Identification of Plastic Behaviour of Sheet Metals in High Strain Rate Tests
In this work, dynamic tension tests have been conducted by an SHB on sheet metals in order to characterize the plastic behaviour of the materials. First of all, the sample geometry and the clamping system were optimized by FEM simulations in order to: (i)
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Identification of Plastic Behaviour of Sheet Metals in High Strain Rate Tests D. Amodio, E. Mancini, M. Rossi, and M. Sasso Abstract In this work, dynamic tension tests have been conducted by an SHB on sheet metals in order to characterize the plastic behaviour of the materials. First of all, the sample geometry and the clamping system were optimized by FEM simulations in order to: (i) reduce impedance disturbance due to the fasteners, (ii) maximize the specimen cross-section to increase the force measurement sensitivity, (iii) reduce the elongation measurement errors due to deformation of the clamping system. Pictures of the samples were acquired during the test by means of a fast camera. On the one hand, this permitted to validate the strain measurement by the classical SHB theory formulas; on the other hand, application of DIC method permitted to obtain the actual strain distribution maps. These strain maps have been used to extract the parameters of a strain hardening constitutive model. Keywords Hopkinson Bar • High strain rate • FastCam • Sheet metals • Plasticity
26.1
Introduction
Nowadays Split Hopkinson Pressure Bar (SHPB) has been vastly used in order to determine the stress-strain curves at strain rates in the range of 102–104 s1. The Hopkinson Pressure Bar was first suggested by Hopkinson as an apparatus to measure stress pulse propagation in a long metal and it was extended by Davies and Kolsky. See [1] for a review of the method. More recently, SHPB is focused not only on compression tests but also on tensile and torsion tests for different classes of materials such as metals, polymers, ceramics, composites and foams [2]. Sheet metals have also been tested at high strain rate. One of the main issues in such experiments is the correct gripping. In some cases, bolt type grips have been used [3]. As well, friction or pin based grips have been employed [4]; the latter one is suggested in the ISO 26203-1:2010 standard. In this work, the direct Split Hopkinson Tension-Compression Bar (SHTCB) developed in [5] has been used to perform dynamic tests on different sheet metals. A clamping system has been designed and optimized in order to avoid excessive sample deformation outside the gauge length; at the same time, restrictive geometrical constraints had to be satisfied, due to the bars size, specimens available lengths, widths and thicknesses. Tensile tests have been conducted and engineering stress and strain data have been collected, by the standard SHB method. A fast camera has been used to acquire images of the samples during the tests to assess the actual deformation within the gauge length.
26.2
Theoretical Background
The Kolsky bar or Split Hopkinson Pressure Bar, consists of a pretension bar, input bar and output bar that sandwich a cylindrical specimen. In this paper, a direct tension/compression Hopkinson bar has been used. The working principle of performing tension test is schematically shown in Fig. 26.1.
D. Amodio • E. Mancini • M. Rossi • M. Sasso (*) Dipartimento di Ingegneria Industria
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