Effect of CO₂ Saturation and Desorption on the Fatigue Life of Polycarbonate

The effect of a cycle of CO₂ saturation and desorption on the yield strength, Young’s modulus, and fatigue life of polycarbonate was investigated. The fatigue life of the saturation-cycled polycarbonate exceeded that of unprocessed polycarbonate by up to a factor of thirty. The fatigue life of these rough-surfaced saturation-cycled specimens was approximately equal to or greater than that of polished smooth bar specimens. The increase in fatigue life is a function of the CO₂ saturation pressure, reaching a maximum at the same saturation pressure at which there is no further reduction in static yield strength. The increased fatigue life is hypothesized to result from one or more of following mechanisms: (1) reduction of shear strength promoting slow-growing shear fatigue cracks: (2) “healing” or blunting of pre-existing flaws or microcracks by swelling strain or by stress relief strain resulting from plasticization by CO₂ during saturation.