FATIGUE CRACK-PROPAGATION IN CRYSTALLINE POLYMERS - EFFECT OF MOISTURE IN NYLON 66

As part of a study of the effects of crystallinity on fatigue crack propagation (FCP) behaviour in crystalline polymers, the FCP response of nylon 66 was examined as a function of both stress intensity factor range (ΔK) and water content. It was found that FCP rates were exponential functions of ΔK, as expected. However, the FCP rates at constant ΔK 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∼ 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. © 1979 Chapman and Hall Ltd.