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cycles to failure (crack initiation longer - once initiated, it will grow like pre-cracked components) -Low-cycle fatigue - maximum stress is above the yield strength; < 104cycles to failure -Stress-related parameters important -Important parameters: -Common testing method - repeatedly bending samples at different levels of stress amplitude until they break High-cycle fatigue -High-cycle fatigue - maximum stress is below the yield strength of2559
S-N Curves -Showing fatigue behaviour for high-cycle fatigue (stress amplitude vs. number of cycles to failure) -Fatigue strength - stress amplitude will cause failure after a given number of stress cycles -Fatigue life - number of cycles to failure at a given stress amplitude -Fatigue limit - limiting stress amplitude, below which fatigue failure will not occur S-N Curves - Mean stress -As mean stress σm increases, the fatigue life will be reduced Effect of mean stress of2659
S-N Curves - Statistics -Fatigue life varies - the time it will take for a crack to initiate different for different samples of material -S-N curves for different probabilities of failure: Low-cycle Fatigue -Low-cycle fatigue - maximum stress is above the yield strength Categories of fatigue -Fatigue of uncracked components - small parts -Fatigue of cracked components - large components due to more likeliness of cracks Crack initiation -Cracks usually initiate at some point of stress concentration -Stress concentration leads to local plastic flow (due to localised increased stress) —> leads to crack initiation of2759
-Examples of stress concentrators: -Sharp corners -Surface scratches or marks -Cracks or defects in the material -Surface holes or notches -Changes in cross-sectional area Preventing crack initiation and growth -Reduce crack initiating by eliminating points of stress concentration: -Design parts to avoid changes in cross-sectional area, sharp corners -Make changes in cross-sectional area occur gradually -Removing surface cracks and marks - smooth surfaces -Slow crack by using surface treatments (shot peening) that introduce a compressive stress in the surface Comet aeroplane disasters -Body of an aeroplane - pressure vessel - pressurised during flight —> each flight corresponds to a stress cycle -Crash: -Recovered wreckage indicated failure caused by crack that started near the corner of windows -Cracks propagated and joined up - large area of skin peeled away from structure -Corner of windows cause stress concentration -But rivet hold near the windows further increased the stress concentration - rivet holes not taken into account -Only one fatigue test performed - but fatigue failure is probabilistic -Fatigue test done on a pre-stressed section - not representative of normal operating conditions Creep/Ethics Creep -At room temperature - strain independent of time -High temperatures - creep can occur -Creep - slow, continuous permanent deformation with time - occurs temp high relative to melting temp Creep temperatures -For metals and ceramics, temperature at which creep occurs depends on melting temp of materials -Generally, creep begins when temperature is: -Metals: > 0.3 - 0.4 Tmof2859