Forum Discussion
ktosv
Oct 18, 2014Explorer
spoon059 wrote:
Basically the argument is that steel needs to have some flex. Without flex it bends. When it bends it becomes fatigued. When it has fatigue, it is more likely to fail. To prevent it from failing, engineers allow for some flex.
Spoon, your understanding of fatigue is flawed . By definition fatigue is the failure of a material due to alternating or cyclical loading. This loading will cause some level of deflection, which would be referred to as flex if it was less than the yield strength and permanent deformation if it is greater than the yield strength. When it flexes back and forth it is actually causing fatigue regardless of the amount of deflection. The material will eventually fail at a load/stress much lower than the materials ultimate strength. Therefore, your statement about engineers allowing for flex to prevent failure (from fatigue) is flawed. They may allow for this flex, but they better be accounting for the fatigue life based on the deflection and cycling.
Your link that you posted even included this information: "and fatigue is the behavior of a material when subjected to cyclically applied stress, which can result in a crack and failure.
“Fatigue is what kills truck frames — not tensile strength or yield strength,” he said. “Fatigue is when you bend something until it breaks. Every material has an elastic zone. If I bend the material and don't exceed the yield strength, it will come back. If I get past the yield strength, I've strained it. It doesn't go back to the original shape.”
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