Failure of automobile seat belts caused by polymer degradation

This paper analyzes the failure of a particular brand of automobile seat be lts. The failures described were part of what nearly became the most expens ive and widespread automobile recall in U.S. history, affecting about 8.8 x 10(6) vehicles and with a potential total cost of U.S. S10(9). The failure s were caused by the degradation and fracture of the seat belts' polymeric release buttons. When fragments break away from the buttons, they can becom e lodged within the seat belt mechanism in a variety of locations, such tha t any one of three distinct failure mechanisms can result: (1) the belt fai ls to latch, (2) the belt will latch but will not unlatch, and (3) the belt appears to be latched but is not. The seat belt mechanism, and the ways in which the degraded button can cause it to fail, are described in detail. T he buttons themselves were found to have been injection molded of ABS and t o have undergone photo-oxidative degradation. This degradation process is d ocumented and described. Conclusions from the analysis and lessons learned from the failures are described, along with the auto industry's short- and long-term solutions to the problem. (C) 1998 Elsevier Science Ltd. All righ ts reserved.