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. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67:1473–1487, 1998