Effects of strain rate on mechanical properties and fracture mechanisms in dual phase steels

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Sharma, Maruwada Sukanya
Gokhale, Arun M.
Thadhani, Naresh N.
Bhat, Shrikant
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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 10-10^2/s during sheet metal forming operations, and strain rates of the order of 10^2−10^4/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 (10^2−10^4/s) have 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)/s to 10^6/s). Three kinds of 980 MPa DP steels with and without protective coatings are investigated. While DP 590 and DP 980 contained different amounts, all three DP 980 steels contained similar volume fractions of martensite. The differences in the volume fractions and connectivity of the martensite in the different DP steels are estimated using quantitative characterization of microstructures. The mechanical properties are measured for strain rates spanning twelve orders of magnitude from 10^(-6)/s (quasi-static strain rate) to 10^6/s (dynamic strain rates). Servo-hydraulic machines, Hopkinson bar and plate impact gas gun experiments are used to generate the different magnitudes of stresses and strain rates. An important aspect of the work performed in this study is that all of the quasi-static and intermediate strain rate experiments on the four DP steels are conducted with the same specimen geometry to eliminate the effects of post-uniform elongation and allow valid comparisons of ductility across different magnitudes of strain rates. The effect of the volume fraction of martensite is discussed both in terms of its effect on the mechanical properties and on the fracture response. Discussions on the effects of adding protective coating layers and the resulting microstructure of the three DP 980 steels are provided to understand the differences in the mechanical properties and fracture response of these steels at both quasi-static and dynamic strain rates. The strain rate sensitivity of both the mechanical properties and fracture response as a function of the underlying microstructure is also explored. 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.
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