ASM Metals HandBook Vol. 8 - Mechanical Testing and Evaluation
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Fig. 12 Micrograph of axial cracking during dynamic uniaxial loading of magnesia partially stabilized
zirconia. Arrows indicate directions from two loadings perpendicular to one another
References cited in this section
9. G. Subhash and S. Nemat-Nasser, Dynamic Stress-Induced Transformation and Texture Formation in
Uniaxial Compression of Zirconia Ceramics, J. Am. Ceram. Soc., Vol 76, 1993, p 153–165
12. G. Subhash and G. Ravichandran, Mechanical Behavior of Hot-Pressed Aluminum Nitride Under
Uniaxial Compression, J. Mater. Sci,Vol 33, 1997, p 1933–1939
13. J. Lankford, Compressive Strength and Microplasticity in Polycrystalline Alumina, J. Mater. Sci., Vol
12, 1977, p 791–796
14. J. Lankford, Mechanisms Responsible for Strain-Rate Dependent Compressive Strength in Ceramic
Materials, J. Am. Ceram. Soc., Vol 64, 1981, p 25–33
15. J. Lankford, Dynamic Compressive Fracture in Fiber-reinforced Ceramic Matrix Composites, Mater.
Sci. Eng. A, Vol 107, 1989, p 261–268
16. J. Lankford and C.R. Blanchard, Fragmentation of Brittle Materials at High rates of Loading, J. Mater.
Sci., Vol 26, 1991, p 3067–3072
17. G. Ravichandran and G. Subhash, A Micro Mechanical Model for High Strain Rate Behavior of
Ceramics, Int. J. Solids Struct., Vol 32 (No. 17/18), 1995, p 2627–2650
18. D.E. Grady and J. Lipkin, Criteria for Impulsive Rock Fracture, Geophy. Res. Lett., Vol 7, 1980, p 255–
258
19. J. Staehler, W.W. Predebon, B.J. Pletka, and G. Subhash, Micromechanisms of Deformation in High
Purity Hot Pressed Alumina, Mater. Sci. Eng., 1999 (in review)
20. J. Lankford, W.W. Predebon, J.M. Staehler, G. Subhash, B.J. Pletka, and C.E. Anderson, The Role of
Plasticity as Limiting Factor in the Compressive Failure of High Strength Ceramics, Mech. Mater., Vol
29, 1998, p 205–218
21. S. Nemat-Nasser and M. Horii, Compression-Induced Nonplanar Crack Extension with Application to
Splitting, Exfoliation, and Rockburst, J. Geophys. Res., Vol 87, 1982, p 6805–6821
22. M.F. Ashby and S.D. Hallam, The Failure of Brittle Solids Containing Small Cracks under Compressive
Stress States, Acta Metall., Vol 34, 1986, p 497–510
23. M.E. Kipp and D.E. Grady, Dynamic Fracture Growth and Interaction in One Dimension, J. Mech.
Phys. Solids, Vol 33, 1985, p 399–415
24. S. Nemat-Nasser and H. Deng, Strain-Rate Effects on Brittle Failure in Compression, Acta Metall.
Mater., Vol 42, 1994, p 1013–1024
Split-Hopkinson Pressure Bar Testing of Ceramics
G. Subhash, Michigan Technological University G. Ravichandran, California Institute of Technology
Conclusions
The inherent limitations of the traditional SHPB technique for testing of brittle materials necessitate
modifications in the SHPB design and testing procedure. In order to obtain accurate and reliable data on the
behavior of ceramics at high strain rates, modifications such as strain gaging the specimen, sandwiching the
specimen between high-strength impedance matching inserts, momentum trapping, and incident pulse shaping
should be employed. Because ceramics exhibit only elastic strains to failure, one should also be aware of the
maximum strain rate that can be achieved using the SHPB technique. This limiting strain rate depends on
specimen size, its elastic properties, incident pulse duration, and the bar diameter. For more accurate
measurement of fracture strains and identification of true failure modes, it is emphasized that close tolerances
should be maintained for flatness and parallelism of the loading surfaces, and the specimens should be carefully
ground and polished to reduce machining-induced damage on the surfaces.
Split-Hopkinson Pressure Bar Testing of Ceramics
G. Subhash, Michigan Technological University G. Ravichandran, California Institute of Technology
References
1. H. Kolsky, An Investigation of the Mechanical Properties of Materials at Very High Rates of Loading,
Proc. R. Soc. (London) B, Vol 62, 1949, p 676–700
2. H. Kolsky, Stress Waves in Solids, Dover, 1963, p 41–94
3. G. Ravichandran and G. Subhash, Critical Appraisal of Limiting Strain Rates for Compression Testing
of Ceramics in a Split-Hopkinson Pressure Bar, J. Am. Ceram. Soc., Vol 77, 1994, p 263–267
4. W. Chen, G. Subhash, and G. Ravichandran, Evaluation of Ceramic Specimen Geometries Used in
Split-Hopkinson Pressure Bar, Dymat, Vol 1, 1994, p 191–210
5. C. Anderson, Jr., P. O'Donoghue, J. Lankford, and J. Walker, Numerical Simulations of SHPB
Experiments for the Dynamic Compressive Strength and Failure of Ceramics, Int. J. Frac., Vol 55,
1992, p 193–208
6. A. Cosculluela, J. Cagnoux, and F. Collombet, Two Types of Experiments for Studying Uniaxial
Dynamic Compression of Alumina, Shock Compression of Condensed Matter, S.C. Schmidt, R.D. Dick,
J.W. Forbes, and D.G. Tasker, Ed., Elsevier Science Publishers, 1991, p 951–954
7. W.R. Blumenthal, High Strain Rate Compression and Fracture of B4C-Aluminum Cermets, Shock-
Wave and High-Strain-Rate Phenomena in Materials, M.A. Meyers, L.E. Murr, and K.P. Staudhmmer,
Ed., Marcel Dekker, 1992, p 1093–1100
8. P.S. Follansbee and C. Frantz, Wave Propagation in the Split-Hopkinson Pressure Bar, J. Eng. Mater.
Technol. (Trans. ASME), Vol 105, 1983, p 61–66
9. G. Subhash and S. Nemat-Nasser, Dynamic Stress-Induced Transformation and Texture Formation in
Uniaxial Compression of Zirconia Ceramics, J. Am. Ceram. Soc., Vol 76, 1993, p 153–165
10. G. Subhash and S. Nemat-Nasser, Uniaxial Stress Behavior of Y-TZP, J. Mater. Sci., Vol 25, 1993, p
5949–5952
11. S. Nemat-Nasser, J.B. Isaacs, and J.E. Starrett, Hopkinson Techniques for Dynamic Recovery
Experiments, Proc. R. Soc. (London) A, Vol 435, 1991, p 371–391
12. G. Subhash and G. Ravichandran, Mechanical Behavior of Hot-Pressed Aluminum Nitride Under
Uniaxial Compression, J. Mater. Sci,Vol 33, 1997, p 1933–1939
13. J. Lankford, Compressive Strength and Microplasticity in Polycrystalline Alumina, J. Mater. Sci., Vol
12, 1977, p 791–796
14. J. Lankford, Mechanisms Responsible for Strain-Rate Dependent Compressive Strength in Ceramic
Materials, J. Am. Ceram. Soc., Vol 64, 1981, p 25–33
15. J. Lankford, Dynamic Compressive Fracture in Fiber-reinforced Ceramic Matrix Composites, Mater.
Sci. Eng. A, Vol 107, 1989, p 261–268
16. J. Lankford and C.R. Blanchard, Fragmentation of Brittle Materials at High rates of Loading, J. Mater.
Sci., Vol 26, 1991, p 3067–3072
17. G. Ravichandran and G. Subhash, A Micro Mechanical Model for High Strain Rate Behavior of
Ceramics, Int. J. Solids Struct., Vol 32 (No. 17/18), 1995, p 2627–2650
18. D.E. Grady and J. Lipkin, Criteria for Impulsive Rock Fracture, Geophy. Res. Lett., Vol 7, 1980, p 255–
258
19. J. Staehler, W.W. Predebon, B.J. Pletka, and G. Subhash, Micromechanisms of Deformation in High
Purity Hot Pressed Alumina, Mater. Sci. Eng., 1999 (in review)
20. J. Lankford, W.W. Predebon, J.M. Staehler, G. Subhash, B.J. Pletka, and C.E. Anderson, The Role of
Plasticity as Limiting Factor in the Compressive Failure of High Strength Ceramics, Mech. Mater., Vol
29, 1998, p 205–218
21. S. Nemat-Nasser and M. Horii, Compression-Induced Nonplanar Crack Extension with Application to
Splitting, Exfoliation, and Rockburst, J. Geophys. Res., Vol 87, 1982, p 6805–6821
22. M.F. Ashby and S.D. Hallam, The Failure of Brittle Solids Containing Small Cracks under Compressive
Stress States, Acta Metall., Vol 34, 1986, p 497–510
23. M.E. Kipp and D.E. Grady, Dynamic Fracture Growth and Interaction in One Dimension, J. Mech.
Phys. Solids, Vol 33, 1985, p 399–415
24. S. Nemat-Nasser and H. Deng, Strain-Rate Effects on Brittle Failure in Compression, Acta Metall.
Mater., Vol 42, 1994, p 1013–1024
Torsional Kolsky Bar Testing
Amos Gilat, The Ohio State University
Introduction
THE TORSIONAL KOLSKY BAR TEST is a reliable technique for testing materials in the 10
2
to 10
4
s
-1
strain-rate regime (Fig. 1). In this technique, shown schematically in Fig. 2, a short material specimen is placed
between two bars. The specimen is loaded by a torsional wave that is generated in one of the bars. Once loaded,
the wave is partially reflected back and partially transmitted to the other bar. The history of load and
deformation in the specimen is determined by monitoring the waves in the bars, which remain elastic
throughout the test. The technique is a variation of a technique proposed by Kolsky (Ref 2), in which the
specimen is loaded in compression. It is described in the article “Classic Split-Hopkinson Pressure Bar Testing”
in this Volume.
Fig. 1 Dynamic aspects of material testing. Source Ref 1
Fig. 2 Schematic of the torsional Kolsky bar apparatus. Angular velocities of the ends of the specimen
are denoted by θ
1
and θ
2
.
References cited in this section
1. U.S. Lindholm, High Strain Rate Tests, in Measurement of Mechanical Properties, Vol 5, Part I,
Techniques of Metals Research, R.F. Bunshah, Ed., Interscience, 1971, p 199–271
2. H. Kolsky, An Investigation of the Mechanical Properties of Materials at Very High Rates of Loading,
Proc. Phys. Soc. London, Vol 62-B, 1949, p 676–700
Torsional Kolsky Bar Testing
Amos Gilat, The Ohio State University
Historical Perspective
In 1949 Kolsky devised an experiment in which short specimens are placed between two long elastic cylindrical
bars that act as wave guides for a loading pulse. The experiment was referred to as the split-Hopkinson bar,
although the purpose of the test, the instrumentation, and the specimen were quite different from those in
Hopkinson's original experiment (Ref 3).
In Kolsky's test, loading is accomplished by a compressive wave propagating along one of the bars and through
the specimen. This wave is partially transmitted into the second bar and partially reflected by the specimen. The
bars in this configuration are used for applying the load to the specimen and as transducers for measuring the
displacements and loads at the specimen ends, which are in contact with the bars. Kolsky showed that the
portion of the incident loading wave that is transmitted through the specimen provides a measure of the axial
stress in the specimen, while the magnitude of the wave that is reflected back is proportional to its strain rate. It
is assumed that the specimen is under a state of uniaxial stress and that the stress and strain are uniform along
the specimen because the wave transit time through the specimen is very short compared to the duration of the
loading wave.
The first torsional Kolsky bar was introduced in 1966 by Baker and Yew, who produced a torsional wave by a
sudden release of a clamped pretwisted end section of an elastic bar (Ref 4). They showed that the stress-strain
curves in shear for the specimen could be obtained by an analysis of the waves measured in the elastic bars on
both sides of the specimen. Originally, a torsional version of the Kolsky bar was developed to eliminate radial
inertia effect in the specimen and in the bars and the radial traction component that can be imposed (if friction
is not eliminated) at the interfaces between the specimen and the bars in the compression test. Today these
reasons no longer seem as important because the results from torsional tests have been found to be consistent
with those of compression. However, torsional loading has other advantages, as described later in this article.
In the early 1970s, several configurations of torsional Kolsky bar were introduced by several investigators (Ref
5, 6, and 7). The various machines differ mainly in the method used for generating the torsional loading wave.
Duffy et al. originally used explosive loading to initiate the loading pulse (Ref 5). Others used a sudden release
of a stored torque. Explosive loading produces a short pulse with a short rise time. A stored-torque loading
system provides a longer pulse, which produces larger strains in the specimen. The pulse, however, has a longer
rise time that depends on the details of the clamp design.
The torsional Kolsky bar has also been modified for studying strain-rate history effects by conducting
incremental and decremental strain-rate tests. In incremental strain-rate tests, the specimen is first twisted quasi-
statically, and then a dynamic strain rate is imposed with no intermediate unloading (Ref 8, 9). In decremental
strain rate tests an initial high strain rate deformation is suddenly reduced and continues at a lower strain rate
(Ref 10, 11). Both strain rates are within the range of the torsional Kolsky bar.
The technique has also been adapted for testing materials at temperatures below and above the ambient (Ref 12,
13). For temperatures below the ambient and above the ambient up to about 150 °C, or 302 °F the technique is
used without any modifications. In tests at higher temperatures (up to 1000 °C, or 1832 °F), the effect of
heating the specimen on the adjacent elastic bars must be addressed.
The torsional Kolsky bar also provides means for studying localization of plastic deformation through the
formation of adiabatic shear bands (Ref 14, 15, and 16). The evolution of the localization is recorded by high-
speed photography and associated temperature increase is measured by detecting infrared radiation. Recently, a
recovery experiment, in which the specimen was loaded up to a predetermined strain and then unloaded and
recovered for microscopic examination, has also been introduced (Ref 17).
References cited in this section
3. B. Hopkinson, The Effects of Momentary Stresses in Metals, Proc. Phys. Soc. London, Vol 74-A, 1905,
p 498–506
4. W.W. Baker and C.H. Yew, Strain Rate Effects in the Propagation of Torsional Plastic Waves, J. Appl.
Mech., Vol 33, 1966, p 917–923
5. J. Duffy, J.D. Campbell, and R.H. Hawley, On the Use of a Torsional Split Hopkinson Bar to Study
Rate Effects in 1100-O Aluminum, J. Appl. Mech., Vol 38, 1971, p 83–91
6. T. Nicholas and J.E. Lawson, On the Determination of the Mechanical Properties of Materials at High
Shear-Strain Rates, J. Mech. Phys. Solids, Vol 20, 1972, p 57–64
7. J.L. Lewis and J.D. Campbell, The Development and Use of a Torsional Hopkinson Bar Apparatus,
Exp. Mech., Vol 12 (No. 11), 1972, p 520–524
8. R.A. Frantz and J. Duffy, The Dynamic Stress-Strain Behavior in Torsion of 1100-O Aluminum
Subjected to a Sharp Increase in Strain Rate, J. Appl. Mech., Vol 39, 1972, p 939–945
9. J.D. Campbell, A.M. Eleiche, and M.C.C. Tsao, Strength of Metals and Alloys at High Strains and
Strain Rates, in Fundamental Aspects of Structural Alloy Design, R.J. Jaffee and B.A. Wilcox, Ed.,
Plenum Press, 1977, p 545–563
10. J. Lipkin, J.D. Campbell, and J.D. Swearengen, The Effects of Strain-Rate Variations on the Flow Stress
of OFHC Copper, J. Mech. Phys. Solids, Vol 26, 1978, p 251–268
11. A. Gilat and Y.H. Pao, High Rate Decremental-Strain Rate Test, Exp. Mech., Vol 28 (No. 3), 1988, p
322–325
12. A.M. Eleiche and J. Duffy, Effects of Temperature on the Static and Dynamic Stress-Strain
Characteristics in Torsion of 1100-O Aluminum, Int. J. Mech. Sci., Vol 17, 1975, p 85–95
13. A. Gilat and X. Wu, Elevated Temperature Testing with the Torsional Split Hopkinson Bar, Exp. Mech.,
Vol 34 (No. 2), 1994, p 166–170
14. L.S. Costin, E.E. Crisman, R.H. Hawley, and J. Duffy, On the Localization of Plastic Flow in Mild Steel
Tubes Under Dynamic Torsional Loading, in Proc. 2nd Int. Conf. Mechanical Properties of Materials
at High Rates of Strain, J. Harding, Ed., The Institute of Physics, 1979, p 287–282
15. A. Marchand and J. Duffy, An Experimental Study of the Formation Process of Adiabatic Shear Bands
in a Structural Steel, J. Mech. Phys. Solids, Vol 36, 1988, p 251–283
16. J.H. Giovanola, Adiabatic Shear Banding Under Pure Shear Loading, Part I: Direct Observation of
Strain Localization and Energy Dissipation Measurements, Mech. Mater., Vol 7, 1988, p 59–71
17. Q. Xue, L.T. Shen, and Y.L. Bai, A Modified Split Hopkinson Torsional Bar in Studying Shear
Localization, Meas. Sci. Technol., Vol 6, 1995, p 1557–1565
Torsional Kolsky Bar Testing
Amos Gilat, The Ohio State University
Comparison with Compression Kolsky Bar
The torsional Kolsky bar was originally introduced in order to verify strain-rate effects that were observed in
tests with the compression Kolsky bar. Concerns have been raised with regard to the effects of inertia and
friction on the experimental results when a compression load is used. Although the compressive and torsional
Kolsky bar techniques are based on the same principles, there are significant differences between the two. In
addition to the difference in the states of stress, the techniques use specimens that have different geometry, and
the boundary conditions at the specimen ends are different. The two techniques are now compared in detail.
Inertia Effects in the Specimen. Due to the Poisson's ratio effect, loading in the compressional test is
accompanied by radial expansion of the specimen. This radial expansion is opposed by a radial inertia that
becomes greater with a shorter pulse and larger amplitude. This effect results in a radial stress component in the
specimen superposed on the axial stress component. Hence, in axial loading, the state of stress in the specimen
is not fully uniaxial. Furthermore, the radial component of stress is difficult to evaluate because it depends on
dynamic material properties that have not yet been determined. In torsional testing, due to the absence of
Poisson's ratio effect, radial expansion or contraction does not occur and inertial effects do not exist.
Friction Effects. In the compression Kolsky bar, radial deformation accompanies the axial motion. Radial
traction is imposed at the interfaces between the specimen and the bars because the Poisson's ratio of the
specimen is not necessarily the same as of the bars, and because the specimen undergoes plastic deformation.
The radial traction causes unknown stresses and barreling in the specimen. Although friction can be minimized
by proper lubrication, it cannot be completely eliminated. In torsional testing, friction does not exist.
Wave Dispersion in the Bars. In the compression Kolsky bar, the elastic waves that travel in the bars undergo
geometric dispersion because different frequency components in the compressive pulse have different
velocities. In particular, the higher frequency components travel at a lower velocity than the main pulse. In
contrast, there is no geometric dispersion when a torsional pulse travels along an elastic bar in its primary
mode; that is, all frequency components of the torsional pulse have the same velocity. Hence, a torsional pulse
does not change its shape as it propagates toward the specimen.
The absence of geometric dispersion in torsion means that a pulse initiated with a short rise time will maintain
this rise time until it reaches the specimen, independent of the length of the Kolsky bar. Also, the torsional
strain gage stations can be located as near or as far from the specimen as desired and still reveal the correct
shape pulse incident on, or reflected from, the specimen. In an axial Kolsky bar, however, a gage placed too
close to the specimen is subject to errors due to three-dimensional end effects, while a gage placed too far from
the specimen may produce unsatisfactory results due to geometric dispersion.
If a torsional pulse is noisy when initiated (i.e., if some high-frequency components are superposed on the main
pulse), this characteristic will be maintained, regardless of the length of the Kolsky bar. As a result, the strain
rate imposed on the specimen will not be constant. In axial Kolsky bar, such high-frequency components
gradually disappear. Hence, an axial pulse tends to become flat—that is, tends to smooth out—as it travels
along an elastic bar. A noisy torsional pulse will retain this characteristic; it will never become flat and, hence,
will not provide deformation at a constant strain rate.
Specimens. Polycrystalline specimens tested in a torsional Kolsky bar experiment are shaped as short, thin-wall
tubes. These specimens require substantially more material and are more expensive to machine than the
specimens used in a compression Kolsky bar, which are shaped as short cylinders. In addition, gripping tubular
specimen is more difficult, and special care must be taken that the specimens are attached firmly to the bars and
that there are no wave reflections due to the gripping method.
Torsional Kolsky Bar Testing
Amos Gilat, The Ohio State University
Description
The torsional Kolsky (split-Hopkinson) bar apparatus is made up of two collinear bars that face each other and
are supported by bearings that allow them to rotate freely. The specimen, which is usually a short, thin-wall
tube, is attached between the two bars. To conduct a test, a torsional wave is generated in one of the bars
(incident bar). The wave propagates toward the specimen, and, when it arrives, the specimen is loaded and the
wave is partially reflected back to the incident bar and partially transmitted to the other bar (transmitter bar).
The apparatus and the specimen are designed such that the amplitude of the incident wave is much larger than
the torque required for deforming the specimen or causing the specimen to fail. Consequently, most of the wave
is reflected back to the incident bar and only a small portion is transmitted to the transmitter bar. As a result, the
end of the incident bar where the specimen is attached rotates with an angular velocity that is much larger than
the angular velocity of the other end of the specimen that is connected to the transmitter bar. The difference in
the angular velocity between the specimen ends causes the specimen, which is very short, to deform at a high
rate. In addition, because the specimen is short, and hence, the time it takes for the wave to propagate through is
very short, the deformation is uniform. The history of load and deformation of the specimen is determined by
monitoring the waves in the bars, which remain elastic throughout the test.
The Bars
The bars typically have a diameter of about 25 mm (1 in.) and are made of aluminum. Steel and titanium have
also been used. The bars must be aligned properly and supported along the length by a series of bearings that
allow free rotation. Aluminum bars are easy to use and are suitable for most applications. For an elastic
torsional wave that propagates in a bar, the relationship between the torque, T, and the angular velocity, , is
given by:
(Eq 1)
where ρ is the density, J is the polar moment of inertia, and c is the torsional wave speed. The maximum torque
that can propagate in an elastic bar is limited by the yield stress of the material, τ
Y
, which is related to the
torque by:
(Eq 2)
where r is the radius of the bar. Substituting T from Eq 2, and c = , where G is the shear modulus, in
Eq 1 gives:
(Eq 3)
Equation 3 shows that, theoretically, a higher angular velocity can be achieved with bars made of a material that
has high yield stress and low density and shear modulus. Titanium is a material that has a high value of
τ
Y
/ . Practically, however, it is difficult to produce a torsion pulse in the incident bar with an amplitude
high enough for the shearing stress to approach the yield stress of alloyed titanium. With this limitation, an
aluminum bar has an advantage because, for the same torque, it will have a higher angular velocity than
titanium.
Generating the Incident Wave
To conduct a test, a torsional wave has to be generated in the incident bar. The wave is characterized by its rise
time, shape, amplitude, and duration. The most common method for generating the wave is a sudden release of
a stored torque. The torsional wave can also be initiated by explosives. Descriptions of both methods follow.
Stored-Torque Torsional Kolsky Bar. A stored-torque Kolsky bar is shown schematically in Fig. 3. The loading
wave in the incident bar is produced by the release of a torque that is initially stored at the section of the bar
between the clamp and the loading end. The stored torque is generated by first tightening the clamp and then
turning the end of the bar. It is important that the section of the bar with the stored torque will be loaded with
pure torque without bending moment or axial force. This can be done by rotating a wheel, attached at the end of
incident bar, with a hydraulic system of a cable and pulleys that applies a pure couple to the wheel. The design
of the clamp is crucial for good results. The clamp must be able to hold the desired torque without slipping and
release the torque rapidly enough to produce a sharp-fronted stress pulse traveling toward the specimen.
Fig. 3 Schematic of a stored-torque torsional Kolsky bar and its wave characteristic diagram. Torsional
wave speed is denoted by c.
Clamping Techniques. The design of the quick-release clamp has a direct influence on the quality of the
incident loading wave. Ideally, the incident pulse should rise instantly to a constant amplitude and then drop off
immediately to zero at the end of the pulse, thus forming a square loading pulse. In practice, a finite time (rise
time) is required to achieve maximum torque in the loading pulse.
A clamp designed by Duffy (Ref 18) is shown in Fig. 4. It consists of two arms that are held together at the top
by a notched pin and are hinged at the bottom to a fixed peg on one side and to a sliding peg on the other. The
clamp is tightened by a hydraulic ram that pushes the lower ends of the clamp arms together. After the desired
torque is loaded between the loading end and the clamp, the hydraulic pressure is increased until the notched
pin at the top fractures, releasing the stored torque.