押込試験による材料強度評価の限界ひずみ

抄録

The dual indentation technique has been proposed in order to evaluate stress-strain curve, and it is very effective for tiny specimen. For a reliable establishment of this technique, it is very important to define the applicable range. This is because the evaluation error is not small enough for stainless steel. Thus the authors proposed a critical strain which shows the effective limit of stress-strain curve obtained by this technique. First, a sharp indenter penetrated into the original power-law material and several materials which assigned perfect plastic behavior after a bifurcation strain, respectively. When the error of the loading curvature between the original material and the modified material became less than 1%, the bifurcation strain of that modified material was defined as the critical strain. If two materials that have different stress-strain relationship in the larger strain range than the critical strain are penetrated, those load-displacement curves show the same behavior. In other words, those materials can not be distinguished by the dual indenter technique. According to the parametric study with the finite element simulations, the critical strain depends on the indenter angle, the work-hardening exponent and the ratio of the Young's modulus and the representative stress. Finally, the authors proposed the equation which gives the critical strain for a wide range of the engineering materials.