High Speed Photography in Dynamic Materials Testing

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  • High Speed Photography in Dynamic Materials TestingR. D. Perkins and S. J. Green Citation: Review of Scientific Instruments 39, 1209 (1968); doi: 10.1063/1.1683621 View online: http://dx.doi.org/10.1063/1.1683621 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/39/8?ver=pdfcov Published by the AIP Publishing Articles you may be interested in HIGHSPEED PHOTOGRAPHY OF DETONATION PROPAGATION IN DYNAMICALLY PRECOMPRESSEDLIQUID EXPLOSIVES AIP Conf. Proc. 955, 857 (2007); 10.1063/1.2833261 Highspeed photography of impact effects in threepoint bend testing of polymers J. Appl. Phys. 67, 4304 (1990); 10.1063/1.344946 HighSpeed Photography Phys. Today 9, 32 (1956); 10.1063/1.3059894 High Speed Photography Phys. Today 4, 32 (1951); 10.1063/1.3067307 An Inexpensive Stroboscope for High Speed Photography Rev. Sci. Instrum. 14, 273 (1943); 10.1063/1.1770186

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  • NOTES 1209

    "C" washers are u~ed so that the Polaroid filters can be removed with ease in the event of damage) it may be placed on a crystal growing furnace. The intensity of the radiation passing through the Polaroid filters is controlled by turning the forward filter housing in either a clockwise or counter-clockwise direction. An L-shaped slot has been machined in the cylinder on the main housing. By applying pressure on this slot, the forward filter housing will be held in the selected position by friction.

    This viewing port is affixed to a door of a Verneuil (flame-fusion) furnace. The Verneuil burner is enclosed in a metal cabinet with front and rear doors. This prevents the inhalation of toxic materials, and with this light in-tensity viewer, the only object seen by the operator is the growing boule. AU-shaped bracket (mounting bracket) was fabricated and fastened to the furnace door, and a 5 cm diam hole cut through. The light intensity viewing port can easily slide in and out of the bracket, and if one desires to view the growing crystal with an optical py-rometer, the port is removed, and the pyrometer can then be focused through the hole.

    The viewing port has been affixed to a variety of crystal growth equipments whose designs are such that visual observation during growth is possible. These include Czochralski and Bridgman-Stockbarger, in addition to the aforementioned Verneuil, apparatus. The use of this device has reduced significantly the strain on the eyes of crystal growers in this Laboratory.

    The author wishes to thank Messrs. Walter Jackson and Robert Maher for their assistance in fabrication of the device.

    1 F. J. Gray & Company, Brooklyn, New York. 2 Polarizing glass laminated discs---i:atalog number 31-52-62-12,

    Bausch & Lomb Inc., Rochester, New York. 3 Tru-Arc Retaining Rings Division, Waldes Kohinoor, Inc., Long

    Island City, New York.

    High Speed Photography in Dynamic Materials Testing

    R. D. PERKINS AND S. J. GREEN Manufacturing Development, General Motors Corporation

    Warren, Michigan 48090

    (Received 16 June 1967; and in final form, 18 March 1968)

    FOR some time experimentalists and theoreticians have studied the influence of dynamic loadings on the behavior of metallic materials. More recently, these studies have been extended to the behavior of polymers, fiber

    WEIGHBAR ANVIL BAR

    FIG. 1. Schematic diagram of experimental apparatus.

    composites, and geologic materials, where load applications frequently produce fracture under dynamic conditions. To study dynamic macroscopic fracture, a simple high speed photographic technique was coupled with the dynamic testing device. Separately, the mechanics of the photo-graphic techniques as well as the dynamic loading tech-niques are not new; however, the simplicity of their com-bined operation provides the experimenter with much information concerning the dynamic benavior of the ma-terial using low cost equipment found in many laboratories.

    The split Hopkinson barl is a technique often used to determine the response of a material to uniaxial stress wave loading at strain rates of about lOS/sec. The device consists of three bars: the driver bar which initiates a stress wave in the impacted bar, the impacted bar or weighbar, and the rear bar or anvil bar. All three bars are required to remain elastic throughout the tests. By measur-ing the response of the weighbar and anvil bar with a specimen sandwiched between, and assuming one-dimen-sional elastic wave propagation theory, the stress-strain behavior of the specimen may be determined. Maiden and Green2 give a complete description of the device used for these tests as well as the data reduction techniques.

    For the photographic equipment almost any type of camera may be used. In these tests, a Calumet View

    FIG. 2. Solenhofen limestone under compression at the rate of 103/sec, loaded to 93% of fracture strength.

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  • 1210 NOTES

    FIG. 3. Solenhofen limestone 8 p.sec after fracture. Specimen loaded in compression at the rate of l()3/sec.

    Camera was used with high speed Polaroid, lOX12.7 cm sheet film, in order to obtain a conveniently large format without enlargement. The lens shutter was operated in the bulb condition. A General Radio Strobotac served as a very short duration (0.8 ~sec), high intensity light source. In order to improve unifonnity of the lighting, the strobe light was mounted in a circular reflector surrounding the bars. The synchronization of the light with the test was provided by the delayed sweep generator in the Tektronix 556 oscilloscope which, in turn, was triggered by a piezo-electric accelerometer attached to the weighbar. The ex-perimental arrangement is shown schematically in Fig. 1.

    One photograph is obtained per specimen test. If repeatability in material behavior is attainable, then several single photographs may be combined to show the defonnation of an average sample during a typical test.

    Solenhofen limestone was one of several materials which has been studied with this technique. At quasistatic rates of loading this rock behaved in an elastic brittle manner with longitudinal slabbing after fracture. Dynamically, however, the" limestone exhibited a higher than static peak stress followed by flow 'of the material at about the quasistatic fracture stress before the load was released. Through photographs, such as shown in Fig. 2, it was detennined that slip lines (previously seen in Solenhofen limestone when deforming ductilely under hydrostatic confining pressure3) cross hatched the specimen about the time the peak stress was reached. The observed flow was developed by the fractured material continuing to slip along these planes as shown in Fig. 3. As shown in this example, much information concerning the observed dy-namic material behavior is available from simple photo-graphic instrumentation.

    1 H. Kolsky, Proc. Phys. Soc. (London) 62B, 676 (1949). 2 C. J. Maiden, and S. J. Green, J. Appl. Mech. 33, 496 (1966). H. C. Heard, Geol. Soc. Am. Mem. 79, 193 (1960).

    Improvement of Instrumentation Leakage Currents in a Low Temperature Cryostat

    DONALD R. ZRUDSKY AND WILLIAM R. SAVAGE

    Department of Physics and Astronomy, The University of Iowa, Iowa City, Iowa 52240

    (Received 4 March 1968)

    IN the course of conducting low temperature specific heat and similar measurements it is often necessary to make

    electrical measurements of voltages developed at high impedance sources. In many situations of this type a serious electrical current leakage problem arose at the cryostat seals which was solved by a simple procedure of use in similar cases.

    Instrumentation wires had to be soldered to glass-to-metal insulated vacuum seals which were mounted in place on the top of the cryostat because of other construction constraints. Most likely, the solder flux residue was highly sensitive to surface moisture, as indicated by a humidity sensitive electrical resistance between the lead wires and the cryostat. High current leakage between the leads and the cryostat was observed although the seals appeared dry and free of condensed water. The usual cleaning techniques for removal of the residue of the neutral (nonacid) flux were without significant effect.

    Our solution was to enclose the external surface of the flange carrying the vacuum seals by attaching a simple hermetically sealed metal junction box to it. The junction box served to mount standard connectors for the electric instrumentation leads. The box was large enough to permit heat stationing of the wires to a large block of copper. By placing a small pan or Petri dish of desiccant, such as calcium perchlorate, inside of this box, surface moisture was effectively controlled and the electrical resistance remained high. If the box remains sealed the drying agent is effective for many months. Periodic checks of electrical resistance indicates the time to replace the desiccant. In addition, the junction box served as an ac electrical shield for the instrUmentation wires.

    In our case, leakages typically less than 106 n with the outer surfaces of vacuum seals exposed to atmosphere were raised to greater than 1010 n by this type of humidity control. This approach proves particularly valuable when an experiment at liquid helium temperatures is in pro-gress with the top of the cryostat cool enough to condense water, which is possible even on a relatively low humidity day.

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