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Fig. 14. Short-term solution to the seat belt failure problem. The plastic guard installed on the seat belt receptacle is
intended to reduce impact damage to the release button.

These acetal copolymers are somewhat more expensive than the ABS materials they replace.
The manufacturing cost of the ABS release buttons might conservatively be estimated at U.S.
$0.05, about 50% of which would be material costs [7]. By using the less expensive ABS material
on the affected vehicles, automakers may have saved a few pennies per car, but it very nearly cost
them a billion dollars in the long run.

5. Conclusions

1. The subject failures resulted from a classic 'failure chain' of events and conditions. These
included a polymer susceptible to environmental degradation (ABS), the presence of repeated,
low energy impacts over a period, perhaps, of years, a hostile environment that included
temperatures up to 75"C, ultraviolet radiation, oxygen, and a design that allowed fragments of
the release button to interfere with the function of the release mechanism. Had any one of the
elements in this failure chain been absent, it is likely that the widespread failures of the subject
seat belts would never have taken place.
2. Each of the three reported modes of failure can be explained in terms of a specific location
within the seat belt mechanism in which a fragment of the release button becomes lodged.
3. The short-term solution to the problem agreed upon by industry and government (that is, the
installation of the plastic guard) was economical, if less than satisfying from a safety standpoint.
4. The longer-term solution arrived at (the specification of a more environmentally resistant
material) breaks the failure chain and appears to be satisfactory.
5. This seat belt release button, with a manufacturing cost of approximately $0.05, nearly resulted
in the most expensive automobile recall in industry history. Thus, the failure of even the
humblest of parts can have grave consequences.