Stress, strain and failure
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Question
Define stress and strain.
Answer
Stress is the force divided by the cross-sectional area carrying it. Strain is the extension divided by the original length, so it has no units.
Question
What are the four landmarks on a stress-strain graph?
Answer
The elastic region, whose gradient is Young's modulus; the yield point where deformation becomes permanent; the ultimate strength at the peak; and fracture at the end.
Question
Why design to yield rather than to ultimate strength?
Answer
Past yield the deformation is permanent. A part loaded to its ultimate strength has not broken, but it has deformed for ever — which for almost every product is already a failure.
Question
Stiffness or strength — which fixes a deflection problem?
Answer
Stiffness, which is the gradient of the elastic line. A stronger material with the same modulus deflects just as much; section depth is what changes deflection.
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Topic 3.2 hub
Structural systems
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