THE SCHOOL OF MATERIALS SCIENCE AND ENGINEERING

GEORGIA INSTITUTE OF TECHNOLOGY

Under the provisions of the regulations for the degree

DOCTOR OF PHILOSOPHY

on Friday, July 12, 2019

10:00 AM
in LOVE 299

will be held the

DISSERTATION DEFENSE

for

Sukanya M. Sharma

"Effects of Strain Rate on Mechanical Properties and Fracture Mechanisms in Dual Phase Steels"

Committee Members:

Prof. Naresh N. Thadhani, Advisor, MSE

Prof. Arun M. Gokhale, Advisor MSE

Dr. Shrikant P. Bhat, ArcelorMittal Global R&D

Prof. Preet M. Singh, MSE

Prof. Kimberly E. Kurtis, CEE

Abstract:

Dual Phase (DP) steels are a class of Advanced High Strength Sheet (AHSS) steels which are used as structural components of an automobile body. They possess a good combination of strength and formability coupled with crashworthiness. The microstructure of DP steels consists largely of ferrite and martensite. These commercial grade steels may also contain a coating layer to protect the steel against atmospheric corrosion. These 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/s can be reached under an automotive crash condition. The fracture mechanisms of these DP steels at slow strain rates are well understood; however, these may not be representative of the material's fracture response under dynamic or high strain rate loading conditions. The mechanical behavior of DP steels under dynamic rates (102-104/s) has been studied in the past but there are no conclusive results on the operative fracture mechanisms. Another important aspect currently lacking is the effect of the protective coating under dynamic rates. Hence, to address these critical gaps, an understanding of the role of all microstructural features (substrate and coating) on the fracture response of DP steels under varying strain rates is required. Thus, the objective of this work is to investigate and quantitatively characterize the fracture surfaces of DP steels generated under a wide range of strain rates and gain an understanding on the microstructure-based fracture mechanisms. Four DP steels, of two nominal strength levels (590 MPa and 980 MPa) are subjected to strain rates spanning twelve orders of magnitude (10-6/sto106/s). Three kinds of 980 MPa DP steels with and without protective coatings are investigated.

The main thrust of the current work is to employ quantitative fractography, a stereological technique, to understand the effects of the quantity, distribution and morphology of the various microstructural constituents of the substrate and coating on the operative fracture mechanisms of DP steels under varying strain rates. For this purpose, the area fractions of various features observed on the fracture surfaces are estimated. Ultimately, hypotheses for the fracture mechanisms of these steels as a function of strain rate are presented. The significance of this study is to help gain a deeper understanding on the differences in microstructure obtained in DP steels of similar nominal strength levels when processed to contain protective coatings, and the effects of these differences on the mechanical properties and fracture response under strain rates representative of automotive crash. The results of the current work will help design better grades for improved forming and higher crash-resistant automobile parts.