Studies have shown that the fatigue life of multidirectional laminated fibre composites is reduced for variable amplitude block loading and random load spectra compared to constant amplitude loading. One of the hypotheses in the literature for this is a cycle mix effect that causes extra damage each time the load increases i.e. load amplitude or mean load. However, no explanation for this effect has been provided in the literature. This work aims at identifying the reasons for the reduced fatigue life of variable amplitude block loading tests compared to constant amplitude loading tests. This is done through white light imaging which is used to detect when intralaminar cracks occur in the off-axis layers and from this to determine crack density evolution, crack density rates, as well as initiation and propagation of intralaminar cracks contributing to the total crack density. Combined, these parameters show that there is a slow transition from a high crack density rate in the high load blocks to a lower crack density rate in the low load blocks. The crack density rate in the low load blocks in a variable amplitude loading test is higher than in a constant amplitude loading test at similar damage states. Finally, the high load blocks initiate new cracks in each block but they also cause all existing cracks to continue their propagation in subsequent low load blocks at increased rates. In conclusion, there is evidence that the block changes from high to low loads cause increased crack propagation in subsequent low load cycles, which is an important contribution to explain the damaging effect of cycle mixing.
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