Effect of Transient Loads on Fatigue Crack Growth in Solution Treated and Aged Ti-62222 at -54, 25, and 175°C

Abstract

Transient loads consisting of single tensile overloads and single tensile overloads followed by single compressive underloads were applied to Ti-62222 solution treated and aged titanium alloy at -54, 25, and 175°C. Tensile overload ratios were 2.0 and 2.5, and the compressive underload ratio was -0.5. Four reference steady state ΔKss values, using constant ΔK testing at R = 0.1, were investigated at each temperature. Cycles of delay, fatigue crack growth during delay, and minimum fatigue crack growth rate during a transient load were obtained for all tests. Cycles of delay ranged from zero to crack arrest. Higher tensile overloads caused greater delay cycles, and underloads were often detrimental. Low and high temperatures were primarily beneficial to delay cycles relative to those at room temperature. Crack growth delay distance was always greater than the pertinent reversed plastic zone size. Fatigue crack growth life predictions were made using FASTRAN III and AFGROW computer programs and produced both conservative and non-conservative results with more than half the predictions being within ± 2 of the experimental results. Macro- and microfractography revealed surface crack closure, Mode II displacements, crack tip blunting, branching, and tunneling contributed to the transient fatigue crack growth behavior.