Modelling combined HCF-LCF fatigue crack growth under the influence of LCF overload

Fatigue crack growth (FCG) rates have previously been reported in forged Ti6Al-4V aero-engine disk material under the conjoint action of LCF and HCF cycles at room temperature. It is found that systematic increases in LCF overload, applied prior to the commencement of the HCF cycles, reduce the contribution of the HCF cycles to crack growth rates. The application of overload also enhances the stress intensity range, ∆Konset, at which the HCF cycles commence to contribute to the crack growth rate. In this paper, a fatigue crack growth prediction code, FASTRAN, based on Newman’s plasticity induced closure model, is employed to facilitate the estimation of crack growth rates under combinations of LCF and HCF loading sequences with and without the overload effect. It is found that FASTRAN gives a reasonable prediction on fatigue crack growth rates for stress intensity ranges ∆KLCF greater than 10 MPa√m. However, the ∆Konset value for the onset of contribution by the HCF cycles to the crack growth is underestimated and the FCG rates near ∆Konset are thereby overestimated. This is considered to be due to the fact that other effects, e.g. a dependence on Kmax of the fatigue threshold and near-threshold crack growth, are ignored in FASTRAN.