Abstract:
A high-g shock-producing device for testing a sample specimen is described which includes a beam and a shock column. The beam is of predetermined length and has at least one end substantially rigidly fixed with the specimen mounted thereon at a position remote from the one end. The shock column is positioned to apply a force causing said beam to bend in a direction transverse to the length. The column is configured to have a buckling failure when exposed to a pressure which is sufficient to bend the beam an amount to provide the desired high-g force to the specimen. The buckling failure causes the force to be suddenly removed from the beam so as to release the beam and produce the high-g shock on the specimen.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates generally to 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 an intended use.  
         [0002]     A test specimen, for example, an accelerometer, may be tested under substantially identical conditions as will be encountered in actual use. One such example is a gun launch test. However, the cost of transporting the specimen to a gun launch test facility and performance of the gun launch testing is very high. In addition, typically it is feasible to conduct only one or two gun launch tests per day. As such, it is economically and logistically beneficial to perform as much laboratory testing as possible, so as to minimize expense and increase convenience, so that many more tests per day can be performed.  
         [0003]     One such laboratory testing apparatus is a bench top high-g shock tester. The shock tester uses a ceramic column to load a beam that is rigidly supported at each end. In one known test setup, the ceramic column is shot out, utilizing a projectile, to release the beam fast enough to cause the beam to resonate and apply high-g loads to any samples attached to the beam. In another known test setup, the ceramic column was replaced with an explosive bolt, which also released the beam fast enough to allow it to resonate.  
         [0004]     Typically aluminum is utilized for the beam due to its low cost and ease of machining. However, some high-g load testing is performed with titanium beams, which can withstand a much higher loading level than the ceramic columns can withstand. The higher loading level also results in an instability of the ceramic columns.  
       BRIEF SUMMARY OF THE INVENTION  
       [0005]     In one aspect, a high-g shock-producing device for testing a sample specimen is provided. The device comprises a beam of predetermined length having at least one end substantially rigidly fixed with the specimen mounted thereon at a position remote from the one end and a shock column. The shock column is positioned to apply a force causing the beam to bend in a direction transverse to its length. The column is further configured to have a buckling failure when exposed to a pressure which is sufficient to bend the beam an amount to provide the desired high-g force to the specimen. The buckling failure of the column causes the force to be suddenly removed from the beam so as to release the beam and produce the high-g shock on the specimen.  
         [0006]     In another aspect, a method of suddenly releasing a beam of a high-g force testing apparatus is provided. The method comprises configuring a shock column with a buckling failure point, the buckling failure point being at a pressure, inserting the shock column between a beam rigidly mounted at least at one end and a pressure producing device, and applying a pressure to the shock column to bend the beam to a desired point, the pressure needed to bend the beam to the desired point being equal to the buckling failure point pressure of the shock column.  
         [0007]     In still another aspect, a shock column for a high-g tester is provided. The shock column comprises a top cap, a bottom cap, and a column portion extending between the top cap and the bottom cap. The column portion is configured to buckle when a specific pressure is applied between the top cap and the bottom cap.  
         [0008]     In yet another aspect, a high-g shock producing device for testing a specimen is provided, The high-g shock producing device comprises a beam having a first end and a second end, and capable of flexing without permanent deformation, rigidly mounted at the ends, and a fastener for mounting the specimen atop the I-beam proximate the center thereof. The device further comprises a shock column comprising a top cap, a bottom cap, and a column portion extending therebetween, and positioned such that the top cap bears against the beam. The shock column portion comprises a feature which causes the column portion to buckle at a specified pressure. The device also comprises a hydraulic ram positioned to produce a directed force on the bottom cap of the column to cause the beam to bend to a position where the pressure is sufficient to cause the column portion to buckle. The buckling of the column portion causes removal of the directed force thereby allowing the beam to suddenly unbend and apply a g-force to the specimen. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  is an illustration of a high-g tester.  
         [0010]      FIG. 2  illustrates oscillations of a beam of the high-g tester of  FIG. 1  after removal of the column under the beam.  
         [0011]      FIG. 3  is an illustration of a column calibrated to withstand a specific load pressure.  
         [0012]      FIG. 4  is an illustration of another column configuration calibrated to withstand a specific load pressure.  
         [0013]      FIG. 5  is an illustration of a third column configuration calibrated to withstand a specific load pressure.  
         [0014]      FIG. 6  is an illustration of a high-g load tester which utilizes the calibrated column of  FIG. 3 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]      FIG. 1  illustrates a high-g shock tester  10 . Tester  10  includes a beam  12  of high strength material, for example, aluminum or titanium, which in one embodiment, is shaped in the form of an I-beam. Beam  12  is shown rigidly connected at both ends  14 ,  16  to a solid structure shown by mounting portions  18  and  20 .  
         [0016]     In one embodiment, high strength aluminum is utilized as a material for beam  12 , 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. In one embodiment, an I-beam configuration is used for beam  12  to provide strength and store energy with as little weight as possible. In general, the greater the weight, the less amplitude of acceleration results.  
         [0017]     A specimen  22  to be tested, which may be any of a variety of devices such as a printed circuit, an accelerometer, or a gyroscope, is fastened utilizing a fastener (not shown) to an approximate middle of beam  12  in preparation of the high-g test of specimen  22 . Specimen  22  is connected to a test apparatus  24  by wires  26  and  28 , or another method of connection, to record or monitor the effects of the high-g test.  
         [0018]     A force, denoted in  FIG. 1  as F, from pre-load producing device  30 , for example, a hydraulic ram or another device capable of providing such a force, is shown as connected to beam  12  by a member  32  to produce an upwardly directed force as shown by the force arrows. Other embodiments are contemplated, for example, where device  30  and member  32  direct a downward force onto beam  12 . Member  32  is preferably a frangible material with high compression strength, such as a ceramic, to allow sudden fracture. In another embodiment member  32  is an explosive bolt, which an operator can activate when a specific pressure onto beam  12  is attained.  
         [0019]     In one embodiment, member  32  is provided with protective ends  34  and  36  to apply the force over a larger area, to help prevent the formation of indentations in beam  12 . As the force is applied, beam  12  is bent upwardly, as shown, by an amount which provides the g-force needed to perform the high-g test, but in no event past the yield point of beam  12 .  
         [0020]     Once the beam is bent the amount needed to perform the high-g test, a projectile  40  or other shattering device is utilized to break or shatter member  32 , as shown by the directional arrow behind projectile  40 . Beam  12  is then suddenly allowed to spring back downwardly, producing the high-g shock wave that subsequently is applied to specimen  22 . At such time, beam  12  is sometimes said to be resonating, or oscillating. Utilization of an explosive bolt results in a similar motion of beam  12  upon activation of the bolt.  
         [0021]     The oscillating action of beam  12  is depicted in  FIG. 2 . Although specimen  22  is not shown, it is understood that when specimen  22  is mounted on beam  12 , specimen  22  is moving down and up with beam  12  until beam  12  quickly damps to a standstill, as does specimen  22 . 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 tester  10 , a damping member (not shown), for example, a dash pot, might be attached to beam  12 . Projectile  40  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 member  32 . The application of a high-g force requires a relatively sudden release of beam  12 , and the magnitude of the force may be adjusted using different amounts of bending for various requirements dictated by the specimen  22 . Specimen  22  is shown attached near a center of beam  12  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 such as gyroscopes and accelerometers.  
         [0022]     However, utilizing a projectile  40  has drawbacks, for example, the testing area should include safety precautions as any projectile should be considered as having an element of danger involved. In addition, when beam  12  is made from titanium, a higher load must be placed on the beam in order for it to flex as described above. The higher loading requirements sometimes cannot be met by ceramic members  32  as currently configured, nor by known explosive bolts.  FIG. 3  illustrates a shock column  50  which is configured to meet the high loading requirements associated with titanium beams. Column  50  has improved stability over columns  32  (shown in  FIG. 1 ).  
         [0023]     As described below, column  50  incorporates features which allow columns  50  to be calibrated to withstand a specific load pressure. The load pressure on a titanium beam directly correlates to shock level applied to a specimen. Column  50  includes a column portion  52 , a top cap  54 , and a bottom cap  56 . Column portion  52  includes a notch  58  formed therein which causes a buckling failure of column portion  52 , and therefore column  50 , and initiates oscillations of a beam at a specific load pressure. The buckling failure of column portion  52  can also eliminate the need to shoot out the ceramic column from under the loaded beam with a projectile as described above.  
         [0024]     As different test specimens are tested at different and various shock levels, a size and depth of notch  58  can be adjusted at manufacture to provide the buckling failure at specific load pressures. In a preferred embodiment, column portion  52  is configured with notch  58  near a center of the span of column portion  52  to a depth calculated to correspond to buckling failure at a specific load level. Top cap  54  and bottom cap  56  are, in alternative embodiments, snug-fitting, threaded, made from a metal, and include a recess  60  into which ends  54  and  56  of column portion  52  are inserted. Utilization of top cap  54  and bottom cap  56  increases vertical stability of column  50  under a load.  
         [0025]     Top cap  54  and bottom cap  56  are shown as having threads  62  which screw onto threaded end portions  64  and  66 , respectively, of column portion  52 . In other embodiments, top cap  54  and bottom cap  56  are configured with deformable, vertical or horizontal ridges, in place of threads, which are press fit onto end portions  64  and  66 . The vertical and horizontal ridges provide a tight fit between column portion  52  and top cap  54  and bottom cap  56 . Deformable vertical or horizontal ridges, also provide an amount of vertical stability for column  50  as a load is applied to beam  12 . For example, the ridges (or threads in the threaded embodiment) are somewhat malleable under the stresses applied to bend a beam, and act to absorb at least a portion of any sideways forces encountered by column  50 .  
         [0026]      FIG. 4  illustrates another embodiment of a shock column  70  which is configured to provide a buckling failure at specific load pressures. Specific components of column  70  which are the same as those described for column  50  (shown in  FIG. 3 ) have the same reference numerals. Column  70  includes a column portion  72  which has a reduced cross-section portion  74 . Reduced cross-section portion  74 , when under the load of forcing a beam to bend, for example, beam  12  (shown in  FIG. 1 ) causes column portion  72  to have a buckling failure when a specific load is reached. Reduced cross-section portions can be made longer, or deeper during manufacture of column portion  72  to provide buckling failures at various load pressures.  
         [0027]      FIG. 5  illustrates still another shock column  90  which is configured to buckle under load pressures. Specific components of column  90  which are the same as those described for column  50  (shown in  FIG. 3 ) have the same reference numerals. Column  90  includes a column portion  92  which has an enlarged cross-section  94  which is greater in diameter than threaded end portions  64  and  66 . The configuration of enlarged cross-section  94  as shown in  FIG. 5  results in a notch portion  96  near each of top cap  54  and bottom cap  56 . Notch portions  96  are a stress point when column  90  is utilized to apply pressure to a beam, as described above, and result in a buckling failure at a specific load level. Enlarged cross-section  94 , and notch section  96  can be configured in different sizes at manufacture of column  92 , to provide failures at different load levels.  
         [0028]      FIG. 6  illustrates a high-g load tester  100  which utilizes shock column  50 , which is described in detail with respect to  FIG. 3 . Specific components of high-g load tester  100  which are the same as those components described for high-g load tester  10  (shown in  FIG. 1 ) as shown utilizing the same reference numerals.  
         [0029]     Load producing device  30  applies a force to beam  12  through shock column  50  to produce an upwardly directed force as shown by the force arrows. As the force is applied, beam  12  is bent upwardly, as shown, by an amount which provides the g-force needed to perform the high-g test. As the force applied by load producing device  30  increases, to the point needed to eventually provide the correct amount of g-force to specimen  22 , the presence of notch  58  within column portion  52  of column  50  causes a fissure  102  to begin to develop in column portion  52 . As fissure  102  develops across column  52 , column portion  52  reaches a breaking point and separates, falling away from load producing device  30  and beam  12 , allowing beam  12  to oscillate and apply the desired g-force to specimen  22 .  
         [0030]     Using shock columns  50 ,  70 , and  90  increases reliability of high-g testing methods as controlling a buckling failure of such columns provides an increased repeatability of the high-g test methods herein described. Further, utilization of shock columns  50 ,  70 , and  90  create a known and predictable failure mode, eliminate one step as compared to the known testing process (firing of a projectile), and increase the safety of the test process by removing an air-pressure propelled projectile from the procedure. In addition, utilization of top cap  54  and bottom cap  56  with deformable ridges or threads which are press fit onto a column portion allow a column assembly to absorb some of the sideways forces such a test setup might experience if a bottom surface of beam  12  or a top surface of load producing device  30  are not exactly parallel to one another.  
         [0031]     While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.