Abstract:
An apparatus and method for calibrating high temperature strain gases which serve for both dead weight and constant deflection measurements. A cantilever support arm allows the test unit to slide into a furnace while one end is subjected to bending strain either by hanging weights upon it or by deflecting it with a push rod. The dual nature of the fixture permits both tests to be run without change of the test specimen or removal from the furnace.

Description:
ORIGIN OF THE INVENTION 
     The invention described herein was made by an employee of the United States Government and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefor. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to strain gage testing and specifically to an apparatus for calibrating strain gages at high temperatures. 
     An established method of calibrating and evaluating strain gages is to mount the gage on a standard test specimen of material whose characteristics are well documented and to thereby check the gage readings against predicted results based on calculations of the known characteristics of the material. Most test fixtures limit the mode of test of the standard specimen to tension for dead load tests and to bending for constant deflection tests. The dead load test is one in which the standard specimen is stressed in tension by adding weights to a fixture which causes elongation. The predictable elongation of the standard specimen is then compared to the reading taken from the strain gage to establish a calibration curve for the gage. Under the typical system of tension calibration, however, the gage calibration cannot be directly compared to results attained with an identical gage tested in a bending mode. 
     It is also of considerable interest to calibrate strain gages in what is termed a constant deflection mode. This is particularly of interest in systems where the gages and test specimens are subjected to varying temperatures. In such calibration system the standard specimen is deflected a fixed amount in a bending mode, usually by an inclined plane wedge, and both the specimen and the strain gage are subjected to heat. The known characteristics of expansion and other thermal effects of the standard specimen can be appropriately discounted in calibrating the changes of strain gage readings with thermal changes. 
     Since conventional tension testing requires a different specimen than constant deflection testing, errors are caused by the difference in strain patterns which result from different standard specimen configurations. Moreover, the different standard specimen configuration requires twice as many standard specimens to be used and requires different test fixtures for each test. 
     It is therefore an object of the present invention to provide an improved test fixture capable of subjecting a single test specimen to both dead weight and constant deflection testing. Another object of the invention is to facilitate the thermal cycling of the test specimen and strain gage while subjecting the test specimen to alternate testing in the dead weight and constant deflection mode. Still another object of the invention is to provide a direct means of measuring the amount of deflection to which the specimen is subjected in the constant deflection mode. 
     SUMMARY OF THE INVENTION 
     These and other objects of the invention are achieved by the use of a test fixture which supports the test specimen from one end of a cantilever arm and operates on the other end of the test specimen to subject it to dead weight and constant deflection forces in the bending mode. 
     In a typical arrangement the cantilever support arm has a cut-out center portion so that the test specimen may be bolted to the outboard end of the arm and yet have considerable motion at the inboard end of the support arm with no mechanical interference throughout the length of the test specimen. The cut-out portion also allows the combination of the support arm and test specimen to display minimum cross section both in height and width and thus permits the combination to slide axially into a tubular furnace chamber of minimum size. 
     At the inboard end of the support arm, the arm is connected to a vertical front plate containing a hole which matches the support arm cut-out and permits a loading adapter bolted to the free end of the test specimen to protrude through the front plate. The loading adapter is the part upon which the actual test forces operate. A weight hanger is hung around the end of the loading adapter and hangs below the support structure for ease of addition or removal of weights for dead weight testing. The loading adapter is also subject to pressure from above by a push rod operated by a lead screw and vernier arrangement. This push rod thus can apply constant deflection in exactly the same direction as the dead weight test, eliminating any need for interpretation of calibration results due to different force actions during the two test modes. 
     The loading adapter length and thermal insulators placed on the front plate and within the length of the loading adapter supply thermal isolation between the furnace and the load force systems. This isolation permits the operating mechanisms to remain below 80°F. while the test specimen and attached gages are operating as high as 1600°F., thus allowing alternate application of each force mode to the specimen at each temperature under study with safety and convenience. A complete set of data can therefore be secured during a single cycling of the furnace. Because the cycling time of a high temperature furnace is so great, the present invention thereby yields dramatic savings in testing time. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partial cross-sectional view of the arrangement of parts in a preferred embodiment of the invention; 
     FIG. 2 is a perspective view of the test specimen and cantilever support arm portion of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Structure of the Preferred Embodiment 
     The preferred embodiment selected for illustration is shown in FIG. 1. Test fixture 10 is held in place upon base 12 by upright support 14 which is connected to base 12 by slidable joint 16 which extends the entire length of base 12. Slidable joint 16 permits the cantilever portion 18 of fixture 10 to be inserted into tubular area 20 of the furnace 22 by simply pushing test fixture 10 along the base 12 to which furnace 22 is rigidly attached by vertical support 24. Heat for furnace 22 is furnished by any conventional means. Support 14 is held in proper alinement with tubular area 20 by attached block 26 which rides within groove 28 in base 12. 
     Support box 30, attached to upright support 14, is an open-sided structure assembled from top plate 32, bottom plate 34, back plate 36, and front plate 38. Cantilever support arm 40, which is attached to front plate 38 extends outward a distance somewhat greater than the length of test specimen 42 to which are bonded gages 44 to be evaluated. Test specimen 42 is attached to support arm 40 by bolts 46 at the outboard end 47 and is held completely clear of side 48 of support arm 40. The center portion of support arm 40 is cut out to a sufficient width to permit test specimen 42 to flex downward without interference. Loading adapter 50 (FIG. 2) bolted to test specimen 42, by bolts 52 extends through hole 55 in front plate 38 to the center of support box 30. 
     As shown in FIG. 2, loading adapter 50 is constructed by bonding porcelain thermal insulator 54 between adapter section 53 and loading rod 56 which is a rod structure with locating groove 58 cut into it to permit weight hanger 60 to hang loosely upon loading adapter 50 without sliding off. The thermal insulation and the length of insulation 54 furnish sufficient isolation between the furnace heat and support box 30 to prevent damage due to excess heat transfer to the load applying mechanism and to permit safe operation. Weight hanger 60 hangs downward from loading adapter 50 through hole 62 in bottom plate 34 to permit ease of replacement of weight 64 for operation in the dead weight mode of testing. 
     Deflection assembly 66 functions for the constant deflection mode of operation to drive push rod 68 against loading adapter 50 and thus deflect test specimen 42 by a prescribed amount. Deflection assembly operates as a conventional threaded drive wherein turning knob 70 causes threaded rod 72 to move deflection plate 74 with a mated threaded hole 75 and attached push rod 68 in a direction parallel to the axis of threaded rod 72. Vernier scale 76 attached to deflection plate 74 and scale 78 attached to fixed plate 80 act together as a vernier measurement system to provide accurate indications of the amount of deflection to which the test specimen is subjected. 
     During calibration of strain gage 44 and subsequent use of the apparatus to test various material specimens, gage 44 is attached by leads 82 to strain gage balancing circuit 84. Balancing circuit 84 is attached to the back side of support 14 and moves along with test fixture 10 so that no disconnection of electrical circuits is required at any time during the test procedure. 
     To furnish protection for the loading mechanisms, balancing circuit, and the test operator from the heat of the furnace, various thermal insulators are attached to the assembly. Front insulator 86 is attached to front plate 38 on the side facing furnace 22. When cantilever support arm 40 is placed into furnace 22, insulator 86 is pushed tightly against furnace 22 to form the basic heat closure. Hole 88 in the center of insulator 86 is made just large enough to give proper clearance for the movements of loading adapter 50. Thermal insulators 90, 92, 94, and 96 are attached, respectively, to inner surfaces of back plate 36, top plate 32, bottom plate 34, and front plate 38 to further protect the loading mechanisms from the furnace heat. 
     OPERATION OF THE PREFERRED EMBODIMENT 
     Before mounting test specimen 42, loading adapter 50 is attached by bolts 52 to the end farthest removed from the part to which gages 44 have been bonded. Turning knob 70 is then backed off to move push rod 68 as far back as possible and test specimen 42 is attached to cantilever support arm 40 at outboard end 47 by use of bolts 46. Lead wires 82 are then connected between strain gages 44 and balancing circuit 84. Test fixture 10 is then pushed into the test position with cantilever support arm 40 and test specimen 42 within furnace 22, and insulator 86 tightly against furnace 22. Furnace 22 is then raised to each temperature of interest and three tests are run at each temperature setting. 
     A no/load, no/deflection test is run to test for changes in gage characteristics due to temperature alone. For this test, weight hanger 60 is removed and push rod 68 is backed off. Balancing circuit 84 is then read to establish the output of gages 44 and further tests on electrical resistance to the test fixture are also performed. 
     For the dead weight test, weight hanger 60 is installed upon loading adapter 50 and various predetermined weights are substituted for weight 64. Balancing circuit 84 is read for each different weight applied. 
     For the constant deflection test it is first necessary to establish a &#34;zero&#34; reference point at approximately 5% of total expected deflection. Vernier scale 76 is read at this point and this reading is assumed to be the zero reading. Test points are then established at various measures of deflection which are of interest and the balancing circuit is read at each point. Repetition of the same readings on the vernier scale when the test specimen is at different temperatures will repeat the same deflection of the test specimen. 
     Each of these three tests, the no/load test, the dead weight test and the constant deflection test is performed separately at each temperature setting of the furnace. The long time required to reach a stable temperature in the typical furnace makes the ability to accurately repeat each test at each temperature a distinct advantage in that only one progressive run through the temperature points need be made rather than three. 
     It is to be understood that the forms of the invention herein shown are merely preferred embodiments. Various changes may be made in the shape, size, or arrangement of parts; equivalent means may be substituted for those illustrated and described and certain features may be used independently from other features without departing from the spirit and scope of the invention. For example, other deflection measurement means such as an electrical slidewire or potentiometer may be used or other heat sealing means may be used instead of insulator 86 attached to front plate 38.