Tensile properties and effect of temperature

Properties from the tensile test

• Young's modulus $E$: slope of the elastic line, a measure of stiffness.
• Yield strength $\sigma_y$: stress at which plastic deformation begins (0.2% offset if no clear yield).
• Ultimate tensile strength: maximum engineering stress, $P_{max}/A_0$.
• Ductility: percentage elongation and percentage reduction in area.
• Resilience: energy absorbed elastically, $U_r = \sigma_y^2/2E$ (area under the elastic part).
• Toughness: total energy absorbed per unit volume up to fracture (total area under the curve).
• Poisson's ratio: ratio of lateral to axial strain (about 0.3 for steel).

Effect of temperature

As temperature rises:

1. Yield strength, UTS and modulus decrease, because thermal energy helps dislocations to move.
2. Ductility (elongation, reduction in area) increases.
3. Toughness generally increases, and the yield point of mild steel becomes less sharp.

 strength / ductility
  |\  strength
  | \_______
  |         \___
  |  ductility  /
  |      _____/
  +-------------------> temperature

At low temperature BCC metals (carbon steel) show a ductile-to-brittle transition: yield strength rises, but fracture becomes brittle. FCC metals (Cu, Al, austenitic steel) stay ductile. At high temperature, time-dependent deformation (creep) also appears under constant load.