Design Aids for Preventing Brittle Failure in Polycarbonate and Polyetherimide

Abstract

This chapter focuses on the prevention of brittle failures. It describes the brittle failure mechanism for polycarbonate and polyetherimide. The work leading up to the development and validation of brittle failure criterion for polycarbonate and polyetherimide has been reviewed. Building upon this understanding of the brittle failure behavior of polycarbonate and polyetherimide, design aids to prevent brittle failure have been presented and described. Subsurface crazing has been shown to precede brittle failure in these two materials. Since crazing precedes brittle failure, a craze initiation criterion can be used as a conservative brittle failure criterion. Using this knowledge of the brittle failure behavior of these two materials, a three step approach has been presented to assist designers in preventing brittle failure. The first step is to look at a material's ductility ratio at the strain rate and temperature of interest. If a material has a ductility ratio of 1.0, then brittle failure is not a concern since the ductility ratio represents the relative ductility of the material in a “worst case” stress state. If the ductility ratio is less than one, then the generic, geometric severity ratios representative of geometric features of the part should be compared to the failure criterion of the material.