Effects of Variable Amplitude on Off-Axis Fatigue Strength of Unidirectional Hybrid Composite:GLARE 2

Two-stress level fatigue strengths of unidirectional GLARE 2 laminates have been studied using various off-axis plain coupon specimens. Tension-tension fatigue tests accompanied by a single change in the stress amplitude were performed for step-up and step-down sequences. History-dependence of the growth of fatigue damage under two-stress amplitude conditions was elucidated through comparisons with the results of constant-amplitude fatigue tests. The residual lifetimes under two-level fatigue loading were significantly dependent on the sequence of fatigue loads and on the off-axis angles of specimens, so that they deviated markedly from those under constantamplitude fatigue loading. To highlight the significance of the non-linear accumulation of fatigue damage under two-stress level conditions the residual lifetimes of the specimens damaged by prior fatigue were compared with the predictions using the Palmgren-Miner's linear accumulation theory. Applicability of existing theories for a non-linear accumulation of fatigue damage was also evaluated on the basis of the experimental results.