An innovative electromagnetic compressive split Hopkinson bar
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An innovative electromagnetic compressive split Hopkinson bar C. M. A. Silva Æ P. A. R. Rosa Æ P. A. F. Martins
Received: 29 October 2008 / Accepted: 12 February 2009 / Published online: 27 February 2009 Springer Science+Business Media, B.V. 2009
Abstract This paper proposes a new design for the compressive split Hopkinson bar that makes use of the intense pressure created in a transient magnetic field formed by the passage of a pulse of electric current through a series of coils. The proposed technology enables to characterize the behaviour of materials under high strain-rates with a small acceleration path length of the striker bar and, because propulsion is purely electromagnetic, the overall performance can be easily controlled and nearly infinitely adjustable. The presentation is focused on the design and fabrication of the mechanical, electrical and electromagnetic components of the new compressive split Hopkinson bar and includes results from stress–strain characterization at high strain-rates to demonstrate the validity of the proposed concept. Keywords Hopkinson bar Electromagnetic High strain-rate
1 Introduction The idea of using a pressure bar for evaluating the behaviour of materials at very high rates of loading was originally proposed in 1872 by John Hopkinson.
C. M. A. Silva P. A. R. Rosa P. A. F. Martins (&) IDMEC, Instituto Superior Tecnico, TULisbon Av. Rovisco Pais, 1049-001 Lisbon, Portugal e-mail: [email protected]
During the 1910s Hopkinson (1914) performed a major step forward onto the original concept by developing a technique to measure the shape of a stress pulse in a long elastic bar. The pressure bar concept continued to evolve and the subsequent most important improvement was due to Kolsky (1949) by introducing strain gauge technology and oscillographic recording techniques. Kolsky also proposed the modification of the original Hopkinson bar concept by introducing a two-bar system, sandwiching the compression test specimen between them. This modification is commonly known as the compressive split Hopkinson bar and it is currently utilized for performing the mechanical characterization of materials at high strain-rates. During the 1960s Lindholm (1964) and Lindholm and Yeakley (1968) further improved the compressive split Hopkinson bar by developing a mechanical spring system to launch the striker bar and introducing modern data acquisition technology. From this time until today the concept kept being developed namely through the development of compressed air launching systems and utilization of new materials for the pressure bars. A good overview on the Hopkinson bar concept and technology can be found elsewhere (Follansbee 1985). In general terms, currently installed compressive split Hopkinson bar apparatuses consist of two long slender cylindrical bars of the same diameter, called the incident and the output bars, and of a shorter cylindrical bar, called the striker bar that is most
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commonly propelled by an air gun against the inciden
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