Some effects of matrix and interface properties on the fatigue of short fiber-reinforced thermoplastics

The fatigue behavior of injection-molded tensile bars of short-fiber-reinforced theromplastics is described and related to the fatigue behavior of the matrices and the strength of the fiber/matrix interface. A brittle matrix system based on polyphenylene sulfide is shown to behave in a similar manner to long-fiber composites. Glass-fiber reinforcement in this matrix gives fatigue sensitivity that correlaes with that of unimpregnated glass fiber strands, while carbon-fiber rein-forcement gives better fatigue resistance. A well-bonded, due-tile matrix system based on nylon 6,6 gives matrix-controlled fatigue sensitivity. Fatigue data for glass- and carbon-fiber-reinfoced nylon 6,6 superimpose on the matrix fatigue data when normalized by the ultimate tensile strength. Another ductile matrix, polyetherther ketone, is very fatigue-resistant, but its composite progressively loses its reinforcing effect in fatigue, apparently due to interface failure. A transitional matrix, polysulfone, shifts from ductile to fatigue-crack-dominated failure as the cyclic stress is reduced. Its composites show an analogous failure mode shift, and the high cycle-fatigue response is correlated with fatigue-crack-growth data.