Damage Tolerant Design of the Aircraft Components
The closed form expression for estimation of the crack initiation life at combined HCF/LCF loading is derived in this paper, 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. So, fatigue design becomes more conservative, broaching the subject of reliability of recent fatigue assessment of the components under variable amplitude loading. Herein, the reshaped crack growth rate formula is applied for the fatigue design of aircraft 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 NLCF = NB cycles at maximum stress and load ratio r = 0, followed by one HCF stress block with nHCF·NB cycles at maximum stress and load ratio r = ( - 2 )/ , the closed form expression is derived for estimating the crack propagation life at combined HCF/LCF loading.
Smith and Haigh diagrams as design tools for estimating the fatigue strengths for designed fatigue life, known load ratio and various number nHCF cycles, are obtained and presented for the parts made of titanium alloy Ti-6Al-4V and subjected to combined HCF/LCF loading.