Effects of Microstructure on Mixed-mode, High-cycle Fatigue Crack-growth Thresholds in Ti-6al-4v Alloy

A B S T R A C T Effect of microstructure on mixed-mode (mode I ‡ II), high-cycle fatigue thresholds in a Ti-6Al-4V alloy is reported over a range of crack sizes from tens of micrometers to in excess of several millimeters. Specifically, two microstructural conditions were exam-inedÐa fine-grained equiaxed bimodal structure (grain size $20 mm) and a coarser lamellar structure (colony size $500 mm). Studies were conducted over a range of mode-mixities, from pure mode I (DK II /DK I ˆ 0) to nearly pure mode II (DK II / DK I $ 7.1), at load ratios (minimum load/maximum load) between 0.1 and 0.8, with thresholds characterized in terms of the strain-energy release rate (DG) incorporating both tensile and shear-loading components. In the presence of through-thickness cracksÐlarge (> 4 mm) compared to microstructural dimensionsÐsignificant effects of mode-mixity and load ratio were observed for both microstructures, with the lamellar alloy generally displaying the better resistance. However, these effects were substantially reduced if allowance was made for crack-tip shielding. Additionally, when thresholds were measured in the presence of cracks comparable to microstructural dimensions, specifically short ($200 mm) through-thickness cracks and microstructurally small (< 50 mm) surface cracks, where the influence of crack-tip shielding would be minimal, such effects were similarly markedly reduced. Moreover, small-crack DG TH thresholds were some 50±90 times smaller than corresponding large crack values. Such effects are discussed in terms of the dominant role of mode I behaviour and the effects of micro-structure (in relation to crack size) in promoting crack-tip shielding that arises from significant changes in the crack path in the two structures. The control of failures owing to high-cycle fatigue (HCF) in turbine-engine components has been identified as one of the major challenges facing the readiness of the US Air Force fleet today. 1±3 In order to address this issue, a consortium of industrial, government and academic institutions has been charged with the task of modifying the existing design methodologies for improving the HCF reliability of these components. 4 One critical issue involves the effects of mixed-mode cyclic loadsÐthat is, the presence of both tensile and shear loadingÐon the critical states of damage for such HCF failures. Indeed, there are many fatigue-critical locations within the turbine engine where such mixed-mode conditions existÐe.g. in the presence of fretting fatigue cracks in the blade±dovetail contact section. 5 The effective crack-driving force here can be considered to be …