Comparison of laminate thickness on the low velocity impact behaviors for Z-pinned composite laminates

The Z-pinning technique can enhance the delamination resistance for composite laminates effectively. In this paper, the effect of laminate thickness on the low velocity impact responses of Z-pinned laminates with [0₂/90₂]_2S, [0₂/90₂]_4S, and [0₂/90₂]_6S layup patterns was explored. These laminates had nominal thickness of 2.1 mm, 4.5 mm and 6.9 mm, respectively. The impact energy was set to 5 J, 10 J, 20 J and 30 J, and the mechanical curves during the experiment were recorded. Afterwards, ultrasonic C-scanner, scanning electron microscope (SEM) and X-ray micro-computed tomography (μCT) were used to detect the impact-induced damage. The theoretical models for predicting the delamination initiation threshold and related threshold impact energy were consistent with the experimental data. The results showed that Z-pins had almost no effect on the delamination initiation, but Z-pinning became significant for delamination suppression as the impact energy exceeded the threshold impact energy. In addition, due to the improved penetration resistance, the maximum central displacements were suppressed with Z-pin implantation as the noticeable fiber breakage occurred. Due to the fact that the dominant failure mode changed from fiber breakage to delamination with the increase of the laminate thickness, the delamination damage and internal defect suppression for thin laminate was inferior to that for the intermediate-thickness laminate. However, the suppression effectiveness became weak for the thick laminate at the same impact energy due to the smaller bridging force with slight Z-pin pull-out.