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

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Abstract
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.