Fatigue damage in a centrally notched composite laminate due to two-step spectrum loading

Abstract

The fatigue response due to tension-tension (T-T) two-step spectrum loading in a certrally notched quasi-isotropic [0/90/±45] 2s T300/5208 graphite/epoxy composite laminate has been investigated. The main emphasis of the work is on experiments, and the test data correlate well with the proposed cumulative damage model. The degradation of elastic modulus is measured non-destructively as a function of loading cycles and the delamination area is inspected carefully by X-ray radiography; both of these are important parameters for expressing the damage caused by cyclic loading. The constant stress amplitudes for various stress ratios are executed first as baseline data and the S-N curve is thus obtained. Low and high stress levels are chosen as 0.65 and 0.85 of ultimate stress, respectively; cumulative damage due to the adverse cases of low-high and high-low two-step spectrum loading is then recorded. The features of our results are as follows. More delamination is found under low stress level as well as a higher modulus reduction due to low stress level for the same fraction of life. The specimen subjected to a high-low spectrum loading is more damaged than that subjected to a low-high spectrum loading. The damage can be reasonably explained by a simple Miner's rule. Finally, the prediction of life is found to be in good agreement with the experimental data.