Lecture24 - ME 382 Lecture 24 FATIGUE FAILURE Fatigue...

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ME 382 Lecture 24 15/iii/06 1 F ATIGUE F AILURE Fatigue failure: accumulation of damage and eventual failure under cyclic loading Old problem in engineering: 1828 mine hoists, mid-1800’s railway axles Very important problem in engineering (a major cause of failure) Must be considered if any possibility of oscillating loads E.g. , Engines Transportation (vibrations in road/rail/flight applications) Marine structures Periodic loading (under-carriage of aircraft, weapons, sports equipment) Two approaches for design against fatigue (i) Propagation of an existing crack (topic of this lecture) (ii) Empirical relationships for total life (including crack initiation) F ATIGUE CRACK GROWTH Essential aspect of fatigue crack growth Cracks grow (slowly), even when K I < K Ic , if subjected to oscillating stress Assume stress varies from σ = σ min to σ = σ max (i) Always check for yield when σ = σ max (ii) Loading parameter for fracture varies from K I min to K I max K I min = f geometry ( ) ! min " a K I max = f geometry ( ) max a Initial crack small: K I max K Ic Fracture does not occur Crack grows because of oscillating stress K I max increases Eventually crack is large enough for (i) K I max = K Ic
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Lecture24 - ME 382 Lecture 24 FATIGUE FAILURE Fatigue...

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