Chapter 29 Image-Based Inertial Impact Tests on an Aluminum Alloy S. Dreuilhe, F. Davis, Clive R. Siviour, and F. Pierron Abstract This paper presents the development of a novel inertial test for the identification of non-linear parameters for elasto-plastic constitutive models at high strain rates. After briefly presenting the principle of the approach using the virtual fields method, the experimental implementation is detailed. Using the virtual fields method, the yield stress and hardening modulus of aluminum 6082-T6 were identified. Keywords High-strain rate • Ultra-high speed imaging • Virtual fields method (VFM) • Grid method • Inertial test 29.1 Introduction A number of experimental techniques have been developed to study the high strain rate response of materials. However, the most popular technique for characterizing material behavior at high strain rates (102–104 s 1) is the Kolsky (or split Hopkinson) bar. In a compression test, the specimen is placed between two long bars, the incident and output bars which are upstream and downstream of the sample, respectively. An impact is used to generate a stress wave in the incident bar. This stress wave then propagates into the sample and the output bar. Assuming that the stress wave is one-dimensional, the stress, strain, and strain-rate in the sample are determined from the stress wave in the incident and output bars. This is readily accomplished by placing strain gauges on the incident and output bars. Embedded in the calculation of stress, strain, and strain rate is the assumption that the deformation of the sample is homogenous. In addition, the forces on the front and back faces of the specimen must be equal for the stress calculations to be valid. These requirements limit the size and impedance of materials that can be tested using a Kolsky bar. Our current understanding of the mechanical response of materials at high strain rates is still hampered by the lack of more robust and detailed experimental data. Recent advances in ultra-high speed imaging have now made it possible to record images at a rate of 5 Mfps with a resolution of 0.67 Mpixels. Using an imaging technique such as digital image correlation (DIC) or the grid method, the time-resolved displacement fields on the specimen surface during dynamic loading can be captured. Further, by applying an inverse identification technique such as the virtual fields method, the material parameters can be identified. This approach has already been implemented to determine the elastic parameters of quasiisotropic composite specimens subjected to inertial impact tests [1]. The approach has also been paired with the classic Kolsky bar test to identify the material parameters for glass epoxy composites [2]. The strength of the virtual fields method is that it does not require any load measurement. Instead the acceleration field, derived from the time-resolved displacement field, is used as a load cell. In fact, this approach also has the advantage of relieving the constraint on specimen size and impedance, as quasi-static force equilibrium is no longer required. The objective of this paper is to use inertial impact tests and the virtual fields method to identify the parameters that describe the elasto-plastic response of aluminum 6082-T6, an aircraft grade aluminum, at high strain rates. First, a brief introduction to the virtual fields method is given. Next, to validate the approach, the inertial impact test was simulated using finite element analysis. The virtual fields method was then used to identify yield stress and hardening modulus from the finite element displacement data. Finally, inertial impact tests were performed in collaboration with the Impact Engineering Group at Oxford on aluminum 6082-T6 samples. An ultra-high speed camera was used to capture the dynamic response of each S. Dreuilhe (*) • F. Davis • F. Pierron Engineering Materials Group, Faculty of Engineering and the Environment, University of Southampton, Southampton SO17 1BJ, UK e-mail: smd2e13@soton.ac.uk; frances.davis@soton.ac.uk; f.pierron@soton.ac.uk C.R. Siviour Department of Engineering Science, University of Oxford, Oxford, UK e-mail: clive.siviour@eng.ox.ac.uk #The Society for Experimental Mechanics, Inc. 2017 S. Yoshida et al. (eds.), Advancement of Optical Methods in Experimental Mechanics, Volume 3, Conference Proceedings of the Society for Experimental Mechanics Series, DOI 10.1007/978-3-319-41600-7_29 219
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