ISSN:
1460-2695
Source:
Blackwell Publishing Journal Backfiles 1879-2005
Topics:
Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
Notes:
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 examined—a fine-grained equiaxed bimodal structure (grain size ∼20 µm) and a coarser lamellar structure (colony size ∼500 µm). Studies were conducted over a range of mode-mixities, from pure mode I (ΔKII/ΔKI = 0) to nearly pure mode II (ΔKII/ΔKI ∼ 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 (ΔG) incorporating both tensile and shear-loading components. In the presence of through-thickness cracks—large (〉 4 mm) compared to microstructural dimensions—significant 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 µm) through-thickness cracks and microstructurally small (〈 50 µm) surface cracks, where the influence of crack-tip shielding would be minimal, such effects were similarly markedly reduced. Moreover, small-crack ΔGTH 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 microstructure (in relation to crack size) in promoting crack-tip shielding that arises from significant changes in the crack path in the two structures.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1046/j.1460-2695.2002.00522.x
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