Papers by Author: M. Bruhis

Abstract: High-manganese austenitic steels are promising emerging automotive steels demonstrating high strength and ductility. The main deformation products observed in these steels are mechanical twins and ε-martensite, where the dominant deformation products vary quite strongly with stacking fault energy (SFE), which in turn is a very strong function of the alloy carbon content. In this research, a Fe-22Mn-0.6C sheet steel was decarburized to achieve a variety of through-thickness C gradients, thereby varying the dominant deformation products through the sheet thickness, with the overall objective of producing unique microstructures and mechanical properties. Microstructural analyses after interrupted tensile testing indicated that the amount of both mechanical twins and ε-martensite increased with increasing true strain, where the deformation products changed from mechanical twins at the higher-C core to ε-martensite at the lower-C surface. The spring-back properties of the C graded steels were also compared with reference to the effect of differential carbon concentration gradient.

Abstract: Forming limit diagram (FLD) is a measure of the formability of a sheet material. The major-minor strain pairs that are closest to the neck on multiple specimens of various strain paths are utilized to construct a boundary between safe and unsafe zones. The challenge to obtain the FLD is the determination of incipient necking. Three approaches to determine the limit strains have been investigated and compared in this research in order to establish the optimal one for implementation: (1) commonly used Bragard criterion ( 1)e Br with periodic grids; (2) tracking the region of large local strains from strain history to locate the instance when critical major strain ( 1)e cr happens; (3) post-processing of strain history to locate the inflection in the major strain rate curve 1 max (e&&) at the onset of localization. The last criterion of inflection in strain rate 1 max (e&&) carries both a numerical and a physical meaning towards developing an understanding of flow localization, formability and fracture.

Abstract: The occurrence of intense shear band is a prelude to failure in many Al-sheet materials. In the present study, a full field optical system measurement technique (digital image correlation) and the finite element method are used to characterize the sequence of deformation in uniaxial tension before and after the intense shear band formation in AA6111-T4. The results indicate good agreement between the measurement and the predictions in terms of shear band width, strain distribution along the gauge length and the failure mode.