Hopkinson bar

Impact resistance is determined usiag flyer plate impact tests, long rod impact tests, Hopkinson bar tests (50), and the Hquid jet technique (51). Impact damage resistance is often quantified by measuring the postimpact strength of the ceramic. 

The principal input and output tests used by the US at the time of publishing NOLTR 1111(1952) included Drop Ball Testers (two types) Sand Bomb Apparatuses 100, 200, 360 500 gram Condenser Discharge Apparatus Torsional Ballistic Pendulums (two types) Drop Ball Test Set Mk 135 with Chronograph Thermocouple Drop Ball Test Set Mk 136 with Chronograph Gas Explosion Chambers (two types) Gas Volume and Impulse Apparatus Pressure Bomb Apparatus and Hopkinson Bar Apparatus (Ref 11, pp 9-2 9-3, Table 9-D... 

An ingenious split Hopkinson bar tester, which imparts tension directly to the specimen without the precompression wave, was developed by Costin and Duffy.40 The compressive wave developed by the explosive charge is... 

The test arrangement is shown diagramatically in Fig. 3. The upper end of the specimen is screwed into a strain-gauged PMMA Hopkinson bar of 19 mm diameter and 1.5 m length, calibrated for short-time response. Pilot tests had shown that the craze lifetime under impact lay within the expected uniaxial stress wave return time of 1.2 ms. 

Fig. 4. Load/time trace from a typical test. Viscoelastic delay of strain recovery in the PMMA Hopkinson bar is compensated for by dynamic calibration.

Regardless of the material model being used, data at intermediate strain rates under controlled laboratory conditions are needed. Such data should be obtainable soon with the new large diameter gas gun and Split-Hopkinson Bar facilities being established at Los Alamos. 

A wide variety of analogous experiments with different conditions of pressure, velocities, surface characteristics, and temperature are required to fully characterize the friction phenomenon at variable sliding speeds. Various research techniques are available for the study of dynamic friction such as pressure-shear plate impact frictional experiments, Split-Hopkinson bar method, pin-on-disk tests, and others, ranging from quasistatic conditions to very low sliding velocities. At low velocities (Vc < 1 m/s), for steel-on-steel non-lubricated contact of sliding surfaces, the friction coefficient demonstrates a strong... 

Fig. 8.4 (a) The dependence of the yield stress of PC on the uniaxial strain rate machine-controlled ( ) and Hopkinson bar (O), at 298 K. (b) Partial storage-modulus scans of PC showing rate-dependent shifts in a- and P-relaxations with strain rate, from e = 10 s to B = 10 s (from Mulliken and Boyce (2006) courtesy of Elsevier). 

Gama BA, Lopatnikov SL, Gillespie JW (2004) Hopkinson bar experimental technique A critical review. Appl Mech Rev 57 223-250... 

Gray GT (2000) Classic split-Hopkinson pressure bar technique. Mech Test Eval, ASM Handbook 8 462-476 Chen W, Zhang B, Forrestal MJ (1999) A split Hopkinson bar technique for low-impedance material. Exp... 

In addition to these standard tests, a considerable amount of research on the impact of adhesives has been carried out and presented by many scientists over the last 4 decades. A special apparatus, developed for testing materials under high strain rate and applied also to adhesives, is the Hopkinson bar, described in a subsequent section. A selection of other special testing methods and the related modeling are described in a further section. 

In addition to the limitations already mentioned, the two tests described in the previous sections present the further restriction that the strain rate attained in the adhesive cannot be much high, since it usually does not exceed 10 s When testing at higher strain rates is needed, the suitable equipment is the so-called Hopkinson bar. 

Use of threaded specimen and split ring to carry out tensile tests with the Hopkinson bar (From Goglio et al. [2008], copyright Elsevier]... 

Goglio et al. (2008) have tested a bi-component epoxy under tension and compression, to assess the effect of the strain rate on the strength of this adhesive. Also, in this work both the Hopkinson bar and a hydraulic test machine have been employed, depending on the desired strain rate (from 10 to 3 x 10 s ). 

There are other specific problems, which affect testing with the Hopkinson bar, that deserve to be mentioned. 

Zhao and Gary (1996), presenting a study related to the measurement of small strains with the Hopkinson bar, have considered two important sources of error. The first is the dispersion (related to the difference between phase and group velocities of the waves), which the authors correct with a numerical procedure. The second is the determination of the time delay between the signals, for which an iterative procedure is proposed. These authors also propose an identification technique to treat the data corresponding to the initial transient of the measurement, when the equilibrium in the specimen is not yet attained. 

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