Cycles to failure crack initiation longer once

This preview shows page 25 - 29 out of 59 pages.

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 ; < 10 4 cycles 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 of 25 59
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 σ m increases, the fatigue life will be reduced Effect of mean stress of 26 59
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 of 27 59
- 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 T m of 28 59

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture