Chapter 34 High Temperature Tension HSB Device Based on Direct Electrical Heating M. Hokka, K. O¨ stman, J. R€am€o, and V.-T. Kuokkala Abstract The effects of strain rate and temperature on the mechanical properties of various engineering materials have been extensively studied within the past few decades. However, the high temperature high strain rate tension Hopkinson Split Bar (HSB) testing is still quite challenging to perform due to the need to fix the sample to the stress bars. Mechanical fixing of a sheet material sample is not very convenient and can produce low quality results. Therefore, the sheet samples are typically glued directly to the stress bars. This glue joint, however, loses strength rapidly if the temperature of the glue joint increases above room temperature, which makes the high temperature testing more difficult. In this paper, we present a tension Hopkinson Split Bar device with a high temperature system that allows the sample to be heated while keeping the glue joint at or close to room temperature. The sample is rapidly heated by a powerful low voltage high amperage DC pulse. When testing stainless steels, test temperatures between 400 and 800 C are reached in less than one second, and even the melting temperature of the material is reached in less than 2 s. The system is fully computer controlled allowing accurate timing and control of the different actions during the test including heating of the sample, pneumatic manipulation of the heating electrodes, releasing of the striker bar, and recording of the test results. The results obtained with the current high temperature system are high quality and the obtained high temperature stress strain curves are essentially oscillation free. Keywords Tension testing • Hopkinson split bar • High temperature • High strain rate • Stainless steels 34.1 Introduction The mechanical behavior and properties of most metallic materials are strongly affected by temperature and strain rate. Many of today’s fabrication methods, such as hot rolling and forging are carried out very rapidly at elevated temperatures. Also various types of impacts, such as car crashes, include material deformation at strain rates higher than 200 s 1, and the temperatures can vary widely from subzero to hundreds of degrees centigrade. The need to increase the scientific understanding as well as the need for reliable material data for modeling the material behavior in high rate loadings at various temperatures drives the development of testing techniques at these challenging conditions forward. The characterization of material behavior at these conditions is, however, scientifically and practically very challenging due to the special nature of the testing. A test typically lasts only less than a millisecond and is normally performed without a closed-loop control. During the recent years, the Hopkinson Split Bar (HSB) devices have become a popular choice for performing high strain rate testing. These devices can be built for tension, compression, shear, and bending testing, and the material behavior can be studied at strain rates ranging roughly from 200 s 1 up to 10,000 s 1 depending on the test type and the properties of the sample material. A typical tension HSB device consists of two stress bars and a system to produce a tension stress pulse for loading of the specimen. The tension wave is usually produced by impacting a striker tube to a flange at the free end of the incident bar or by preloading the incident bar and releasing the preload with a clamp-release mechanism. The specimen needs to be fixed to both bars prior to the test so that the tension loading can be applied on the sample. The specimen is typically fixed to the stress bars by gluing or by mechanical clamping. Mechanical clamping is a convenient solution for round samples that can be fixed to the threaded holes machined to the ends of the stress bars. M. Hokka (*)•K.O¨stman•J.R€am€o • V.-T. Kuokkala Tampere University of Technology, P.O.B. 589, 33101 Tampere, Finland e-mail: mikko.hokka@tut.fi B. Song et al. (eds.), Dynamic Behavior of Materials, Volume 1: Proceedings of the 2014 Annual Conference on Experimental and Applied Mechanics, Conference Proceedings of the Society for Experimental Mechanics Series, DOI 10.1007/978-3-319-06995-1_34, #The Society for Experimental Mechanics, Inc. 2015 227
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