Machine Design: Why Parts Break Below Their Strength
Fatigue, stress concentrations, bolts, springs, bearings, gears and fits: the hidden rules that decide whether a machine lasts or breaks.
Try 3 questions
1. Why do ductile metals like mild steel yield according to shear-based theories such as von Mises?
- Yielding is driven by atomic planes sliding
- Ductile metals have no tensile strength
- Steel always fails by crushing first
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Answer: Yielding is driven by atomic planes sliding
Plastic flow happens when dislocations glide along crystal planes, a shear process, so distortion energy predicts it well.
2. For brittle materials like cast iron or glass, which failure criterion fits best?
- Distortion energy (von Mises)
- Maximum shear stress
- Maximum normal stress in tension
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Answer: Maximum normal stress in tension
Brittle materials fail by cracks opening under tension, so the largest tensile principal stress governs, with much higher strength in compression.
3. A small round hole is drilled in a wide plate under tension. Roughly how much does it raise the stress at its edge?
- Not at all, the hole is small
- By about 10 percent
- To about three times the average
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Answer: To about three times the average
The theoretical stress concentration factor for a circular hole in a wide plate is 3, however small the hole.
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