Measurement and Analysis of Heterogeneous Strain Fields in Uniaxial Tensile Tests for Boron Steel Under Hot Stamping Con
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RESEARCH PAPER
Measurement and Analysis of Heterogeneous Strain Fields in Uniaxial Tensile Tests for Boron Steel Under Hot Stamping Conditions R. Zhang 1 & Z. Shao 1
&
J. Lin 1 & T.A. Dean 2
Received: 25 June 2019 / Accepted: 10 August 2020 # The Author(s) 2020
Abstract Background A significant amount of uniaxial tensile tests has been carried out using Gleeble systems to investigate the viscoplastic deformation of boron steel (22MnB5) under hot stamping conditions. However, due to heat loss through the end clamps, a temperature gradient in the reduced parallel section of dog-bone shaped specimens is inevitable. Objective In the work reported in this paper, the effect of temperature gradient on measured outcomes is examined. Methods Uniaxial tensile tests on 1.5 mm thick boron steel specimens are carried out, under hot stamping conditions and strain fields are quantified using the digital image correlation (DIC) technique. The effect of gauge length on the properties of boron steel, as calculated from observed test results, is determined. Results Compared with the test at room temperature, a bell-shaped strain distribution occurs within the gauge length even before the appearance of the maximum load. Also, average strain within the gauge length, especially in the later stages, changes with gauge length within the investigated range, and thus, different engineering stress-strain curves and fracture strains are determined. In addition, normalized strain rate is significantly dependent on gauge length, which results in over 16% difference among the computed flow stresses by using a unified constitutive model. Conclusions The characterized properties of the material are dependent on gauge length and thus, a testing standard for measuring thermal-mechanical data of materials by using a Gleeble need to be defined. Keywords Boron steel . Hot stamping . Temperature gradient . Uniaxial tensile test . Digital image correlation
Introduction In the vehicle manufacturing industry, an increasing demand to improve strength to weight ratio of metallic panel components has stimulated technological development in sheet metal forming [1]. The hot stamping and cold die quenching process (abbreviated as ‘hot stamping’) has been developed for the manufacture of high-strength steel components [2, 3] and complex-shaped light alloy components [4, 5]. Boron steel is the most commonly used steel grade for hot stamping applications for automobiles [6]. The sheet is heated above its Ac3temperature followed by soaking for a prescribed time to * Z. Shao [email protected] 1
Departmental of Mechanical Engineering, Imperial College London, London SW7 2AZ, UK
2
Departmental of Mechanical Engineering, University of Birmingham, B15 2TT, Birmingham, UK
ensure transformation of its microstructure into austenite and is then formed and quenched simultaneously, between cold dies, into a complex-shaped component with a fully martensitic structure with ultra-high strength [7]. To enable accurate material and process modelling of hot stamping,
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