The influence of glass fibers content and testing conditions (frequency and mean load) on the fatigue crack propagation rate in injection molded neat and fiber reinforced polypropylene has been investigated. The resistance to fatigue crack propagation increases as the fiber volume fraction increases. The fatigue resistance of either neat and fiber reinforced polypropylene is tremendously dependent on the crack propagation direction, with higher values for crack propagating transversely to the melt flow direction in the molded plaques. A dramatic increase of the crack growth rate per cycle by decreasing frequency, at any given crack length was found for all the materials investigated. Analysis of data obtained at various frequencies and applied mean loads suggests that crack propagation is determined mostly by viscoelastic creep processes at the crack tip, the role of pure fatigue appearing quite secondary.
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