Failure of plastic press release buttons in automobile seat belts

In the late 1980s and early 1990s Honda, Nissan, Mitsubishi, Mazda, Suzuki, Subaru, Isuzu and Daihatsu and other automobile manufacturers modified their restraint system seat belt buckle assemblies in several models of vehicles. These automobile manufacturers incorporated plastic seat belt buckle assemblies manufactured by Takata Corporation as the 52 and A7 model series [National Highway Transportation Safety Administration (NHTSA), 1997, p. 1]. These seat belt models were introduced as early as 1986 and soon it was reported that numerous complaints of the malfunctions of the seat belt locking and release mechanisms had occurred. The National Highway Transportation Safety Administration (NHTSA) issued voluntary Recall No. 95V-103.001 on May 24, 1995, concerning 1986–1991 Honda Accords with model 52 and A7 series Takata seat belt buckles after there were numerous reported failures of these buckle assemblies. This issue has been the subject of numerous engineering investigations and failure analysis and, to date, the root cause of the seat belt failures has been attributed to the selection of an inappropriate material of construction for the seat belt press release button (acrylonitrile–butadiene–styrene plastic or “ABS”); however, the evidence presented has been inconclusive and had been debated in the literature. This paper will present failure analysis of the seat belt buckle assembly, including polymer materials property review, optical microscopy and scanning electron microscopy, Fourier transform infrared spectroscopy, pyrolysis gas chromatography/mass spectroscopy and Finite Element Analysis modeling. Prior failure analysis has concluded that the root cause of the failures was that the ABS press release buttons were fracturing due to a loss of material strength as a result of photolytic degradation (exposure to sunlight) [Eng. Fail. Anal. 1999 6 (l) 13]; however, this paper presents failure analysis that not only casts doubt on this root cause of failure but also provides evidence that for an alternative root cause of the failures, specifically that the design of the ABS press release buttons without the inclusion of rib reinforcement provided inadequate strength for the button to withstand the stress exerted from the insertion of the latch plate.