Fatigue Crack Propagation in Nylon 66 Blends

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Abstract
Fatigue crack propagation (FCP) rates were measured in nylon 66, an impact-modified nylon 66, and in blends of the two resins as a function of the stress intensity factor range ΔK and cyclic frequency. Significant reductions in FCP rates were observed with increasing additions of the impact-modified polymer to pure nylon 66, though the relative ranking of the various blends was found to depend on the magnitude of ΔK and test frequency. These differences were related to specimen heating associated with certain blend compositions and high ΔK levels. Crack tip temperatures increased with increasing ΔK level and were as high as 125°C (257°F) in the impact-modified nylon 66 that had been moisture equilibrated at room temperature. Other tests revealed that this material represents the first polymeric solid to exhibit higher fatigue crack growth rates with increasing cyclic frequency.

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