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Mechanical properties and fracture

Take about 0.4 times the melting point (in absolute temperature) as the rough temperature above which creep can no longer be ignored. For aluminum (melting point 660 °C), which value is closest to this temperature?

  1. AAbout 100 °C
  2. BAbout 264 °C
  3. CAbout 373 °C
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Answer: A. About 100 °C

This rule of thumb works in absolute temperature, so with Tm = 660 + 273 = 933 K, 0.4Tm ≈ 373 K, about 100 °C. About 264 °C is the mistake of computing 0.4 × 660 in °C, and about 373 °C mistakes 373 K for °C. This ratio (the homologous temperature) varies between materials, roughly 0.3–0.5, but for aluminum alloys creep and loss of strength must be considered from about 100 °C upward, and low-melting solders creep even at room temperature. Creep proceeds through three stages, primary, secondary (steady-state) and tertiary, before rupture, and the steady-state creep rate is the key quantity in design.

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