◆Ⅳ. Strain harden ing( or work hardening The increase of strength of materials with strain is called strain hardening. It is shown by o-scurve 2. Hall-Petch formula: 0=0 +kd 2 Where o and k are material constant d: The size of grains(diameter) do hardening rate de◆ Ⅳ.Strain hardening (or work￾hardening) 1. The increase of strength of materials with strain is called strain hardening. It is shown by curve. 2. Hall-Petch formula: Where and k are material constant. d: The size of grains. (diameter)  − 2 1 − = + k d  ys  o   hardening rate d d  