Crack propagation life prediction for Ti-6Al-4V based on striation spacing measurements

Abstract

A combined experimental and numerical study was conducted on forged Ti-6Al-4V titanium alloy representative of a turbine engine fan blade material. The experimental information on striation spacing in the given material was converted into d a/d N–Δ K data and further extrapolated towards lower crack growth rates assuming a linear relationship between d a/d N and Δ K when plotted in log–log coordinates. The NASA Crack Analysis Code was used with several different sets of the crack growth rate input to predict the number of load cycles required to propagate a semi-circular surface crack from a certain initial size to final fracture in a round bar of nominal diameter 5 mm. For an initial flaw size of 10 to 50 μm, predictions based on the d a/d N–Δ K test results from both compact tension and surface flaw specimens greatly overestimated the fatigue crack propagation life. On the contrary, very good agreement with available experimental data has been shown to exist for predictions based on extrapolated striation spacing measurements.