Toughness of Reinforced Ductile Thermoplastics

Abstract

Fillers and reinforcements are widely used to enhance the strength and rigidity of plastics. These improvements are often obtained at the expense of toughness. This paper discusses the effects of major material parameters such as concentration and shape of the reinforcement, matrix morphology, and interfacial strength on the toughness of two reinforced ductile thermoplastics, namely polypropylene and PET/PBT blend. The toughness of these composites is strongly governed by the competition between shear yielding and crazing in the matrix. It has been shown in previous work that shear yielding occurs around the reinforcement, due to the debonding at the reinforcement/matrix inter face, and results in a stretching effect of the matrix which leads to an increase in the crack growth resistance of the composite. Crazing occurs in the matrix and dissipates much less energy. This paper demonstrates that the reinforcement content can be optimized in order to maximize the matrix stretching. The interfacial strength affects the stress distribution in the matrix and consequently the magnitude of shear yielding and crazing. The dimensions of the reinforcement are another major parameter in the fracture process. Two- dimensional fillers (flakes) behave the same as three-dimensional fillers (spheres), and for these classes of reinforcement, a unique qualitative relationship between the maximum crack growth resistance and the average distance between the surfaces of two adjoining particles has been observed, regardless of the nature of the filler. In the case of fibers, there is a critical fiber length which results in lowest crack growth resistance. Below the critical length, the matrix stretching is dominant and above the critical length, fiber pull- out enhances the dissipation of energy in the fracture process.