A material under loading will deform and if the load increases, the material will eventually fail by fracturing. Fracture may be either brittle or ductile. In a ductile fracture, failure occurs only after a considerable amount of plastic deformation “necking” of the material will occur. The stress level in the necked portion builds up and cracks begin to form. Finally, fracture occurs by shear failure at an angle of about 45° to the loading direction. This is because shear stress is at a maximum at 45° to the line of direct stress. In a brittle failure, there is little or no plastic deformation prior to fracture.
The type of failure depends largely on the mechanical properties (ductile or brittle) of the material but is also affected by other factors such as the rate of application of load (slow or impact), the type of load (tensile or lateral), the temperature and the environmental conditions.
The failure behavior of a material is affected by the geometry and surface conditions of the component. A sudden change of section or a surface notch will act as a point of stress concentration where cracks form mare easily.
Fatigue refers to the failure of a material after a large number of repeated or periodic loading cycles. The material fails even though the maximum stress in any one cycle is considerably less than the failure stress of the material as determined by “static” loading test (e.g. tensile test). In practice, many engineering components are subjected to periodic or fluctuating loading cycles (e.g. shafts, structures under wind loads). In the loading cycles, the stress may be alternating in sign(tensile and compression). In a fatigue test of a material to determine its fatigue strength, periodic alternating stress cycles with different maximum stress levels but with a mean stress value of zero is usually applied to a specimen. The number of cycles, N, before the specimen fails is noted and plotted against the maximum stress level, S, in the cycle.
The S-N curves of most steels show a clear fatigue limit and the strength at the fatigue limit is about one-half of the tensile strength. Non-ferrous metals usually show no definite fatigue limit. It is then only possible to design for a limited life of the material and a life of 106 or 107 cycles is usually used.
Fatigue is due to crack initiation at a low stress level and the propagation of cracks. The fatigue strength of a material is affected by many factors. These include surface condition, component design and the environmental conditions. Specimens for fatigue testing are usually prepared with a polished surface and this condition will give the best fatigue performance. If there is a scratch or notch on the surface or the surface is roughly grounded, the fatigue strength will be reduced. A sharp change in section provides points for stress concentration and crack initiation and therefore leads to a poor fatigue performance.
Under a constant continuous load, a material will show additional deflection slowly with time. This is called “creep”. For most metals at normal temperatures, creep is negligible. At raised temperatures, creep becomes more significant. A plot of strain against time shows three phases of creep: primary, secondary and tertiary creep. In secondary creep, strain increases at a steady rate with time. Tertiary creep will eventually lead to failure.
Thursday, 15 January 2009