Limiting the inﬂuence of friction on the split Hopkinson pressure bar tests by using a ring specimen M. Alves a, * , D. Karagiozova b , G.B. Micheli a , M.A.G. Calle a a Group of Solid Mechanics and Structural Impact, Department of Mechatronics and Mechanical Systems Engineering, University of São Paulo, Av. Prof. Mello Moraes, 2231, 05508-900 São Paulo, SP, Brazil b Institute of Mechanics, Bulgarian Academy of Sciences, Acad. G. Bonchev St., Block 4, Soﬁa 1113, Bulgaria article info Article history: Received 2 September 2011 Received in revised form 25 April 2012 Accepted 26 April 2012 Available online 7 May 2012 Keywords: Hopkinson pressure bar Ring specimen Friction Ductile materials Material characterization abstract The deformation of a ring under axial compression is analyzed in order to estimate a favorable ring specimen geometry capable of limiting the inﬂuence of friction on the stressestrain curve obtained from SHPB tests. The analysis shows that the use of a ring specimen with a large inner diameter and a small radial thickness offers some advantages comparing with the traditional disk sample. In particular, it can improve the reliability of the test results for ductile materials in the presence of friction. Based on the deformation analysis of a ductile ring under compression, a correction coefﬁcient is proposed to relate the actual material stressestrain curve with the reading from the SHPB. It is shown using ﬁnite element simulation that the proposed correction can be used for a wide range of conventional ductile materials. Experimental results with steel alloys indicate that the correction procedure is an effective technique for an accurate measurement of the dynamic material strength response. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction The split Hopkinson pressure bar (SHPB) test is an experimental technique widely used to obtain the dynamic strength properties of various materials. Although dynamic tensile tests can be performed [1], the concern here is with dynamic compression tests, with the tested sample, usually in a cylindrical shape, placed between input and output long cylindrical bars. Upon axial compression, the tested sample expands radially and it has been determined that an important condition to have a reliable test data is that the specimen must deform uniformly. Such a behavior is opposed mainly by the radial and longitudinal inertia and by friction, which naturally opposes the radial motion of the specimen between the input and output pressure bars interfaces. The validity and applicability of the assumption made in the 1D Hopkinson bar theory are discussed in Ref. [2]. Fundamentals of the Hopkinson bar experimental procedure are outlined, including bar calibration, specimen design, pulse shaping, and data analysis including correction for dispersion. In addition to the elasticeplastic metals, methodologies for soft and hard materials are also discussed. The errors due to both longitudinal and radial inertia have been analyzed [3e6]. For a cylindrical specimen of height L 0 and diam- eter D 0 , longitudinal and radial inertias have opposite responses each other and a ratio of L 0 =D 0 ¼ ﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ 3n s =4 p (1) was suggested by Davies and Hunter [3] which minimizes these effects. Here, n s is the Poisson ratio such that, for metals, L 0 / D 0 z 0.5. Due to this relation, the cylindrical sample normally takes a form of a thick disk. However, this ratio is smaller than the one which is determined to be the most favorable for the minimization of the errors due to friction between the bars and the cylindrical specimen, which speciﬁes that [7], 1:5  L 0 =D 0  2: (2) Therefore, a L 0 /D 0 ratio of 0.5 will not introduce large errors due to inertia only when friction is negligible. Thus, lubrication at the specimen/pressure bars interfaces is required. Inertia effects become particularly important in the range of small strains [8] for brittle materials. The initial spike in the stressestrain curve for ductile materials is also inﬂuenced by radial * Corresponding author. E-mail address: maralves@usp.br (M. Alves). Contents lists available at SciVerse ScienceDirect International Journal of Impact Engineering journal homepage: www.elsevier.com/locate/ijimpeng 0734-743X/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijimpeng.2012.04.005 International Journal of Impact Engineering 49 (2012) 130e141