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 • Property-to-cost ratio • design for stiffness and strength where low overall price is important  example: children’s toys, non-critical parts of home appliances 

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 sectionalarea 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 sample’s 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 isremoved.

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. • Stiffmaterials have a high modulus the deformation (strain) resulting from the applied force (stress) is low. • Flexiblematerials 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 strengthsY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. • For these standard specimens a loading rate of 0.127 cm (0.05 in/min) is normally used. Flexural test experimental set-up

18. 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  Scorresponds 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 = specimen’s length of span between supports (m) Calculated entities: • 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 • Highly crosslinked polymers (thermosets) are typically brittle materials  since chain movement is severely restricted, they do not usually yield, but fail in a brittle manner. 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. • The friction performance of polymers varies extensively, the value of m ranging from 0.2 to 0.7 and increasing surface roughness tends to increase friction.

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. • Agiven 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. Ashort 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. • These are good indicators that failure is in progress and that fracture is following. In other cases, rupture can take place without any signs of warning.

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 part’s 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 • flaw density (number of flaws per unit volume) • component size • batch • other factors which make the prediction of fatigue failure in terms of time or number of cycles subject to the mathematical laws of probability.

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: