The split Hopkinson bar

In contrast to the other techniques described above, which can only be used to evaluate Young's modulus of elasticity in either tension or compression, the split Hopkinson bar can be used to evaluate both simultaneously on the same specimen.

In the test a flat-faced tablet is sandwiched between two cylindrical steel rods (10 mm diameter) which are aligned horizontally as shown schematically in Fig. 8a (Al-Hassani etal., 1989). The two rods are supported on four adjustable V-slot knife edges which minimize frictional effects and thus prevent disturbance of the stress wave. The porous tablets are held in place using standard strain gauge adhesive which allows the transmission of stresses through the specimen. In the experiment a short compressive loading pulse is initiated by striking the free end of the input bar with a short 10 mm diameter aluminium rod. At the specimen-bar interface the pulse divides into transmitted and reflected components, or compressive and tensile stress waves, respectively. The compressive wave travels to the free end of the output bar, reflecting as a tensile wave to load the specimen

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Output bar

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Fig. 8 Schematic diagram of the Split Hopkinson bar with space-time diagram showing pulse partition (adapted from Al-Hassani eta/., 1989).

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Fig. 8 Schematic diagram of the Split Hopkinson bar with space-time diagram showing pulse partition (adapted from Al-Hassani eta/., 1989).

in tension, as shown schematically in Fig. 8b. In time numerous internal reflections occur in the bar and specimen resulting in complex waveforms which are disregarded in the analysis. The stress waves are detected by four strain gauges at each position on the input and output bars, thus maximizing sensitivity and allowing only the measurement of axial strains. The first compressive and tensile incident pulses and their associated reflected and transmitted pulses are used to calculate the elastic constants.

The theory underlying the method is fully described by Al-Hassani et al. (1989). Young's modulus either in compression, Ec, or in tension, Eā€ž can be calculated using the appropriate strain signals using the expression:

where ER and Es are the Young's modulus of elasticity of the bar material and specimen, respectively, A and As the cross-sectional area of the bar and specimen, respectively, ls is the specimen thickness, er and es are the

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