The fatigue behaviour of notched polyethylene as a function ofR

The fatigue behaviour of linear polyethylene with a shallow 0.4mm notch was investigated at 0.5 Hz under plane strain conditions with both square-wave and sine-wave loading. The maximum stress was constant at 10 MPa andR was varied from +1 to –1.5. The rate of damage was monitored by measuring the crack opening displacement,. The rate of damage during most of the lifetime could be represented by the equation d/dN= whereN is the number of cycles, exhibited a minimum with respect toR. The time to failure varied inversely with and exhibited a maximum atR 0.5 for a square-wave loading and 0.1 for the sine-wave. Generally, the square-wave produces faster damage than the sine-wave. The results are quantitatively explained in terms of two mechanisms; (1) the time under load, which disen tangles the fibrils and (2) the unloading which bends and crunches the fibrils in the damaged zone. A quantitative model was obtained for describing the dependence of onR.