Crack Closure in the Near-threshold Region in Metallic Materials

Abstract Resistance to fatigue crack propagation is divided into the intrinsic (effective) component and the extrinsic component, represented by crack closure, which is very significant in the near-threshold regime. Crack growth rate measurements were done on the compact tension specimens made of the ferritic-pearlitic steel for automotive at the stress ratio R = 0.1 and R = 0.8. Crack closure was quantified from the experimentally measured data using the effective crack growth rate curve da/dN-ΔKeff and a new expression of the crack closure ratio Rcl = Kcl/Kmax depending on plasticity-induced crack closure, roughness-induced crack closure, oxide-induced crack closure and the exponent of the Paris power law. The presented methodology helps to analyse the crack closure mechanisms and to calibrate parameters used in the crack closure models.