Prediction of Ductile Fracture Propagation of High Strength Steels in Automotive Structures 2019-01-1097

Initiation and propagation of ductile fractures in crashed automotive components using high strength steels are investigated in order to understand the mechanism of the fracture propagation. The fracture is often prone to occur at sheet edge in strain concentration zone at crash deformation. The fracture is extended intricately to inside of the structure, with the influence local stress and strain field. It is very important to predict the fracture initiation and propagation under crashed deformation for the reliable design of the light-weight structural by using ultra-high strength steels. In this study, 3-point bending tests are carried out with a hat-shaped part which have a notch and a hole to induce fracture. Three types of notch are machined by wire cut electric discharge process. The hole is located near the notch to investigate the influence on fracture propagation. Automotive steel sheets with tensile strengths of 590MPa and 1180MPa are examined to investigate the effect of material properties on the fracture behavior. In 3-point bending test, force-stroke curves are recorded and the progress of fracture are captured by digital camera or high-speed video camera. The experimental results indicate that the location of the hole have influence on direction of fracture propagation. The force-stroke curves and fracture propagation for 590MPa and 1180MPa steels are analyzed. The force is dropped at the moment when the fracture occurs. 1180MPa steel shows the sudden drop of force, which indicates the rapid propagation of fracture. The fracture drawn toward the hole in case of near location of hole (15mm-offset). The 1180MPa steel shows smaller tuning of fracture due to the rapid propagation of fracture. Based on the experimental results, FE simulation is carried out for the 3-point bending deformation as shown in Fig.3. The simulation shows good agreement with experimental results in force-stroke curves. The stress distributions around the notch and hole at the fracture point is investigated to understand mechanism of the fracture initiation. A nonlocal XFEM method is applied to predict the fracture propagation. The modeling of failure criteria is determined by tensile test with strain measuring system of digital image. The XFEM shows fracture propagation from the notch to inside of the hat part. The advantage of nonlocal XFEM is shown by comparison with a conventional FEM method. These studies on numerical modeling of the fracture propagation promises the improvement of crash simulation for the full size vehicle.