Fatigue mechanisms in poly (methyl methacrylate) at threshold: effects of molecular weight and mean stress

Abstract

Fatigue tests were conducted on three linear poly (methyl methacrylate) (PMMA) resins having weight average molecular weights (M _w) of 82 000, 205 000 and 390000 and on a fourth, cross-linked sample (M _c=3337 g mol⁻¹). Fatigue threshold test conditions included a constant load ratio (R ^c=0.1) and a constant maximum stress intensity level (K ^c_max =0.52 MPa m^1/2). TheR ^c=0.1 test results demonstrated that fatigue resistance increased with increasingM _w, and that the cross-linked sample possessed a higher fatigue threshold than the linear Iow-M_w material. However, the K ^c_max test results revealed the opposite trend, with fatigue resistance decreasing with increasingM _w and chemical crosslinking. The marked change in relative fatigue resistance of the PMMA resins investigated under high mean stress conditions is believed to be a consequence of the competition between two molecular deformation mechanisms: chain scission and chain slippage. The presumed shift in operative mechanism as a function of theR level is reflected in differences noted on the fracture surfaces of the PMMA resins studied. Discontinuous growth band formation, which is indicative of large amounts of chain slippage, is favoured by lowM _w and lowR ratios, but disappears in association with high-M _w and highR-ratio test conditions.