EBB 220/3 FAILURE IN POLYMERS

1
EBB 220/3FAILURE IN POLYMERS

• DR AZURA A.RASHID
• Room 2.19
• School of Materials And Mineral Resources
  Engineering,
• Universiti Sains Malaysia, 14300 Nibong Tebal, P.
  Pinang
• Malaysia

2
Importance of mechanical properties of materials
in engineering

• Need to acquire knowledge of the properties of
  materials ? The correct selection of a material
  for a given application.
• Mechanical properties data were used to predict
  the response of materials under mechanical loads.
• Expressed in terms of forces which may deform
  materials or even cause them to fail completely.
• To avoid failure and keep deformation under
  control so the individual system components
  remain functional as parts of a whole ? need a
  various considerations
• Is stiffness / rigidity important? (i.e. minimum
  deformation under a given load)
• Is strength essential? (for maximum tolerance of
  loads before failure)

3

• The questions we may have to ask are
• What is the nature of the load?
• Continuous and uniform or rising steadily
• IMPACT (e.g. hammering action, accidental drop)-
  Alternating (periodic application of a force)
• FATIGUE (e.g. vibration, rotation in loaded
  components)
• The geometry of the loaded component ? can be
  designed to deal with these conditions.
• The physical nature of the material ? has to
  ensure that the component can survive in service.
  
• Cost and component weight ? when evaluating and
  selecting materials, with the use of indices such
  as
• Modulus-to-density ratio
• design for stiffness, in weight-critical
  applications ?example an aircraft

4
Fundamental concepts for mechanical properties

• Below are some terms we find in dealing with
  materials in relation to structural applications
  
• Stress
• Strength
• Strain
• Stress-strain relationships
• Modulus
• Concept of deformation
• Deformations can be produced by forces ? which
  cause a body to be stretched, compressed, twisted
  or sheared.
• These forces can also be combined to produce more
  complex types of deformation ? for example
  flexural.

5
Unloaded
Stretched (Tension)
Squeezed (Compression)
Twisted (Torsional shear)
Cut (Simple shear)
6

• Extension by stretching in one direction ?the
  simplest type of deformation that can be used to
  explain key concepts in mechanics

Rectangular specimens subjected to different
loads in tensile mode
7
Stress

• Stress is the force exerted on a body per unit
  cross sectional area.
• By stretching a body using a force (the force is
  weight), the tensile stress (in the direction of
  elongation)?
• If the force applied is 100 N (Newtons), and the
  cross sectional area measures 0.0004 m2 (square
  metres), the stress becomes
• or 250 KN/m2, or 0.25 MN/m2. If the force doubles
  (200 N), stress will increase accordingly to 500
  kN/m2.
• We could also double the level of stress by
  reducing the cross sectional area to half of its
  original value, i.e. to 0.0002 m2.

8

• If the same weights were placed on the
  rectangular specimens to cause a contraction in
  the longitudinal direction ? the resulting stress
  would be called compressive stress.
• The other common type of stress is shear stress.
• This relates to the force which distorts rather
  than extends a body ? example where a solid
  section is sheared,
• Shear forces can also result in failure.

Cylindrical specimen subjected to simple shear,
e.g. during cutting.
Everyday example of shear failure
9
Strength

• Concept of strength ? the influence of the
  cross-sectional area on the force which
  ultimately causes the material to fail.
• Strength defined ? the highest stress that a
  material can withstand before it completely fails
  to perform structurally.
• If the applied force is tensile (stretch) ? the
  ultimate stress is known as tensile strength
  (i.e., maximum tensile stress that the material
  can tolerate).
• Others types of strength are related to the mode
  of the applied force ?compressive, shear,
  torsional and flexural.
• Use the following expressions
• A strong material ? can withstand a very high
  force per unit area before it fails.
• A weak material ? markedly deteriorates or fails
  at relatively low levels of applied forces.

10
Strain

• To understand the effect of specimen size on the
  amount of deformation resulting from force ?use
  the concept of strain.
• Strain ? the change in one dimension produced as
  a result of an applied force and it is expressed
  as the ratio of the amount of deformation to the
  samples original dimension.
• In the case of tension,
• Strain is often expressed as i.e. the strain
  multiplied by 100.
• Assuming the force applied causes the original
• length of 0.5 m to extent to a new length of 0.9
  m ?
• then the strain becomes

11
Stress-strain relationship (below failure
conditions)

• Materials deform elastically or inelastically.
• During elastic deformation ? the stress in a body
  is directly related to the strain, and
  vice-versa.
• When the force is removed (i.e. when stress
  becomes zero) then strain returns to zero.
• The plot of stress against strain produces a
  straight line ?
• the stress can be increased or decreased, and
• stress and strain are always proportional to
  each other.

Linear elastic stress-strain relationship
12

• For ductile materials ? increasing the stress
  above a certain limit will give rise to inelastic
  deformations, known as yielding.
• when the stress is removed ? the strain does
  not return to zero (and the original shape is not
  fully restored)
• some deformation has permanently set in.
• The stress level at which this occurs is referred
  to as the yield stress or yield point.

• The applied force takes the material
• beyond the linear elastic region.
• Continued loading causes permanent
• deformation.

The amount of permanent deformation is evident
after the force applied is removed.
13
Modulus

• The relationship between stress and strain is
  expressed in terms of a property called the
  Modulus (or Young Modulus).
• The linear portion of the stress-strain curve can
  be used to determine the modulus ? correspond to
  the slope of the curve before the yield point, up
  to which all deformation is elastic and
  recoverable.
• In other words,
• The slope (modulus) ? at any point in the linear
  portion of the line gives the same result.
• The modulus ?denotes stiffness or rigidity for
  any kind of applied load, i.e. tension,
  compression or shear.
• Stiff materials have a high modulus ? the
  deformation (strain) resulting from the applied
  force (stress) is low.
• Flexible materials have a low modulus ? undergo
  large deformations with relatively low applied
  forces.
• Modulus of Elasticity ? for materials deformed
  in tension or compression.
• Modulus of Rigidity ?used to express the
  resistance to shear or torsion.

14
Assessment of mechanical properties

• The simple tests used to measure mechanical
  properties are described in standard test
  methods.
• The most widely used are the ASTM tests ?
  nowadays these are gradually being replaced by
  ISO procedures
• The most common types of test performed on
  plastic materials
• Tensile properties
• Flexural properties
• Impact strength

15
Tensile properties

• Tensile properties are determined using
  dumbbell-shaped specimens.
• The type defined in the ASTM D-638 standard is
  as shown in the diagram below
• In a tensile experiment the specimen is gripped
  firmly by mechanical jaws at the wide portion on
  either side and extended by means of a tensile
  testing machine
• The pulling is normally carried out at a constant
  rate of 0.50, 5.0 and 50 cm/min, depending on the
  type of plastic being tested.
• The low speeds ? to test rigid materials
• the higher speeds ? to test flexible materials.

16

• Calculated entities
• Tensile stress ? measured the force at any time
  divided by the original cross sectional area of
  the waist portion.
• Tensile strain ? the ratio of the difference in
  length between the length marked by the gauge
  marks and the original length,
• Yield strength sY ?ultimate tensile strength
  (strength value prior to fracture), st
• Elastic modulus, E ? ultimate elongation (strain
  value at fracture), et

Typical stress-strain curves for a brittle
material (1) and a ductile material (2)
Note that in the diagram above yield stress
is only specified for the ductile material ?as
the brittle material fails catastrophically
without reaching the yielding conditions.
17
Flexural properties

• Flexural properties are important in assessing
  the resistance of materials to bending.
• A typical experimental set-up is as the one shown
  in the schematic below
• Specimen dimensions may vary but the use of bars
  with a cross section measuring 1.27 0.32 cm and
  span of 5.0 cm.

Flexural test experimental set-up
18
Calculated entities

• The maximum stress caused by bending is
  calculated by the following formula
• where
• S stress (N/m2)
• F load or force at break or at yield (N)
• L span of specimen between supports (m)
• b width (m)
• d thickness (m)
• If the load recorded corresponds to the value at
  failure occurs ? S corresponds to the flexural
  strength.

• The maximum strain due to bending (compression
  and tensile is estimated by
• where
• e strain (dimensionless i.e., no units)
• D deflection at the centre of the beam (m)
  see schematic below
• d thickness (m)
• L specimens length of span between supports
  (m)

• The flexural modulus from the recorded load (F)
  and deflection (D) is

19
Impact strength

• The energy used by the pendulum hammer to
  fracture the specimen (see diagram) is given by
  the reduction in the height of the hammer in its
  swing after fracturing the specimen ?
• Where
• m mass of pendulum hammer
• g acceleration due to gravity (9.8 m/s2)
• ho initial height of pendulum hammer (m)
• hf height of the pendulum hammer after
  fracturing specimen
• The specimen geometry is taken into account in
  terms of the cross-sectional area which has
  undergone fracture.
• The impact strength is defined as the energy
  divided by the area ? joules/m2.
• Note Because the distance from the notch tip to
  the edge of the specimen is constant, sometimes
  the impact strength is expressed as the energy to
  fracture per unit thickness.

20
Charpy test configuration
Apparatus to measure impact strength
Izod test configuration
21
Deformation of polymers

• Permanent deformations ? Yielding
• Mechanical properties at the surface ? Hardness,
  Friction, Wear
• Special issues in designing with polymers ?Creep
  and Stress Relaxation
• Factors that determine the resistance of
  polymeric components to deformation
• Enhancement of the resistance of polymers to
  deformation

22
Yielding of polymers

• Yielding is a phenomenon closely related to the
  onset of permanent deformation, i.e. an
  irreversible process.
• This is due to molecular chains unfolding and
  becoming aligned in the direction of the applied
  load.
• Yielding under a tensile load is shown below

• The progress of the yielding process for a
  specimen under tension 
• A prior to loading
• B onset of necking in the waist
• region after the yield point
• C neck propagation ("cold drawing")
• D neck extension and fracture

23

• In non-crystalline (amorphous) polymers ?
  yielding occurs by molecular uncoiling.
• At the yield point ? a neck forms which is
  followed by an overall drop in stress.
• At the neck region ? the folded chains become
  aligned.
• Macroscopically ? because of the thinning down
  in cross section,
• the stress rises locally and any deformation
  occurs preferentially there.
• This helps the neck propagate along the waist of
  the specimen under a steady load ?a process known
  as cold drawing
• Any deformation produced beyond the yield point
  ?is not recoverable.
• In a crystalline polymer ?
• the unfolding of chains begins in the amorphous
  regions between the lamellae of the crystals.
• this is followed by breaking-up and alignment of
  crystals

24
Alignment of molecular chains in polymer
crystals progress A-D same as aforementioned
25

• Points to note
• Yielding is a ? phenomenon which is responsible
  for ductile deformations,
• as opposed to brittle fracture.
• the degree of ductility of a polymer ? often
  controlled by a number of variables

26

• The deformation behaviour of polymers is time and
  temperature dependent, ? specimen may be ductile
  or brittle, according to the testing conditions
  strain rate and temperature.
• If the temperature is sufficiently high and/or
  the strain rate is slow enough ?
• the specimen is ductile and will yield
  extensively.
• The yield stress and stiffness increase and
  ductility decreases with lowering the temperature
  or increasing the strain rate.
• Under extreme strain rates, as under impact
  conditions ? specimen may be unable to undergo
  cold drawing and become brittle

Tensile stress-strain behaviour at high strain
rate and/or low temperature(A) low strain rate
and/or high temperature (B)
27
Hardness, Friction Wear

• These three surface-related properties are less
  frequently dealt with in theoretical
  interpretations than fundamental properties such
  as modulus, viscoelasticity and yielding,
• but they are very important in applications that
  involve sliding contact and frictional motions.
• Gears, bearings, piston rings and seals are
  examples of applications where these properties
  are of great significance.
• The properties are
• Hardness
• Friction
• Wear

28
Hardness

• Hardness ? more appropriately described as
  resistance to abrasion, cutting, machining or
  scratching.
• Related to fundamental bulk properties ? such as
  yield strength and modulus.
• Standardized techniques to measure hardness ?
  based on the degree of penetration into a
  specimen by hard indenters of conical or
  spherical shape.

The hardness test
29
Friction

• Friction is the resistance offered by a surface
  to the relative motion of objects in contact.
• The frictional force opposing movement is
  described by the formula
• The coefficient of friction, m, ? is a property
  of the material which determines its resistance
  to sliding action against another surface.
• Friction arises from temporary adhesive contacts
  between the two surfaces
• It is overcome through the rupture of these
  contacts by local plastic deformations.
• Compressive yield strength shear strength of
  the contacting materials are important in
  friction abrasion.
• In viscoelastic polymers ? local rises in
  temperature resulting from shearing at higher
  loads and sliding velocities cause the
  coefficient to increase.
• In bearing applications ? where a metal and a
  thermoplastic are in contact, increases in
  pressure and the sliding velocity will increase m
  and limited by the conditions during service.

30
Wear

• Wear occurs ? when material is lost from the
  interface between the contact surfaces during
  relative motion.
• At low temperatures ? primary mechanism for wear
  damage is adhesive wear, whereby fine particles
  are removed from the surface.
• Since polymers overheat through friction ? more
  severe damage can result as larger volumes of
  locally melted material can be extracted from the
  surface.
• Temperature is also expected to adversely affect
  the wear rates.
• High-strength ductile engineering thermoplastics
  such as nylon and acetal, offer good wear
  performance ? can be further improved with the
  addition of internal lubricants or reinforcing
  additives
• Fibre reinforcements (e.g., glass fabric) and
  mineral fillers (e.g., calcium carbonate (CaCO3)
  may be compounded into the base polymers to
  improve their load-carrying capacity ? but can
  increase friction and give rise to more
  detrimental abrasive wear.
• Very high molecular weights have a positive
  effect in reducing wear ? UHMWPE (Ultra High
  Molecular Weight Polyethylene).

31
Creep Stress relaxation

• A serious challenge when designing products to be
  made from polymeric materials is the prediction
  of performance over long periods of time.
• The amount of deformation after short or long
  term loading has to be known reasonably
  accurately in advance, i.e. at the design stage.
• During long term service, creep and stress
  relaxation are the main deformation mechanisms
  that can be cause for concern.

32
Creep

• Creep phenomena are particularly common in
  polymers.
• Creep occurs when a force is continuously applied
  on a component ? causing it to deform gradually.
• For polymers,
• the delayed response of polymer chains during
  deformations ?cause creep behaviour
• Deformation stops when the initially folded
  chains reach a new equilibrium configuration
  (i.e. slightly stretched).
• This deformation is recoverable after the load is
  removed,
• but recovery takes place slowly with the chains
  retracting by folding back to their initial
  state.
• The rate at which polymers creep depends not only
  on the load, but also on temperature.
• In general, a loaded component creeps faster at
  higher temperatures.

33
Time dependence

• If a load is slowly applied to a polymeric body ?
  the chains in the polymer have time to unfold and
  stretch.
• There are three main ways of presenting creep
  data to be presented as
• Creep curves Strain versus the logarithm of time
  elapsed (various curves at constant load, or
  stress)
• Isochronous curves Stress versus strain (various
  curves at constant time of duration of load)
• Isometric curves Stress versus the logarithm of
  elapsed time (various curves at constant strain
  values)

34
Temperature dependence

• The temperature at which a polymeric body is
  loaded ? very important to its mechanical
  behaviour.
• Low temperatures ? imply low internal energy
  within the molecules.
• Polymer chains are less energetic (more sluggish)
  and also more reluctant to move under a force.
• Makes it more difficult for them to unfold ?their
  ability to undergo large deformations is
  suppressed.
• In this state ? polymers are more likely to
  resist the applied load and stiffer.
• Higher temperatures ?the energy level of chains
  favours their movement, so unfolding is easier.
• A given amount of deformation requires a lower
  force and a force of a given magnitude produces a
  larger deformation.
• Rising temperature and above the glass transition
  temperature, Tg, ?solid polymers become softer
  and progress through the rubbery state to finally
  become a viscous melt capable of flow.
• The term "rubbery" ? refers to the ability to
  deform sluggishly, but the deformations recover
  when the load is removed.
• The term "glassy ? relates to the hardness,
  stiffness and brittleness of the polymer at low
  temperatures. 

35
The diagram below describes the variation of the
deformability of polymers over a wide range
of temperatures
Typical effect of temperature on the
deformability (reverse of stiffness / rigidity)
of a polymer
36
Stress Relaxation

• Stress relaxation is almost exclusively a
  characteristic of polymeric materials and is a
  consequence of delayed molecular motions as in
  creep.
• stress relaxation occurs when
• deformation (or strain) is constant and
• manifested by a reduction in the force (stress)
  required to maintain a constant deformation.

37
Failure in Polymers

• Modes of mechanical failure
• Types of mechanical failure Creep Rupture,
  Fatigue, Impact
• Factors that determine the mode of failure of
  polymers
• Enhancement of the resistance of polymers to
  failure

38
Modes of Mechanical Failures

• Failure analysis and prevention ? important
  functions to all of the engineering disciplines.
• The materials engineer ? plays a lead role in the
  analysis of failures, whether a component or
  product fails in service or if failure occurs in
  manufacturing or during production processing.
• Must determine the cause of failure to prevent
  future occurrence, and/or to improve the
  performance of the device, component or
  structure.
• Failure in a product implies ? the product no
  longer functions satisfactorily.
• Mechanical failure in polymer materials ? caused
  by
• Excessive deformation
• Ductile failure
• Brittle failure
• Crazing

39

• Excessive deformation
• Very large deformations are possible in
  low-modulus polymers ? are able to accommodate
  large strains before failure.
• Such deformations could occur without fracture ?
  design features and other considerations might
  only tolerate deformations to a prescribed
  ceiling value.
• The case in rubbery thermoplastics, such as
  flexible PVC or EVA, for pressurized tubing.
• Ductile failure
• Encountered in materials that are able to undergo
  large-scale irreversible plastic deformation
  under loading, known as yielding, before
  fracturing.
• Yielding marks the onset of failure ? setting the
  upper limit to stress in service to be below the
  yield point is common practice.
• Estimate loading conditions ?likely to cause
  yielding (yield criteria), in order to design
  components with a view to avoid it in service.

40

• Brittle failure
• This is a type of failure ? involves low strains
  accompanied by negligible permanent deformation
  and is frequently characterized by "clean"
  fracture surfaces.
• It occurs in ? components that contain
  geometrical discontinuities that act as stress
  concentrations.
• These physical features ? the effect of locally
  raising stress. Effective stress concentrating
  discontinuities are usually in the form of
• cracks,
• badly distributed or
• oversized additive particulates,
• impurities etc.
• Contrary to ductile failures ? plastic
  deformation provides a warning signal for the
  ultimate fracture,
• Brittle failures can occur without prior warning,
  ?except for the formation of crazes, as in glassy
  thermoplastics.
• Because of this ? design specifications based on
  fracture strength data tend to be conservative
  (e.g., will incorporate very large safety
  margins) with respect to the maximum stress
  levels allowed relative to the strength.

41

• Crazing
• Crazing is a phenomenon that often occurs in
  glassy polymers before yielding,? i.e. for
  deformation at temperatures below the glass
  transition.
• It occurs at ? a strain level which is below the
  level required for brittle fracture and although
  undesirable, this type of "failure" is not
  catastrophic.
• Crazing is often observed in highly strained
  regions during bending.
• Crazes are made up of microcavities whose
  surfaces are joined by highly oriented, or
  fibrillar, material.
• They are initiated near structural
  discontinuities, such as impurities, and are
  collectively visible at the strained surface
  because they become large enough to reflect
  light.
• Crazes are not cracks and can continue to sustain
  loads after they are formed.
• However, they can transform into cracks via the
  breakage of the fibrils.

42
A short film illustrates tensile tests on
plastics. The transparent sample is polystyrene
and shows the formation of crazes, as the
horizontal lines across the width of the specimen
before fracture.
43
Types of Failures

• Because of the viscoelastic character of
  polymers ? no failure can be described
• entirely ductile or
• entirely brittle.
• The proportion of each type of fracture involved
  in polymer failure depends on many factors
• the speed (and time) of loading and
• the temperature of the sample.
• The type of stress, for instance, whether static
  or dynamic (fluctuating), determine the mode of
  failure.
• Below are links to the most common of rupture
• Creep Rupture
• Fatigue Failure
• Impact Failure

44
Creep rupture

• Creep rupture is the culmination in the
  deformation process of creep.
• The result of creep is a slow increase in
  deformation, which ultimately leads to fracture
  when the polymer chains can no longer accommodate
  the load.
• The level of stress,
• the service temperature,
• the component geometry,
• the nature of the material and
• any defects induced by the fabrication process
• are all decisive factors in determining the
  time taken for fracture to occur.
• Although the precise details of the failure
  mechanism that precedes rupture in creep are
  unclear ? it is known that locally,
• stress reaches high enough levels for
  microcracks to form.
• These propagate in a slow stable manner,
  gradually reducing their ability to sustain the
  load.
• It is worth noting that the ultimate failure in
  creep may be preceded by shear yielding, i.e. the
  creation of a neck, or by crazing.

45
Fatigue failure

• Fatigue is a failure process ?which a crack grows
  as a result of cyclic loading.
• This type of loading involves ? stresses that
  alternate between high and low values over time.
• The stress values may be entirely positive
  (tensile), entirely negative (compressive), or a
  combination of the two (see diagram).

Cyclic stress that gives rise to fatigue in
materials
46

• However, the effect of fatigue increases with
  higher tensile Cyclic stress that gives rise to
  fatigue in materials
• Once a crack is initiated ? it propagates by
  small steps during the tensile portion of a
  stress cycle.
• The crack grows slowly but steadily up to the
  point where the remaining area of the parts
  section is unable to support the load.
• The subsequent failure is invariably brittle.
• Failure prediction
• The stresses involved in fatigue are ? much lower
  than the value required to cause outright
  failure.
• Final failure is only possible by cumulative
  damage.
• The initial crack from which the damage starts is
  either
• pre-existing (i.e., mechanically generated or
  fabrication imperfection) or
• initiated by high local stress at weak regions in
  the material.
• A suitably large flaw or weak enough region lies
  in an adequately stressed region of loaded
  components may vary according to

47

• The nature of stress in fatigue
• The amplitude of the stress ? the variation in
  stress between the maximum and minimum values,
  affects the speed of propagation of the crack,
  because
• it determines the amount by which a crack makes a
  step forward during each stress cycle.
• higher stress amplitudes with a high positive
  mean stress decrease the time, or cycles, to
  failure.
• The frequency of the stress ? stress alternates
  between maximum and minimum, also affects the
  time to failure as it causes the step-like
  propagation of the crack to advance more rapidly.

48

• Parameters in cyclic (alternating) stress
• The fatigue in polymers is subject to
  complications because of viscoelasticity in
  polymers.
• This causes damping of the alternating load, a
  process which itself creates heat.
• This heat is dissipated with difficulty because
  of the generally low thermal conductivity of the
  polymers.
• The rate of heat production due to an increase in
  stress amplitude
• and/or frequency becomes lower than the rate of
  heat dissipation, and so stored heat causes the
  temperature in the material to rise.
• At sufficiently high temperatures the polymer may
  overheat and fail not through fatigue but rather
  through creep or heat softening,
• whereby the modulus decreases to the extent that
  the material is unsuitable for its intended use.

49
Impact failure

• The type of loading that constitutes an impact is
  what could be described as a "knock" or "blow",
• a force applied very fast, capable of causing
  failure by brittle fracture.
• Is achieved is through the transfer of the energy
  of impact to defects in the structure ? then grow
  rapidly.
• Accidental occurrence of impact makes resistance
  to this type of abuse an important one ?
  especially for materials used in critical
  applications.
• Impact strength is the typical parameter quoted
  in order to characterize resistance to impact.
• However the conditions under which impact is
  experienced are crucial to the relevance of this
  data.

50
In general, resistance to fracture through impact
is affected by the following
51

• Factors relevant to the ductility of polymers
  have the same effect on impact resistance ? the
  time and temperature dependence of polymers limit
  the ability of chains to "give" under impact
  (very high strain rate) conditions by undergoing
  compensating molecular motion.
• An important exception to the ductility and
  impact toughness is ? use of fibre reinforcement
  in composites, where impact strength is improved.
  
• the energy of impact is expended on diverting the
  crack along the fibre-matrix interface.
• Although some debonding of fibres occurs in the
  process ?catastrophic failure is largely
  prevented.
• The factors that increase the possibility of
  embrittlement ? lead to decreases in impact
  strength.
• The presence of notches ?lowers the energy
  requirements of fracture by highly concentrating
  the stress of impact locally ? ? stress
  concentrations.
• The size and shape of the notch (i.e., whether
  blunt or sharp) is critical ? in determining the
  impact strength obtained from tests.
• Polymers such as rigid PVC, polycarbonate, some
  members of the polyamide family, polymethyl
  methacrylate (acrylic) ?significantly affected by
  the notch condition and are often described as
  notch sensitive.

52
Factors that affect the mode of polymer Failure

• The following factors affect polymer fracture
  behavior adversely by promoting the brittle type
  of mechanism
• Loading Conditions
• Environmental
• Material structure aspects
• 1. Loading conditions
• Very fast loading as in the case of impacts
• Triaxiality of stress the development of
  stresses in more directions relative to the one
  from which a load is applied
• triaxial stresses promote brittle failure in
  materials.
• this 3D type of stress system appears at
  discontinuities (stress concentrations) within a
  component.
•  

53

• 2. Environmental
• Low temperature
• can bring a transition in fracture mode from
  ductile to brittle ? experienced by a material
  when the temperature falls below a point known as
  the ductile-brittle transition temperature, TDB.
• Deterioration of physical properties ? as a
  result of chemical changes to molecular structure
  through
• Oxidation reactions with substances such as
  oxidizing acids and water moisture
• Weathering the combined effect of exposure to
  u.v. radiation and oxygen
• Degradation ?due to exposure to excessive heat,
  particularly in the presence of oxygen
• Environmental Stress Cracking ingress to defect
  sites within the material of normally
  non-aggressive liquids (mostly organic) that
  promote fracture at low levels of stress and over
  short periods of time.
• 3. Material structure aspects
• Discontinuous microstructure ? arising from the
  presence of
• particulate additives
• crystallinity in the polymer

54
Improving the resistance of polymers to failures

• To minimize the risk of catastrophic failure ? a
  material needs to be tough as well as ductile.
• The mechanical design ? has a role in avoiding
  the incorporation of features that promote the
  likelihood of brittle fracture.
• The following guidelines ? to identify the steps
  to enhance the failure resistance of polymers in
  service
• Design considerations
• Material Selection
• Material Modification
•  
• Design considerations
• Design for a particular set of stress conditions
  anticipated in service ? example
• attention to section thicknesses, and
• utilisation of material data obtained under
  conditions relevant to service (creep, fatigue,
  impact)
• Elimination of the majority of stress-concentratin
  g design features ?abrupt changes in section,
  holes, notches

55

• Material Selection
• Should be based structural aspects affecting
  failure, as well as physical and chemical issues
  arising from the use of polymers in a particular
  environment such as the effect of temperature,
  oxidants and aggressive liquids.
• Given that the most important properties
  affecting resistance to brittle fracture are
  toughness and ductility,
• key material data to be used in design in order
  to minimise the likelihood of brittle fracture
  should include
• ductility indicators (e.g., energy absorption
  values obtained directly by measuring the area
  under load-extension curves obtained in tensile
  tests which are carried out to failure (see
  schematic).
• Energy absorption values derived from impact
  tests

Energy absorbed during extension
56

• Material Modification
• Toughening through microstructural modification
  of thermoplastics
• Based on the principle that the energy which
  contributes to brittle fracture can be dissipated
  by localized yielding ahead of the crack tip ?
  possible to produce toughened thermoplastic
  polymers by the incorporation of a partially
  compatible rubbery phase.
• This is typically accomplished
• (a) at the polymerisation stage by
  copolymerisation, and by
• (b) direct blending (e.g. mixing acrylic rubber
  with PVC or with PBT.
• The success of the toughening of thermoplastics
  by rubber modification depends on
• the rubber existing as well dispersed discrete
  particles
• the interfacial adhesion between the
  thermoplastic matrix and the rubber being at an
  optimum level (i.e., neither too strong nor too
  weak)
• the glass transition temperature of the rubber
  phase ? lower than the service temperature.

57
Example of the exams question

• The failures of polymeric materials can be affect
  by a few factors. Discuss two of this factors.
  failure?
• There are a few types of failures in polymeric
  materials such as creep rupture, fatigue and
  impact. Based on your understanding, discuss two
  of this mechanical failures and how this failures
  can be describe as brittle or ductile deformation

58
(No Transcript)