Effects of molding variables, glass-fiber reinforcement and aging on the fatigue behavior of polycarbonate in aqueous and saline environments
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The effects of molding temperatures, glass-fiber reinforcement, stress concentration, aging and saline environment on the fatigue behavior of injection molded polycarbonate were investigated. Fatigue tests were made with specially designed machines in which statistical samples of specimens were subjected to alternating fluid loading. The fluid loading permitted temperature control, high testing speeds, and aqueous or other environments. The fatigue lives fell on Weibull distributions with nonzero minimum life parameters. S-N equations werg fitted at 5%, 50% and 95% probabilities of failure. Lower cylinder temperature during molding resulted in substantially higher fatigue strengths for glass-reinforced polycarbonate. This was attributed primarily to orientation. The increased viscosity at lower temperature caused steeper velocity gradients for flow in the mold. The resulting shearing strains oriented both reinforcing fibers and molecular chains. Higher mold temperature produced slightly higher fatigue strengths for glass-reinforced polycarbonate. This was explained by premature cooling of the surface layer in the colder mold, so that orientation was incomplete. The fatigue strength of glass-reinforced polycarbonate increased with increase in percentage of reinforcing fibers. However, 40% glass-reinforced polycarbonate required higher cylinder temperatures than 20% glass-reinforced. Also, the higher fiber content tended to inhibit orientation. As a result, highly oriented 20% glass-reinforced polycarbonate had fatigue strengths almost as high as 40% reinforced. The notch sensitivity of glass-reinforced polycarbonate was quite low. This was primarily due to increased orientation in the notched specimens, resulting from steeper velocity gradients during flow through the abrupt change in cross-section. It was possible to separate the effects of fiber orientation and molecular orientation by means of annealing. In the specimens without stress concentration, fiber orientation caused an increase in fatigue strength of about 40%. Molecular orientation caused an additional increase of about 25%. Thus the total increase in strength due to orientation was about 65%. In the notched specimens, total increase in strength due to orientation was about 90%. There was no evidence that residual stresses had any significant effect on fatigue strength of reinforced polycarbonate. Aging for periods of up to 5 years did not cause crazing or other degradation of polycarbonate resin specimens. The fatigue strengths of polycarbonate resin specimens were substantially reduced by cyclic stressing in sodium chloride solution. The reduction in fatigue strength was attributed to solvent action of the saline solution on certain polycarbonate molecular bonds.
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