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Low-cycle fatigue is the greatest potential cause of failure in critical aeroengine components. To prevent premature failure, the fatigue life of these components needs to be predicted accurately. This requires knowledge of crack nucleation and growth under fatigue loading. Initial crack propagation is not continuous as a result of obstacles such as grain boundaries. Laboratory observation has shown that cracks decelerate rapidly as they approach these boundaries. Further growth may be impeded for a considerable number of cycles if there is a significant difference in orientation between the cracked and adjacent grains. This behavior effectively disappears at longer crack sizes. A detailed quantitative appraisal of crack growth rates is vital for component life predictions based on fracture mechanics. Cracks that nucleate during engine operation experience very high stresses that may exceed the material yield stress. Established linear elastic fracture mechanics may not be applicable due to loss of similitude caused by violation of the small-scale yielding criterion. These concepts are also based on continuum fracture mechanics and therefore cannot describe the early stages of crack growth. This paper considers an empirical model for short crack growth in a β heat-treated near-α titanium alloy at room temperature. Crack growth rate equations are expressed in terms of crack length and effective stress range and were derived from experimental crack length versus cycles data. These data were generated at constant peak stress and various stress ratios. The expressions are shown to give an excellent representation of experimental crack growth rate versus crack length data. In producing the model, procedures have been established to determine crack opening loads and critical crack sizes above which continuum fracture mechanics apply.
low-cycle fatigue, fracture mechanics, short fatigue cracks, titanium alloys
Higher scientific officer, Defence Research Agency (Pyestock), Farnborough, Hampshire