Ford Aerostar rear axle fatigue failures (1985)

The shock to Ford when high-strength steel substitution for fuel-economy weight reduction failed in fatigue — Ford's design engineers didn't know what bridge engineers had known for forty years, that welded joints get no fatigue benefit from higher strength steels.

Now this was sort of a shock to Ford in the mid 80s. They were starting to build the Aerostar van in 1985 in the St. Louis plant, and they were starting to use high-strength steels to get lighter weight to improve fuel economy. And they were finding that people's axles were fatiguing — not a good thing — on the rear axles. So they had a huge program in the late 80s to look at fatigue strength of welds at Ford. Other people in the construction business of bridges and things like that already knew this and had known this for about forty years. But the design engineers at Ford didn't know about it. They went ahead figuring, oh, we'll be on this curve, higher-strength steels means higher fatigue life, when in fact they were on this curve — higher-strength steels gave you nothing in terms of fatigue. So the welds on the axles were fatiguing. They've since learned how to design for some of those things.

Welded rear axle of HSLA steel, designed assuming fatigue strength proportional to tensile strength. Brackets welded onto axle; field failures in first six months. Big product recall. Triggered industry-wide work on fatigue of welded HSLA sheet.

To give you an example, in 1985 Ford came up with the Aerostar van, and they were using high-strength low-alloy steel for the first time instead of plain carbon sheet steel. They designed the rear axle to have a fatigue strength proportional to the tensile strength of the better steel — that's what their designers knew. That would be true if it weren't welded, but the rear axle had brackets welded onto it, and they found people throwing axles, fatigue-failing them in the first six months. This was not a great day for Ford. Big product recall. There was a tremendous amount of work done after that on the fatigue strength of sheet metal welds in high-strength low-alloy steels.