Influence of LCF overloads on combined HCF/LCF crack growth

Abstract

Fatigue crack growth rates have been studied in forged Ti–6Al–4V aero-engine disc material under the conjoint action of LCF and HCF cycles at room temperature. An overload has been introduced into the LCF cycle component of the test sequence, which has been based on a ratio of HCF to LCF cycles of 1000:1. Systematic increases in the overload, applied prior to the commencement of the HCF cycles, clearly demonstrated a diminution in the contribution of the HCF cycles to crack growth rates. Accompanying the reduction in crack growth rates is an increase in the stress intensity range at which the HCF cycles begin to contribute to the crack growth rate. For three HCF cycle stress ratios ( R=0.7, 0.8 and 0.9), the severity of the overload required to negate the effect of the HCF cycles has been determined. Additionally, threshold values for the same stress ratios have been found and used to predict the onset of HCF cycle contribution to crack growth, and the effect of multiple vs. single overloads has been investigated. Modelling to predict crack growth rates and the onset of minor cycle damage has been undertaken.