A mild steel specimen is under uni-axial tensile stress. Young's modulus and yield stress for mild steel are 2 x 105 MPa and 250 MPa respectively. The maximum amount of strain energy per unit volume that can be stored in this speciment without permanent set, is

• 156 Nmm/mm3
• 15.6 Nmm/mm3
• 1.56 Nmm/mm3
• 0.156 Nmm/mm3