Comparison of the fracture and failure behavior of polypropylene composites reinforced by long glass fibers and by glass mats

Abstract

The static fracture and related failure behavior of injection-molded long-glass-fiber-reinforced polypropylene (LGF-PP) and hot-pressed glass-fiber-mat-reinforced polypropylene (GMT-PP) with the same reinforcement fraction (V f ≈ 0.20) has been studied and compared. It was established that GMT-PP is superior to LGF-PP when fracture toughness and energy are considered under both static and dynamic conditions. The failure mode and sequence of the composites were determined from acoustic emission (AE) analysis by relating AE signal characteristics (amplitude and energy) to the individual failure events. GMT-PP failed in a stepwise manner related to the successive breakdown of the GF mat incorporated. This resulted in a more extended damage zone than for LGF-PP. Size and shape of the damage zone were estimated by mapping the located AE events and by considering infrared thermographic (IT) pictures showing heat evolution in the crack tip region. The damage zone size deduced from AE analysis proved to be lower than that obtained from IT studies. This was attributed to differences in the damage assessment: AE informs us about the development of the full damage zone at every stage of the loading, while IT is sensitive for the process zone (being a part of the damage zone) in the crack growth range. It may be stated that the energy-related toughness of GMT-PP is superior to that of the LGF-PP at the same reinforcement content.