The Effect of Temperature Gradients on Elastic Wave Propagation in Split Hopkinson Pressure Bars
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REVIEW ARTICLE
The Effect of Temperature Gradients on Elastic Wave Propagation in Split Hopkinson Pressure Bars Stephen M. Walley1 Received: 30 July 2019 / Accepted: 27 March 2020 © The Author(s) 2020
Abstract If it is desired to obtain high rate mechanical data of materials at non-ambient temperatures using the split Hopkinson (Kolsky) bar technique, it is necessary either to consider what effect a temperature gradient has on the propagation of elastic waves along a metallic rod or to design a mechanism that minimises the exposure of the Hopkinson bars to heating or cooling. Two main mechanical systems have been devised: the first where the bars are brought into contact with the specimen a short time (less than one second) before the specimen is dynamically loaded; the second where the specimen is moved into position just before it is dynamically loaded. As these mechanisms are complex to design and build, many researchers choose the simpler option of heating (or cooling) the ends of the bars as well as the specimen. This review summarises issues that should be considered if this option is taken. Keywords Hopkinson bar · Temperature gradient · Elastic wave
Introduction Some materials such as water ice only exist at cryogenic temperatures. Some may be subjected to impact when in use at high temperatures, such as the alloys used in turbines [1, 2]. Yet others may be subjected to a heat pulse at the same time as a shock in, for example, blast loading of concrete [3] or rock [4]. If such events are going to be accurately modelled, a full constitutive relation for the material of interest is needed and this in turn requires mechanical data to be obtained for that material over a wide range of temperature and strain rate [5–10]. Thus it is necessary to be able to perform high rate split Hopkinson bar (SHPB) tests at both high and low temperatures. Heating and cooling techniques for achieving this have been reviewed by Chen & Song [11]. So the emphasis of this article will not be on the experimental methods for accomplishing this but on the effect of temperature gradients on elastic wave propagation in the bars themselves. For a comprehensive overview of the SHPB technique and its applications, the reader is referred to the book by Chen & Song [12]. * Stephen M. Walley [email protected] 1
PCS Fracture and Shock Physics Group, Cavendish Laboratory, J.J. Thomson Avenue, Cambridge CB3 0HE, UK
If the specimen (and hence the Hopkinson bar ends) are at a different temperature to ambient, there are several problems that have to be addressed.
First The elastic modulus, E, of the Hopkinson bar rods (and hence their mechanical impedance) changes with temperature (Fig. 1). As can be seen from this figure, temperature has a significant effect even for a metallic alloy such as Inconel 718 whose elastic properties are a weak function of temperature (the decrease in modulus of about 40 GPa for a temperature rise of 600 K corresponds to a change in impedance of about 10%). The problem is much worse for Hopkinson bars made f
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