Split-Hopkinson pressure bar
The Split-Hopkinson pressure bar, named after Bertram Hopkinson, sometimes also called a Kolsky bar, is an apparatus for testing the dynamic stress-strain response of materials.
History
The split-Hopkinson pressure bar was first suggested by Bertram Hopkinson in 1914[1] as a way to measure stress pulse propagation in a metal bar. R.M. Davies and H. Kolsky refined Hopkinson's technique by using two Hopkinson bars in series, now known as the split-Hopkinson bar, to measure stress and strain. Later modifications have allowed for tensile, compression, and torsion testing.
Compression testing
For compression testing, two symmetrical bars are situated in series, with the sample in between. The first bar is the incident bar, which is struck by a striker bar during testing. The striker bar is fired from a gas gun. The second bar is the transmitted bar, which collides with a momentum trap (typically a block of soft metal). Strain gages are mounted on both the incident and transmitted bars.[2]
Tension testing
Tension testing in a Split Hopkinson Pressure Bar (SHPB) is more complex due to a variation of loading methods and specimen attachment to the incident and transmission bar[3]. The first tension bar was designed and tested by Harding et al. in 1960; the design involved using a hollow weight bar that was connected to a yoke and threaded specimen inside of the weight bar. A tensile wave was created by impacting the weight bar with a ram and having the initial compression wave reflect as a tensile wave off the free end[4] Another breakthrough in the SHPB design was done by Nichols who used a typical compression setup and threaded metallic specimens on both the incident and transmission ends, while placing a composite collar over the specimen. The specimen had a snug fit on the incident and transmission side in order to bypass an initial compression wave. Nichols setup would create an initial compression wave by an impact in the incident end with a striker, but when the compression wave reached the specimen, the threads would not be loaded. The compression wave would ideally pass through the composite collar and then reflect off the free end in tension. The tensile wave would then pull on the specimen[5] The next loading method was revolutionized by Ogawa in 1984. A hollow striker was used to impact a flange that is threaded to end on an incident bar. This striker was propelled by using either a gas gun or a rotating disk. The specimen was once again attached to the incident and transmission bar via threading [6].
Torsion testing
As with tension testing, there exist a variety of methods for specimen attachment and loading when subjecting materials to torsion on a SHPB. A common setup involves clamping the midsection of the incident bar while a torque is applied to the free end. To begin the test the clamp is suddenly released, which causes a torsion wave (called the incident wave) to propagate from the clamp toward the specimen. Upon reaching the specimen, the incident wave breaks into two component waves. The first wave propagates through the specimen (called the transmitted wave), while the other is reflected back toward the clamp (called the reflected wave). [7]
See also
References
- ^ B. Hopkinson, “A Method of Measuring the Pressure Produced in the Detonation of High Explosives or by the Impact of Bullets,” Philos. Trans. R. Soc. (London) A, 213, pp. 437-456, 1914.
- ^ G. T. Gray, Classic Split Hopkinson Pressure Bar Technique ASM V8 Mechanical Testing (1999) 17-20
- ^ T. Nicholas, Tensile Testing of Materials at High Rates of Strain, Exp. Mech. 21 (1981) 177-188
- ^ J. Harding, E. O. Wood and J. D. Campbell, Tensile Testing of Materials at Impact Rates of Strain, Journal of Mechanical Engineering Science 2 (1960) 88-96
- ^ T. Nicholas, Tensile Testing of Materials at High Rates of Strain, Exp. Mech. 21 (1981) 177-188
- ^ K. Ogawa, Impact-tension Compression Test by Using a Split-Hopkinson Bar, Exp. Mech. 24 (1984) 81–86
- ^ A. Gilat, Y.H. Pao, High-Rate Decremental-Strain-Rate Test, Experimental Mechanics, 1988, Vol. 28, p. 322 – 325