General Motors and Ford minivan spot-weld fatigue failures

Mid-1980s lightweighting initiative. GM and Ford substituted high-strength steel and assumed fatigue life would scale with ultimate strength. The spot welds — stress concentrations — did not benefit, and minivan owners experienced rattles within a couple of years.

Now General Motors and Ford sort of ran into this in the mid-80s when they were trying to lightweight automobiles. They said, oh, we'll save weight on the cars, we'll use high-strength steel. And they did, and they spot-welded the high-strength steel, and they assumed that they would get the same proportional strength increase in the weld as they got in the base metal strength. But you don't. You get the same proportional strength in the solid piece of base metal, but if you have a stress concentration like just a simple hole, you don't get a 45 degree slope — you get something less. That's just the base metal, just the stress concentration. You put in a weld, you have an even sharper stress concentration. It turns out there's no advantage.