Modeling crack closure and damage in variable amplitude fatigue using smooth specimen fatigue test data

• Published in: International journal of fatigue Vol. 33; no. 2; pp. 223 - 231
• Main Authors: El-Zeghayar, M., Topper, T.H., Conle, F.A., Bonnen, J.J.F.
• Format: Journal Article
• Language: English
• Published: Kidlington Elsevier Ltd 01.02.2011; Elsevier
• Subjects: Amplitudes; Applied sciences; Crack closure; Crack opening stress; Crack propagation; Cracks; Effective strain–life curve; Exact sciences and technology; Fatigue; Fatigue crack closure; Fatigue failure; Fatigue tests; Mathematical models; Mechanical properties and methods of testing. Rheology. Fracture mechanics. Tribology; Metals. Metallurgy; Steady state crack opening stress; Stresses; Variable amplitude fatigue loading; Fatigue crack closure; Variable amplitude fatigue loading; Steady state crack opening stress; Crack opening stress; Effective strain–life curve; Fatigue test; Effective strain-life curve; Variable load; Crack closure effect; Mechanical properties; Fatigue; Modeling; Fatigue crack
• ISSN: 0142-1123; 1879-3452
• DOI: 10.1016/j.ijfatigue.2010.08.010

Modeling changes in crack opening stress level and fatigue damage using data derived from periodic underload fatigue tests of smooth specimens. [Display omitted] ► In constant amplitude fatigue loading the crack opening stress increases in an exponential manner until it reaches a steady state level. The steady state crack opening stress levels follow the trends predicted by model proposed. ► Deriving the steady state crack opening stresses from constant amplitude and underload fatigue data for smooth specimens gave a good fit to the crack opening stresses measured using crack growth specimens. ► Using the test procedure suggested in this paper, we were able to obtain data for crack opening stress recovery to a steady state level after underloads (and the associated crack closure parameter) from smooth specimen tests. ► The effective intensity versus crack growth rate curve parameters obtained from smooth specimen tests give a good fit to the experimental crack growth data. During overloads in variable amplitude fatigue, local stresses at small cracks growing from notches reach yield stress magnitude. Such high stress levels result in a large decrease in crack opening stress and an increase in the fatigue damage of subsequent smaller stress cycles. This paper presents a methodology for modeling changes in crack opening stress level and fatigue damage using data derived from periodic underload fatigue tests of smooth specimens. Predicted crack closure stress levels agree well with those obtained from crack growth observations made with a high magnification microscope.