Failure and fracture mechanisms of injection-molded plastic products

The effect of geometry transitions on the mechanical load-carrying ability of specimens has been studied. Special attention has been given to a potentially positive influence of the injection molding process. Tensile tests, three-point bending tests, and low cycle fatigue tests were performed on specimens with either drilled or molded-in holes. Tests were conducted at various temperatures and deformation rates. Two commercial grades of poly(methyl methacrylate) have been applied. To obtain a better understanding of the fracture mechanism, the fracture surface morphology was related to the molecular orientation investigated by the birefringence method and the results of a finite element method analysis. The extent of redistribution of stresses seemed considerable, even in the case of a brittle material like poly( methyl methacrylate). As a result, the linear theory is a safe, but very conservative, approach for the load-carrying ability of plastic products. It also seemed that injection molding may have a favorable influence on load-carrying ability. This result could be related to the fracture mechanism. (C) 1998 John Wiley & Sons, Inc.