Date of Award

2013

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Mechanical, Automotive, and Materials Engineering

First Advisor

Green, Daniel E.

Second Advisor

Stoilov, Vesselin

Keywords

Applied sciences, Characterization, Dual phase steels, Electrohydraulic froming, Failure, High strain rate, Hyperplasticity

Rights

CC BY-NC-ND 4.0

Abstract

In this research, three commercial dual phase steel sheets, i.e. DP500, DP780 and DP980, were formed under quasi-static and high strain rate conditions using the Nakazima test and Electrohydraulic Forming (EHF), respectively. In EHF, as a result of a high-voltage electrical discharge between two electrodes in a water chamber, a shock wave was produced which travelled through the water and formed the sheet into the final shape. When a 34° conical die was used in EHF, significant formability improvement, known as hyperplasticity, was achieved in the specimens compared to the specimens formed in the Nakazima test. In this research, hyperplasticity as well as failure in the specimens were characterized at different scales of observation. Quantitative metallography showed relative deformation improvement of around 20% in ferrite and 100% in martensite when formed under EHF. Dislocations in ferrite and deformation twinning in martensite were found to be responsible for the significant improvements of deformation in the constituents under EHF. As a mechanism of failure, voids were found to nucleate in the ferrite/martensite interface due to decohesion. However, under EHF, the significant deformation improvement of martensite enhanced the plastic compatibility between ferrite and martensite. Consequently, the strain gradient across the ferrite/martensite interface, i.e. decohesion, was reduced and nucleation and growth of the interfacial voids was suppressed. Furthermore, quantitative analysis of the voids showed that void growth in the specimens formed under EHF was slower than in the specimens formed in the Nakazima test. The reason was attributed to impact of the sheet against the die that generates significant compressive and shear stresses which act against void growth. Therefore, under EHF, coalescence of the voids to form micro-cracks was postponed to higher levels of strains which resulted in suppression of failure. Fractography of the specimens showed ductile fracture as the dominant type of fracture under both quasi-static and high strain rate forming conditions. In addition, limited quasi-cleavage fracture was observed in DP780 and DP980 specimens. Shear fracture was also observed in the specimens formed under EHF.

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