Hopkinson bar test

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. 

Testing with the Hopkinson bar has been applied also to nonconventional cases, to assess the material response in special conditions. For instance, Martinez et al. (1998) have studied the behavior of a confined adhesive, i.e., when it cannot freely expand laterally under compression. The interest for such a case is related to the use of an adhesive to join, in an armored panel, the front ceramic layer to the metal backing plate. To this aim, the adhesive specimen in the Hopkinson bar tests has been surrounded by a confinement tube. The study, carried out on several types of adhesives, has evidenced an optimum value of the adhesive thickness in the armor, in order to ensure enough strength to contain the ceramic fragments and the minimum... 

The method of the Hopkinson bar testing has also been applied to specimen types other... 

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... 

Hopkinson s Pressure Bar Test. See under Physical Tests for Determining Explosive and Other Properties in Vol 1 of Encycl, p XVI... 

Hopkinson s Pressure Bar Test. The quantitative measurement of the press developed by expls over small intervals of time, which is a measure of expl violence, was made possible by application of the method conceived by Prof B. Hopkinson. The application of Hopkinson s principle to a wide field of research such as iaitiation of deton, properties of the deton wave, and the design of detonators fuses and the design of a variety of instruments were carried out by physicists of the Res Dept, Woolwich, England... 

British Tests. See Physical Tests for Determining Explosive and Other Properties Vol 1 and specifically the following British tests a)Ballistic Pendulum Test, p VII b)Exudation(ot sweating) Tests, p XI c)F7 lest(Figure of Insensitiveness Test) p XII Fragment Gun, p XII d)Friction Sensitivity Tests, p XIII e)Hopkinson s Pressure Bar Test, p XVI and i)Silvered Vessel Test or Waltham Abbey Silver Vessel Test, p XXIV... 

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.

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... 

Ross, C. A., Thompson, R Y., Tedesco, J. W. (1989). Split-Hopkinson Pressure-Bar Tests on Concrete and Mortar in Tension and Compression. Aci Materials Journal, 86,475-481. 

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. 

Schematic of the modified Hopkinson bar used by Sato and Ikegami (1999) to test butt joints of tubes under tension and torsion...

This chapter has described the main tests used to assess the impact strength of adhesives and joints, considering both those foreseen by international standards and those appeared in research papers. The main types of test rigs used in the experiments are pendulum, falling weight, and Hopkinson bar or evolutions of these. 

When the mechanical properties of the adhesive at high loading rate (strain rate 10 s ) are sought, the appropriate test rig is the Hopkinson bar. This apparatus is suitable to test specimens of adhesive under compression in straightforward manner tests under compression can be carried out with modifications of the basic scheme. Also simple joints can be tested by properly adapting the ends of the bars. Although the Hopkinson bar is still not very common in the laboratories, due to the interest recently shown by the researchers it will likely become more popular in future. 

Forrestal MJ, Wright TW, Chen W (2007) The effect of radial inertia on brittle samples during the split Hopkinson pressure bar test. Int J Imp Eng 34 405-411... 

---