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In this paper, the ballistic behavior of multi-layer Kevlar ® aramid fabric/polypropylene (PP) composite laminate (CL) and plain layered aramid fabric (AF) impact specimens was investigated. It was found that the thermoplastic PP matrix increases the ballistic performance of CL targets when compared to AF targets with similar areal density, resulting in less aramid fabric needed to obtain the same level of protection when the PP matrix is incorporated. It was found that the improved ballistic performance of CL targets is due to the fact that the thermoplastic matrix enables energy-absorbing mechanisms such as fabric/matrix debonding and delamination. The ballistic limit and penetration threshold energy of the CL configurations, which were predicted using an empirical model, were found to be higher than those of the AF targets. These results show that aramid fabric/PP laminates should be further studied for improved ballistic performance at lower costs.

The mechanical characterization of plain foamed concrete (PFC) and fiber-reinforced foamed concrete (FRFC) with a density of 700 kg/m3 was performed with compression and tension tests. FRFC was reinforced with the natural fiber henequen (untreated or alkaline-treated) at volume fractions of 0.5%, 1% and 1.5%. Polypropylene fiber reinforcement was also used as a reference. For all FRFCs, the inclusion of the fibers enhanced the compressive and tensile strengths and plastic behavior, which was attributed to the increase of specimen integrity. Under compressive loading, after the peak strength, there was no considerable loss in strength and a plateau-like regime was observed. Under tensile loading, the fibers significantly increased the tensile strength of the FRFCs and prevented a sudden failure of the specimens, which was in contrast to the brittle behavior of the PFC. The tensile behavior enhancement was higher when treated henequen fibers were used, which was attributed to the increase in the fiber–matrix bond produced by the alkaline treatment. The microscopic characterization showed that the inclusion of fibers did not modify the air-void size and its distribution. Higher energy absorption was observed for FRFCs when compared to the PFC, which was attributed to the enhanced toughness and ductility by the fibers. The results presented herein warrant further research of FRFC with natural henequen fibers for engineering applications.

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Scaling effects in the low velocity impact response of a polypropylene-based fiber-metal laminate (FML) structure have been investigated. The FML was based on a 2024 aluminum alloy, a self-reinforced polypropylene composite and a polypropylene film acting as an interlayer adhesive. The study focuses on the assessing the possibility of using scale model tests for predicting the full scale low velocity impact response of FMLs based on [Aln, 0°/90°n]s and [Al, 0°/90°]ns configurations. These two systems were used to scale four different sample sizes (n = 1/4, 1/2, 3/4 and full scale). The impact load-displacement traces were normalized and found to collapse onto a single curve, suggesting that the laminates obey a scaling law. Attention also focused on characterizing the resulting damage in these multi-layered systems, where it was shown that the deformation modes and failure mechanisms were similar in all four scaled sizes. Other parameters such as the maximum impact force and the time to maximum load showed little sensitivity to scale size. This evidence suggests that data collected from tests on small FML plates can be used to predict the low velocity impact response of larger, more representative structures.