Fatigue characterization of a hydroxyapatite-reinforced polyethylene composite. II. Biaxial fatigue.

Previously, the uniaxial fatigue behavior of 40 volume % hydroxyapatite-reinforced polyethylene composite (HAPEXtrade mark) at 37 degrees C in saline was studied. Fatigue limits between 37 and 25% of the ultimate strengths of the material were established. This study investigates the biaxial fatigue behavior of the same material using various combinations of axial and torsional stresses. The addition of torsional to axial loading significantly reduced the fatigue life. When torsion at 25% of the ultimate strength was applied in addition to axial loading at 25% of the ultimate tensile strength, the fatigue life remained more than 1 million cycles. Out-of-phase loading was less detrimental to the fatigue life of the composite than in-phase. Fatigue damage was monitored by hysteresis loops, the increase in dissipated energy, the reduction in tangent modulus, and the increase in strain values. Copyright 2000 John Wiley & Sons, Inc.