Abstract:
A test fixture for simultaneously testing two material test samples is provided. The fixture provides substantially equal shear and tensile stresses in each test specimens. By gradually applying a load force to the fixture only one of the two specimens fractures. Upon fracture of the one specimen, the fixture and the load train lose contact and the second specimen is preserved in a state of upset just prior to fracture. Particular advantages of the fixture are (1) to control the tensile to shear load on the specimen for understanding the effect of these stresses on the deformation behavior of advanced materials, (2) to control the location of fracture for accessing localized material properties including the variation of the mechanical properties and residual stresses across the thickness of advanced materials, (3) to yield a fractured specimen for strength measurement and an unfractured specimen for examining the microstructure just prior to fracture.

Description:
GOVERNMENT RIGHTS 
       [0001]    This invention was made with government support under Contract No. DE-AC05-00OR22725 awarded by the U.S. Department of Energy. The government has certain rights in the invention. 
     
    
     FIELD 
       [0002]    This disclosure relates to the field of material strength testing. More particularly, this disclosure relates to tests for shear and tensile strength. 
       BACKGROUND 
       [0003]    The assessment of advanced materials often benefits from an analysis of stresses that result from different operational conditions. Often these stresses are induced by a combination of shear forces and tensile forces. Often these stresses are different in different portions of articles made from such materials. Current test fixtures typically are not able to assess various combinations of these characteristics. What are needed therefore are improved test fixtures for assessing the mechanical strength of materials. 
       SUMMARY 
       [0004]    The present disclosure provides a fixture for imposing tensile and shear stresses on a material. The fixture includes a first block disposed adjacent a base surface. The first block has a first test specimen mounting portion with a first attaching mount for receiving a first test specimen. The first block also has a second test specimen mounting portion that is disposed distal from the first test specimen mounting portion. The second test specimen mounting portion has a second attaching mount for receiving a second test specimen. The first test specimen mounting portion and the second test specimen mounting portion are substantially immovably fixed with respect to each other. There is a second block disposed between the first test specimen mounting portion and the second test specimen mounting portion. The second block has a third test specimen mounting portion having a third attaching mount for receiving the first test specimen and a fourth attaching mount for receiving the second test specimen. When the first test specimen is disposed in the first attaching mount and the third attaching mount and the second test specimen is disposed in the second attaching mount and the fourth attaching mount the second block is disposed offset from the base surface, and the second block moves with respect to the first block when a force is applied to the second block. 
         [0005]    Also provided is a method of imposing near fracture tensile and shear stresses in a material test specimen. The method includes a step of loading two substantially identical test specimens in a test fixture that is configured to impose substantially identical shear and tensile stresses in each test specimen. The method also includes a step of loading the two test specimens with substantially identical shear and tensile stresses until only one of the test specimens fractures thereby allowing for the examination of the unfractured specimen just prior to fracture. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    Various advantages are apparent by reference to the detailed description in conjunction with the figures, wherein elements are not to scale so as to more clearly show the details, wherein like reference numbers indicate like elements throughout the several views, and wherein: 
           [0007]      FIG. 1  is a somewhat schematic front view of a controlled shear/tension test fixture. 
           [0008]      FIG. 2  is a somewhat schematic top view of the test fixture of  FIG. 1   
           [0009]      FIG. 3  is a somewhat schematic perspective view of a portion of the test fixture of  FIG. 1  and  FIG. 2 . 
           [0010]      FIG. 4  is a somewhat schematic perspective view of a portion of a test fixture. 
           [0011]      FIG. 5  is a somewhat schematic perspective view of a portion of a test fixture. 
           [0012]      FIG. 6  is a force vector diagram illustrating tensile and shear stresses induced in a test specimen. 
           [0013]      FIG. 7  is a somewhat schematic view of a controlled shear/tension test fixture. 
           [0014]      FIG. 8  is a somewhat schematic top view of the test fixture of  FIG. 7 . 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings, which form a part hereof, and within which are shown by way of illustration the practice of specific embodiments of fixtures for imposing tensile and shear stresses on a material. It is to be understood that other embodiments may be utilized, and that structural changes may be made and processes may vary in other embodiments. 
         [0016]    For many advanced materials the ability to control the location of shear fracture enables a systematic study of the effects of varying compositions, microstructures, and residual stresses on the yield and fracture strengths of such materials. Also, it is often very useful to examine the state of the materials just prior to fracture. This is especially true for non-ductile materials such as glass or ceramics.  FIG. 1  illustrates a front view of a test fixture that is useful for these purposes. The fixture  10  includes a first block  20  that is disposed adjacent a base surface  30 . In the embodiment of  FIG. 1 , the base surface  30  is a surface on a test platform  40 . In other embodiments the base surface  30  may be a surface on the first block  20 . 
         [0017]    The first block  20  includes a first test specimen mounting portion  50  with a first attaching mount  60  for receiving a first test specimen  70 . The first block  20  also includes and a first reference surface  80  that is disposed at a first angle  90  to the base surface  30 . Typically the first angle (e.g., the first angle  90 ) is an acute angle. The first block  20  also includes a second test specimen mounting portion  150  that is disposed distal from the first test specimen mounting portion  50 . The second test specimen mounting portion  150  has a second attaching mount  160  for receiving a second test specimen  170  and a second reference surface  180  that is disposed at a second angle  190  to the base surface  30 . In the embodiment of  FIG. 1  the second angle  190  is supplementary to the first angle  90 . (That is, the sum of the first angle  90  in degrees plus the second angle  190  in degrees equals 180 degrees.) In the embodiment of  FIG. 1  the first angle  90  is an acute angle and the second angle  190  is an obtuse angle. In some embodiments the first angle  90  and the second angle  190  may each be substantially 90° angles. In the embodiment of  FIG. 1  the first reference surface  80  and the second reference surface  180  are substantially straight, flat surfaces; in other embodiments the first reference surface  80  and the second reference surface  180  may not be straight, flat surfaces. In most embodiments the first test specimen mounting portion  50  and the second test specimen mounting portion  150  are substantially immovably fixed with respect to each other. 
         [0018]    The test fixture  10  also has a second block  200  that is disposed between the first test specimen mounting portion  50  and the second test specimen mounting portion  150 . The second block  200  has a third test specimen mounting portion  210 . The third test specimen mounting portion  210  has a third attaching mount  220  for receiving the first test specimen  70  and a fourth attaching mount  230  for receiving the second test specimen  170 . The third test specimen mounting portion  210  also has a third reference surface  240  that is disposed adjacent the first reference surface  80  at substantially the first angle  90  from the base surface  30  and a fourth reference surface  250  that is disposed adjacent the second reference surface  180  at substantially the second angle  190  from the base surface  30 . The combination of the two reference surfaces  80  and  240  is referred to herein as a first tilt interface  260 , and the combination of the two reference surfaces  180  and  250  is referred to herein as a second tilt interface  270 . In the embodiment of  FIG. 1  the third reference surface  240  and the fourth reference surface  250  are substantially straight, flat surfaces; in other embodiments the third reference surface and the fourth reference surface may not be straight, flat surfaces. In some embodiments the first test specimen  70  has a first longitudinal axis  280  and the first longitudinal axis  280  is disposed normal to the first tilt interface  260 . In preferred embodiments the second test specimen  170  has a second longitudinal axis  290  and the second longitudinal axis  290  is disposed normal to the second tilt interface  270 . 
         [0019]    In the embodiment of  FIG. 1 , the first test specimen mounting portion  50  and the second test specimen mounting portion  150  are formed as two congruent right-angle trapezoidal structures at opposing ends of the fixture  10 . The third test specimen mounting portion  210  is formed as a single isosceles trapezoidal structure. The first attaching mount  60 , the second attaching mount  160 , the third attaching mount  220 , and the fourth attaching mount  230  allow for two test specimens (test specimens  70  and  170 ) to be loaded simultaneously. 
         [0020]    When the test specimens  70  and  170  are mounted in the fixture  10 , the second block  200  protrudes slightly above the first block  20  as shown in  FIG. 1 . This facilitates the process of applying a downward loading force  300  on the second block  200  only. The second block is disposed slightly above (by offset  310 ) the base surface  30 . When the downward loading force  300  is applied normal to the base surface  30 , and the first angle  90  and the second angle  190  are supplementary angles, the first test specimen  70  and the second test specimen  170  each receive equal tensile and shear forces until one or both specimens fracture. If the first test specimen  70  and the second test specimen  170  are substantially identical in composition and geometry, fracturing may be limited to only one of the two specimens by gradually increasing the downward loading force  300 . Upon fracture of the one specimen, the second block  200  and the load train lose physical contact so no further force is applied to the un-fractured specimen. This has the effect of removing the load force prior to fracture of the test specimen that is not fractured. Thus the un-fractured specimen is preserved in a state of upset that exists just prior to fracture. 
         [0021]    A further beneficial characteristic of the fixture  10  is that during the process of applying the downward loading force  300 , gaps are established between the third test specimen mounting portion  210  and the first test specimen mounting portion  50  and between the third test specimen mounting portion  210  and the second test specimen mounting portion  150 . Consequently there is no contact (or friction) between these components. This is important because it ensures that the downward loading force  300  is applied totally on the two test specimens. Hence, while the fractured specimen gives the strength data, the un-fractured specimen can be used to study the microstructure just prior to fracture. 
         [0022]      FIG. 2  illustrates a top view of the fixture  10 . In the embodiment of  FIGS. 1 and 2 , the first block  20  is fabricated with a backing plate  320 . The first test specimen mounting portion  50  and the second test specimen mounting portion  150  are fabricated from two elements that are attached to the backing plate  320  at joint lines  330 . In other embodiments the first block  20  may be machined or cast or otherwise formed as a single piece of material. However, removably attaching a first test specimen mounting portion (e.g., the first test specimen mounting portion  50 ) and a second test specimen mounting portion (e.g., the second test specimen mounting portion  150 ) to a backing plate (e.g., the backing plate  320 ) is preferred because it facilitates removal of the test specimens  70  and  170  after testing. Such removable attachment may, for example, be accomplished with attachment means such as bolts, studs, or clamps. 
         [0023]      FIG. 3  illustrates a perspective view of the first block  20 . For simplicity of illustration the first attaching mount  60  and the second attaching mount  160  are not depicted.  FIG. 3  illustrates a further view of the first reference surface  80  and the second reference surface  180 .  FIG. 3  also illustrates a recessed area  340  in the first block  20  and the first reference surface  80  and the second reference surface  180  are disposed in the recessed area  340 . 
         [0024]      FIG. 4  illustrates a perspective view of a first block  20 ′. The first block  20 ′ is similar to the first block  20  of  FIGS. 1 ,  2 , and  3 , except that the first block  20 ′ has a first attaching mount  60 ′ that is a slightly different shape than the first attaching mount  60  of the first block  20 , and the second attaching mount  160 ′ of the first block  20 ′ is a slightly different shape than the second attaching mount  160  of the first block  20 .  FIG. 4  illustrates how the first attaching mount  60 ′ comprises a first test specimen mounting recess  64 ′ in the first test specimen mounting portion  50 ′ and the second attaching mount  160 ′ comprises a second test specimen mounting recess  164 ′ in the second test specimen mounting portion  150 ′. 
         [0025]      FIG. 5  illustrates a perspective view of a second block  200 ′. The second block  200 ′ is similar to the second block  200  of  FIGS. 1 ,  2 , and  3  except that the second block  200 ′ has a third attaching mount  220 ′ that is a slightly different shape than the third attaching mount  220  of the second block  200 , and the second block  200 ′ has a fourth attaching mount  230 ′ that is a slightly different shape than the fourth attaching mount  230  of the second block  200 . The third attaching mount  220 ′ has a third test specimen mounting recess  224 ′ in the third test specimen mounting portion  210 ′ and the fourth attaching mount  230 ′ has a fourth test specimen mounting recess  234 ′ in the third test specimen mounting portion  210 ′. 
         [0026]    The first attaching mount  60 , the first attaching mount  60 ′, the second attaching mount  160 , the second attaching mount  160 ′, the third attaching mount  220 , the third attaching mount  220 ′, the fourth attaching mount  230 , and the fourth attaching mount  230 ′ are referred to as “half-dog-bone shaped mounting recesses.” Half-dog-bone shaped mounting recesses are characterized as having a neck portion (e.g., neck portion  226 ′ in  FIG. 5 ) adjacent a reference surface (e.g., third reference surface  240 ′ in  FIG. 5 ) and a flared portion (e.g., flared portion  228 ′ in  FIG. 5 ). In other embodiments, other attaching mount shapes for other test specimen geometries may be used. The half-dog-bone-shaped recesses of the first attaching mount  60 , the second attaching mount  160 , the third attaching mount  220 , and the fourth attaching mount  230  are generally preferred over the first attaching mount  60 ′, the second attaching mount  160 ′, the third attaching mount  220 ′, and the fourth attaching mount  230 ′ because the curved shape of the neck portions of the first attaching mount  60 , the second attaching mount  160 , the third attaching mount  220 , and the fourth attaching mount  230  reduce stress concentration in the test specimens. 
         [0027]    Referring again to  FIG. 1 , upon applying a downward loading force  300  on the top surface of the second block  200 , a downward motion of the central block results in a shear displacement parallel to each of the two tilt interfaces  260  and  270 . Fracture of one of the test specimens is expected to occur either between the first reference surface  80  and the third reference surface  240  or between the second reference surface  180  and the fourth reference surface  250 . Because the location of the shear fracture is substantially controlled by the fixture (e.g. fixture  10  of  FIG. 1 ) test specimens may be fabricated to cause fracturing at a specific area of interest in a test material, such as at a reinforced region, or at a weak region, or at another structural transition zone in a material, or at various locations subjected to different residual stresses. This is very useful for assessing localized material properties. 
         [0028]    As illustrated in  FIG. 6 , the resolved forces on each test specimen are determined by the inclined angles  90  and  190 , where φ is the angle  90 . For an applied downward loading force  300  of 2 P, the resolved tensile and shear loads on the specimen are P cos φ and P sin φ, respectively, as shown in  FIG. 6 . Different ratios of the tensile to shear load on the specimen may be achieved by varying the inclined angle, φ (i.e., angle  90 ) of the tilt interface. Thus, this fixture is particularly beneficial in that it can impose different stress ratios on the specimen to characterize the mechanical behavior of materials. Note that varying the included angle φ (i.e., angle  90 ) of the tilt interface results in a corresponding variation in the angle  190 , because the first angle  90  and the second angle  190  are supplementary. 
         [0029]    A further embodiment of a controlled shear/tension fixture is the test fixture  410  depicted in  FIG. 7  (front view) and  FIG. 8  (top view). In this embodiment the inclined angle, φ ( 490 ) is 90°, and the test specimens  470  and  570  are subjected to shear force only. In the embodiment of  FIG. 7 , the test specimens  470  and  570  have a simple rectangular shape. 
         [0030]    The fixture  10  of  FIGS. 1 and 2  and the fixture  410  of  FIG. 7  may be used to perform a method of imposing near fracture tensile and shear stresses in a material test specimen. This may be accomplished by loading two substantially identical test specimens in a test instrument such as fixture  10  or fixture  410  that is configured to impose substantially identical shear and tensile stresses in each specimen. Then the two test specimens may be loaded with substantially identical shear and tensile stresses. However, due to either intentional design considerations or variability due to manufacturing tolerances, the two specimens will have cross sectional areas that are not exactly identical. In general, the specimen with the smaller cross sectional area will fail first and the other specimen remains un-fractured. The test specimen that did not fracture may then be examined for the effects of near fracture tensile and shear stresses. 
         [0031]    In summary, embodiments disclosed herein describe various test fixtures for assessing the mechanical strength of materials where both shear stresses and tensile stresses may be applied simultaneously to test specimens. The foregoing descriptions of embodiments have been presented for purposes of illustration and exposition. They are not intended to be exhaustive or to limit the embodiments to the precise forms disclosed. Obvious modifications or variations are possible in light of the above teachings. The embodiments are chosen and described in an effort to provide the best illustrations of principles and practical applications, and to thereby enable one of ordinary skill in the art to utilize the various embodiments as described and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.