Title Page
Abstract
Contents
Chapter 1. Introduction 13
1.1. Study Background 13
1.2. Purpose of Research 15
1.3. References 15
Chapter 2. Experimental measurements and 3D microstructure 20
2.1. Introduction 20
2.2. Mechanical tests and microstructure characterization 21
2.3. Reconstruction of 3D microstructure 26
2.4. Conclusion 35
2.5. References 35
Chapter 3. Dual-scale finite element model with dislocation density-based crystal plasticity model 37
3.1. Introduction 37
3.2. Simulation model for macroscale 38
3.3. Simulation model for microscale 44
3.3.1. Description of crystal plasticity theory 44
3.3.2. Parameters identification in the model 47
3.4. Deformed microstructures in tensile and shear test 56
3.5. Conclusion 67
3.6. References 68
Chapter 4. Microstructure-based fracture criterion and prediction of hole expansion ratio 71
4.1. Introduction 71
4.2. Microstructure-based fracture criterion 76
4.3. Prediction of HER by constructed fracture criterion 85
4.4. Conclusion 91
4.5. References 91
Chapter 5. Summary 94
국문 초록 97
Table 2.1. Mechanical properties obtained from the results of the uniaxial tensile test of the RD, DD, TD direction on the DP780 steel sheet. 24
Table 3.1. Measured Hill parameters of the DP780 steel sheet. 41
Table 3.2. Optimized parameters of hardening equation in the macroscale model. 42
Table 3.3. Initial dislocation density measured by the W-H method. 53
Table 3.4. Selected and optimized parameters of the crystal plasticity model in the microscale model. 54
Table 4.1. Fitted parameters for the constructed fracture criterion. 83
Figure 2.1. Stress-strain curves measured from the uniaxial tensile test of the RD, DD, TD directions on the DP780 steel sheet. 23
Figure 2.2. EBSD maps on the investigated dual-phase steel. (a) ND-oriented inverse pole figure map. (b) Grain-average image quality map. (c) Grain average... 25
Figure 2.3. SEM (left) and EBSD IPF map (right) for the first and last sections of the investigated specimen. Yellow-dashed line denotes the area of EBSD scanned. 30
Figure 2.4. (a) ND-oriented IPF map and (b) phase map of the reconstructed 3D microstructure. The phase fraction is indicated with colored phase. 31
Figure 2.5. Image quality distribution of the reconstructed 3-D microstructure. The green and red colors indicate ferrite and martensite, respectively. 32
Figure 2.6. ND-oriented IPF map of the 3D microstructure (a) before coarsening and (b) after coarsening. (c) RVE with the phase indication of the coarsened 3D microstructure. 33
Figure 2.7. The orientation distribution function (φ₂ = 45˚) obtained from (a) the EBSD measurement on the large area (total 1969 grains). (b) the 3D microstructure... 34
Figure 3.1. (a) Contour of Mises stress for the tensile test model in macroscale. (b) Experimental and simulated stress-strain curves of the investigated DP780 sheet.... 43
Figure 3.2. X-ray diffraction patterns (upper) and FWHM (lower) of the initial state of the specimen. 52
Figure 3.3. (a) FE model to simulate the nanoindentation test and (b) the Mises stress contour map at the maximum load (4mN). (c) Load-displacement curves of... 55
Figure 3.4. Stress-strain curves of the experimental and simulated data of RVE in (a) RD and (b) TD directions, respectively. Experimental data are plotted until the... 61
Figure 3.5. Pole figures of the (a) initial, (b) ε-p=0.05, (c) ε-p=0.1, and (d) ε-p=0.15 tensile-deformed specimens from experimental and simulated in 〈011〉...[이미지참조] 62
Figure 3.6. (a) Geometry of the specimen to impose shear deformation on the steel sheet. (b) Load-displacement curves of the shear deformation from experimental... 63
Figure 3.7. Pole figures of the (a) initial, (b) F=4kN, (c) F=4.7kN, and (d) F=5.1kN shear-deformed specimens from experimental and simulated in 〈011〉 and... 64
Figure 3.8. Kernel average misorientation maps on the (a) initial, (b) ε-p=0.05, (c) ε-p=0.1, and (d) ε-p=0.15 tensile-deformed specimens from experimental (upper)...[이미지참조] 65
Figure 3.9. IPF maps on the (a) initial, (b) F =5.1kN of the shear-deformed specimens. Kernel average misorientation maps on the (c) initial, (d) F =5.1kN of... 66
Figure 4.1. (a) Schematic diagram of hole expansion test. (b) Fractured HE specimen. The diameter of this state is considered to be a final diameter. 75
Figure 4.2. Contour map of equivalent plastic strain from macroscale simulation results of hole expansion test on DP780 steel. The local strain history is extracted... 80
Figure 4.3. 40,000x-magnified electron microscopy in order of increasing strain from left to right. (left: initial, center: ε-pε-p=0.05, right: ε-pε-p=0.1). (a-c)...[이미지참조] 81
Figure 4.4. Fracture criterion curve obtained from three different deformation on the investigated DP780 steel. Scattered plots are the relation between accumulated... 82
Figure 4.5. Constructed fracture line and the relation between accumulated shear strain and stress triaxiality for determining the fracture of 6R-notched tensile... 84
Figure 4.6. Contour map of equivalent plastic strain from macroscale simulation results of hole expansion test on DP780 steel. The local strain history is extracted... 87
Figure 4.7. Local strain history of the HET specimen at the region of RD-directed (a) top, (b) middle, and (c) bottom, and TD-directed (d) top, (e) middle, and (f)... 88
Figure 4.8. Constructed fracture line and the relation between accumulated shear strain and stress triaxiality for determining the fracture of each region in HE... 89
Figure 4.9. HER value from the experimented and simulated results. 90