Microstructural efficiency and fracture toughness of short fiber/thermoplastic matrix composites

Abstract

This paper outlines the fracture behavior of composites with thermoplastic matrices of different fracture toughness K cm (increasing in the order PPS → PET ( I) → PET ( II) → ETFE → PC). In particular, the way in which the fracture toughness of these composite systems, K cc, is affected by the volume fraction, orientation and distribution of short glass fibers across the plaque thickness (fiber length ≈ 200 μm, fiber diameter ≈ 10 μm), and by the quality of their interfacial bonding to the matrix is discussed. SEM studies were carried out to define the microstructural details and the dominant mechanisms of energy adsorption during breakdown of the composites. In general, an increase in composite toughness can be expected with increasing extent of reinforcement if the matrix is in a brittle condition (here also verified by K c-tests at lower temperatures) and if the fibers are well bonded and mostly oriented perpendicular to the crack front. An opposite tendency may occur for matrices which behave in a highly ductile manner even in the presence of fibers. The probability of this behavior is favored in poorly bonded systems. The results are discussed in terms of a ‘microstructural efficiency factor’ M, which mainly considers the relative contributions of fiber and matrix related mechanisms to energy dissipation during breakdown of a composite (‘energy absorption ratio’ n) as well as the reinforcement content and its arrangement in the matrix (‘reinforcing effectiveness parameter’ R).