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Study on the Fracture Strain of DP780 Sheets Considering the Strain Rate and Loading Path (변형률속도와 하중경로를 고려한 DP780 강재의 파단변형률에 관한 연구)
Fracture strain data in proportional loading conditions are essential to develop and construct ductile fracture models which can predict onset of fracture in metals accurately. To evaluate the possibility of fracture in real forming processes, the strain rate effect should also be taken into account. In the present work, the hybrid experimental–numerical method and the 2-dimensional digital image correlation (2D-DIC) method are introduced to measure the fracture strain of DP780 sheets in the wide range of the loading path in terms of the stress triaxiality and the Lode angle, and in the intermediate strain rates.
Firstly, uniaxial tensile tests are carried out in the rolling, transverse, and diagonal directions in order to measure r-values and construct the yield function which will be used for finite element analysis. To obtain hardening curves with strain rates, uniaxial tensile tests in the rolling direction are conducted for six strain rates from 0.001/s to 100/s.
Four types of specimen shapes are chosen to induce various loading paths to the material point where the maximum equivalent plastic strain is observed under uniaxial tensile loading. For the pure shear condition, a diagonally double notched specimen is slightly modified. A specimen with a central hole is used for the uniaxial tension condition along with a dog bone specimen because its loading path is kept nearly constant through the entire deformation process in spite of necking. Plane strain tension condition is achieved by a flat grooved specimen. The fourth type has two symmetric cutouts which can control the initial loading path. The cross-head speed of each test is determined such that the average strain rate of the fracture initiation point reaches the target strain rate. The loading histories of each type of a specimen are thoroughly investigated with finite element analysis and average loading paths are calculated.
Fracture tests are carried out using INSTRON 5583 and high speed material testing machine with deformation images captured by the high speed camera. For the quasi-static case, both the hybrid experimental-numerical method and the 2D-DIC method are employed in order to measure the fracture strain. By comparing 3D fracture surfaces and 2D fracture loci constructed based on the measured data with each method, applicability of the 2D-DIC method to measuring the fracture strain of DP780 sheets are explored. Thus, for higher strain rates, only is the DIC method adopted to measure the fracture strain because reliable true stress–true strain curves beyond necking at various strain rates are difficult to determine.
Based on the acquired fracture strain data and calculated average loading paths, the modified Mohr–Coulomb fracture loci are constructed and represented with respect to the strain rate. Also, the loci are transformed into fracture forming limit diagrams which is relatively easy to understand and compare with conventional forming limit diagrams.