Effects of rolling reduction and direction on fatigue crack propagation in commercially pure titanium with harmonic structure

Abstract

Cold rolling followed by heat treatment was performed on commercially pure titanium with a bimodal harmonic structure, which is defined as a coarse-grained region (Core) surrounded by a network structure of fine grains (Shell), and fatigue crack propagation tests were conducted to clarify the effects of the rolling reduction, rolling direction, and force ratio. From the relationship between the crack propagation rate da/dN and the stress intensity factor range ΔK, da/dN for the L-T orientation was always higher than that for the T-L orientation, and da/dN was higher for a higher force ratio and a higher rolling reduction for either rolling reduction. Crack closure resulting from the roughness of the fracture surface can be partially explained by the above effects; however, the relationship between da/dN and the effective stress intensity factor range ΔKeff also depended on the same factors, while the effects were smaller than those for the da/dN–ΔK relationship. The crack opening stress intensity factor Kop,th and effective stress intensity factor range ΔKeff,th under the threshold condition linearly increased with the square root of the average grain size in the Shell region, which decreased with rolling reduction. Thus, the threshold condition of the harmonic structured material is considered to be determined by the average grain size in the Shell region.