Page 332 - Failure Analysis Case Studies II

Failure Anaiysis Case Studies 11
D.R.H. Jones (Editor)
0 200 1 Elsevier Science Ltd. All rights reserved 317

Failure of automobile seat belts caused by polymer

degradation

J.M. Henshaw".", V. Wood", A.C. Hallb

The University of Tulsa, Department of Mechanical Engineering, 600 South College Avenue, Tulsa, OK 74104, U.S.A.
The University of IIIinois, Materials Science and Engineering, Urbana, IL 61801, U.S.A.

Received 30 July 1998; accepted 9 September 1998

Abstract

This paper analyzes the failure of a particular brand of automobile seat belts. The failures described were
part of what nearly became the most expensive and widespread automobile recall in US. history, affecting
about 8.8 x IO6 vehicles and with a potential total cost of U.S. $lo9. The failures were caused by the
degradation and fracture of the seat belts' polymeric release buttons. When fragments break away from the
buttons, they can become lodged within the seat belt mechanism in a variety of locations, such that any one
of three distinct failure mechanisms can result: (1) the belt fails 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. The buttons themselves were found to have
been injection molded of ABS and to have undergone photo-oxidative degradation. This degradation process
is documented 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. 0 1998 Elsevier
Science Ltd. AH rights reserved.

Keywords: Photo-oxidative degradation; Polymer degradation; ABS; Seat belts

1. Background

In the spring of 1995, a major news story in the United States recounted what was potentially
the largest formal automobile recall in the history of the industry. While news reports in the
popular press were lacking in technical detail [ 1, 21 it was noted that 'Apparently part of a plastic
release button deteriorates. . .' 113 sometimes causing seat belts to malfunction. It was further noted
that about 8.8 x lo6 vehicles were affected from the model years 1986-1991, including models from
Honda, Nissan, Mazda, Ford, GM, and Chrysler, among others. (About 75% of the affected
vehicles were from Honda and Nissan. Toyota alone among the U.S. and Japanese carmakers was

* Corresponding author.

Reprinted from Engineering Failure Analysis 6 (l), 13-25 (1 999)