Failure of Polyacetal Latch Assemblies

Components of a latch assembly used in a consumer safety restraint exhibited a relatively high failure rate. The failures were occurring after installation but prior to actual field use when failure could result in severe injury. Cracking occurred within retaining tabs used to secure a metal slide on an older design, whereas newer components showed no signs of failure. The latch assembly components were injection molded from an unfilled commercial grade of a polyacetal copolymer. Investigation of failed parts (including visual inspection, a specially designed proof load test, 59x SEM images, micro-FTIR in the ATR mode, and DSC/TGA/MFR analysis) showed no evidence of contamination or degradation from the molding process. The conclusion was that the parts failed via brittle fracture associated with stress overload. The stress overload was accompanied by severe apparent embrittlement resulting from a relatively high strain rate event and/or significant stress concentration. A relatively sharp corner formed by a retaining tab on the older design was shown to be a primary cause of the failures.