Fatigue strength for small shallow defects/cracks in torsion

Fatigue strength in non-homogenous materials is controlled by the threshold condition of small cracks which nucleate at the defect tip Therefore the fatigue limit is not a limit stress for crack nucleation but rather a threshold condition of small cracks emanating from the defect In order to investigate fatigue crack propagation under Mode III torsional fatigue tests with shallow micro-notches (root area < 1000 mu m) were carried out on a SAE 5135 gear steel An initial series of tests showed the formation of Mode III non-propagating cracks ahead of the micro-notches on run-out specimens A second series of tests were then carried out after a Mode I pre-cracking (at stress ratio R = -2) procedure results show a Mode III co-planar crack growth for Delta K(III) > Delta K(th I) even on specimens whose failure is controlled by Mode I branches Fatigue tests at different Delta K(III), levels showed two types of crack advance a discontinuous growth in the near-threshold region and a stable co-planar growth at higher Delta K levels The Delta K(III), levels which control the formation of discontinuous crack (Delta K(th) III) and the onset of stable co-planar growth (Delta K(III) > Delta K(th III)) clearly show a small crack effect (C) 2010 Elsevier Ltd All rights reserved