Long fatigue life critical crack lengths

• Published in: Fatigue & fracture of engineering materials & structures Vol. 33; no. 5; pp. 320 - 330
• Main Authors: PLUMTREE, A., UNTERMANN, N.
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Publishing Ltd 01.05.2010; Blackwell; Wiley Subscription Services, Inc
• Subjects: Applied sciences; closure; Cracks; critical crack lengths; Exact sciences and technology; Fatigue; Fatigue life; fatigue threshold; long life fatigue; Mechanical properties and methods of testing. Rheology. Fracture mechanics. Tribology; Metals. Metallurgy; strain distribution; Stress analysis; Fatigue life; Crack length; Mechanical properties; critical crack lengths; Strain distribution; Fatigue; Fatigue threshold; closure; long life fatigue; Fatigue crack
• ISSN: 8756-758X; 1460-2695
• DOI: 10.1111/j.1460-2695.2010.01444.x

ABSTRACT A model based on surface strain redistribution and crack closure is presented for prediction of the endurance or fatigue limit stress by determining the threshold stress and critical length of short cracks that develop under microstructural control. The threshold stress first decreases with crack size to a local minimum then increases to a local maximum corresponding to the fatigue limit stress. This occurs at the critical crack length corresponding to about four grain diameters. The model is capable of determining the threshold stress range and depth of propagating and non‐propagating surface cracks as a function of stress ratio, material and grain size. The microstructure is shown to be particularly significant in the very long life regime (Nf ≈ 109 cycles). When the surface cracks become non‐propagating, internally initiated cracks continue growing slowly, eventually reaching the critical crack length with failure occurring after a very high number of cycles (107 < Nf < 109 cycles).