Chapter 6 Evaluating Path of Stress Triaxiality to Fracture of Thin Steel Sheet Using Stereovision D. Kanazawa, S. Chinzei, Y. Zhang, K. Ushijima, J. Naito, and S. Yoneyama Abstract A stereovision technique based on digital image correlation is applied to the evaluation of the stress triaxiality and fracture strain of thin steel sheet. A tensile testing specimen with notches made of high strength steel sheet is loaded and the surface displacements are measured from both sides of the specimen surface using two stereovision systems. Not only the in-plane strains but 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. Keywords Stereovision • Digital Image Correlation • Stress Triaxiality • Fracture Strain • Through-thickness Strain 6.1 Introduction Thin and high strength steel sheets are widely used for automobile body frames to realize lightweight and strong bodies. In order to design reliable body frames, the predictions of deformation and fracture phenomenon of the thin steel sheet at the time of a crash have become important. The fracture of metal materials occurs after the necking developing. It is known that the onset of the necking of the materials depends on the deformation and the stress state of the materials [1]. Similarly, the fracture strain depends on the stress state of the materials. The stress triaxiality is one of the parameters that show the stress state of materials. Therefore, the evaluation of the stress triaxiality as well as the fracture strain has become essential for predicting the fracture. Ostuka et al. [2] evaluated the fracture strains and the stress triaxialities at the time of the fracture experimentally with Bridgman’s formula and revealed that the fracture strains were dependent on the stress triaxiality. On the other hand, Ma et al. [4, 5] used FEM analysis for evaluating the stress triaxiality and reported that the value of the stress triaxiality varied with the deformation of materials. Therefore, the variation of the stress triaxiality with deformation as well as the stress triaxiality at fracture should be evaluated as one of the material properties for the precise prediction of the fracture phenomena. Bai and Wierzbicki [6] proposed a method for evaluating the stress triaxiality from the increments of principal plastic strains. However, it is considered that strain increments obtained by experiment are suffered from experimental errors because measured displacements have a lot of errors in their derivatives. In this study, the variation of the stress triaxiality and the fracture strain of thin steel plates are evaluated experimentally. A stereovision technique based on digital image correlation [7] is used for this purpose. A tensile testing specimen with notches made of high strength steel sheet is loaded and the surface displacements are measured from both sides of the D. Kanazawa • Y. Zhang • S. Yoneyama ( ) Department of Mechanical Engineering, Aoyama Gakuin University, 5-10-1 Fuchinobe, Sagamihara, 252-5258, Japan e-mail: yoneyama@me.aoyama.ac.jp S. Chinzei • J. Naito Mechanical Engineering Research Laboratory, Kobe Steel, LTD, 1-5-5 Takatsukadai, Nishi, Kobe, 651-2271, Japan K. Ushijima Department of Mechanical Engineering, Tokyo University of Science, 6-3-1 Niijyuku, Katsushika, Tokyo, 125-8585, Japan © The Society for Experimental Mechanics, Inc. 2018 L. Lamberti et al. (eds.), Advancement of Optical Methods in Experimental Mechanics, Volume 3, Conference Proceedings of the Society for Experimental Mechanics Series, DOI 10.1007/978-3-319-63028-1_6 37
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