Fracture behavior of injection-molded short and long glass fiber—polyamide 6.6 composites

Abstract

Relationships between the fracture toughness, K Q, and microstructure of chopped short (SGF) and long glass fiber (LGF) reinforced injection-molded polyamide 6.6 composites have been studied. K Q and elastic modulus, E, of the composites were determined on compact tension specimens as a function of temperature, T, and crosshead speed, v. The microstructure of the composites was characterized by the dimensionless reinforcing effectiveness parameter, R, which was extended in this work for LGF reinforcement. R takes into account not only the processing-induced fiber layer structure, the fiber alignment and the fiber volume fraction but also the aspect ratio and aspect ratio distribution of the reinforcement. The semi-empirical linear relationship between fracture toughness of the composite, K Q,C, and that of the matrix, K Q,C, established for SGF-reinforced plastics, i.e. K Q,C = MK Q,M = (a + n R)K Q,M still exists if the newly defined modified R is used. Both the matrix stress condition factor, a, and the energy absorption coefficient, n, have been determined under different testing conditions and tabulated together with K Q,M . This allows an estimate of K Q,C for any given R. Normalized fracture maps in form K Q vs (E,T) have been constructed. Failure mechanisms of both the matrix and the composites which have been revealed by scanning electron microscopy are discussed and summarized in failure maps indicating changes of breakdown processes as a function of T and v.