DEFORMATION AND FRACTURE OF URETHANE-METHACRYLATE RESINS

The tensile properties of three urethane-methacrylate resins that vari ed in the soft segment content of the urethane were characterized. The strain birefringence at a circular hole was observed during loading-u nloading cycles to progressively higher displacements. The shear strai n distribution at the hole was calculated from the isochromatic fringe contours and compared with results from linear elastic analysis. When the onset of nonlinearity, and the subsequent appearance of residual strain at the root of the hole, were correlated with features of the m acroscopic stress-displacement curves, three regions of prefracture de formation were defined. A region of linear elastic behavior was observ ed at the lowest strains. The maximum shear strain at the linear limit was the same in all the resins, and appeared to correlate with the yi eld condition at the hole. When the shear strain at the hole exceeded about 2.8%, the fringe patterns started to deviate from the elastic pr ediction. However, strain was fully recoverable in this region as indi cated by the absence of residual birefringence at the hole after unloa ding. This region of nonlinear, recoverable deformation extended to pr ogressively higher strains as the amount of urethane soft segment incr eased. This feature was attributed to the network structure of the ure thane-methacrylate resins. A region characterized by nonrecoverable de formation at the hole followed at higher strains; the urethane soft se gment content had a major effect on the amount of permanent deformatio n sustained before fracture. The fracture surfaces exhibited features typical of brittle fracture without crazing. (C) 1995 John Wiley & Son s, Inc.