Abstract:
An in-plane shear and multi-axial tension or compression testing apparatus having four-bar linkages pivotable to two sleeves on an opposite vertices with the sleeves of each vertex rotationally attached to each other. Lateral links of each linkage are pivotally attached to load transfer plates in which the plates secure a test specimen. Each linkage is rotatable to the other linkages while the vertices are subjected to a compression or tensile load. The vertices are also capable of rotation by a testing machine for shear testing. During compression or tension of the vertices of the apparatus, the plates respectfully move toward or away from each other thereby applying compression or tension to the specimen. The bars of one linkage can be rotated with respect to the other, thereby applying torsional loading to the specimen.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor. 
    
    
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates to a combined in-plane shear loading and multi-axial tension or compression testing apparatus in which the apparatus is capable of determining the mechanical properties of metals, plastics, woods, fabrics, elastomers and other materials. 
     (2) Description of the Prior Art 
     Plain-woven fabrics are widely utilized as structural materials in air-inflated structures and rapidly deployable structures such as temporary shelters, tents, temporary bridges and space structures. Unlike metallic structures, these structures are primarily designed to be lightweight, self-erecting and deployable to volume-storage ratios that may be 1000-to-1. Air-inflated structures utilize pressurized fabric tubes and pressure-stabilized beams (known as air beams) as load-carrying members. 
     Although, the structures are well-known in the art, the technology for the structures has not been refined such that reliable structures can be analytically designed. Specifically, this analysis has gained in importance due to advancements in the material of the structural fiber and the weaving/braiding of the structural fiber, both of which have improved the load carrying capacity of the structures. Accordingly, there is a recognized need to model the mechanical properties of woven fabrics. 
     Presently, modeling the mechanical properties of woven fabrics results in complex responses because of the complex microstructures of the composite materials used. Unlike traditional composite materials, plain-woven fabrics used in inflated structures exhibit high non-linearity with a dependence on internal pressure and contact interactions within the woven fabric. 
     Accordingly, there is a need for a testing apparatus which measures the elastic and shear moduli of air beams as a function of inflation pressure. To measure the elastic modulus of the fabric, a multi-axial loading has been shown to be preferable and to measure the shear moduli of the fabric; an in-plane shear loading has been shown to be preferable. As such, there is a need for a testing apparatus capable of combining in-plane shear and multi-axial loading. For non-orthogonal composite or fabric materials, such as braids or knits, there is a further need for a testing apparatus capable of loading the specimen in varying non-orthogonal positions. 
     While biaxial testing apparatuses with compression and tension loading or in-plane shear testing apparatuses exist in the prior art, there are no apparatuses that exist with a combined feature of in-plane shear and compression/testing capabilities. Also, a testing apparatus does not exist that is capable of applying non-orthogonal multi-axial loading. 
     Additionally, testing apparatuses of the prior art employ two or more separate actuators in complex test fixtures or pressurization techniques for applying a biaxial load to a test specimen. An apparent disadvantage is the need for two or more loading devices and the associated high cost of equipment. 
     In regard to specific references, Lynch et al. (U.S. Pat. No. 3,776,028) describes an apparatus requiring three independent loading mechanisms. Holt (U.S. Pat. No. 4,192,194) describes an apparatus for biaxial loading of a specimen by pressurizing the inside surface of a cylinder. A restrictive disadvantage of the apparatus is the requirement of the cylindrical shape of the specimen and a high cost associated with pressurization of the cylinder. Additionally, the disadvantages include restriction to orthogonal loads about the axial, hoop and radial directions and an apparatus that is not capable of applying an in-plane shear stress to the specimen. 
     Mathiak et al. (U.S. Pat. No. 5,144,844) describes a cruciform planar specimen for biaxial material testing which has the disadvantage of being limited to use in two loading directions. Ward et al. (U.S. Pat. No. 5,279,166) describes an apparatus for self-alignment of a biaxial loading device. The apparatus requires that the two axial loading directions be orthogonal with a maximum of two loading directions. The apparatus also has no capability for applying an in-plane shear load to the specimen. 
     Tucchio (U.S. Pat. No. 5,448,918) describes an apparatus with an X-shape that is only used for compression load. Clay (U.S. Pat. No. 5,905,205) describes an in-plane biaxial test apparatus comprising linkages to transfer the load to an orthogonal direction of loading. A disadvantage of this apparatus is that it is not capable of applying in-plane shear to the test specimen. Another disadvantage of this apparatus is that the biaxial loading is limited to an orthogonal configuration. 
     As noted above, none of the references are capable of combining the in-plane and compression/tension loading of a specimen while only using one loading system. As such, there exists a need for an apparatus capable of applying a combined in-plane shear and tension/compression load to a specimen. Such an apparatus would be cost-effective due to reduced space and a reduced amount of equipment normally needed for material testing. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a general purpose and primary object of the present invention to provide an apparatus for testing a specimen with a multi-axial in-plane tension or compression loading. 
     A further object of the present invention is to provide an apparatus for applying a non-orthogonal multi-axial tension or compression loading to the specimen. 
     A still further object of the present invention is to provide an apparatus for applying in-plane shear loading to the specimen when the specimen is subjected to multi-axial loading. 
     A still further object of the present invention is to provide an apparatus for employing a loading system compatible with conventional prime mover testing equipment. 
     A still further object of the present invention is to provide an apparatus for applying an unequal and multi-axial loading to the specimen. 
     To attain the objects described, there is provided an apparatus for simultaneous or independent in-plane shear and tension or compression loading of a test specimen such as metals, plastics, composites, woods, fabrics or anisotropic materials. The loading of the test specimen can orthogonal or non-orthogonal. 
     With the apparatus, the uniaxial tensile or compressive  11  load of a test machine can be converted to an equal or unequal stress state on a planar test specimen and an orthogonal or an oblique (non-orthogonal) stress state on the specimen by the use of load transfer systems comprising four-bar linkages movable to define the borders of varying rhombus-shapes. 
     The apparatus also provides flexibility by the ability to apply an unequal stress state or multi-axial load to the test specimen by utilizing load transfer plates of different lengths. Additionally, the apparatus provides flexibility by the ability to apply a non-orthogonal multi-axial loading by utilizing a different angle, other than exact angles between the vertices of the four-bar linkages. The angle of rotation of the linkages between the vertices can be measured directly by the test machine through load cells or other conventional instrumentation. 
     More specifically in structure, the apparatus comprises two four-bar linkages for biaxial testing and capable of the addition of other four-bar linkages for testing along additional axes. The four-bar linkages defining a rhombus-shape are pivotally connected to one another at opposing ends or vertices by sleeve bearings positioned at each vertex. The sleeve bearings at each vertex are axially connected to one another with a pin and thrust bearings between the sleeve bearings  11  thereby allowing the sleeve bearings to rotate freely with respect to one another while connected in the axial (vertical direction). Load transfer plates are pivotally attached to lateral links for each of the four-bar linkages. A securing clamp for the specimen is attached to the distal end of each of the load transfer plates. 
     When testing a specimen, an exposed end (two ends for uniaxial loading, four ends for biaxial loading, six ends for triaxial loading etc.) of the specimen is rigidly secured by the clamp. The vertices of the apparatus are attached to the crossheads of a conventional uniaxial tensile/torsion machine. Upon a movement of the vertices of the linkages toward each other, their lateral links will extend outward thereby increasing the distance between the corresponding load transfer plates of each linkage. This movement applies planar tension on the specimen. Additionally, by rotating one linkage with respect to the other, the specimen will be subjected to the in-plane shear. 
     Similarly, upon movement of the vertices of the two linkages away from each other, their lateral links contract inward; thereby, decreasing the distance between the corresponding load transfer plates of each linkage. This movement applies planar compression on the specimen. Additionally, by rotating one linkage with respect to the other, the specimen will be subjected to the in-plane shear. 
     An added feature of the invention would be affixing a camera or optical recording device to a vertex of the apparatus. Another added feature would be the affixing a draping or puncturing mechanism to a vertex of the apparatus to conduct drape and/or puncture tests on the specimen. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the invention and many of the attendant advantages thereto will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein: 
         FIG. 1  depicts a perspective view of the multi-axial testing apparatus of the present invention with a specimen secured by the apparatus such that the specimen is subjected to multi-axial tension; 
         FIG. 2  depicts a cross-sectional view of a joint assembly of the apparatus of the present invention with the view taken from reference line  2 — 2  of  FIG. 1 ; 
         FIG. 3  depicts a view of the joint assembly of the apparatus of the present invention with an additional set of extending arms for a third linkage in support of tri-axial loading; 
         FIG. 4  depicts a tensile wedge clamp attached to a load transfer plate of  FIG. 1 ; 
         FIG. 5  depicts a compressive wedge clamp attached to the load transfer plate of  FIG. 1 ; 
         FIG. 6  depicts a tongue-and-groove clamp attached to the load transfer plate of  FIG. 1 ; 
         FIG. 7  depicts a pre-tensioning clamp attached to the load transfer plate of  FIG. 1 ; 
         FIG. 8  depicts a side view of the apparatus of the present invention; 
         FIG. 9  depicts a perspective view of the multi-axial testing apparatus of the present invention detailing the measurement system for testing the specimen; and 
         FIG. 10  depicts a top view of the apparatus of the present invention with the view taken from reference line  10 — 10  of  FIG. 1  in which the specimen is positioned for non-orthogonal loading of the test specimen by varying the angle between the vertices of the linkages. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings wherein like numerals refer to like elements throughout the several views, one sees that  FIG. 1  depicts a preferred embodiment of a testing apparatus  10  of the present invention. As shown in the figure, the apparatus  10  for biaxial loading generally comprises four-bar linkages  12  and  14  defining a perimeter of a variable rhombus shape, a first (or as shown) a top joint assembly  16 , a second (or as shown) a bottom joint assembly  18 , load transfer plates  20 ,  22 ,  24  and  26  and an associated strain and displacement measurement system  28 . 
     The linkage  12  includes two pairs of oblong and rigid members  30  and  32 . Each end of each member is rigidly connected to a bracket  34  in which each bracket is pivotally connected to the top joint assembly  16  and the bottom joint assembly  18 . The linkage  14  includes two pairs of oblong and long rigid members  36  and  38 . Each end of each member is rigidly connected to a bracket  40  in which each bracket is pivotally connected to the top joint assembly  16  and the bottom joint assembly  18 . 
     Each of the load transfer plates  20 ,  22 ,  24  and  26 , is pivotally connected to lateral links  42 ,  44 ,  46  and  48  of the linkages  12  and  14  and each secured by a pin  50 . Each load transfer plate  20 ,  22 ,  24  and  26  includes a clamp of a type known to those skilled in the art, either a clamp with a first wedge  52  for tensile loading (described further below), a clamp with a second wedge  54  for compressive loading (described further below), a clamp with a tongue and groove  56  for loading of fabric or similarly flexible materials (described further below) or a clamp  58  with a pre-tensioning roller  59  for loading  11  of fabric or similarly flexible materials (described further below). Each of the clamps secures a test specimen  60  by clamping or attaching to an exposed side of the specimen. 
     Referring now to the cross-sectional view of  FIG. 2 , the top joint assembly  16  comprises a first sleeve  62  and a second sleeve  64 , a first thrust bearing  66 , a second thrust bearing  68 , a connecting rod  70  and a pin  72 . The second sleeve  64  includes apertures  74  at the distal end of its extending arms as pivoting connecting points for the rigid members  36 . Similarly, the first sleeve  62  includes apertures  76  at the distal end of its extending arms as pivoting connecting points for the rigid members  30 . During loading for a test, the height of the apertures  74  and  76  of the top joint assembly  16  are on a horizontal plane  77  in which the horizontal plane is allowed by the second sleeve  64  having upward extending arms  78  and the first sleeve  62  having downward extending arms  79 . As shown in  FIG. 3 , an additional pair of extending arms  80  are positioned on the second sleeve  64  to support a third four-bar linkage (not shown) in which tri-axial loading can be accomplished by load transfer plates of the third four-bar linkage. In this manner, additional linkages can be added to the second sleeve  64  for further multi-axial loading. 
     For the configurations of both figures, the pin  72  restrains the vertical motion of the sleeves  62  and  64 , yet allows rotation of one sleeve with respect to the other. A crosshead  81  of a testing machine (not shown) is rigidly connected to the top joint assembly  16  by the pin  72 . 
     Referring again to  FIG. 1 , the bottom joint assembly  18  is similar to the top joint assembly with the bottom joint assembly comprising a first sleeve  82 , a second sleeve  84 , a first thrust bearing  86 , a second thrust bearing  88  and a pin  90 . An additional pair of extending arms can be positioned on the first sleeve  82 , similar to the positioning of extending arms  80  to support the other end of the third four-bar linkage. In this manner, additional linkages can be added to the first sleeve  82  for further multi-axial loading. 
     Similar to the top joint assembly  16 , the vertical motion of the sleeves  82  and  84  is restricted by a pin  90 , yet the sleeves rotate with respect to the other. A crosshead  92  of a testing machine (not shown) is rigidly connected to the bottom joint assembly  18  by the pin  90 . 
     During a setup of the apparatus  10 , the crosshead  81  and crosshead  92  are rigidly connected to a testing machine by the pins  72  and  90 . An exposed section of the specimen  60  is rigidly secured by the clamps of the load transfer plates  20 ,  22 ,  24  and  26 . For tensile loading of the specimen  60  and preferable if the specimen is a planar solid, the first wedge  52  attached to the load transfer plate  20  of  FIG. 4  is used to secure the specimen. For compressive loading of the specimen  60 , the second wedge  54  attached to the load transfer plate  20  of  FIG. 5  is used to secure the specimen. For loading of fabric or other bendable material as the specimen  60 , the tongue and groove clamp  56  attached to the load transfer plate  20  of  FIG. 6  is used to secure the specimen. Alternatively, for loading of the specimen  60 , the clamp  58  with the pre-tensioning roller  59  and attached to the load transfer plate  20  of  FIG. 7  is used to secure the specimen. By a series of fasteners or by other fastening means known to those skilled in the art, wedges  52  and  56  as well as clamps  56  and  58  can be rigidly attached to the individual load transfer plates  20 ,  22 ,  24  and  26 . 
     During a test and depicted by the configuration of the assembly  10  in  FIG. 8 , the downward or compressive movement in direction “A” of the crosshead  81  causes lateral links  42 ,  44 ,  46  and  48  of the linkages  12  and  14  to move outward from a longitudinal axis  96  thereby increasing in distance from each other to the assembly configuration of FIG.  1 . More specifically, a compressive force is transmitted from the crosshead  81  by the rigid and oblong members  30 ,  32 ,  36  and  38  to vary the rhombus shape defined by the linkages  12  and  14 . By rotation of the members  30 ,  32 ,  36  and  38  on the lateral links  42 ,  44 ,  46  and  48 , the linkages  12  and  14  move outward. The increase in distance by the linkages  12  and  14  reflects the conversion of the compressive load by the crosshead  81  into a biaxial tension in the specimen  60 . By positioning the third four-bar linkage in the same direction “A”, tri-axial tension on the specimen  60  can be accomplished by the load transfer plates of the third four-bar linkage. 
     Separately or combined with the movement of the crosshead  81 , the upward or compressive movement in direction “B” of the crosshead  92  causes lateral links  42 ,  44 ,  46  and  48  of the linkages  12  and  14  to move outward from the longitudinal axis  96  thereby increasing in distance from each other to the assembly configuration of FIG.  1 . The increase in distance by the linkages  12  and  14  reflects the conversion of the compressive load by the crosshead  92  into a biaxial tension in the specimen  60 . Similarly, by positioning the third four-bar linkage in the same direction “B”, tri-axial tension on the specimen  60  can be accomplished by the load transfer plates of the third four-bar linkage. 
     Conversely, the upward or tensile movement of the crosshead B 1  in direction “C” in  FIG. 1  causes the lateral links  42 ,  44 ,  46  and  48  of the linkages  12  and  14  to move toward the axis  96  thereby decreasing a distance from each other. More specifically, a separating force similar to a tensile movement is transmitted from the crosshead  81  by the rigid members  30 ,  32 ,  36  and  38  to vary the rhombus shape defined by the linkages  12  and  14 . By rotation of the members  30 ,  32 ,  36  and  38  on lateral links  42 ,  44 ,  46  and  48 , the linkages  12  and  14  move to the axis  96 . The decrease in distance between the linkages  12  and  14  reflects the conversion of the tensile load by the crosshead  81  into a compressive biaxial load in the plane of the specimen  60 . Similarly, by positioning the third four-bar linkage in the same direction “C”, tri-axial compression on the specimen  60  can be accomplished by the load transfer plates of the third four-bar linkage. 
     Separately or combined with the movement of the crosshead  81 , the downward or tensile movement of the crosshead  92  in direction “D” in  FIG. 1  causes the lateral links  42 ,  44 ,  46  and  48  of the linkages  12  and  14  to move toward the axis  96  thereby decreasing a distance from each other. The decrease in distance between the linkages  12  and  14  reflects the conversion of the tensile load by the crosshead  92  into a compressive biaxial load in the specimen  60 . Similarly, by positioning the third four-bar linkage in the same direction “D”, tri-axial compression on the specimen  60  can be accomplished by the load transfer plates of the third four-bar linkage. 
     Additionally, upon rotation of the crosshead  81  in direction “E”, the first sleeve  62  of  FIG. 2  rotates with respect to the second sleeve  64  thereby rotating the linkage  12  with respect to the linkage  14 . This rotation thereby rotates the load transfer plates  24  and  26  of the linkage  12  with respect to the load transfer plates  20  and  22  of the linkage  14  such that an in-plane shear or torsional stress is applied to the specimen  60 . 
     Separately, upon rotation of the crosshead  92  in direction “E”, the first sleeve  82  rotates with respect to the second sleeve  84  thereby rotating the linkage  14  with respect to the linkage  12 . This rotation thereby rotates the load transfer plates  20  and  22  of the linkage  14  with respect to the load transfer plates  24  and  26  of the linkage  12  such that an in-plane shear or torsional stress is applied to the specimen  60 . 
     During any of the testing described above, the measurement system  28 , typical of measurement systems known to those skilled in the art, measures the multi-axial displacements due to compression or tensile loading of the specimen. The measurement system  28  includes a conventional displacement wire transducer  98  placed on the load transfer plate  22 . By a connecting wire  100 , the transducer  98  is rigidly attached to a hook  102  on the load transfer plate  20  parallel to one transverse axis  104  of the biaxial loading. For a second transverse axis  106  of the biaxial loading, a separate transducer  98  and a separate connecting wire  100  (shown in  FIG. 9 ) are positioned on the bottom surface of the load transfer plates  24  and  26 . For a third transverse axis of a triaxial loading, a separate transducer and a separate connecting wire (not shown) may be on an alternate plane from the connecting wires  100  for the axis  104  and  106  in order not to interfere with either. Strain gauges  108  are placed on the sidewalls of the load transfer plates  20 ,  24  and on (but not shown) load-transfer plates  22 ,  26  to directly monitor the loading of the specimen  60 . 
     To visually record the deformation of the specimen  60 , a camera or another optical recording device  110  may be affixed to the second sleeve  64  of the joint assembly  16 . Another feature would be the affixing of a puncturing or the shown draping mechanism  112  to the second sleeve  84  of the joint assembly  18  to conduct puncture and/or drape tests on the specimen  60 . 
     As shown and described above, the specimen  60  is subject to an equal biaxial loading wherein the length of the load transfer plates  20 ,  22 ,  24  and  26  are equal. In a first variant of the embodiment of the present invention, an unequal biaxial loading of the specimen  60  is capable. To have an unequal biaxial loading ratio, the length of the load transfer plates  20  and  22  of the linkage  14  would differ from those of the load transfer plates  24  and  26  of the linkage  12 . The displacement relationship caused by the unequal biaxial loading can easily be extracted by using the Pythagorean theorem. 
     In a second variant of the embodiment of the present invention, the apparatus  10  is also capable of non-orthogonal (oblique) multi-axial loading of the specimen  60 . Non-orthogonal multi-axial loading is particularly important for testing of braided or knitted fabrics and other non-orthogonal composite materials. As depicted in  FIG. 10  for a test, the angle  120  between the axis&#39;s  104  and  106  of the linkages  12  and  14  can be varied by rotating either linkage to match an angle defined by non-orthogonal fiber directions. 
     As is obvious in view of the prior description of the movements of the apparatus  10 , the apparatus is capable of loading the specimen  60  for uniaxial tension, uniaxial compression, biaxial tension, biaxial compression, in-plane shear, biaxial tension with in-plane shear, biaxial compression with in-plane shear and unequal biaxial compression with in-plane shear as well as any other loading and resultant testing derivable by those skilled in the art. 
     Thus by the present invention its objects and advantages  4  are realized and although preferred embodiments have been disclosed and described in detail herein, its scope should be determined by that of the appended claims.