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gm19_20notes2 - M19 Fatigue and Fracture Reading Ashby and...

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M19 Fatigue and Fracture Reading: Ashby and Jones ch. 15, 16 Fatigue is the process of crack initiation and growth under cyclic loading. This has particular significance for aerospace structures which are typically light weight and highly stressed and exposed to oscillating and vibratory loads. There have been some notable fatigue failures in aerospace history and design against fatigue is particularly important in all aircraft: The De Havilland Comet (the world’s first jet airliner) http://www.centennialofflight.gov/essay/Commercial_Aviation/Opening_of_Jet_era/Tran6G1.htm
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Basic mechanisms: Fatigue crack initiation: stress cycling drives dislocations on slip planes. Leads to dislocation “pile up” and formation of “persistent slip bands” Image adapted from: Suresh, S. Fatigue of Materials . Cambridge University Press, 1998. 1 1 2 2 A C D Descriptions of the critical annihilation distance for screw and edge dislocations. Mechanism of extrusion formation by combined glide and dislocation annihilation. slip band in a state of compression. indicate repulsive forces on interfacial dislocations and smaller arrows denote forces caused by the applied load. (After Essmann, Gosele & Mughrabi, 1981.) S LH y s A A B X Matrix Matrix PSB Y Interface dislocation B B' A' y e S RH . . B S S A B B' A' b A B C D Irreversible slip in the PSB creating effective interfacial dislocations which put the The combined effects of applied stresses and internal stresses. Bigger arrows
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Persistent slip bands initiate cracks, initially in shear and then grow under tensile load. Engineering Materials 1 σ σ Stage 2 crack Stage 1 cracks Slip planes How cracks form in low-cycle fatigue. Image adapted from: Ashby, M. F., and D. R. H. Jones. . Pergamon Press, 1980.
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Cracks continue to grow by local plasticity at crack tip: Engineering Materials 1 New surface 4 ar y r y /2 K max δ δ δ δ π π Image adapted from: Ashby, M. F., and D. R. H. Jones. . Pergamon Press, 1980. How fatigue cracks grow.
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Initiation of cracks due to local plasticity, often caused by stress concentrations, local surface roughness, wear damage at contacts etc. Lower yield stress materials more prone to fatigue at a given stress level. Also, welded joints can be sources of residual stresses and lower yield stress – tend to avoid welds in aerospace structures.
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