Abstract:
A test machine for testing the tension or compression properties of a test specimen is provided. The test machine utilizes magnetic force to prevent the fixture of the machine from rotating while placing the test specimen in tension or under compression, which accordingly prevents the test specimen from rotating as well as the source of such rotation. Unintended forces are thereby minimized, enabling a user to obtain more accurate test results.

Description:
BACKGROUND 
       [0001]    The present invention relates to a material testing system capable of applying loads to a test specimen. More particularly, the invention relates to a testing system that utilizes a magnetic force to substantially prevent the specimen from rotating while the load is applied to it. 
         [0002]    Various physical properties of materials may be tested. In the field of elastomeric materials, such physical properties may include, shear strain, tensile strength, elongation, compressive strain, among others. In the testing of each particular property, it is beneficial to isolate the forces applied to the test specimen so that only the desired force or forces are being applied to the test specimen. For example, if the shear properties of a specimen were being tested, it would be undesirable to apply an unknown elongation force to the specimen during the shear testing. 
         [0003]    The same is true of tensile and compressive testing of a test specimen. It has come to the attention that during the tensile or compression testing of a test specimen, at times the specimen may unintentionally rotate or twist. This rotation of the test specimen introduces an undesirable variable into the test results. Therefore, there is a need to develop a testing apparatus and a test method which will isolate the test specimen, such that unintended forces do not interfere with the physical property testing being conducted. 
       BRIEF DESCRIPTION 
       [0004]    A test machine is provided. The test machine includes an actuator and a fixture in communication with the actuator, wherein the fixture contacts a test specimen. The machine further includes one or more magnets. The magnets are aligned to substantially prohibit the fixture from rotating during the testing. In one embodiment, the testing may include the application of tensile or compressive forces being applied to the test specimen. 
         [0005]    A method of testing the tension or compression properties of a test specimen is also provided. The method comprises securing a test specimen between two fixtures, wherein one fixture is in communication with the actuator. The method further includes moving the fixture in the direction of the longitudinal axis of the machine in order to place the specimen in tension or under compression and substantially preventing the fixture from rotating while moving the fixture in the direction of the longitudinal axis of the machine by the use of magnetic force. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is a perspective view of a material testing machine; 
           [0007]      FIG. 2  is a perspective view of a material testing machine which includes an embodiment of the present invention. 
           [0008]      FIG. 3  is a top view of the relationship of the magnets in one embodiment of the invention. 
           [0009]      FIG. 4  is a top view of a material testing machine which includes another embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    In the various figures described herein, like reference numerals or the same reference numerals are used to describe like or similar components of the embodiments described herein. 
         [0011]    During the tensile or compressive testing of an elastomeric article, it has become apparent that once the article is placed in tension or compressed, the article may exhibit a tendency to undesirably rotate. It has come to the attention of the inventor that the source of the rotation of the article is the rotation of an actuator that is in communication with a fixture gripping the article. Thus, the inventors have developed an apparatus and a technique to inhibit such rotation. Advantageously the technique does not interfere with the ability of the apparatus to apply a compressive force on the test specimen or to place the test specimen in tension. It is preferred that the technique does not mechanically engage the fixture or the test specimen. Mechanically engaged is used herein to describe at least the situation when the two items would come in physical contact. 
         [0012]    Examples of mechanical testing equipment which the invention may be applicable are described in the following U.S. patents, which are hereby incorporated by reference in their entirety U.S. Pat. Nos. 4,478,086, 4,869,112, 5,005,424, 5,361,640, 5,425,276, 5,693,890, 5,719,339, 6,526,837, and 6,679,124. A commercially available example of such an apparatus is the MTS 831 available from MTS Systems Corporation of Eden, Minn. 
         [0013]      FIG. 1  illustrates an exemplary material testing apparatus  10  for applying loads to a test specimen. The apparatus  10  includes an upper fixture  14 A and a lower fixture  14 B that hold the test specimen along a longitudinal axis  15 . The lower fixture  14 B is connected to an actuator  16  through which loads are applied to the test specimen and reacted against a reaction structure generally indicated at  18 . Optionally, the apparatus  10  may include more than one actuator. For example, a second actuator may be located proximate of fixture  14 B. 
         [0014]    As appreciated by those skilled in the art, the upper fixture  14 A and lower fixture  14 B, of apparatus  10 , can take many forms. Any suitable fixture may be used to practice the invention. Examples of other such fixtures are illustrated in the aforementioned U.S. patents. Preferably, each fixture is able to grip a portion of the test specimen with sufficient retentive force that the fixture is able to retain the test specimen during the desired testing. In one alternate example, fixtures  14 A and  14 B are capable of retaining the test specimen for compressive testing and fixtures  14 A and  14 B comprise plates. 
         [0015]    In the embodiment illustrated, the material testing apparatus  10  includes a frame  20  having a base  22 . A pair of support members  24  extend upwardly from the base  22  and are joined together by a crossbeam  26 , which provides a stable support surface. A pair of stationary support columns  28  extends upwardly from the crossbeam  26  to an adjustable crosshead  30 . A support  36  extends from crosshead  30  to a load cell  32 . Load cell  32  joins the upper fixture  14 A to reaction structure  18 . The load cell  32  provides a representative signal indicative of tension/compressive forces applied to the test specimen. Alternatively, the load cell may be located in communication with fixture  14 B (not shown) instead of fixture  14 A, as shown. A further alternative is that apparatus  10  may include more than one load cell. In one of the various embodiments of the apparatus, it is preferred that the actuator or actuators are aligned with an upper or lower fixture and that the load cell or load cells are aligned with the fixture which the actuator is not aligned. 
         [0016]    Apparatus  10  further includes an actuator  16 . Actuator  16  may be powered by any type of drive system such as an electrical system, a pneumatic system, or a hydraulic system. Support  34  extends from actuator  16  to lower fixture  14 B. Preferably actuator  16  is in communication with fixture  14 B and actuator  16  may be used to move fixture  14 B to apply a tensile force or compressive force to a test specimen. 
         [0017]    Optionally apparatus  10  may include a control system that provides control signals along a signal line to actuator  16  (or actuators if the system includes more than one actuator) and receives signals along a control line from load cell  32  which are proportional to the forces measured by the load cell (or load cells if the system includes more than one load cell). Examples of a commercially available control system are the various FLEXTEST® control systems available from MTS Systems Corporation. FLEXTEST is a registered trademark of MTS Systems Corporation. 
         [0018]    With respect to the apparatus  10 , it was discovered that during the application of either a tensile or compressive force, that actuator  16  exhibited a tendency to rotate which in turn would twist fixture  14 B and the test specimen. Typically, upper fixture  14 A would not rotate. Therefore, the rotation of actuator  16  would result in an unknown torquing force applied to the test specimen. An aspect of the invention is to prohibit fixture  14 B from rotating, more preferably prohibiting both fixture  14 B and actuator  16  from rotating. Preferably the rotation of fixture  14 B is prohibited without a physical structure coming in contact with fixture  14 B. In a preferred embodiment, magnetic force may be used to prohibit the rotation of fixture  14 B, as well as, actuator  16 . 
         [0019]    With reference to  FIG. 2 , the apparatus  10  includes a magnet  40  or  42  on each support column  28  respectively. Also a magnet  44  or  46  attached to each side of fixture  14 B respectively. Preferably magnets  40  and  42  extend along support columns  28  at a length that at least corresponds with the desired displacement of the test specimen. Also, it is preferred that the magnetic force exerted by magnets  42  and  46  are opposing and the same is preferred for magnets  40  and  44 . Preferably magnets  42  and  46 , as well as  40  and  44 , are spaced apart such that an equilibrium is formed between magnets  42  and  46  and magnets  40  and  44 , such that the distance between the opposing magnets D remains substantially constant as magnets  44  and  46  vertically pass by magnets  40  and  42  respectively. Distance D may be defined as the distance between the opposing magnets in the Z-direction. 
         [0020]    Preferably the strength of magnets  40 ,  42 ,  44 , and  46  are sufficient to prohibit fixture  14 B from rotating in either direction, more preferably sufficient to prohibit both fixture  14 B and actuator  16  from rotating in either direction. With respect to magnets  40 ,  42 ,  44 , and  46 , a preferred range of strength for each magnet may be about 5 to 15 lbs. The invention is not limited to any particular type of magnet, as any particular material which has the ability of attracting a like material may be used. For example various types of magnets such as electro-magnets, rare earth magnets, and combinations thereof may be used to practice the invention. 
         [0021]    With reference to  FIG. 3 , magnets  40  and  42  are placed along support columns  28 . Magnets  44  and  46  are attached to each side of fixture  14 B. In the embodiment shown, magnets  44  and  46  are placed on structure  48 , which in turn is attached to fixture  14 B. The magnetic force exerted by magnets  42  and  46  are opposing to each other, and the same is preferred for magnets  40  and  44 . Preferably magnets  42  and  46 , as well as  40  and  44 , are spaced apart such that an equilibrium is formed between magnets  42  and  46  and magnets  40  and  44 , such that the distance between the opposing magnets D remains substantially constant as magnets  44  and  46  vertically pass by magnets  40  and  42  respectively. Distance D may be defined as the distance between the opposing magnets in the Z-direction. 
         [0022]    Another embodiment of the invention is illustrated in  FIG. 4 . In this particular embodiment, apparatus  10  may include one or more magnets. Preferably, in this embodiment, one magnet  50  is attached to arm  52  which extends from structure  48 . Preferably, magnet  50  is located at an equilibrium position between structures  60  and  62  so that a substantially equal magnet force is applied toward each structure and the force prohibits the aforementioned rotation of fixture  14 B. The single magnet embodiment is not limited to the particular embodiment shown in  FIG. 4 . In an alternate embodiment, the magnet may extend from an arm attached to support  34  instead of a structure connected to lower fixture  14 B. 
         [0023]    A further embodiment contemplated may include a hybrid of the embodiments, illustrated in  FIGS. 2 and 4 . In this particular embodiment, magnet  50  may be attached to fixture  14 B or attached to a structure which is attached to fixture  14 B as described above. Magnets  40  and  42  may be attached to columns  28  as illustrated in  FIG. 2 . Preferably magnet  50  would exert a magnetic force outward in the x direction toward magnets  40  and  42  and magnets  40  and  42  would exert an opposing magnetic force back in the direction of magnet  50 , thereby locating magnet  50  at an equilibrium location between magnets  40  and  42 . 
         [0024]    A method of testing the tension properties of a test specimen is also provided. The test specimen is secured between fixtures  14 A and  14 B. The test specimen is moved a selected distance by actuator  16  in a direction that coincides with the longitudinal axis  15  of the machine to place the specimen in tension. A load cell  32  measures the forces applied to the test specimen. Preferably, a system capable of monitoring the force measurements from load cell  32  is provided (not shown). 
         [0025]    Magnetic force is used to prevent fixture  14 B, and more preferably fixture  14 B and actuator  16 , from rotating while fixture  14 B moves in the direction which coincides with longitudinal axis  15 . Preferably, magnets  40  and  42  are placed on each support column  28 , respectively. More preferably, magnets  40  and  42  extend along support columns  28  at least the distance of the displacement of fixture  14 B. Magnets  44  and  46  are attached to each side of fixture  14 B, respectively. Alternatively, magnets  44  and  46  may be attached to fixture  14 B by attaching magnets  44  and  46  to a structure and attaching the structure to fixture  14 B. In an alternative embodiment, magnets  44  and  46  may be attached to support  34  by attaching magnets  44  and  46  to a structure and attaching the structure to support  34 . Preferably, the magnetic force exerted by magnets  42  and  46  are opposing to each other, and the same is preferred for magnets  40  and  44 . It is also preferable that magnets  42  and  46 , as well as  40  and  44 , are spaced apart such that an equilibrium is formed between magnets  42  and  46  and magnets  40  and  44 , such that the distance between the opposing magnets D remains substantially constant as magnets  44  and  46  vertically pass by magnets  40  and  42  respectively. This may be done by placing magnets  46  and  44  substantially equidistance apart from fixture  14 B or support  34  in the plane perpendicular to longitudinal axis  15 . 
         [0026]    The above method may be modified to practice the aforementioned single magnet or three magnet embodiments also. In the single magnet embodiment, magnet  50  is attached to arm which extends from either of a structure attached to fixture  14 B or support  34 . Preferably magnet  50  exerts a sufficient magnetic force on both of structures  60  and  62  so to prohibit the rotation of fixture  14 B. In three magnet alternate embodiment, the rotation of fixture  14 B is prohibited by aligning magnet  50 , shown in  FIG. 4 , between opposing magnets  40  and  42 , as illustrated in  FIG. 2  and described above. 
         [0027]    The test specimen may be any shape to test the tension properties. To allow for easier retention by fixtures  14 A and  14 B, a rectilinear shaped test specimen is preferred when testing the tension properties. 
         [0028]    A method of testing the compression properties of a test specimen is also provided. When testing compression properties, in one embodiment, fixtures  14 A and  14 B are preferably plates that oppose each other (not shown). The test specimen is placed between fixtures  14 A and  14 B. Actuator  16  engages fixture  14 B to move fixture  14 B a predetermined distance to apply a selected compressive force on the test specimen. Load cell  32  may be used to measure the compressive force applied. Preferably, a system capable of monitoring the force measurements from load cell  32  is provided (not shown). 
         [0029]    When testing the tension or compression properties, the test specimen may be any material. Examples of such materials include rubbers, textiles, plastics, metals, and combinations thereof. 
         [0030]    The exemplary embodiment has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the exemplary embodiment be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.