Polymer Fracture And Failure Flashcards
What are the causes of polymer failure?
Incorrect material selection
Chemical and environment interactions - leads to rapid crazing, cracking, fracture and product failure
Response to long term loads - creep and fatigue, strength and stiffness of polymers in service conditions usually a lot lower than on a data sheet
Processing errors - incorrect processing leads to high residual stresses, degradation, embrittlement, inhomogenity, introduction of faults, defects or contaminants
Inappropriate design - long term loads, chemicals, high speed loading, fatigue loads - shortens service life
Effect of additives - too much our too little or incorrect additive, non-uniform dispersion, migration to surface, unanticipated secondary effects
What are some examples of unintentional additives?
Extraneous lint, dirt etc.
Residual solvent
Water
Ionic impurities from water
Trace metal from extruder barrel
Impurities in intentional additives
What are the two natures of loads?
Impact - hammering, accidental drop
Fatigue - vibration, rotation in loaded components
What are the types of mechanical failure in polymers?
Excessive deformation
Ductile failure - encountered in materials that can undergo large irreversible plastic deformation before fracturing (tend to design products with a yield criteria in mind as yielding is the onset of failure)
Brittle failure - low strains and negligible permanent deformation, occurs in components with geometrical discontinuities (stress concentrations)
(No failure is entirely brittle or ductile, fraction depends on speed of loading and temperature of sample)
Crazing - occurs at strain level below that required for brittle fracture, isn’t catastrophic but undesirable
What is crazing?
Crazes are made up of micro-cavities whose surfaces are joined by highly oriented material
They initiate near structural discontinuities like impurities and are collectively visible at strained surface as they become large enough to reflect light
They are not cracks and can sustain loads but can become cracks if the fibrils break.
What is LEFM?
Linear elastic fracture mechanics
Observing linear elastic behaviour in brittle polymers, fibre-reinforced materials, specimens of great thickness or below Tg
What is SIF?
Stress intensity Factor (K)
Describes the linear elastic area in front of the crack tip
Critical value of SIF is called fracture or crack toughness Klc (static loading) or Kld (dynamic loading)
Kii or Kiii refer to crack opening modes, mode I is highest importance
What are the three crack opening modes?
Mode 1 - opening like splitting
Mode 2 - in-plane shearing (pushing back or forward)
Mode 3 - out of plane shear (splitting sideways)
In LEFM, how does stress and strain grow?
They increase linearly together until crack initiation when there is an abrupt drop of load
What’s the formula for stress intensity factor K?
K = external stress x diameter of flaw
What is EPFM?
Elastic Plastic Fracture Mechanics
Observing toughness for non-negligible elastic-plastic material and extensive plastic area in front of crack tip
What is CTOD and what is a measure of it?
Crack tip opening displacement
Based on assumption that for ductile materials, fracture process is not controlled by stress intensity but by plastic deformation in front of the crack tip
Measure of it is widening at crack tip
What is chain scission?
Fracture on an atomic level - bonds breaking
Caused by high stress concentrations, non-uniform loads causing some bonds to have a high load
Non-uniformity is more noticeable in amorphous polymers - crystalline tend to distribute stress more evenly
What is chain disentanglement?
Molecules separate from each other intact.
Likelihood depends on length of molecules and degree to which they are entangled
What are the two types of failure of polymers like type I and type II?
Type I - tend to fail by crazing if craze initiation stress is less than yield stress
Type II - tend to fail by shear yielding if yield stress is less than craze initiation stress
If both stresses are equal then both mechanisms can occur simultaneously
What are some properties of Type I and II polymers?
Type I - brittle just below Tg, low initiation and crack propagation energies so low unnotched and notched impact strength
Type II - high crack initiation and propagation energies so high unnotched impact strength but low notched, shows a brittle to ductile transition Tbt
What materials are most prone to crazing?
Amorphous, glassy polymers such as PS, SAN, PMMA, PVC
Semi crystalline polymers (PE, PP, PETP)
Epoxy resins
What are the 3 stages of craze propagation?
Craze initiation/nucleation - usually on surface grooves or imperfections in bulk of polymer
Craze propagation
Craze breakdown
What indicates that crazing is time dependent?
There can be significant lag time between load application and first visible craze.
Why do crazes tend to form at the surface of polymers?
Defects are usually found at surface
Injection moulding and machining usually results in a different surface structure to bulk material
Environmental attack like diffusion, degradation etc usually starts and penetrates from surface
Explain the process of craze nucleation
Plastic deformation starts at a local stress concentration
Yield is non-linear and polymer glasses soften with strain so there will be localised deformation as the strain increases
Material further is undeformed so some lateral stresses develop
EITHER: Strain hardening response of material will stabilise the strain localisation process OR micro shear band will spread out
If lateral stresses become high enough, material in deformation zone will cavitation and craze nucleation occurs
Explain the process of craze propagation
Craze grows by:
Craze tip displaces into increased material by forming voids ahead of the craze tip or by coalescence with a separate, smaller craze
OR
Craze widens while extending craze matter eg. Molecular strands may rupture or disentangle giving rise to a crack
What can lead to craze slowing?
If a craze hits an obstacle such as a zone of higher orientation or end of specimen
If a craze has grown to such an extent that the initiating crack is blunted and rendered uncritical
If a craze interacts with another craze or the environment which can change the conditions that have led to its growth or initiation (eg. Stress relaxation or penetrating into a lower stress zone adjacent to another craze)
What causes a craze to turn into a crack?
Molecular strand degrades and eventually fails under the tensile load
What causes craze breakdown?
Slippage and disentanglement of molecular coils leading to fibril rupture
Breakage of entangled chains leading to fibril rupture
Fibril-matrix separation
What criteria in terms of molecular weight states that a material will be brittle?
If molecular weight is lower than double the molecular weight for entanglement
How does molecular weight effect craze behaviour and an example?
Fibrillated crazing tends to occur in only high molecular weight polymers
In lower molecular weight polymers, cracks without fibrillation occurs because of the missing intermolecular connections and strength
Eg. In a PS blend of 50-50 of over 2Me and under 2Me, the craze happens only in the higher molecular weight regions and crack in the lower ones
What is rubber toughening?
On loading, elastomeric phase concentrates and makes the stress distribution more complex in surrounding matrix
Yield stress is reduced and toughening occurs by the resultant plastic deformation either by crazing or formation of shear bands
How does crazing cause rubber toughening?
Dispersed rubber particles act as craze initiators.
The rubber particles must be:
- small enough to prevent a crack propagation
- large enough not to be engulfed by a craze
- in the order of microns
How does shear yielding mechanism cause rubber toughening?
Dispersed rubber particles act as initiations for the shear bands.
Need to be less than a few tenths of a micron
Shear bands are barriers to craze or crack propagation so delays failure
How can a rubbery second-phase increase toughness?
Makes craze/shear yielding initiation easier by providing sites for nucleation, lowering the stress required for craze and shear yielding formation
What factors denote the contribution of either crazing or shear yielding?
Rubber particle size, dispersion and concentration
Matrix
Temperature
How could you tell the difference between crazing and shear yielding?
Dilatometry - crazes increase volume, shear yielding doesn’t change it
Visibly - crazes show a stress whitening caused by the light scattering due to the refractive index difference in crazed and uncrazed areas
What are some selection criteria for rubber toughened plastics?
- rubber should exist as a discrete second phase, usually as spheroids
- rubber phase should be dispersed effectively
- adhesion between rubber and matrix polymer should be optimised
- optimum particle size and distribution may vary according to yield characteristics of matrix
- elastomer Tg should be well below minimum expect service temperature of the toughened plastics
What is the microstructure of HIPS and how does rubber toughening benefit the material?
Continuous polystyrene (PS) matrix with lots of dispersed salami-like domains which each contain many spherical PS sub domains separated by a polybutadiene phase
The polystyrene component provides the rigidity while the polybutadiene phase brings five times higher toughness than pure PS
The hierarchical salami morphology can accommodate displacements to avoid formation of flaws.
What is the common trade off with rubber content in toughening?
More rubber content = higher impact strength but lower tensile and bending strength which is a common conflict
By controlling size of rubber domains to produce a bimodal size distribution, surface of larger particles are more likely to initiate crazing while smaller particles can reduce average ligament thickness between particles which increases toughness
What is the structure of ABS and how does rubber toughening benefit it?
Continuous polystyrene-co-acrylonitrile glassy phase with droplets of dispersed polybutadiene latex
Emulsion polymerisation is used industrially as it yields a fine graft control and good distribution which toughens more.
What factors influence the mechanical performance of ABS polymers?
Matrix composition and molecular weight
Type of rubber used
Rubber phase volume fraction
Rubber particle size and internal structure
Rubber cross linking density
How does bimodal size distribution help the toughness of ABS?
Big particles increase toughness, small particles increase surface finish so a mix of a range of sizes is good (ideal for ABS is 0.3-0.3um)
How does the rubber phase increase the toughness of ABS?
Stress concentrations in equatorial regions of rubber particles cause deformation processes
With increasing deformation, stresses are restricted and more mechanical energy can be dissipated before fracture causing multiple crazing
Only if temp is over 70C does shear deformation start to dominate
What is the effect of temperature and strain rate on deformation of polymers?
Temperature:
Tg - glass transition - below is typically brittle so have limited deformation and above, polymer chains are more mobile so there is increased ductility and deformation.
Tm - melting temp - above this, chain mobility is increased significantly so viscosity decreases a lot and allows substantial deformation
Strain rate:
Viscoelastic - polymers are viscous and elastic.
Low strain rates - more elastic behaviour, allowing for significant deformation and recovery
High strain rates - more viscous behaviour, permanent deformation
What are the causes of premature polymer product failure?
Incorrect material selection - temp limits, mech properties, service life, durability
Environmental factors - temperature, humidity, UV, chemicals etc can cause degradation, promote oxidation, thermal expansion or physical damage
Manufacturing defects - poor processing or quality control can leave improper moulding, voids, inclusions, uneven additives which affects structural integrity
Mechanical stresses - excessive mechanical stresses beyond material capability
Response to long term loads - creep or fatigue, strength in service lower than data sheet
Inappropriate design of all of these
Describe the yield process for a thermoplastic
Stress is applied, polymer chains slide past each other - plastic deformation
Yield point is when enough stress is applied that it makes the polymer chains move causing permanent deformation
May exhibit strain hardening or softening dependent on the material
What is the mechanism for craze formation?
Occurs in high tensile stress regions, localised not bulk
Polymer chains are aligned and stretched in the direction of applied stress
Microvoids are formed and connect, causing a network of fibrillation structures called crazes
What’s the difference between a craze and a crack?
Crazes are fine, interconnected micro cracks or fibrils — Cracks are larger, more distinct fractures
Crazing is reversible deformation, shape can be recovered — Cracks are irreversible, shape can’t be recovered
Crazes propagate in a network pattern — Cracks propagate linearly and extend throughout material
Crazing can enhance toughness and strength — Cracking decreases toughness and strength
How can you distinguish crazes and cracks?
Optical Microscopy - observe deformation pattern in material (craze will be fine, interconnected fibrils and cracks will be larger, more distinct features)
Scanning Electron Microscopy - provides higher magnification of surface morphology
What is the effect of temperature, strain rate and microstructure on a rubber toughened polymer?
Temperature - Tg affects brittle or ductile
Strain rate - better energy dissipation and relaxation at low strain so better toughness
Molecular structure - long, flexible chains have higher toughness as they can absorb energy, prescience of
What is the rubber toughening mechanism in ABS?
Soft rubbery phase is introduced into a brittle matrix material which improves the impact toughness
Due to energy dissipative and deformation process initiated at the particles - as there are stress concentrations at the particles
How do we achieve the optimised rubber toughening effect?
Consider:
- rubber phase selection to ensure a good compatibility with matrix (should have good plastic deformation)
- rubber particle size and distribution (smaller will improve toughness by increasing crack deflection)(well distributed means better consistency in toughening)
- compatibility and adhesion using surface modification or coupling agents (crucial for efficient stress transfer and energy dissipation)
- rubber content (higher will be better toughness)
