Long glass fiber reinforced thermoplastic composites have been increasingly used in automotive parts due to their excellent mechanical properties and recyclability. However, the effects of strain rates on the mechanical properties and failure mechanisms of long glass fiber reinforced polypropylene composites (LGFRPPs) have not been studied systematically. In this study, the effects of strain rates (from 0.001 s−1 to 400 s−1) on the mechanical properties and failure mechanism of LGFRPPs were investigated. The results showed that ultimate strength and fracture strain of the LGFRPPs increased obviously, whereas the stiffness remained essentially unchanged with the strain rates from low to high. The micro-failure modes mainly consisted of fibers pulled out, fiber breakage, interfacial debonding, matrix cracking, and ductile to brittle (ductile pulling of fibrils/micro-fibrils) fracture behavior of the matrix. As the strain rates increased, the interfacial bonding properties of LGFRPPs increased, resulting in a gradual increase of fiber breakage at the fracture surface of the specimen and the gradual decrease of pull-out. In this process, more failure energy was absorbed, thus, the ultimate strength and fracture strain of LGFRPPs were improved.

In this article, the mechanical properties, flame retardancy, and thermal stability of basalt fiber reinforced polypropylene composites (BFRPPs) and polypropylene (PP) were investigated and compared. The combustion performance and thermal stability of BFRPPs and PP were evaluated by limiting oxygen index (LOI) test, cone calorimeter test (CCT), and thermogravimetric analysis (TGA). The results showed that basalt fibers could enhance mechanical properties, flame retardancy, and thermal stability of PP. With the increase of basalt fiber content, the strength and stiffness of BFRPPs increased significantly, and the elongation decreased obviously. Adding basalt fibers into PP could improve the LOI value. BFRPPs burned appreciably more slowly than PP under the same oxygen concentration. Simultaneously, BFRPPs indicated a better antimelt dripping effect than PP matrix. The heat release rate (HRR), total heat release (THR), rate of smoke release (RSR), and total smoke release (TSR) of BFRPPs decreased compared to PP matrix. Moreover, the addition of basalt fibers to PP could contribute to the formation of a more compact and continuous char layer, which effectively reduced the transfer of heat and oxygen, resulting in a better flame retardancy of BFRPPs.

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