Fatigue damage of designed T-type specimen under different proportion repeating Two-Step variable amplitude loads

Abstract

In order to investigate the fatigue damage evolution of typical joints of River-Sea-Going Ship (RSGS) under a repeating two-step variable amplitude (VA) loading condition, and to estimate the influence of the damage of low amplitude load below the fatigue limit and the interaction effect between the high and low amplitude loads on the fatigue life of the structure, a series of fatigue experiments of designed T-type specimen (DTS) under different repeating two-step VA loading conditions were carried out in this work, which are designed according to hull structure characteristics and actual service conditions of RSGS. The experimental results show that the interaction damage value (Dnl) firstly increases and then decreases with increasing the cycle ratio of low to high amplitude load from 1 to 5. The value of Dnl is negative when the cycle ratio is relatively large, not less than 4.5 in the current study, which indicates that the effect of loading interaction not only doesn’t cause damage but has a beneficial effect on the fatigue life of the DTS. The X-ray Diffraction (XRD) analysis was carried out subsequently at the fatigue crack initiation region with the aim of confirming the material characteristic at the microcrack tip changing from work-hardening state to work-softening state with the cycle ratio increasing.