A B S T R A C TThe multiaxial fatigue behaviour of a high-density polyethylene was investigated at room temperature and constant frequency. As a consequence of the mode of failure, an end-oflife criterion for fatigue tests is discussed in the first part of the work, in order to define the number of cycles to failure. Based on force controlled fatigue tests under tension, compression and torsion at two stress ratio, a multiaxial fatigue criterion including the stress-ratio effect is proposed for the fatigue design of this polymer. This criterion is based on the maximum and mean values of the second invariant of the stress tensor.A = parameter of the proposed fatigue criterion c = parameter of the proposed fatigue criterion F = axial force J 2 = second invariant of the stress tensor J 2max = maximum value of the second invariant of the stress tensor J 2mean = mean value of the second invariant of the stress tensor l 0 = initial value of the gauge length of the sample l = gauge length of the sample M = torque N m = number of cycles to failure N m (when reaching the hydraulic limits of the machine) N i = number of cycles at the inflexion point of the time evolution of the maximal strain N f = number of cycles at the end of the stationary stage of the time evolution of the maximal strain R = stress ratio (defined as the minimum stress divided by the maximum stress over the fatigue cycle) R ext = external radius of the pipe sample in the minimum diameter cross-section R 0 ext = initial value of the external radius of the pipe sample in the minimum diameter cross-section R int = internal radius of the pipe sample in the minimum diameter cross-section R 0 int = initial value of the external radius of the pipe sample in the minimum diameter cross-section R med = median radius of the pipe sample in the minimum diameter cross-section z = longitudinal axis Correspondence: A. Berrehili.
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