Multiaxial fatigue damage assessment of repaired composite cruciform specimens using stress ratio dependent fatigue damage accumulation model

Abstract

The paper presents a combined experimental and numerical study of multiaxial fatigue damage accumulation in scarf-repaired cruciform specimens subjected to fatigue loading. The existing fatigue analysis module is improved by including the local stress ratio and mode mixity dependent fatigue damage initiation description to capture the change in fatigue damage accumulation rate due to stress redistribution. Multiple stress ratios defined in the local ply material system are used to account for the distinct rate of fatigue damage accumulation under mixed-mode loading. A three-stage cycle jumping approach is implemented to characterize the local stress ratio dependent fatigue damage accumulation: 1) conducting FE analysis for one full cycle to establish a trendline, 2) extrapolating the trendline spanning many cycles, and 3) using the extrapolated state as the initial state for subsequent full-cycle calculation. A static analysis associated with the first cycle of the loading block is performed based on a mechanism-driven progressive damage accumulation model under multiaxial loading. The minimum and maximum stress components associated with the matrix, fiber, and delamination failure are determined for each element at the critical zone along with its corresponding stress ratios. A constant life diagram (CLD) is constructed to determine the S-N curve at a given stress ratio (R) based on the limited test data associated with a few applied load ratios. The interpreted S(N) data at a given R ratio and fatigue growth data are used to determine the evolved cohesive properties where the cohesive strength is degraded based on S-N and the shrunk post-peak cohesive behavior is driven by the fatigue damage growth model. The fatigue tests of cruciform specimens with and without repair are performed under biaxial fatigue loading. The cruciform specimens with and without scarf repair are fabricated and tested under biaxial fatigue loading of different levels of applied peak loads. The detected fatigue damage accumulation data and measured stiffness against cycle curves are compared with the model predictions for the cruciform specimens.