regular creep curve of a metal is shown in the left with the creep curve of A plastic or an FRP shown on the right . The tertiary creep rupture characteristicOf metals is not prominent in plastics.

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Fracture Mechanics

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Fracture Mechanics

Fracture energy in plane stress

K ( SIF ) in MPa √M = Y σ √π a, where Y~1.12 , σ ( Fracture stress) and a= semi crack length If in the centre and crack length if an edge notch

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Single Edge Notch Test

Double Edge Notch Test

Double Cantilever Beam

Tapered DCB

Wedge opening Load

Three Point Bend

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Fracture Mechanics Testing

Mixed mode testing of compositematerials is a recent trend as causes are multiple during failure in real conditions

Fracture mechanics tests to evaluate fracture toughness and strain energy release rates have found quite a few standards in ASTM

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Double Cantilever Beam Test

Modified Beam Theory

Compliance Theory

Modified Compliance Theory

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End Notched Flexure Test

Modified Beam Theory

Compliance Theory

Modified Compliance Theory

ASTM Standards

Russel’s Theory

Direct Beam Theory

Cohesive Zone Models

Mixed Mode Models ………

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Glass/ Epoxy MWCNT Multiscale Composites

The mode 2 fracture toughness/ SIF can also be found out byUsing the ILSS specimens in the flexural testing using an edge notch.

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Advances in Sandwich Composites

•Develop fracture mechanics test methods for sandwich composites•Focus on facesheet core delamination•Both Mode I and Mode IISuitable for ASTM standardization

Flexural Failure of Vacuum Bagged PIR foam / aluminium skin sandwich composite

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Fractography and Failure Analyses

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Steps for Failure Analyses

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Shear Banding of Polystyrene

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Glass/Epoxy Sheared Sample

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Fractography of Fatigue

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Fatigue Striations

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Fatigue Damage in Carbon/Epoxy

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Mode I and Mode 2 Fracture of FRPs

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Impact of Epoxy-Polycarbonate Blends

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Adhesively bonded joints

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Common Failure Modes

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Bolted Joints Failures

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Bolted and Bonded Composite Joints

Joint testing of a composite lug

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Natural and Bio Derived Plastics and Composites

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Cashew Nut Shell Oil as Composite Matrix Material

• Cashew nut shell oil can be polymerized using acids, toluene as inhibitor and formaldehyde at 120 celsius.

• A tough and strong maroon coloured matrix !

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CNSL Matrix Material

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Fracture mechanics of CNSL-Glass Fibre Composites

The single edge notch fracture mechanics test of Glass fabric/CNSL CompositeProves that CNSL can be tougher than basic epoxies ( and termite resistant with Low moisture absorption and degradation properties.)

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Chemical Composition of Some Vegetable Fibres

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Main physical properties of cellulose based fibres

compared with conventional synthetic fibres

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Natural Fibre Cross Section

Confocal Laser Scanning Microscope (CLSM) images

Non-uniform cross sectionsprovide interesting interfacialproperties and other mechanicalproperties , different ROM !

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Silk fibre properties

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Spider Silk

Spider silk is sometimes stronger than silkworm silk.It may be 1.4 GPa in tensile strength compared to 500 MPa for the mulberry silkworm produced silk. It is a myth that natural fibres are weak !

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Spider Silk

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Self Reinforced Natural Composites

• The same material as the fibre and the pulp matrix

• The fibre matrix-interface is interesting• Weight and cost savings• Interesting Properties !• Bio derived self reinforced polyethylene

from sugar cane

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Positive Hybrid Effect

• Synergy in Properties• Cellulosic Interfaces• Silane and Other Interfaces• Shear to Tensile Strength Ratios• Fracture Behaviour• Crack tip blunting, Fracture energy• Underlying Mechanisms

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Failures and Multiple Causes• In nature, most of the real environment failures

are due to multiple causes.• As enough standard procedures are available

for failures due to a single cause, the obvious move is to develop the understanding as well as standard procedures for failures due to multiple causes.

• An understanding of these failures leads to a better fracture control that aids superior designs with advanced composite materials and processing.

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Temperature Distribution

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Thermal – Structural Results

Displacement Vector sum Von mises stress

Stress intensity XY Shear stress

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Failure theory for 4x4mm 24L TQLMP

Peak conditioning i.e 125°C for 24 hrs X stress in MPa = 70.509Y stress in MPa = 98.811XY shear stress in MPa = 45.793Tensile strength in MPa =27.01Compressive strength in MPa = 28.47Shear strength in MPa = 13.5

Tsai – Wu Failure theory: 22.29> 1Azzi-Tsai-Hill theory: 22.047(Tensile) / 21.083(compressive) > 1Design is unsafe at 125ºC but safe at MOT.

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SAM Picture for 24L TQLMP after preconditioning, No Plasma* Cleaning

Red areas show delaminations in IC Packages

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Thermal Shock Test (JESD22 – A106B)

• Purpose of this test is to determine the resistance of the part to sudden

exposures of extreme changes in temperature and alternate exposures

to these extremes as well as its ability to withstand cyclical stresses

• Here the IC packages are baked in an oven for 125ºC/24 Hrs and the

temperature is spiked to 260ºC for lead free product and 240ºC for

leaded product for 5 to 10 minutes.

• If the baking temperature is higher than the glass transition temperature

at this extreme heat the package tends to delaminate or fail. This failure

or delamination can be viewed using SAM (Scanning Acoustic

Microscopy)

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Non Destructive Evaluation

Ultrasonic C-Scan NDT can do depth profiling of composites delamination and damage profiling

Scanning acoustic microscope can sense delaminations of micron dimensions ( shown as red areas) in TQLMP IC packages. FEA techniques are also NDE techniques.

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FEA as an NDT • Numerical method used for solving problems

that cannot be solved analytically (e.g., due to complicated geometry, different materials)

• Well suited to computers• Originally applied to problems in solid

mechanics• Other application areas include heat transfer,

fluid flow, electromagnetism

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Failure Theories

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Azzi-Tsai-Hill Failure Theory

Where, σ11 = X ( tensile / compressive) stress in MPa σ22 = Y ( tensile / compressive) stress in MPaτ12 = Shear stress in MPaSLt = Longitudinal tensile strength in MPaSTt = Transverse tensile strength in MPaSLts = In-plane shear strength in MPa

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Tsai – Wu Failure Theory

F1 σ11 + F2 σ22 + F6 τ12+ F11 σ112+ F22 σ22

2+ F66 τ122+2 F12 σ11σ22 = 1

Where,Other parameters / symbols appear on the previous slide,SLc = Longitudinal compressive strength in MPaSTc = Transverse compressive strength in MPa.

Narayanaswamy and Adelman have suggested F12 = 0

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Hygrothermal Behaviour • Fibre reinforced plastics are known for environmental

attacks that reduce their function.• Mechanical properties degrade over time !• Moisture plays havoc at elevated temperatures, in the

presence of voids, defects and in low Tg plastics . So do UV, IR, alkali , acid and marine environs.

• Diffusion and osmotic pressure are the driving mechanisms for hygrothermal attack

• Evaluation methods and surface preservation • Important in marine, biomedical, aeronautical, electronic

and automobile applications

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Moisture Absorption

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Maximum Moisture Content

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Variation of Tg with Moisture

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ASTM STP D 5229 M Rule• The MOT( Maximum Operating Temperature)

of the material, device/component should be at least 25 º Celsius lower than the lowest Tg (normally wet) of the material attained after hygrothermal equillibration. All the polymeric materials and their composites must satisfy this rule in order to qualify for certification for reliability and durability.

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Current Research

Machining

You start machining a monkey and due to structural change end up with a parrotAnd report the properties as that of a monkey ! Machining induced fracture and failure depends on structure and then the machining parameters. Any optimization be should based on this truth !

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Failure Processes and Feedback

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The Process of Failure Analyses

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Boeing FALN

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Fracture Mysteries

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Bibliography• P,K. Mallick, Fibre reinforced composites, Marcel and

Dekker Inc., New York .• Derek Hull and T.W. Clyne, ` An Introduction to

composite materials’, Cambridge solid state science series, 1996.

• E.J. Barbero, `Introduction to composite materials design’, Taylor and Francis ,MI.

• J.K. Kim and Y.W. Mai, `Engineered interfaces in fibre reinforced composites ‘, Elsevier, 1998.

• www.wikipedia.org• Rao Tummala, Microsystems Packaging,McgrawHill.

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Bibliography• Sanjay K Mazumdar, Composites Manufacturing, CRC

Press, 2002.• Geoffrey Pritchard, Reinforced Plastics Durability,

Woodhead publishing,Cambridge, England, 1999.• ASTM Standards Handbooks Vols: 08.01,08.02 and

08.03, PA, USA.• www.astm.org• ANSYS v.14 Analysis Manuals, 2013.• Rayner M Mayer, Design with Reinforced Plastics,

Design Council, London.• Characterization and Failure Analysis of Plastics, ASM,

2003, USA.

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Bibliography• Padmanabhan Krishnan, Linked In Web Page. • Introduction to Polymer Science and Chemistry, Manas

Chanda, Taylor and Francis, 2006, USA.• K. Padmanabhan, S.Subeesh, K. Balaguru and T.

Karthik , ` An Analyses of Reliability and Hygrothermal Effects in IC packages’, in ANSYS Users’ Conference CD, 6 & 7 November 2008, Bangalore. BEST PAPER AWARD

• K. Padmanabhan, D. Sanjay and S Subeesh,` Design and electro-hygrothermo-mechanical reliability analyses of a leadless quad IC package’, in the

• PADMANABHAN K, SASHIDARA. S and KISHORE , “ STUDIES ON FLEXURE OF TWO DIFFERENT WOVEN FABRIC KEVLAR/EPOXY COMPOSITES.” , MATERIALS FORUM, 15 (1991), p354-359

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Bibliography• PADMANABHAN K AND KISHORE, “INFLUENCE OF CIRCULAR DEFECTS

ON THE FLEXURAL STRENGTH OF KEVLAR/EPOXY COMPOSITES.” JL. OF REINFORCED PLASTICS AND COMPOSITES ( American society of composites) , VOL: 11 (1992), p 211- 219.

• PADMANABHAN K and KISHORE , ‘ FLEXURAL STUDIES ON ASYMMETRIC HYBRID KEVLAR/EPOXY COMPOSITES. ‘ JL. MATER. SCI., 27 ( 1992 ) p 4282-4286 .

• PADMANABHAN K, “ FLEXURAL BEHAVIOUR OF UNMODIFIED AND MODIFIED FIBRE REINFORCED EPOXY BASED COMPOSITES.” JL. OF INDIAN INSTITUTE OF SCIENCE, 72 (1992) p464-466.

• PADMANABHAN. K AND KISHORE , `LOOPING BEHAVIOUR OF KEVLAR FIBRES’, SCRIPTA MET et MATER. 28 (1993) p367-370.

• PADMANABHAN K and KISHORE , “ FIBRE MATRIX INTERFACIAL FAILURE SEQUENCES IN TRANSLAMINAR FLEXURE OF GLASS/EPOXY COMPOSITES.” JL. MATER. SCI .29 (1994) p 33-38.

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Bibliography• PADMANABHAN K and KISHORE, “ ON THE ELASTIC -

PLASTIC BEHAVIOUR OF WOVEN FABRIC KEVLAR/ EPOXY COMPOSITES ”., INTL. JL OF FRACTURE, VOL:65, No: 3, (1994) R59-61.

• K PADMANABHAN and KISHORE , “ INTERLAMINAR SHEAR BEHAVIOUR OF WOVEN FABRIC KEVLAR/ EPOXY COMPOSITES. IN THREE POINT LOADING” MATER. SCI. & ENGG. PART A ,197 ( 1995) p113-120

• . M. MATHESWARAN , K PADMANABHAN and KISHORE, “ STATIC AND IMPACT BEHAVIOUR OF THERMOPLASTIC MODIFIED GLASS/EPOXY COMPOSITES ”. JL. OF MATER. SCI. LETT. 14 ( 1995) p 951-953.