Issue 33

Y. Wang et alii, Frattura ed Integrità Strutturale, 33 (2015) 345-356; DOI: 10.3221/IGF-ESIS.33.38 345 Focussed on multiaxial fatigue Critical plane approach to multiaxial variable amplitude fatigue loading Yingyu Wang Key Laboratory of Fundamental Science for National Defense-Advanced Design Technology of Flight Vehicle, Nanjing University of Aeronautics and Astronautics, Nanjing, 210016, China yywang@nuaa.edu.cn Luca Susmel Department of Civil and Structural Engineering, the University of Sheffield, Sheffield S1 3JD, UK l.susmel@sheffield.ac.uk A BSTRACT . A new critical plane approach based on the modified Manson-Coffin curve method (MMCCM) is presented in this paper for predicting fatigue lifetime under variable amplitude (VA) multiaxial fatigue loading. The critical plane is assumed to coincide with that material plane experiencing the maximum variance of the resolved shear strain. Fatigue damage is hypothesized to be a function of both the amplitude of the resolved shear strain and the so-called critical plane stress ratio. The latter quantity depends on the mean value and the variance of the stress perpendicular to the critical plane as well as on the variance of the shear stress resolved along the direction experiencing the maximum variance of the resolved shear strain. Load cycles are counted from the resolved shear strain time history by using the classic rain flow counting method. Palmgren-Miner’s linear damage rule is applied to estimate cumulative fatigue damage. The accuracy and reliability of the proposed approach is checked by using several experimental data taken from the literature. The estimated fatigue lives based on the new approach are seen to be in sound agreement with the experimental results. K EYWORDS . Multiaxial fatigue; Variable amplitude loading; Critical plane; Fatigue life prediction. I NTRODUCTION atigue life prediction approaches based on the concept of the critical plane are generally considered to be accurate enough to estimate fatigue lifetime under multiaxial load histories. The critical plane concept is based on the physical observation that cracks initiate and grow on some specific material planes. We believe there are three key aspects needed to be considered when applying the critical plane approach to estimate fatigue lifetime under multiaxial variable amplitude loading, i.e.: (i) determining the orientation of the critical plane, (ii) counting the fatigue cycles and (iii) calculating the amplitude and mean value of the stress/strain components relative to the critical plane. As to the available critical plane approaches, Findley [1] determined the critical plane by maximizing a linear combination of the shear stress amplitude and the maximum normal stress. Brown and Miller [2] defined the critical plane as the plane experiencing the maximum shear strain amplitude. Wang and Brown [3] have also proposed a modified version of their criterion by defining the critical plane as the one experiencing not only the maximum shear strain range, but also the largest value of the normal strain excursion. Fatemi and Socie [4] assumed that the critical plane is the plane of maximum shear strain amplitude when the crack initiation process is Mode II governed. From the brief review presented above, it F