Effect of Off-Axis Ply on Tensile Properties of [0/θ](ns) Thin Ply Laminates by Experiments and Numerical Method

The effect of off-axis ply on the tensile properties of unbalanced symmetric [0/θ](ns) laminates was explored through experimental and numerical analysis. Six CFRP [0/θ](2s) plies with different off-axis angles θ were fabricated for tensile tests. In situ observations of the damage to the laminates were conducted to investigate the initiation and progressive growth of the laminates during the tension tests. The fiber fractures, crack initiation, and progressive propagation were analyzed by observing the free edge of the laminates, and the difference in damage behavior caused by different off-axis angles was investigated. All the six [0/θ](2s) plies with off-axis angles θ ranging from 15° to 90° showed approximate linear stress–strain responses in the tensile tests. Matrix cracks were not observed prior to the final catastrophic failure in the off-axis layers of the [0/θ](2s) laminates with a θ in the range of 15–60°. Finite element analysis (FEA) of the [0/θ](s) plies was conducted using a 3D micromechanical model, in which matrix cracking and fiber-matrix debonding in the off-axis layer were simulated using a cohesive interface element. Three micromechanical crack-free, cohesive interface, and initial crack models were analyzed to predict the influence of the matrix cracks inside the off-axis layer on the damage behavior of the [0/θ](s) laminates. The numerical results from the initial crack micromechanical model show a lower bound of the tensile strength of the [0/θ](s) plies. A high stress concentration is observed adjacent to the cracked off-axis layer, inducing a tensile strength loss of about 20%.