Abstract
Abstract The effect of mean stress on the fatigue threshold in poly(methylmethacrylate) (PMMA) and polycarbonate was investigated using both low and high constant R-ratio (Rc), constant Kmean (Kmeanc), and constant Kmax (Kmaxc) threshold testing procedures. While ΔKth for commercial PMMA under Rc = 0.1 conditions was found to be 0.35 MPa√m, no threshold condition was obtained during the Kmaxc procedure for ΔK as low as 0.1 MPa√m due to the high mean stress which exists in the latter test procedure. Similar results were obtained for two other PMMA resins. Creep tests conducted at room temperature on PMMA clearly demonstrated the synergistic relation which exists between fatigue and creep induced damage during the Kmaxc test procedure. Kmaxc experiments also confirmed the beneficial mean stress effect in polycarbonate, while high temperature testing (65°C) revealed that this mean stress effect can be almost entirely suppressed. Finally, it was shown that a K-gradient as high as −0.2 mm−1 can be used to decrease test time and material required for the determination of ΔKth; this gradient compares favorably with the ASTM maximum recommended value of −0.08 mm−1, established for metals.
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