Mechanical properties of composite laminates reinforced with rectangular z-pins in monotonic and cyclic tension

This paper presents an experimental study investigating the static and fatigue tensile properties of unidirectional and quasi-isotropic carbon fiber/epoxy laminates reinforced with rectangular and circular z-pins. All z-pinned laminates showed reduced tensile stiffness and strength compared to unpinned specimens. The insertion of rectangular z-pins that were aligned lengthwise to the fiber direction of a laminate ply was observed to cause minor microstructural damage, such as in-plane fiber waviness. Therefore, the use of rectangular z-pins led to significantly higher tensile stiffness and strength in both unidirectional and quasi-isotropic laminates. Unpinned laminates showed only minor fatigue effects. The insertion of z-pins resulted in a decrease in the fatigue performance of the tested unidirectional and quasi-isotropic laminates. Since this deterioration was primarily caused by the initial knockdown of the static tensile strength of the z-pinned laminates, rectangular z-pins showed superior fatigue performance in both unidirectional and quasi-isotropic laminates.