Page 341 - Failure Analysis Case Studies II

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Fig. 13. Scanning electron micrograph of the button in Fig. 12. The crazing cracks are seen to extend through the surface
of the material.

since the butadiene phase is rubbery throughout the range of usage temperatures for an automobile
interior (approximately -40°C to + 75°C). However, polymers are much more susceptible to
environmental degradation above T, since the diffusion of, for example, dissolved oxygen is so
much more rapid above T,.
The operating conditions inside an automobile are relatively harsh for many polymers, including
ABS. Because of solar loads, the temperature inside a parked car with its windows closed can
reach 75°C. Visible and ultraviolet radiation from the sun can also degrade most of the plastic
components inside a car, including the seat belt release buttons. Oxygen and moisture are of course
present as well.
The failure of the release buttons involved a combination of (1) repeated, low-level impact
damage and (2) degradation of the material due to the combined effects of radiation and oxidation
(photo-oxidative degradation). Because of the design of the seat belt receptacle in question, it is
relatively easy for the release button to be subjected to impact loads from the clasp since there is
no barrier between the button and the entrance for the clasp. These impacts result when the seat
belt wearer is slightly off with his or her aim when attempting to insert the clasp in the receptacle,
thus striking a blow to the receptacle housing and/or the release button. Evidence of impact
damage to the release button and receptacle housing is visible in Fig. 1.

3.3. Photo-oxidative degradation of ABS
‘Weathering’ of polymers refers to all the possible effects that may occur when polymers are
exposed to the outdoor environment (which is taken here to include the interior of an automobile).
The effects of weathering on polymers can include discoloration, loss of surface gloss, surface
chalking, and reductions in mechanical properties such as tensile and impact strength.
The molecular mechanism for photo-oxidation of ABS has been studied and is described by
Grassie and Scott [4]. Their description details how the combined effects of radiation and oxygen