Stress-strain curve estimation method

A stress-strain curve estimation method for estimating a relationship between stress and strain of a metal material, the method comprising:

• applying a first torsional torque T1 around a first axis of a first round-bar specimen having a first reduced section of a round bar-shape having a first radius R1 to perform a first torsion test for shear deformation of the reduced section, and acquiring a first relationship between the first torsional torque T1 and a first torsional angle Φ1;
• applying a second torsional torque T2 around a second axis of a second round-bar specimen having a second reduced section of a round bar-shape having a second radius R2 smaller than the first radius R1 to perform a second torsion test for shear deformation of the second reduced section, and acquiring a second relationship between the second torsional torque T2 and a second torsional angle Φ2;
• obtaining a third relationship between a sheared stress τ and a shear strain γ of surface on a first surface of the first reduced section by using the first relationship and the second relationship;
• and
• converting the sheared stress τ and the shear strain γ of surface into a stress σ and a strain ε in simple tension to estimate a fourth relationship between the stress σ and the strain ε in the simple tension,
• wherein the obtaining includes
• obtaining the sheared stress τ using the first torsional torque T1 at the first torsional angle Φ1 and a third torsional torque T2′ when the second torsional angle Φ2 is aligned with the first torsional angle Φ1, and
• obtaining the shear strain γ of surface using the first torsional angle Φ1.