Dissertation Proposal Defense - Sukanya M. Sharma
Prof. Naresh N. Thadhani, Advisor, MSE
Prof. Arun M. Gokhale, Advisor, MSE
Dr. Shrikant P. Bhat, ArcelorMittal
Prof. Preet M. Singh, MSE
Prof. Kimberly E. Kurtis, CEE
"Effects of Strain Rate on Mechanical Properties and Fracture Mechanisms in Dual Phase Steels"
Dual Phase (DP) Steels are a class of Advanced High Strength Sheet (AHSS) Steels used as the structural components of an automobile owing to their good combination of strength to weight ratio, formability and crashworthiness. The microstructure of DP steels consists mostly of ferrite and martensite. In order to provide corrosion protection, DP steels may also have a galvannealed layer. Additionally, to improve the bendability of galvannealed DP steels, some commercial grades may also contain a ”decarburized” surface layer. This leads to a gradient in the carbon content and in turn in the microstructure along the thickness direction. Thus, mechanical properties of these steels depend on the geometric attributes of the bulk microstructure and the chemistry, and the thickness and microstructure of the decarburized layer and the protective galvannealed coating. DP steels are exposed to strain rates of the order of 10 − 102/s during sheet metal forming operations, and strain rates of the order of 102 − 104 can be reached under an automotive crash condition. Hence, it is of interest to study how microstructure, microstructural gradients, and surface coating affect the strain rate dependence of fracture micromechanisms and mechanical response of DP steels.
Several studies have been conducted on the effects of strain rate on the mechanical behavior of DP steel. However, none of the investigations have examined the effects of microstructural gradients and the galvannealed layer on the strain rate dependence of mechanical response and fracture micromechanisms. Furthermore, the bulk microstructure has only been quantified for the volume fraction of martensite and the uses of statistically unbiased techniques such as quantitative fractography and stereology have been absent. These techniques can be used to correlate microstructural and fractographic parameters to the strain rate dependence of fracture micromechanisms and mechanical properties. Additionally, most of the earlier studies have utilized different specimen geometries for the tensile tests performed at low (using servo hydraulic machines) and intermediate (using Hopkinson Bar tests) strain rates, and to the best of authors’ knowledge, there are no experimental data on mechanical response of DP steels at strain rates of 104 /s or higher.
The proposed research involves a combination of testing mechanical properties over a wide range of strain rates (10−4 /s to 104 /s), quantitative characterization of microstructure and fracture surfaces using stereology and quantitative fractography, and computer simulations to understand the strain rate dependence of microstructure-properties relationships of interest in DP 980 steels. To establish a baseline, preliminary experiments have been performed on DP 600 steel specimens in the strain rate range of 10−4 /s to 103 /s. A few experiments have also been performed on DP980 specimens. Microstructure and fracture surfaces of the tested specimens have been quantitatively characterized to demonstrate utility of these techniques for establishing how microstructure affects strain rate dependence of fracture path, fracture micromechanisms and the mechanical response of DP steels.
The intellectual merit of the proposed research is that it will lead to the scientific understanding of how microstructure, microstructural gradients, and surface coatings affect strain rate dependence of fracture path, fracture micromechanisms and mechanical response of DP steels. The results of this work will be useful for design and development of DP steels with superior formability and crashworthiness.