Abstract
The influence of steady mode III loading on the fatigue crack growth behaviour of a Ti-5Al-2·5 Sn alloy subjected to mode I fatigue loading has been determined using circumferentially notched specimens. It was found that the superimposition of a constant torque (mode III) on the alternating tensile load (mode I) always gave rise to a very significant decrease in crack propagation rate. This decrease could reach two orders of magnitude for the highest values of KIII/∆KI. Moreover, a change in fracture path was shown to be associated with combined mode loading. While under mode I loading the crack was essentially transverse to the tensile axis, under combined mode I + mode III loading the crack front divided into facets inclined to the tensile loading axis. This change in fracture path is tentatively explained in terms of the influence of KI max and ∆KI on the fatigue crack growth rate of this alloy. Several factors which could contribute to the reduction in crack propagation rate observed under combined mode loading are described.
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