Initiation and propagation of ductile fractures in crashed automotive components made from high strength steels are investigated in order to understand the mechanism of fracture propagation. Fracture of these components is often prone to occur at the sheet edge in a strain concentration zone under crash deformation. The fracture then extends intricately to the inside of the structure under the influence of the local stress and strain field.
In this study, a simple tensile test and a 3-point bending test of high strength steels with tensile strengths of 590 MPa and 1180 MPa are carried out. In the tensile test, a coupon having a hole and a notch is deformed in a uniaxial condition. The effect of the notch type on the strain concentration and fracture behavior are investigated by using a digital imaging strain measurement system. For the 3-point bending test, hat-shaped parts having various types of notches and a hole located near the notch are examined to simulate ductile fracture in a crashed automotive part. The experimental results indicate that the shape of the notch and material properties influence the speed of fracture propagation. The location of the hole also influences the direction of propagation of the ductile fracture.
Finite Element Method (FEM) is applied to predict the fracture propagation in 3-point bending deformation. In the numerical calculation, general FEM method and extended finite element method (nonlocal XFEM) are used to investigate the possibilities of the nonlocal XFEM in the crash simulation. The numerical results for both the 590 MPa and 1180 MPa steels, are validated in the force-stroke curves and ductile fracture propagation behavior by comparison with the experimental results. The difference of the general FEM and nonlocal XFEM are discussed for the prediction of ductile fracture propagation of high strength steels.