Delayed Fracture of Class 12.8 Bolts in Automotive Rear Suspensions

After over two years service in the “snow belt,” class 12.8 bolts in GM “A” ear rear suspensions began to fail, leading to the recall of 6.4 million cars. Analysis of the failures showed that the cause was corrosion induced hydrogen assisted cracking. The long delay before failure resulted from a decarburized surface through which corrosion had to penetrate before a crack could propagate. The fracture morphology of the field failures was duplicated in laboratory salt spray tests. This type of testing also showed that hardness was the principal factor affecting durability in a corrosive environment.