FATIGUE DAMAGE IN A CENTRALLY NOTCHED COMPOSITE LAMINATE DUE TO 2-STEP SPECTRUM LOADING

The fatigue response due to tension-tension (T-T) two-step spectrum lo ading in a centrally notched quasi-isotropic [0/90/+/-45]2s T300/5208 graphite/epoxy composite laminate has been investigated. The main emph asis of the work is on experiments, and the test data correlate well w ith the proposed cumulative damage model. The degradation of elastic m odulus is measured non-destructively as a function of loading cycles a nd the delamination area is inspected carefully by X-ray radiography; both of these are important parameters for expressing the damage cause d 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 ul timate stress, respectively; cumulative damage due to the adverse case s of low-high and high-low two-step spectrum loading is then recorded. The features of our results are as follows. More delamination is foun d 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 b y a simple Miner's rule. Finally, the prediction of life is found to b e in good agreement with the experimental data.