Page 337 - Failure Analysis Case Studies II
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Fig. 6. ‘Mode two’ failure, showing a fragment of the release button lodged just to the left of the locking slider. In this
case, the fragment keeps the locking slider from sliding to the unlocked position when the release button (not shown) is
pressed.
3. Degradation and failure of the release button
3.1. Characterization of the button material
Once the three failure modes described above had been discovered, attention was focused on
the degradation and eventual fracture of the release button. SEM studies of a fractured surface of
the seatbelt, solubility tests, and a flame test of the seatbelt button revealed that the release button
in question is injection molded from ABS (acrylonitrile-butadiene-styrene) copolymer. While
there are numerous different grades of ABS, all consist of a continuous styrene-acrylonitrile (SAN)
copolymer phase throughout which a discontinuous butadiene phase is dispersed. The rubbery
butadiene phase serves to toughen the relatively brittle SAN phase. The SAN phase is chemically
bonded to the butadiene phase by a graft copolymerization mechanism. The greater the amount
of butadiene phase, the tougher the ABS material will be. The tradeoffs for this increased toughness
are decreased strength and modulus of elasticity, and a potential reduction in environmental
resistance.
Figure 8 shows a release button along with several fragments. This figure also shows (at left) a
release button that is still intact. Both buttons came from Honda Accords, although from different
model years, which accounts for small differences in the designs. A closeup of the fracture surface
of the button from Fig. 8 is shown in Fig. 9.
A scanning electron micrograph of a fracture surface of a degraded release button reveals a two
phase material, as shown in Fig. 10. The small round holes on the fracture surface are typical of a
rubber-modified polymer such as ABS. The material was further characterized as ABS through
solubility and flame tests, using techniques described in [3]. Release button shavings placed in four
different solvents (ethanol, acetone, dichloromethane, and benzene) behaved very similarly to
