FATIGUE MECHANISMS IN POLY (METHYL-METHACRYLATE) AT THRESHOLD - EFFECTS OF MOLECULAR-WEIGHT AND MEAN STRESS

Fatigue tests were conducted on three linear poly (methyl methacrylate ) (PMMA) resins having weight average molecular weights (M(w)) of 8200 0, 205000 and 390000 and on a fourth, cross-linked sample (M(c)=3337 g mol-1). Fatigue threshold test conditions included a constant load ra tio (R(c)=0.1) and a constant maximum stress intensity level (K(max)c = 0.52 M Pa m1/2) The R(c)=0.1 test results demonstrated that fatigue resistance increased with increasing M(w), and that the cross-linked s ample possessed a higher fatigue threshold than the linear low-M(w) ma terial. However, the K(max)c test results revealed the opposite trend, with fatigue resistance decreasing with increasing M(w) and chemical cross-linking. The marked change in relative fatigue resistance of the PMMA resins investigated under high mean stress conditions is believe d to be a consequence of the competition between two molecular deforma tion mechanisms: chain scission and chain slippage. The presumed shift in operative mechanism as a function of the R level is reflected in d ifferences noted on the fracture surfaces of the PMMA resins studied. Discontinuous growth band formation, which is indicative of large amou nts of chain slippage, is favoured by low M(w) and low R ratios, but d isappears in association with high-M(w) and high R-ratio test conditio ns.