The influence of two-stage loading on the longitudinal splitting of unidirectional carbon/epoxy laminates

A review of variable fatigue loading of carbon fibre/epoxy laminates indicates that for two-stage loading, low-to-high loading is generally more damaging than high-to-low loading. Damage accumulation under two-stage loading appears to produce linear damage in some CFRP materials and non-linear behaviour in others. The behaviour of those materials exhibiting linear damage may be predicted by the Palmgren-Miner rule. A model is presented for determining the longitudinal split growth in notched unidirectional carbon fibre/epoxy test coupons. The model is able to predict the non-linear split growth for low-to-high loading whilst the linear damage accumulation for high-to-low loading may best be determined by use of the Palmgren-Miner rule.