Abstract:
An apparatus and method for use in testing devices under high-g environments is disclosed in which an elastic beam, rigidly fastened at least at one end, carries the device under test; the beam being pre-loaded to a bent position by a force producing member which may be suddenly removed to allow the stored energy of the beam to be released, and to apply a high-g force to the device.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to the field of shock testing and more particularly to an apparatus and method for subjecting a test specimen to a high-g shock in the laboratory to simulate the conditions the specimen might encounter in actual use.  
           [0003]    2. Description of the Prior Art  
           [0004]    Of course, a test specimen may be tested at a Proving Ground under substantially identical conditions as will be encountered in actual use. However, the cost of transporting the specimen to the Proving Ground is very high and only one or two tests per day can be performed. Accordingly, it is desired that a laboratory test be provided so as to minimize expense and increase convenience, so that several tests per day can be performed.  
           [0005]    Laboratory shock testing apparatus, utilizng a device identified as a Hopkinson bar is known in the art. Such apparatus is described in a paper entitled “ The Use of a Beryllium Hopkinson Bar to Characterize a Piezoresistive Accelerometer in Shock Environments ” presented by Vesta I. Bateman, Fred A. Brown, and Neil T. Davis of Sandia National Laboratories in Albuquerque, N. Mex., in the 1996  Proceedings of the Institute of Environmental Sciences  on pages 336-343. This prior system employs a Hopkinson bar, i.e., a perfectly elastic homogeneous bar of constant cross-section which has first and second end surfaces substantially perpendicular to the length. A test specimen, in this case an accelerometer, is mounted on the surface at the first end and the bar is then impacted on the surface at the second end by a projectile that is fired by an air gun down a long tube to produce a shock wave that travels the length of the bar and applies a high-g force to the specimen. The prior art has several disadvantages, among which is the fact that to produce a sufficiently high-g force, a relatively large projectile traveling at high speed must be used (the higher the force desired, the greater the size and/or speed of the projectile). This requires a significantly long tube (e.g., up to 40 feet) for the projectile to reach the desired speed necessary to produce a high-g force (for example, say above 10,000). Thus, a great deal of laboratory space needs to be provided, which is costly and inconvenient. Furthermore, the greater size and speed of the projectile introduces greater danger in performing the test. Also, the period or duration of the high-g shock varies inversely with the amplitude of the shock (i.e., the higher the g-force desired, the shorter the duration of the shock; usually considerably less than one second).  
         SUMMARY OF THE INVENTION  
         [0006]    The present invention uses step relaxation of a very stiff spring to attain the high-g levels. The spring, a fixed beam which, in a preferred embodiment comprises an I-beam of high strength aluminum, is used to mount the test specimen. The beam is put under high strain, such as by applying a large force, tending to bend the beam to near capacity (i.e., its yield point), and then the stored energy in the beam is released by suddenly removing the force to produce a high-g shock that has a significantly long duration (for example about one second) which is independent of the magnitude of the shock. Furthermore, in the present invention it is simple, inexpensive, and does not require excessive laboratory space. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    [0007]FIG. 1A shows a side view of the beam with a specimen attached;  
         [0008]    [0008]FIG. 1B shows the beam of FIG. 1A with an applied force bending the beam upwardly;  
         [0009]    [0009]FIG. 1C shows the beam of FIG. 1B with the applied force suddenly removed;  
         [0010]    [0010]FIG. 1D shows the beam of FIG. 1C under a high-g shock;  
         [0011]    [0011]FIG. 2A shows an alternate arrangement of the beam of FIG. 1 with an applied force bending the beam downwardly;  
         [0012]    [0012]FIG. 2B shows the beam of FIG. 2A with the applied force suddenly removed; and,  
         [0013]    [0013]FIG. 2C shows the beam of FIG. 2B under a high-g shock. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0014]    In FIG. 1A, a beam  10  of high strength aluminum, which preferably is shaped in the form of an I-beam, is shown rigidly connected at both ends to a solid structure shown by the cross hatched portions  12  and  14 . High strength aluminum has been chosen in the preferred embodiment because of its high yield point (i.e., its ability to flex without permanent deformation), its low cost, and the ease with which it may be machined. Alternately, titanium and other high-yield-point materials may be used but generally at a higher cost. An I-beam configuration is used to provide strength and store energy with as little weight as possible. In general, the greater the weight, the less amplitude of energy stored.  
         [0015]    In FIG. 1A, a specimen  18  to be tested, which may be any of a variety of devices such as a printed circuit, an accelerometer, or a gyroscope, is fastened to the middle of the beam  10  for purposes to be explained below. The specimen  18  is connected to test apparatus  20  by connectors such as wires  22  and  24  to record or monitor the effects of the high-g test.  
         [0016]    In FIG. 1B, the apparatus of FIG. 1A is repeated with the same reference numerals and, in addition, a force, or pre-load producing device  26  which may be a hydraulic ram, is shown connected by a member  28  to produce an upwardly directed force shown by an arrow  30 . Member  28  is preferably a frangible material with high compression strength, such as a ceramic, to allow sudden fracture. In the preferred embodiment, member  28  is provided with protective ends  32  and  34  to apply the force over a larger area, to help prevent the formation of indentations in the aluminum beam  10 . The beam  10  is bent upwardly by an amount depending on the g-force required, but in no event past the yield point.  
         [0017]    In FIG. 1C, the apparatus shown in FIG. 1B is repeated with the same reference numerals but, in FIG. 1C, a projectile or other shattering device shown by an arrow  40  is depicted as breaking or shattering the member  28  so that beam  10  is suddenly allowed to spring back downwardly, producing a high-g shock wave applied to the specimen  18 .  
         [0018]    This action is depicted in FIG. 1D where the specimen is shown moving down and up until it is quickly damped to a standstill, as in FIG. 1A. The high-g force, the maximum of which occurs during the first full cycle, is in the form of a damped sinusoid. If it was desirable to change the damping characteristics of the system, some damping member, such as a dash pot, might be attached to the beam  10 . The projectile or shattering device may be relatively small, and may be propelled by a pneumatic device and a relatively short coiled tube (not shown). Since the projectile does not impart the shock wave to the bar, its size and speed need only be great enough to shatter the ceramic. This minimizes the danger and space requirements of the prior art. The application of a high-g force requires a relatively sudden release of beam  10 , and the magnitude of the force may be adjusted using different amounts of bending for various requirements dictated by the specimen  18 . The specimen  18  is shown attached near the center of beam  10  so that the g-force is directed primarily upwardly, and secondary g-forces in other directions are minimized. This is especially desirable for testing inertial devices.  
         [0019]    FIGS.  2 A-C show an alternative embodiment where the force applied to the beam is downwardly, rather than upwardly. FIG. 2A shows the same structure with the same reference numerals as in FIG. 11B except that a pre-load device  50 , which again may be a hydraulic ram positioned to operate in the opposite direction, is shown pulling beam  10  downwardly, as shown by arrow  52  through a member  54 . Since ceramic does not have good tensile strength, a material such as steel may be used, and a separation device such as a sheared cable or, preferably, an explosive bolt  56  detonated by an electric signal through wire  58 , may be used to cause the sudden disconnect of member  54 .  
         [0020]    In FIG. 2B, the explosive bolt  56  of FIG. 2A has been activated, which suddenly separates member  54  to allow beam  10  to move upwardly and apply the high-g force to the specimen. Although an explosive bolt  56  is shown, other sudden separation devices that can cause the rupture of the member  54  may also be used.  
         [0021]    [0021]FIG. 2C is the same as FIG. 1D and shows the beam  10  and specimen  18  moving up and down until brought to a standstill by the damping.  
         [0022]    While in the preferred embodiments the beam  10  has been shown rigidly connected at both ends, in some cases connecting the beam  10  at only one end, in cantilever fashion, could be used. In such a case, the movement of the beam  10  would have an angular component that could be tolerated for testing devices that do not require purely linear motion as do most inertial devices.  
         [0023]    In a preferred embodiment, the beam  10  is about 12 inches long and about ¾ inches wide. The yield point is such that bending the beam by about ⅛ inch produces no permanent deformation, and g-forces up to about 17,000 have been produced. Of course, lesser g-forces can be attained by bending the beam less than ⅛ inch and by using different dimensions and different materials. It is also possible to produce forces in excess of 17,000 g with proper choice of materials, dimensions, and bending.  
         [0024]    It is therefore seen that we have provided a simple, inexpensive, and space saving testing device which produces a desired high-g shock force with smaller, less dangerous equipment, that does not depend on a large, high speed projectile to provide the shock. Furthermore, our invention provides a greater duration of shock and is able to vary the amount of g-force produced with a simple bending adjustment that was not available in the prior art.  
         [0025]    Many changes or modification to the invention described herein will occur to those skilled in the art. As mentioned, different materials and different methods of applying the bending force may be substituted, as well as devising different ways of causing the sudden release of the energy in the beam. Accordingly, we do not intend to be limited to the specific structures used to describe the preferred embodiments. The scope of the invention may be determined in accordance with a reasonable interpretation of the appended claims.