The document discusses fracture and failure analyses of plastics and reinforced plastics, highlighting definitions, structures, and properties of materials, as well as testing methods and failure theories. It covers various types of materials, including Kevlar and natural fibers, along with their mechanical behavior under different conditions. The document aims to provide insights into improving design and manufacturing processes based on failure feedback and analysis.

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Fracture and Failure Analyses of
Plastics and Reinforced Plastics
Dr. K. Padmanabhan
Professor & 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 have
Influence 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 Fibre
www.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 characteristic
Of 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 composite
materials 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 by
Using 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 II
Suitable 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 Composite
Proves 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 sections
provide interesting interfacial
properties and other mechanical
properties , 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 = 70.509
Y stress in MPa = 98.811
XY shear stress in MPa = 45.793
Tensile strength in MPa =27.01
Compressive strength in MPa = 28.47
Shear strength in MPa = 13.5
Tsai – Wu Failure theory: 22.29> 1
Azzi-Tsai-Hill theory: 22.047(Tensile) / 21.083(compressive) > 1
Design 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 MPa
τ12 = Shear stress in MPa
SLt = Longitudinal tensile strength in MPa
STt = Transverse tensile strength in MPa
SLts = In-plane shear strength in MPa

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Tsai – Wu Failure Theory
F1 σ11 + F2 σ22 + F6 τ12+ F11 σ11
2
+ F22 σ22
2
+ F66 τ12
2
+2 F12 σ11σ22 = 1
Where,
Other parameters / symbols appear on the previous slide,
SLc = Longitudinal compressive strength in MPa
STc = 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 parrot
And 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.