Near‐threshold behaviour of shear‐mode fatigue cracks in metallic materials

Abstract

This review paper presents a brief state‐of‐the art of the research on long fatigue shear‐mode cracks and describes some recent results on effective crack growth thresholds and mode I branching conditions achieved by the authors for ARMCO iron, titanium with two different microstructures, nickel and stainless steel. A special technique for preparation of fatigue precracks enabled us to substantially suppress the crack closure (friction) effects at the beginning of the experiment, and the measured threshold values could be considered to be very close to the effective ones. In all investigated materials, the effective thresholds under the remote mode II loading were found to be about 1.7 times lower than those under the remote mode III loading. Effective thresholds under mode II loading of investigated materials were found to follow a simple formula assembled by the shear modulus G, the magnitude of Burgers vector b and a goniometrical function nα of the mean deflection angle that depends on the number of available crystallographic slip systems. These quantities determine the intrinsic material resistance to mode II crack propagation at the threshold. A simple criterion for mode I branching in terms of effective threshold values well reflects a transition from the shear‐mode to the opening‐mode controlled crack propagation at the threshold. The associated transition deflection angle of 40° is a material independent constant.

Propagation of Long Fatigue Cracks under Remote Modes II and III in Ferritic-Pearlitic Steel

“…Indeed, the measured ratio ∆K IIIeff,th /∆K IIeff,th ≈ 1.7 was found for all of the four investigated single-phase materials (see Table I) and, simultaneously, the validity of Eq. (1) was also conrmed [7]. However, a majority of engineering materials exhibit a multi-phase microstructure and, therefore, a ferritic-pearlitic steel was experimentally studied in this work.…”

“…3) which corresponds to the ratio of the local SIFRs ∆K I /∆K II ≈ 2, i.e., to a propagation under a dominant opening mode [7]. This is signicantly dierent from the mean angle α IIm,A ≈ 18…”

“…Based on this relation one can check a simple criterion, Eq. (3), for growth of this mode I branch when knowing the eective thresholds ∆K IIeff and ∆K Ieff,th [7]:…”

“…and α IIm is the mean deection angle of the crack front from the plane of the maximum shear stress [7]. For the investigated single-phase materials, the angle α IIm increased with decreasing number of available easy slip systems in the crystal lattice and n α,bcc ≈ 0.9, n α,hcp ≈ 0.6, and n α,fcc ≈ 0.4 for bcc, hcp, and fcc metals, respectively.…”

“…[6,7], this unpleasant situation was improved by obtaining experimental data on ∆K IIeff,th and ∆K IIIeff,th for several single-phase materials see Table I. The data for ∆K IIeff,th were well reproduced by the following simple formula:…”