Influence of microstructure on the multiaxial plasticity and fracture of dual phase steels: experiments and multiscale computational modeling
Open Access
- Author:
- Qin, Shipin
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- March 06, 2020
- Committee Members:
- Allison Michelle Beese, Dissertation Advisor/Co-Advisor
Allison Michelle Beese, Committee Chair/Co-Chair
Zi-Kui Liu, Committee Member
Hojong Kim, Committee Member
Charles E Bakis, Outside Member
John C Mauro, Program Head/Chair - Keywords:
- Dual phase steels
Representative volume element
Multiaxial fracture
Stress state dependent
Fracture mechanism - Abstract:
- As a class of advanced high strength steels (AHSS), dual phase (DP) steels are widely used in the automobile industry for their high strength to weight ratio. DP600 is a type of DP steel that have a tensile strength of 600 MPa or higher, and its microstructure is comprised of a soft ferrite phase and a hard martensite phase. The thesis focuses on identifying the multiaxial plasticity and fracture behavior of DP600 at the continuum level and understanding the influence of the heterogeneous microstructure on the macroscopic behavior. The multiaxial plasticity behavior of DP600 was determined through macroscopic mechanical tests under five stress states. A continuum-level plasticity model was developed based on the experimental results. The plasticity model was found to accurately predict the multiaxial mechanical response of the material through macroscopic simulations. A representative volume element (RVE) model based on the observed microstructures was built, which was able to predict the macroscopic multiaxial plasticity behavior of the material from the microstructural level. The stress state dependent fracture behavior of DP600 steel at the continuum level was investigated through a combined experimental and computational approach. A range of specimen geometries were used to probe the fracture behavior of the material under different stress states. Using an isotropic J2 plasticity framework, finite element simulations of all experiments captured the experimental force displacement curves, and provided information on the evolution of equivalent plastic strain, stress triaxiality, and Lode angle parameter with applied deformation at the location of eventual fracture initiation. The calculated local failure strain as a function of stress state was used to calibrate the modified Mohr-Coulomb (MMC) fracture model. At the microstructure level, fracture can initiate within the ferrite phase, the martensite phase, or at the interfaces between these phases, and the dominant fracture initiation mechanism is expected to depend on a number of factors, including the phase and interface properties as well as the applied stress state. An idealized RVE model containing a circular martensite particle was loaded under five different stress states. The RVE model incorporated a ductile fracture criterion for ferrite, a brittle fracture criterion for martensite, and a cohesive zone model (CZM) for the ferrite/martensite interface. A parametric study was performed to determine the relative influence of fracture properties of each constituent and stress state on the failure initiation behavior, and to identify the conditions under which the fracture initiation behavior was stress state dependent. To study the fracture behavior in a more realistic microstructure, an RVE model based on the microstructure of DP600 was built. Fracture models for ferrite and martensite were incorporated into the RVE model for fracture simulations. Five RVE level failure criteria were presented, and their ability to predict the macroscopic stress state dependent fracture behavior of the material was discussed. To isolate the effect of microstructural heterogeneity on stress state dependent behavior, simulations were performed with the ferrite fracture model to be either stress state dependent or stress state independent. In both cases, the RVE strain to failure was stress state dependent, indicating that the microstructural inhomogeneity, which resulted in strain localization in the microstructure, plays an important role in the stress state dependent fracture behavior of DP600. Simulations revealed that in DP600, microcracks initiated from martensite first, regardless of the global stress state. Under low stress triaxiality loading, the material fails by the propagation of these martensite microcracks into ferrite, accompanied by new microcracks initiation from both phases; while under high stress triaxiality loading, new microcrack initiated from ferrite subsequently, and the material fails by ferrite microcrack propagation.