Term fatigue introduced by Poncelet (France) 1839 progressive fracture is more descriptive Fatigue 1. Minute crack at critical area of high local stress (geometric stress raiser, flaws, preexisting cracks) 2. Crack gradually enlarges (creating “beach marks”) 3. Final fracture (suddenly, when section sufficiently weakened) Fatigue: no or only microscopic distortion static failure: gross distortion 3. Final fracture (rough zone) 1. Fatigue origin 2. Beach marks (velvety zone)
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Fatigue - egr.msu.edu · 1. Minute crack at critical area of high local stress (geometric stress raiser, flaws, preexisting cracks) 2. Crack gradually enlarges (creating “beach
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Term fatigue introduced by Poncelet (France) 1839progressive fracture is more descriptive
Fatigue
1. Minute crack at critical area of high local stress(geometric stress raiser, flaws, preexisting cracks)
Fatigue strength about 10% lesseccentric loads about 20…30% less
CG=0.7…0.9gradient factor
Reversed Torsional Loading
reversed biaxial stressdistortion energy theory 58%
CL=0.58load factor
maximum stresses on surfaceshear stresses fatigue start
Endurance Limit
Fatigue StrengthJuvinall p.312
Fig. 8.12
Steel Sus=0.8 Su
Other ductile material Sus=0.7 Su
Fatigue StrengthInfluence of SurfaceSo far special “mirror polish” surface (only in laboratory ! )Minimizes 1.) surface scratches (stress concentration)
2.) differences of surface & interior material3.) residual stresses from finishing
Commercial surfaces have localized pointsof greater fatigue vulnerability.