Abstract
AbstractResidual stresses can have either beneficial or detrimental effects on the mechanical properties of plastics. Manufacturing processes frequently impose residual stresses on plastics. In this study, controlled thermal processes were used to impose surface compressive stresses on polycarbonate beam samples (6.4 by 12.5 by 80 mm). Resistance strain gage and photoelastic techniques were developed to measure the magnitude of these stored stresses. The compressive surface stresses were found to be between 14 MPa (2000 psi) and 31 MPa (4500 psi) and to vary with process method and cooling rate. The mean fatigue life (in bending) of the treated beam samples was found to improve by a factor of 10 over that of untreated samples. The increase in the fatigue life of the treated samples appears to be directly related to the magnitude of the surface compressive residual stress in the samples. The imposed residual stress, as determined by photoelastic measurements, has not appreciably relaxed after 1 year of storage at room temperature.
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