Fatigue Assessment for Combined HCF/LCF Loading

The closed form expression for estimation of the crack initiation life at combined HCF/LCF loading is derived, and the way of reshaping the crack growth rate formulae in the form enabling their use in fatigue design at non-stationary loading is demonstrated. This new derived formula suggests an additional damage increase when crossing from one stress block to another. It is proposed to call this effect as “block crossing effect”. Herein, the reshaped crack growth rate formula is applied for the fatigue design of structures and components made of titanium alloy Ti-6Al-4V and subjected to combined HCF/LCF loading. For the stress history simplified in the way that it consists of one LCF stress block with number of cycles equal to number of start-up in-service operations, at load ratio r = 0, followed by one HCF stress block at load ratio r > 0 with summed-up all HCF cycles, the closed form expression is derived for estimating the crack propagation life at combined HCF/LCF loading. Smith and Haigh diagrams as design tool for estimating the fatigue strengths for designed fatigue life, known load ratio and various number of HCF cycles per one combined stress block, are obtained for same material and same loading.