AbstractThe influence of molecular weight on fatigue and fracture behavior in nylon 66 (N66) and polyacetal (PA) is examined. Fatigue crack propagation (FCP) resistance and apparent fracture toughness (Kcf) in these two semicrystalline polymers increase with increasing molecular weight in a manner consistent with that reported for another semicrystalline polymer (HDPE) as well as for several amorphous polymers. The improved FCP resistance with increasing molecular weight is attributed to the development of a molecular entanglement network that more effectively resists cyclic‐load‐induced breakdown. A type of discontinous crack growth is identified in PA at 100Hz and in N66 (2.6 H2O) at 50 Hz and compared with that observed in amorphous polym
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