Designing Ply‐Thickness Hybridization in Carbon Fiber Reinforced Polymer Composite Laminates for Enhanced Delamination Resistance

To mitigate delamination due to out‐of‐plane loading, this study designed and fabricated several types of double helicoidal laminates with thin plies inserted between two standard‐thick plies. This stacking microstructure has two pitch angles: the rotation angle of the standard plies and the interply angle between two adjacent plies. The progress of delamination owing to out‐of‐plane indentation was successively observed using ultrasonic C‐scanning. As the load increased, the delamination spread spirally in all the laminates. Delamination occurred discretely in some double helicoidal laminates, in contrast to the laminate consisting of standard plies with a 45° interply angle. Delamination resistance varied depending on the stacking pattern, and mechanisms affecting the growth rate of the projected delamination area were investigated. The growth rate was primarily correlated with the magnitude of the tensile‐bending coupling stiffness of the repeating lamination unit. In the double helicoidal microstructure with opposite rotation directions for the standard and thin plies, delamination occurred at the upper interface of the standard ply. Together with the damage suppression effect of the thin plies, the delamination was not connected by matrix cracks generated in the standard plies and was discretely distributed. Such damage progression suppressed the growth rate of the projected delamination area.