Effects of loading history on fatigue crack growth in high density polyethylene and toughened poly(methyl methacrylate)

Abstract

AbstractFatigue crack growth measurements are made on two grades of high density polyethylene (HDPE), of density 940 and 958 kg/m3, and on one transparent grade of toughened poly(methyl methacrylate) (PMMA). In both grades of HDPE, prolonged cycling is necessary to initiate a crack from a razor notch. Relationships between crack growth and stress intensity factor K are studied in all three materials. The results show that Paris plots of log (da/dn) against log ΔK are linear provided that: (a) the loading pattern applied to the specimen remains constant; and (b) growth has taken place for a sufficient period to eliminate the effects of previous loading history. However, da/dn is strongly dependent upon the ratio of minimum to maximum stress and upon the stress amplitude during previous cycles. Overloads cause accelerated growth which is followed by crack arrest when the stress amplitude is restored to its original level. The use of a transparent polymer enables a more detailed study of crack growth phenomena to be made during fatigue testing.