DEVELOPMENT OF A HIGH FIDELITY ANISOTROPIC-VISCOPLASTIC CONSTITUTIVE MODEL FOR FINITE ELEMENT ANALYSIS OF COLD-ROLLED SHEET METALS
Steel sheet is one of the most common types of material applied as the primary material for vehicle components. In the manufacturing process of these components, the anisotropy resulting from the steel sheet forming is essential to note. Another crucial mechanical aspect of being concerned is strain rate sensitivity or also known as viscoplastic behavior. Various efforts have been made to evaluate these behaviors largely based on trial-and-error approaches, which usually come with an immense price. Therefore, the availability of a high-fidelity predictive numerical tool is crucial to simulate the forming process and generate a cost-effective process. In this dissertation, a novel 3D anisotropic-viscoplastic constitutive model is proposed to investigate the behavior of cold-rolled sheet metals. The Hill 48 yield function, isotropic hardening model, and Cowper-Symonds viscoplastic model were holistically implemented as a user-defined material subroutine in ABAQUS software (VUMAT). This constitutive model development process only requires a small number of experimental works, namely tensile test and Lankford test. The model was validated through tensile test numerical simulations at quasi-static and various strain rates. The numerical simulation results could follow the experimental results with an excellent agreement, with a difference of less than 2%. Furthermore, numerical simulations using the developed constitutive model and Swift’s flat-bottom cup drawing experimental works were carried out. The numerical simulations could predict the earing height in the cup very accurately with the difference between simulation and experiment of less than 4%. This result is better than the work conducted by Ahn et al. (2019), with a difference of 10%. In addition, some novel relationships were successfully established through the extended use of the validated numerical model. It was found that the plastic strain ratio variation affects the earing height and equivalent stress in the cup. The viscoplastic effects were also observed in the earing height, equivalent stress, punch peak force, and required energy to move the punch in the cup drawing. These novel findings have successfully expanded the understanding of the plastic strain ratio variation effect and viscoplastic behavior in deep drawing. This dissertation contributes to making the steel sheet material remain appealing as a lightweight structure solution for vehicle industries by proposing a robust and easy-to-use numerical model to predict the response when given a load. The simplicity of testing required in the modeling process will make it more appealing for industries.