38 D. Kanazawa et al. specimen surface using two stereovision systems. Not only the in-plane strains but also the through-thickness strains are evaluated from the measurement results of the displacements on the both surfaces of the specimen. The variation of the stress triaxiality at an evaluation point is evaluated from the measured strains. The fracture strain is also evaluated from the strain measurement results. Experimental results show that the stress triaxiality and the fracture strain of thin steel sheet can be evaluated by the surface strain measurement. The results can be utilized for simulating deformation and predicting fracture of a component made of thin steel sheet. 6.2 Experimental Procedure The shape and the size of the specimen used in this study are shown in Fig. 6.1. Three types of high strength steel, the tensile strengths of more than 1470 MPa (TS1470MPa), 980 MPa (TS980MPa), and 590 MPa (TS590MPa) are used as the test materials. Notched thin sheet specimens made from above materials are loaded by a tensile testing machine, and the deformation of the both surfaces of the specimen are measured using two stereo digital image correlation systems, as show in Fig. 6.2. For digital image correlation, the specimen is painted with black ink on the surface, and a white dot pattern Fig. 6.1 Shape of specimen Fig. 6.2 Experimental setup
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