Forming Limit Diagrams of Low-Carbon Steels Obtained Using Digital Image Correlation Technique and Enhanced Formability
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JMEPEG https://doi.org/10.1007/s11665-020-05048-6
Forming Limit Diagrams of Low-Carbon Steels Obtained Using Digital Image Correlation Technique and Enhanced Formability Predictions Incorporating Microstructural Developments Basavaraj H. Vadavadagi, H.V. Bhujle, and Rajesh Kisni Khatirkar (Submitted August 19, 2019; in revised form July 22, 2020) In the present work, drawing quality (DQ) and interstitial-free (IF) steel sheets were subjected to limiting dome height tests for determining strain-based forming limit diagrams (FLDs) experimentally and effect of dynamic (variable) material properties, such as work hardening ÔnÕ and plastic anisotropy (r), on FLDs has been studied and simulated by PAMSTAMP finite element software. Dot prints on the steel specimens and an efficient optical strain measurement system GOM which works on the digital image correlation technique principle were used to measure limiting strains accurately instead of conventional circle grid analysis technique and strain measurement by traveling microscope. The dynamic (variable) material properties (n) and (r) were estimated by studying the microstructural developments in terms of changes in grain average misorientation and crystallographic texture, respectively, at different strains and strain paths, during deformation. In our proposed work, a novel technique of incorporating dynamic (variable) material properties (n and r) in FE simulations was carried out during FLDs predictions. Though marginal but improved predictions in FLDs were observed in both IF and DQ steels. In addition to strain-based FLDs, stress-based FLDs were also determined for both grades. Interestingly, in both cases it was noticed that IF steel had higher formability than DQ steel. Keywords
FE simulations, forming limit diagram, plastic anisotropy, texture, work hardening
1. Introduction Formability of a material can be assessed by determining forming limit diagrams (FLDs) (Ref 1-3) which indicates the ability of the metal to be formed into desired shape free from defects. FLD is a useful concept where limiting strains will be determined at the failure of materials through various strain paths. It is a useful diagram in die design and optimization phases during manufacturing practices. Many FLDs determined and reported in the literature (Ref 1-3) so far are strain-based FLDs. FLD is a two-dimensional (2D) plot of logarithm of major and minor strains, which demarcates safe and unsafe regions. In unsafe region, instability and/or necking occurs. FLDs can be determined by both experiments and theoretical investigations/simulations. Two most significant techniques for experimental determination of FLDs (Ref 4-7) are (1) out-ofplane method and (2) in-plane method. In the recent past, digital image correlation (DIC) technique using optical strain measurement systems has been widely employed in many Basavaraj H. Vadavadagi and H.V. Bhujle, SDM College of Engineering and Technology, Dhavalagiri, Dharwad, Karnataka 580002, India; and Rajesh Kisni Khatirkar, Visvesvaraya National
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