Design Aids for Preventing Brittle Failure in Polycarbonate and Polyetherimide

Chapter 8 Failure Prevention Design Aids for Preventing Brittle Failure in Polycarbonate and Polyetherimide Joseph T. Woods And Ronald P. Nimmer GE Corporate Research And Development, Schenectady, NY 12301, USA INTRODUCTION When assessing structural part performance, failure is an important concern. This is particu- larly true in impact events where impacted parts are required to absorb a certain impact energy without failing. Automotive bumper impacts and drops of electronic enclosures are good examples. Added to this concern is the possibility of two different failure modes, ductile and brittle. In a ductile failure the part fails in a slow, noncatastrophic manner in which additional energy is required to further spread the damage zone. In contrast, a brittle failure is characterized by a sudden and complete failure which, once initiated, requires no further energy to propagate. The failure criteria for the two failure modes differ. Generally, effective stress (von Mises stress) is used to assess when plastic (permanent) deformation has initiated. If some permanent deformation is acceptable, then a strain to failure criterion may be used as the ductile failure criterion indicating when tearing is expected to occur. Brittle failure criterion for many polymers have not been firmly established and can vary from one material to the next. However, maximum principal stress has been identified and validated as a reasonable brittle failure criterion for polycarbonate and polyetherimide. Since brittle failures are more catastrophic, they will be the focus of the remainder of this paper. A brief review will be given describing the brittle failure mechanism for polycarbonate and polyetherimide. Next, the work leading up to the development and validation of brittle failure criterion for polycarbonate and polyetherimide will be reviewed. Building upon this understanding of the brittle failure behavior of polycarbonate and polyetherimide, design aids to prevent brittle failure will be presented and described in the final section.

Design Aids for Preventing Brittle Failure in Polycarbonate and Polyetherimide

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