Fatigue Crack Propagation in Crystalline Polymers: Effect of Moisture in Nylon 66.
LEHIGH UNIV BETHLEHEM PA MATERIALS RESEARCH CENTER
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As part of a study of the effects of crystallinity on fatigue crack propagation FCP behavior in crystalline polymers, the FCP response of nylon 66 was examined as a function of both stress intensity factor range delta K and water content. It was found that FCP rates were exponential functions of delta K, as expected. However, the FCP rates at constant delta K decreased as the water content was increased to about 3 at saturation 8 water, the FCP rates were higher than that observed in dry specimens. Results were interpreted in terms of the incorporation of tightly bound water up to a maximum of one water molecule per two amide groups at approx. 2 water, followed by the incorporation of loosely bound water at saturation. The tightly bound water evidently toughens the polymer, while the loosely bound water has a predominantly weakening effect. Examination of fracture surfaces revealed a transition from terminal unstable crack growth at water contents up to 3 to tearing at saturation. Classical fatigue striations were observed only in the specimens containing 2 water--the first time such striations have been unequivocally identified for any crystalline polymer. Author