Some Aspects of Fatigue Crack Closure in Two Contrasting Titanium Alloys

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Abstract
Fatigue crack growth at intermediate rates was measured in compact tension specimens of the microstructurally contrasting alloys Ti-6Al-4V (Imperial Metal Industries, Ltd. [IMI] titanium 318) and Ti-6Al-5Zr-0.5Mo-0.25Si (IMI titanium 685). Sinusoidal loading was used with a ratio of minimum load to maximum load R of 0.1. The incidence of crack closure was examined during fatigue with both direct current potential drop and crack-opening-displacement (COD) methods; COD was found to be more reliable for closure measurement than potential drop. In contract to the fine-grained IMI 318, the coarse-structured IMI 685 showed substantial load transfer across the crack faces during fatigue and this was attributed to mismatch of the fracture faces. It was shown that the large differences in crack growth rate between the two alloys could be substantially accounted for by an effective stress intensity factor range concept. Marked reductions in growth rate produced by variable amplitude loading of the IMI 318 could not be explained in terms of crack closure.

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