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Understanding factors that cause shaft failures

Publication date: 
March 2004
Cyndi Nyberg Esau, Former EASA Technical Support Specialist

Shaft failures are not an everyday occurrence, but when they come in, it can be an interesting challenge to determine the cause of failure. Regardless of what caused the shaft to fail, what actually happens when it bends or breaks? To understand shafts and why they fail, you need to understand the relationship between stress and strain for steel. Stress is the force carried by a material per unit area, measured in psi (pounds per square inch) or Mpa (Megapascals or Mega Newtons per square meter). If a material is under tension, the stress is acting to pull apart the molecules that make it up, making it longer; if the material is under compression, the stress is pushing the molecules together, causing the material to get shorter (and fatter as the compressed material "bulges" outward) if enough stress is applied. Strain is the change in the length, or elongation per unit length, of a material under a tensile stress

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