Fracture Toughness & Fatigue Week 3

Behaviour of Materials in Service A material or structure is deemed to have failed when it is unable to satisfy the original design function. Failure may be due to: Plastic buckling. Dimensional change with time. Loss of material through corrosion, erosion or abrasion. Fracture - partial or complete.

Failure by Yielding Metals exhibit both elastic and plastic behaviour. As the level of stress is increased so the amount of elastic strain increases in direct proportion to the stress applied up to a certain limit – elastic limit. To minimise the possibility of excessive stress factors of safety are applied.

Failure by Fracture The terms, tough, ductile, brittle, or fatigue are frequently used to describe the fracture behaviour of a material. Tough or ductile fracture – failure is preceded by excessive plastic deformation often detectable. Brittle or non-ductile fracture - involves little or no plastic deformation – often Catastrophic.

Fracture Type The type of fracture which occurs is largely dependant upon the type and condition of the material. Other factors include: the type of stress applied. the rate of stress application. temperature and environmental conditions. component geometry. size and nature of internal flaws.

Fracture Mechanics This is the study of the relationships between crack geometry, material strength and toughness and stress systems as they affect the fracture characteristics of a material. The aim of fracture mechanics is to determine the critical size of a defect necessary for fast fracture to occur. That is catastrophic crack propagation and failure under service loading.

Fracture Toughness To improve fracture toughness there is a need to avoid excessive elastic deflections & plastic yielding. Fast fracture can occur which causes catastrophic failure. E.g. Welding of ships, oil rigs, bridges, pipelines, pressure vessels.

Fast Fracture All related to cracks, flaws or defects. Fast fracture caused by growth of these defects which suddenly become unstable & propagate at the speed of sound.

Why Does This Happen? There appears to be a critical pressure (stress) related to the size of the internal flaw. There is therefore a need to determine the critical stress.

Sample with Flaw If the flaw is increased by δa , then: F Work done by loads ≥ Change in elastic energy + energy absorbed at the flaw tip. i.e. F Thickness, t F Fig. 1

Definitions

Fast Fracture at Fixed Displacements Consider a plate clamped at both ends under tension as in fig.1

Increase in Load Consider a small cube of material of unit volume due to load (F)

Cont’d

Cont’d

Fast Fracture at Fixed Loads Load acts in a more complex way. As the flaw grows material becomes less stiff & relaxes. The applied forces move & do work. W is therefore finite & positive.  Uel is now positive & the final result for fast fracture is found to be the same (some of W goes to increase the strain energy). F

Cont’d

Stress Intensity Factor

Toughness & fracture toughness values for some metals Material Toughness Gc (kJ m-2) Fracture toughness Kc (MPa m½ or MNm-3/2) Steels 30 – 150 80 – 170 Cast Irons 0.2 - 3 6 - 20 Aluminium alloys 0.4 - 70 5 - 70

Cont. No material is free from defects, so it is essential that any crack-like defects are relatively harmless. Using values of fracture toughness it is possible to calculate the size of defect or magnitude of stress required to cause failure.

Cont.

Correction Factors

Sellotape Increase the load to the value M that just causes rapid peeling. For this geometry, the quantity Uel is small compared to the work done by M & can be neglected (tape has comparatively little ‘give’). t  a  a M M

Cont’d

Example

Solution

Tutorial Exercise 1 Determine the fracture strength for a high strength steel assuming a crack of 0.2mm & kc=55MNm-3/2. Also if the yield strength is 1550 MPa determine the size of crack that will cause failure at this stress.

Solution

Ceramics The strength of ceramics varies considerably from 0.69MPa to about 7x103MPa. As a class of materials, few have tensile strengths above 172MPa. There are also large differences in compressive strength, usually between 5 & 10 times higher than the tensile strength. Many ceramics are hard & have low impact resistance – reflecting their ionic/covalent bonding.

Deformation Mechanisms Generally lack of plasticity in crystalline ceramics due to bonding. Bonding directional, dislocations are narrow & do not move easily. Deformation is mainly due to ionic bond, but this is brittle. Cracks form at grain boundaries.

Factors Affecting Strength Mainly occur from structural defects: Surface cracks. Voids (porosity). Inclusions. Large grains. These are produced during processing.

Cont’d Once a crack forms - no energy absorbing processes – crack propagates until fracture – general decrease in strength. Size & volume fraction important. Flaw size can be related to grain size – finer grain size ceramics have smaller size flaws at their grain boundaries.

Cont’d Strength is determined by chemical composition, microstructure, surface condition. Additionally – temperature, environment & type of stress & how it is applied.

Ceramic Abrasive Materials High hardness makes them useful for grinding, cutting & polishing other lower hardness materials. Fused alumina & silicon carbide are two commonly used ceramic abrasives. Products – sheet & wheels manufactured by bonding individual ceramic particles together Bonding materials include fired ceramics, organic resins & rubbers.

Cont’d Ceramic particles must be hard with sharp cutting edges. Must have a certain amount of porosity to provide channels for air or liquid to flow through – act to cool the abrasive & removes debris. Aluminium oxide grains tougher than silicon carbide ones, but not as hard.

Cont’d By combining with zirconium oxide improved abrasives have been developed. Another important ceramic abrasive is cubic boron nitride – borazon- almost as hard as diamond, but better heat stability.

Fracture Toughness of Ceramics Due to their combination of covalent & ionic bonding, ceramics have inherently low toughness. Toughness can be improved by processes such as hot pressing with additives & reaction bonding.

Tutorial Exercise 2 The maximum sized internal flaw in a hot-pressed silicon carbide ceramic is 30micron. If this material has a fracture toughness of 3.9MNm-3/2, what is the maximum stress that this material can support?

Solution

Effect of Mean Stress – Goodman & Soderberg Fatigue Effect of Mean Stress – Goodman & Soderberg

Introduction It been estimated that 75-80% of all failures in engineering components, machines, vehicles, structures & bridges, aircraft, ships,etc..are due to fatigue. Fatigue is the general failure which occurs after several cycles of loading to a stress level below the ultimate tensile stress & is generally due to localised plastic deformation at the metal surface, culminating in the nucleation of sharp discontinuities, which once formed continue to grow into cracks.

Cont’d Pre-existing cracks, imperfections & defects, small cracks usually at the surface, weld defects, machining marks, & scratches act as stress concentrators & the failure is characterised as a smooth portion of surface with a ‘clamshell’ or ‘beachmark’ appearance, with a concentric series of lines recording each cyclic advance of the fracture.

Cont’d This abruptly changes to a granular portion which corresponds to the rapid crack propagation at the point of catastrophic failure. Fig. 1 shows the conditions for fatigue loading.

Cont’d Fig. 1 The conditions for fatigue loading. STRESS RANGE FLUCTUATING LOAD TIME MEAN STRESS STRESS RANGE Fig. 1 The conditions for fatigue loading. - STRESS + PULSATED OR REPEATED LOAD TIME MEAN STRESS STRESS RANGE - STRESS + ALTERNATING LOAD TIME STRESS RANGE MEAN STRESS

Conditions Fluctuating load - the mean stress is greater than the stress range. Pulsating or repeating load – the mean stress is equal to half the stress range. Alternating load – the mean stress is zero.

S/N Diagrams Typical S/N diagrams for pieces subject to alternating loads are shown in fig. 2. Goodman & Soderberg investigated the relationship between stress amplitude, mean stress & fatigue. Goodman & Soderberg diagrams are shown in fig.3.

S/N Diagrams Fig. 2 Typical S/N diagrams for pieces subject to alternating loads

S/N Diagrams Fig. 3 Goodman & Soderberg investigated the relationship between stress amplitude, mean stress & fatigue.

Perfect Alternation When the mean stress is zero (perfect alternation), the fatigue limit is at a maximum value before failure occurs. If a steady state of stress is superimposed on the cyclical stress then this must also be taken into account – this steady state stress is the mean stress.

Goodman Diagram In the Goodman diagram the fatigue limit is zero when the mean stress is equal to the tensile strength of the material, since the material will fail at this value before any cyclical loading can commence.

Cont’d Therefore, if the point representing the stress amplitude & the mean stress for any given set of conditions lies within the area bounded by the axes & the ‘Goodman Line’( the shaded area), then according to the Goodman relationship the material should not fail by fatigue.

Soderberg Diagram In the Soderberg diagram the fatigue limit is zero when the mean stress is equal to the yield stress of the material. Again, the point representing the stress amplitude & the mean stress for the material must lie within the shaded area bounded by the axes & the ‘Soderberg Line’, if failure by fatigue is to be avoided.

Note Since perfect alternation (zero mean stress) rarely occurs in practice, S/N curves should not be used alone without consideration of the mean stress. Care must also be taken in using even the Goodman & Soderberg diagrams since they tend to give a low value of the fatigue limit for ductile materials & high value of fatigue limit for brittle materials.