Experimental investigation on the double-position impact responses and damage mechanism for Z-pinned composite laminates

Z-pins can enhance the resistance to delamination for composites effectively under impact loads. In this paper, the effect of Z-pinning technique on double-position low velocity impact behaviors for composite laminates were explored. The two impact positions had the same distance from the center of the laminates with the impact distances of 0 mm, 10 mm, 20 mm and 40 mm in between. The two positions were sequentially impacted with 5 J, 10 J and 20 J impact energy, respectively. The mechanical curves and the delamination damage projected area (DDPA) were recorded during the impact tests. Afterwards, scanning electron microscope (SEM) and X-ray micro-computed tomography (μCT) techniques were performed for characterizing the damage statuses and internal defects. Experimental results demonstrated that Z-pinning was unable to delay the delamination initiation at low impact energy, which led the similar damage statuses of double impact positions by comparing to the unpinned laminates. But at high impact energy, Z-pinning could suppress the delamination damage and internal defects, which weakened the damage interference. In addition, at high impact energy, dominated fiber breakage damage mode occurred for the second impact at the impact distance of 0 mm, accompanied by larger overlapped delamination area with the first impact and lower proportion of internal defects. This resulted in similar values of maximum central displacement and peak energy moment between the unpinned and Z-pinned laminates.