Abstract:
A test fixture for use with a Dynamic Mechanical Analyzer (DMA) restrains a hollow cylindrical tube for purposes of performing either a tensile or transverse/bending load test. The fixture includes a clamp that is configured to restrain the tube without imparting a preload or changing a mechanical property of the tube.

Description:
[0001]    This application claims the benefit of U.S. Provisional Application Ser. No. 61/086,100, filed Aug. 4, 2008, the contents of which are incorporated by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to mechanical analysis of a material used to make a medical device or portions thereof intended for implantation within a body; more particularly, this invention relates to mechanical analysis of a polymeric material represented by a hollow tube having dimensions approximating the dimensions of a medical device, such as a stent. 
         [0004]    2. Background of the Invention 
         [0005]    A Dynamic Mechanical Analyzer (DMA) is a precision instrument designed to measure the viscoelastic properties of a material, such as a polymer material in a dry or wet stage. A DMA may be used to measure changes in a sample material resulting in changes in temperature and/or external forces. Applied external forces may be represented by enforced displacements on the sample, in which case material properties are determined from a measured reaction force. The external forces may be time-varying, e.g., sinusoidal. Prior to testing, a sample of the material is mounted in a clamp, one part of which is stationary and the other part is moving and connected to a motor drive. 
         [0006]    The sample can be in a bulk solid, film, fiber, gel or viscous form depending on the fixture used. The motor drives the sample to a selected strain or amplitude. As the sample undergoes deformation, a linear variable differential transformer mounted on a driving arm or rod measures such quantities as a static or time-varying strain amplitude as feedback control to the motor. Interchangeable fixtures are used to measures quantities such as an elastic modulus, toughness, damping, stress relaxation, creep, and softening points. See e.g.,  Introduction to Dynamic Mechanical Analysis  ( DMA )— A beginner&#39;s Guide,  PerkinElmer® Inc. 
         [0007]    One quantity of importance in analyzing material used to make implantable medical devices is the glass transition temperature, T g . The “glass transition temperature,” T g , is the temperature at which the amorphous domains of a polymer change from a brittle vitreous state to a solid deformable or ductile state at atmospheric pressure. In other words, the T g  corresponds to the temperature where the onset of segmental motion in the chains of the polymer occurs. When an amorphous or semicrystalline polymer is exposed to an increasing temperature, the coefficient of expansion and the heat capacity of the polymer both increase as the temperature is raised, indicating increased molecular motion. As the temperature is raised the actual molecular volume in the sample remains constant, and so a higher coefficient of expansion points to an increase in free volume associated with the system and therefore increased freedom for the molecules to move. The increasing heat capacity corresponds to an increase in heat dissipation through movement. T g  of a given polymer can be dependent on the heating rate and can be influenced by the thermal history of the polymer. Furthermore, the chemical structure of the polymer heavily influences the glass transition by affecting mobility. 
         [0008]    The sample and the fixture restraining the sample is enclosed within a thermal isolation chamber which can heat the sample and the fixtures to temperatures above normal ambient temperatures or cool the sample and the fixtures to temperatures below normal ambient temperatures. The temperature is generally varied dynamically, e.g., at a constant heating or cooling rate. The stiffness and damping of a sample may be calculated as a function of temperature from force, displacement and phase data, using well-known mathematical relationships which separate the applied load into the components due to movement of the mechanical system and the components due to deformation of the sample. The phase relationship between the force applied to the sample and the resultant displacement allows the sample deformation force component to be further divided into an elastic component and a viscous component. The elastic and viscous components are used to determine the elastic modulus and damping through the use of model equations for the particular sample geometry and deformation mode. These equations are well-known in the field, e.g., Theory of Elasticity, S. P. Timoshenko and J. N. Goodier, McGraw-Hill (3rd ed. 1970). Currently, there are two classes of fixtures for DMA—tensioning and non-tensioning. Tensioning fixtures include the 3-point bend, tension/film, tension/fiber, compression, compression and penetration fixtures, while non-tension fixtures include single/dual cantilever and shear sandwich fixtures. 
         [0009]    As mentioned above, existing fixtures for restraining movement of a sample in a DMA are intended for analysis of material in bulk solid, film, fiber, gel or viscous form. Unfortunately, there exists no ability to test a hollow cylindrical tube, in particular, a thin walled tube having dimensions corresponding to a tube that is formed into a stent. A fixture suited for a film or fiber, for example, cannot restrain a hollow tube in a wet or dry environment, as needed, because either the sample cannot be adequately held by a clamp, the clamp induces unwanted preloads into the sample, such as torsion, or collapses the tube walls when the clamp is tightened down on the ends of the tube in order to hold it in place. Other problems with existing fixtures are they are difficult to assemble or modify to accommodate the special needs of a hollow tube. 
       SUMMARY OF THE INVENTION 
       [0010]    According to the invention, a fixture for a DMA is capable of adequately restraining a hollow cylindrical tube made from a polymer and having dimensions of an implantable medical device, such as a stent. When installed in the fixture, the hollow tube is not pre-stressed as a result of bending, tensile loading or torque applied to the tube by the fixture which effects the ability to accurately measure mechanical properties of the extruded material. The tube is supported at its ends to prevent collapse of the relatively thin walls of the tubing. 
         [0011]    According to one aspect of invention, a fixture adapted for performing a tensile or bending load test on a hollow cylindrical tube is provided. In some embodiments, the testing of the hollow tube includes a measurement of the glass transition temperature for a polymer hollow cylindrical tube have approximately the dimensions of a stent, examples of which are discussed in U.S. Pub. No. 20080147165. 
         [0012]    According to another aspect of the invention, a clamp portion of the fixture is configured so that no torque preload is applied to the tube when the tube is secured in the clamp. The clamp may be custom made to hold a tube having a prescribed outer diameter. 
         [0013]    According to another aspect of the invention, an improved assembly procedure for placing a hollow tube in a fixture is disclosed. The assembly procedure can be applied to either a dry or wet test setup procedure and can be rapidly reproduced for different size tubes. 
         [0014]    According to another aspect of the invention, there is a method for assembly, method of testing and/or apparatus for performing dynamic mechanical analysis on a material using DMA, the material taking the form of tubular body that has the dimensions of an extruded polymer tube that will be formed into a stent. In some embodiments, the tube has cylindrical walls. In other embodiments, the tube walls has a stent pattern characteristic of a balloon expandable stent. 
         [0015]    According to one embodiment of the invention, a fixture for tensile or bending load testing using a DMA is capable of restraining a hollow cylindrical tube without causing collapse of tube walls or inducing a preload in torsion for a tube having an outer diameter in the range of about 0.4-0.2 inches, and an inner diameter in the range of about 0.01-0.18 inches. 
         [0016]    According to another aspect of the invention, an apparatus for determining the mechanical properties of a material includes a base platform including a programmable drive configured to move relative to a stationary post, a test specimen including a hollow cylindrical tube substantially formed from the material and having a first end and a second end, and a test fixture, including: a first mount coupled to the drive and extending upwards therefrom, the first mount including at an upper end thereof a first member adapted for applying pressure about the circumference of the hollow tube first end, and a second mount coupled to the post and disposed over the drive, the first mount including at an upper end thereof a second member adapted for applying pressure about the circumference of the hollow tube second end. 
         [0017]    According to another aspect of the invention, a method for predicting a physical property of a material using a mechanical analyzer having a drive and a stationary post includes the steps of coupling one end of a hollow cylindrical tube to a first clamp, the hollow cylindrical tube being substantially formed from the material, coupling the first clamp to the post, coupling a second clamp to the drive, coupling the other end of the hollow cylindrical tube to the second clamp, applying a known load or deflection to the hollow cylindrical tube through the drive, and then measuring the force or displacement of the one end of the hollow cylindrical tube relative to the other end in response to the applied load. 
         [0018]    According to another aspect of the invention, an apparatus includes a clamp capable of restraining a polymer hollow cylindrical tube under an idealized boundary condition, e.g., no net torque, during a mechanical analysis, the clamp including a flexible coupling that couples a plurality of fingers to a pair of arms. The plurality of fingers form an approximately circular bearing surface when the arms are brought adjacent to each other, and the flexible coupling is arranged to produce the desired load distribution about the circumference of the tube. The clamp may be a unitary clamp. The flexible coupling may be a ring. 
         [0019]    According to another aspect of the invention, an apparatus for determining the mechanical properties of a material includes a testing platform including a drive configured to move relative to a post, a hollow cylindrical tube substantially formed from the material and having a first end and a second end, and a test fixture including means for coupling the hollow tube to the drive and the post without preloading the tube in torsion. The means may include a ring surrounding a plurality of fingers having a bearing surface. The bearing surface is brought into contact with the outer surface of the hollow tube when the tube is secured in the clamp. The ring may have a non-constant thickness, or different thickness, among the fingers for modifying the load distribution from the tube surface to the ring given the applied loading external loading. In this case, the stiffness characteristics of the ring are such that when the fixture is subjected to a predetermined applied load, there can be a uniform loading of the tube surface about its circumference, such as no net torque applied to the tube. 
         [0020]    According to another aspect of the invention, a method of restraint suited for determining the properties of a material based on a reaction to an external loading includes the steps of: (i) providing a test base including a moving drive and a stationary post, and a platform, test bed, hollow cylindrical tube having ends and being substantially formed by the material and first and second members adapted for gripping the respective tube ends; (ii) securing one end of the hollow cylindrical tube to the first member; (iii) connecting the platform to the drive; (iv) placing the test bed over the drive and platform; (v) connecting the first member to the test bed; (vi) after step (v), connecting the second member to the platform such that the second member is positioned above the test bed; and (vii) after step (vi), securing the other end of the tube to the second member. 
       INCORPORATION BY REFERENCE 
       [0021]    All publications and patent applications mentioned in this specification are herein incorporated by reference to the same extent as if each individual publication or patent application was specifically and individually indicated to be incorporated by reference. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]      FIGS. 1A and 1B  are schematic illustrations of a fixture for applying a tensile loading on a hollow cylindrical tube using a DMA.  FIG. 1A  shows the fixture and tube prior to moving a drive relative to a stationary post.  FIG. 1B  shows an elongation of the tube after the drive has moved relative to the post. 
           [0023]      FIGS. 2A and 2B  are schematic illustrations of a fixture for applying a transverse or bending load on a hollow cylindrical tube using a DMA.  FIG. 2A  shows the fixture and tube prior to moving a drive relative to a stationary post.  FIG. 2B  shows an elongation of the tube after the drive has moved relative to the post. 
           [0024]      FIG. 3  shows a perspective view of a portion of a DMA with a first stage assembly of a fixture for performing a mechanical analysis of a hollow cylindrical tube. This first partial assembly may be used to construct a fixture for performing either a tensile or transverse load test on the tube. The portions of the DMA include a base, drive and stationary posts. 
           [0025]      FIGS. 4A and 4B  show perspective views of a second stage of assembly of a fixture for performing a tensile load test of the tube after the first stage. 
           [0026]      FIG. 5  shows a perspective view of a first clamp assembly with one end of the hollowing cylindrical tube (TS) secured in the clamp. 
           [0027]      FIG. 6A  shows a plate for holding a second clamp. 
           [0028]      FIG. 6B  shows the assembly of the plate of  FIG. 6A  and the second clamp. 
           [0029]      FIGS. 7A and 7B  show perspective views of the assembled fixture for performing a tensile load test on the tube. 
           [0030]      FIG. 8  shows a perspective view of a second stage of assembly of a fixture for performing a transverse or bending load test of the tube after the first stage of  FIG. 3 . 
           [0031]      FIG. 9  shows a perspective view of a third second stage of assembly of a fixture for performing a transverse or bending load test of the tube after the first stage of  FIG. 3 . 
           [0032]      FIG. 10A  shows a plate for holding a clamp. 
           [0033]      FIG. 10B  shows the assembly of the plate of  FIG. 10A  and the clamp. 
           [0034]      FIG. 10C  shows the tube secured to the clamp and plate of  FIG. 10B . 
           [0035]      FIG. 11  shows a perspective view of the assembled fixture for performing a transverse or bending load test on the tube. 
           [0036]      FIG. 12  shows a perspective view of the plate and clamp of  FIG. 6B . 
           [0037]      FIG. 13  shows a planar view of the clamp of  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0038]    The description proceeds as follows. First, various aspects of fixtures according to the disclosure will be described with reference to  FIGS. 1-2 , which are psuedo-schematic representations of test fixtures according to the disclosure. These illustrations and accompanying text are intended to provide a simplification of the coupling loads between and/or among structure. Specifically, the drawings are intended to illustrate in accordance with the disclosure load paths for equilibrating loads carried by a structure during a test of a hollow cylindrical tube, in terms of a moving and stationary of a testing apparatus, e.g., a DMA, for wet (immersion) testing or dry testing of the hollow tube. Following this description, the discussion turns to a description of more specific examples of structure, namely, two embodiments of test fixtures described with reference to  FIGS. 1 and 2 . Examples of embodiments of more specific structure are discussed with reference to  FIGS. 3-13 . 
         [0039]    Test Fixtures  1  and  2   
         [0040]      FIGS. 1A and 2A  are schematic representations of two embodiments of a test fixture  1  and  2 , respectively, secured to a test bench  3  that provides a moving rod  4  (or drive assembly) and posts to mount a fixture. As alluded to above, these drawings are not intended to illustrate or even suggest actual dimensions or relative sizes of parts for a fixture according to the disclosure. Rather, as will be appreciated, they represent only a simplification of the load paths that are provided between the rod  4  and the posts  5 . The components of the fixtures depicted in  FIGS. 1 and 2  may, therefore, be considered as rigid bodies during the following discussion. 
         [0041]    The first test fixture  1  is configured for restraining a wet or dry test sample (TS), i.e., a hollow cylindrical tube, when subjected at an axial load. The second test fixture  2  is configured for restraining the wet or dry TS when subjected to a transverse or bending load when both ends  11   a ,  11   b  of the tube are fixed in rotation (i.e., the slope at each end is unchanged when the TS is loaded). Other idealized boundary conditions for testing a hollow cylindrical tube, e.g., cantilever, dual cantilever, pinned at both ends, etc. become possible as well in view of this disclosure.  FIGS. 1B and 2B  show respective deformed states for the TS for each of the loading conditions. In some embodiments, parts may be common to both the fixture  1  and the fixture  2  as discussed in greater detail, below. 
         [0042]    Test bench  3  may correspond to the test bench or chamber described in U.S. Pat. No. 5,710,426 (Reed). Bench  3  includes stationary or nonmoving posts  5   a ,  5   b  surrounding a rod  4  that is coupled to a drive mechanism (not shown) which displaces the rod  4  linearly. Examples of posts  5  and rod  4  are post  16  and drive rod  14 , respectively, described in Reed. Rod  4  is part of a linear actuator configured to apply a predetermined static or time-varying (i.e., dynamic) displacement δ, which results in the deformed test specimen (TS′) depicted in  FIGS. 1B  (axial deformation) and  2 B (transverse deflection). Mechanical properties of material represented in TS when the TS is subjected to a static load, dynamic, i.e., time-varying load, and/or thermal loading can then be measured or predicted in terms of, e.g., measuring resistance forces in response to an enforced displacement. Thus, bench  3  may be enclosed within a thermal isolation chamber and may include appropriate instrumentation to measure forces acting on the rod  4 . 
         [0043]    Test fixtures  1  and  2  each include a pair of mount assemblies, namely,  12   a  and  12   b , and  14   a  and  14   b . Mount assembles  12   a ,  14   a  are coupled to the drive rod  4 , while mount assemblies  12   b / 14   b  are coupled to the stationary posts  5 . Mount assembles  12   a / 14   a  may include a beam  20 , standoffs  30   a  and  30   b , a top plate  50  and an upper clamp  40 . Mount assemblies  12   b / 14   b  may include a platform  60  and a lower clamp  80 . 
         [0044]    Mount assemblies  12   a / 14   a  are coupled to the end of the rod  4  by way of a connecting portion of the beam  20 , e.g., a dovetail fitting  4   a . Standoffs  30   a ,  30   b  may be secured at their lower ends by removable fasteners, e.g., screws, to ends  20   a ,  20   b  of beam  20 . The upper ends of the standoffs  30   a ,  30   b  support the plate  50 , which may be secured to the standoffs  30   a ,  30   b  at its ends  50   a ,  50   b , respectively, by removable fasteners. The clamp  40  may be affixed to the top plate  50  by removable fasteners. The clamp  40  is orientated to face downward so that a lower end  11   a  of TS faces the rod  4 . In the case of test fixture  1  (tensile test), the clamp  40  is located on the plate  50  such that when the TS is secured to the clamp  40 , the TS center axis falls on or near the line of action of the rod  4 . The term “line of action” or LOA refers to the straight line that a force acts along. If the equilibrating force is located on this LOA then no equilibrating moment is needed. Thus, by locating the restraining force for end  11   a  along the rod  4  LOA this will ensure that the external, equilibrium forces acting on the TS, i.e., external forces acting on the TS free body, during the test are limited to axial loads as desired for a tensile axial load test. In the case of test fixture  2  the location of, e.g., the removable fastener that secures the clamp  40  to the plate  50  may fall on or near the LOA of the rod  4 . In this case, the bending moments induced at end  11   a  are limited to only the one plane. 
         [0045]    Mount assemblies  12   b / 14   b  are coupled to the stationary posts  5   a ,  5   b , by way of, e.g., removable fasteners connecting ends  60   a ,  60   b  of the platform  60  to the upper ends of the posts as shown. Platform  60  provides support for the lower clamp  80  when the rod  4  is displaced upwards (FIGS.  1 B/ 2 B) or downwards and may be formed to hold a volume of fluid in which the TS is immersed during testing. As indicated in  FIG. 1 , platform  60  also provides passageways  62   a  and  62   b  for standoffs  30   a ,  30   b . This passageway may be formed as, e.g., through holes slightly larger than the cross-sectional dimensions of standoffs  30 . In any event, the passageways  62   a / 62   b  permit unhindered displacement of the standoffs  30  relative to the platform  60 . As such, with TS removed, the mount assembly  12   a / 14   a  can move freely up/down relative to the mount assembly  12   b / 14   b  (bearings or bushings may be provided to allow free travel within the passageways  62   a / 62   b , especially in the case where a transverse load is applied to the TS). With this arrangement, the only structure that couples mount assembly  12   a / 14   a  (beam  20 , standoffs  30 , clamp  40  and plate  50 ) to mount assembly  12   b / 14   b  (frame  60  and clamp  80 ) is TS. 
         [0046]    A travel distance or upwards/vertical clearance of “h” is created between the top of the beam  20  and bottom of the frame  60  when the unloaded test specimen is secured in place in the fixture, i.e., when TS is attached to clamps  40  and  80  as shown in  FIGS. 1   a  and  2   a . This distance h may represent the maximum amount of deflection δ by rod  4  permitted before beam  20  abuts the lower surface of the frame  60 .  FIGS. 1 and 2 , showing exaggerated views of TS′, depict the relationship between the deflection and movement of parts of the fixture relative to each other. 
         [0047]    In comparing the locations of clamps  80  and  40  between mount assemblies  12   a  and  14   a , the lower clamp ( 80 ) is situated to face left-to-right, and the upper clamp  40  right-to-left in frame  60  for a transverse loading, and up/down for a tensile loading. Assembly of the fixture  1  may proceed by securing the TS in the lower clamp  80  first, securing the plate  60  and then securing the end  11   a  to clamp  40 . For the fixture  2 , the TS may be attached to clamp  40  then to clamp  80  (as described in greater detail, below). 
         [0048]    Test Fixture  100  (Tensile Test) 
         [0049]      FIGS. 3-7  illustrate various partial assembly views of a test fixture  100  and components thereof that embody features of test fixture  1  just described with reference to  FIG. 1A-1B . Test fixture  100  includes a first and second mount assembly. The first mount assembly includes a beam  120 , spacers  130   a ,  130   b , top plate  150  and top clamp  140 . The first mount assembly for fixture  100  embodies features of the first mount assembly  12   a  discussed earlier in connection with  FIG. 1A . The second mount assembly includes a platform  160  (formed by a frame  162  and bed  164 ), and a lower clamp  180 . The second mount assembly for fixture  100  embodies features of the second mount assembly  12   b  discussed earlier in connection with  FIG. 1A . 
         [0050]      FIG. 3  illustrate a first step of assembly for the test fixture  100 . This is a perspective view of a test base  103  with the beam  120  and spacers  130   a ,  130   b  (tensile load) attached to ends  120   a ,  120   b , respectively, by fasteners, e.g., screws. The beam  120  is secured to the top of the drive rod  104  by a dovetail connection  104   a  (may also include tightening a dome-top set screw which connects the beam  120  directly to the dovetail head formed at the end of the rod  104 ). There are four stationary posts  105   a ,  105   b ,  105   c  and  105   d  surrounding the drive rod  104 . The base  103  is located within a thermal isolation chamber of a DMA, e.g., the TA Instruments, Inc. “Q800 DMA” described online at http://www.tainstruments.com/product.aspx?n=1&amp;id=12&amp;siteid=11. The spacers  130   a ,  130   b  are tapped at their upper ends, which is where they will attach to the clamp assembly (clamp  140 , plate  150 ) as depicted in  FIG. 7A . 
         [0051]      FIGS. 4A-4B  illustrate a second step of assembly for the test fixture  100 . According to this embodiment the test platform  160  is formed by a frame  162  supporting a bed  164 . The frame  162  includes four bores which align with the tapped upper ends of the posts  105   a ,  105   b ,  105   c ,  105   d . The frame  162  may be secured to the bed  164  prior to securing it to the posts  105 . The lower surface of the frame  162  is disposed above the upper surface of the beam  120  when the fixture is fully assembled ( FIG. 7B ). The platform  160  may be suspended from the posts  105  and thus rest above the beam  120  so that the beam  120  can be moved upwards, thereby causing elongation of the TS without abutting the lower surface of the frame  162 . 
         [0052]    The bed  164  is formed with walls  164   c  and a base  164   b  which together define a volume for a liquid in which a TS can be immersed during testing. A material immersed in a liquid can exhibit different mechanical properties, e.g., glass transition temperature (T g ), then when its properties are measured in air. In some embodiments a tensile or bending test is conducted with the TS immersed in a liquid, such as a PBS buffer. The bed  164  includes an access  164   a  for accessing a tightening screw for the upper clamp  140  ( FIG. 7B ). In other embodiments, the bed  164  may have more shallow walls, or a bed may not be used at all if the TS is loaded in air (as opposed to immersion testing). The bed  164  may be structural or non-structural with respect to load transfer from the clamp  180  (by TS when in tension) to the stationary posts  105 . A structural embodiment may have the clamp  180  and TS secured to the base  164   b  portion of the bed  164  and the bed  164  attached separately to the frame  162 . A non-structural embodiment (preferred) of the bed  164  may have the clamp  180  secured directly to the frame  162 , i.e., the fastener  184  portion of the clamp  180  attached directly to the frame  162 , while the bed base  164   b  is disposed between the frame  162  and the clamp  180 . 
         [0053]    The clamp  180  is shown in perspective view in  FIG. 5  and secured to the frame  162  in  FIG. 4B . As illustrated, the lower end  11   a  of the TS is received in an opening  186  and held therein by a plurality of fingers  300  formed along an annular portion  181  of the clamp  180 . In some embodiments, the fingers  300  have a design such that little or no net torque is applied to the TS when the fingers  300  are pressed into the TS (examples of fingers  300  are discussed in greater detail, below). The pressure applied to the TS by the fingers  300  is controlled by a fastener  182 , e.g., a hex-head screw, which connects two opposing arms  185   a ,  185   b . These arms extend from the annular portion  181 . When the arms  185  are brought together, e.g., by turning the fastener  182  clockwise, the fingers  300  bear down upon the lower end  11   a  of the TS. When the arms  185   a ,  185   b  are moved apart, e.g., removing the fastener  182 , the pressure on the lower end  11   a  is relieved and the TS can be removed. 
         [0054]    Referring to  FIG. 5 , as a part of the step  2  assembly the TS is placed within the clamp  180  and secured thereto using the fastener  182  ( FIG. 5 ) before the clamp  180  is secured within the bed  162 . The clamp  180  may include pins and/or set screws extending from its base,  188   a  and  188   b,  as shown in  FIG. 5 , which are received within corresponding slots, holes or depressions formed in the base  164  (hidden from view in  FIG. 4B ). As will be appreciated, the pins  188  cooperate with the fastener  182  to close the clamp  180 / 140  (pins  188  hold one of the clamp arms in place when the screw  182  is turned clockwise). According to some embodiments, the entire tensile load transferred through the TS during testing is carried in bending and tension through the fastener  184 . After the TS end  11   a  is secured in the clamp  180 , the clamp  180  is secured to the frame  162  by extending the fastener  184  through a hole formed in the base of the bed  164 . 
         [0055]    Step  2  of the assembly for fixture  100  may proceed as follows: secure the bed  164  to the frame  162 , secure the frame  162  to the posts  105 , connect the lower end  11   a  of the TS to the clamp  180 , place the set screws  188  in the holes/slots provided in the bed  164   b , and then secure the clamp  180  in the frame  162 . After completing Step  2 , the upper clamp pieces, i.e., clamp  140  and plate  150 , are assembled. 
         [0056]      FIGS. 6A-6B  show structure for the clamp  140  and the plate  150 .  FIG. 6A  shows the plate  150  only and  FIG. 6B  shows the plate  150  with the clamp  140  secured thereto. For some embodiments, e.g., the illustrated embodiment, the clamp  140  may be identical to the clamp  180  just described. In other embodiments, the clamps  140  and  180  may be different. For example, the location of fastening or set screws, e.g., pins, for the clamp  180  may be different from clamp  140  in order to provide more convenient access points for assembly of the fixture  100 . 
         [0057]    In the illustrated embodiment the clamp  140  is identical to the lower clamp (clamp  180 ). Accordingly, the same reference numerals will be used to refer structure when the structure is the same. Referring to  FIG. 6A , the perspective view shows the face of the plate  150  that faces the base  164   b  of the bed  164  ( FIG. 7A  shows the opposing face of the plate  150 ). The plate  150  includes a tapped hole  156   a  that receives the captive screw  184 , and two slots that receives the pins  188 ,  188   b . Bores  152   a ,  152   b  align with the top of the spacers  130   a ,  130   b  to secure the plate  150  to the spacers  130 . The plate  150  includes a central portion  155  that is sized to fit into the opening of the bed  164  (see  FIG. 7A ) for purposes of alignment of the bores  152   a ,  152   b  with the top of the spacers  130 . The central portion  154  includes a peg  154  that is received within the bore of the upper end  11   b  of the TS. The peg  154  aligns with the center of the opening  186  of the clamp  140  when the clamp  140  is fit onto the plate  150 . Thus, when the TS upper end  11   b  is received in the opening  186 , it will be placed between the contacting surfaces of the fingers  300  (discussed below) and the peg  154 . The assembled view of the upper clamp assembly is shown in  FIG. 6B , which also shows the opening  182   a  in the arms  185  for the tightening screw. The tightening screw  182  is used to secure the end  11   a  of the TS after the plate  150  has been secured to the top of the spacers  130 . 
         [0058]    After the plate  150  and clamp  140  have been assembled ( FIG. 6B ), the rod  104  of the base  103  is moved to the top of its travel and locked in place. After the arm  104  has been locked in place, the plate  150  is connected to the top of the spacers  130  by threaded fasteners that are passed through the bores  152   a ,  152   b  of the plate  150 , as shown in  FIG. 7A . As the plate is being placed onto the spacers  130 , the end  11   b  of the TS central axis should be aligned with the opening  186  of the clamp  140 . After the plate  150  is secured in place, the rod  104  is unlocked and the end  11   b  is allowed to pass into the opening of the clamp  186  as the rod  104  moves downward towards the base  103 . The end  11   b  of the TS may abut the base of the peg  154 . After this step, the tightening fastener  182  is inserted into the opening  182   b  and turned to cause the fingers  300  to grip the upper end  11   b  of the TS (in one example, the TS (i.e., a hollow cylindrical tube) has a length of 1 inch (+/−0.05 inches)). Referring to  FIG. 7B , the access opening  164   a  provided by the walls  164   c  of the bed  164  allows, e.g., a wrench, to be inserted between the walls  164   c  of the bed  164  and the plate  150  so that the fastener  182  and be tightened/loosened. After the assembly is complete ( FIG. 7B ), an opening  169  to the bed  164  may be used to fill the bed  164  with the liquid used to immerse the TS for an immersion testing of the TS. 
         [0059]    Test Fixture  200  (Transverse Load Test) 
         [0060]      FIGS. 8-11  illustrate various partial assembly views of a test fixture  200  configured for performing a transverse or bending test on the TS. The test fixture  200  and components thereof embody features of the test fixture  2  previously described with reference to  FIG. 2A-2B . Test fixture  200  includes a first and second mount assembly, which may share many of the same components as the first and second mount assembly used for test fixture  100 . The shared components may include the beam  120 , spacers  130   a ,  130   b , frame  162  and clamps  140  and  180 . The remaining components, i.e., the top plate and the bed differ between the fixture  100  and fixture  200 . In the following description, the structure that is the same between fixture  100  and  200  will also use the same reference numerals. 
         [0061]    The first mount assembly for fixture  200  embodies features of the first mount assembly  14   a  discussed earlier in connection with  FIG. 2A . The first mount assembly includes the beam  120 , the spacers  130 , a top plate  150 ′ and the top clamp  140 . The second mount assembly for fixture  200  embodies features of the second mount assembly  14   b  discussed earlier in connection with  FIG. 2A . The second mount assembly includes a platform  160 ′ (formed by the frame  162  and bed  164 ′), and the lower clamp  180 . 
         [0062]    Step  1  of the fixture  200  assembly is the same as before, i.e., secure the beam  120  and spacers  130  to the test base  103 . Step  2  is depicted in  FIG. 8 . The frame  162  and a bed  164 ′ are secured to the base  130  in similar fashion as described earlier in connection with platform  160 . In the case of fixture  200 , the bed  164 ′ takes on a different form from bed  164  since the TS will be loaded by a traverse loading. Bracket  166 , having mounting holes  166   a  for securing clamp  180  thereto (as before), is attached at a wall of the bed  164 ′. The mounting holes  166   a  are arranged so that the opening  186  of the clamp (for receiving an end of the TS) will face left to right (as opposed to bottom to top). 
         [0063]    Step  2  of the fixture  200  assembly is illustrated in  FIG. 9 . In this step the clamp  180  is attached to the bracket  166 . In contrast to fixture  100 , the TS is not secured in the clamp  180  at this point. Rather the TS is attached to the upper clamp  140  first, then to the lower clamp  180  during the final assembly. As can be appreciated from inspection of  FIG. 9 , the bed  164 ′, bracket  166  and/or clamp  140  may cooperate so that the hole  182   a  for receiving the tightening fastener  182  can be easily accessed after the top plate and clamp assembly (described next) are attached to the tops of the spacers  130 . 
         [0064]    Step  3  of the fixture  200  assembly is illustrated in  FIGS. 10A-10C .  FIG. 10A  illustrates a perspective view of a top plate  150 ′. In step  3  the clamp  140  is secured to the top plate  150 ′ ( FIG. 10B ) then the TS upper end  11   b  is secured in the clamp  140  ( FIG. 10C ). Plate  150 ′ includes an arched portion  155   a ′ and an extension  155   b ′. Arched portion  155   a ′ includes the bores  152   a ,  152   b  at the ends. The peg  154  (where the end  11   b  of the TS is received) is located at the end of the extension  155   b ′. The holes  156  for receiving the pins and the captive screw  184  are shown. The captive screw  184  is shown inserted into the extension  155   b ′ in  FIG. 10B . The location of the opening  186  and fingers  300  of the clamp  140  are depicted relative to the bores  152   a  and  152   b . In this case, the opening  186  is located such that when the plate  150 ′ is secured to the top of the spacers  130  the opening  186  associated with the lower clamp  180  will align with the TS held by the upper clamp  140 , i.e., their centers lie on the same axis, so that no transverse pre-load is applied to the TS when end  11   b  is secured to clamp  140 . Again, the arm  104  position may be adjusted (i.e., moved to the top of its travel) in order to locate this position of clamp  140  with respect to clamp  140 . 
         [0065]    In step  4  of the fixture  200  assembly the top plate  150 ′ and clamp  140  are secured to the spacers  130  using a fastener received in the bores  152   a ′,  152   b ′, as shown in  FIG. 11 , which shows a final assembly of the fixture  200 . As can be appreciated from this top perspective view, the arched portion  155   a ′ allows the clamp  140  (coupled to the drive  104 ) to be set back or recessed from the clamp  180  (coupled to the posts  105 ). This arrangement has at least one possible advantage. It allows the effective length of the TS to be greater, thereby allowing a test to be conducted on long beams, which means that more of a bending stress condition can be imposed on the 
         [0066]    TS, i.e., end loading a long slender beam, as opposed to shear stress condition, i.e., end loading a short and wide beam. Further, it enables a tube of a specific length, such as the intended length of a stent, to be examined. 
         [0067]    Tube Clamp 
         [0068]    The following discussion provides description of embodiments of a tube clamp according to another aspect of the disclosure. In particular, embodiments of the tube clamps  180 / 140  discussed below enable a hollow cylindrical tube, e.g., an extruded polymer tube having a 0.064″ outer diameter (0.021″ inner diameter) or 0.136″ outer diameter (0.124″ outer diameter), to be firmly held without crimping or buckling the ends, without applying a torque pre-load, and without requiring an excessive length of the tube to secure the clamping surfaces sufficient to prevent pull-out during testing. In other words, a relative small portion of the tube is clamped. 
         [0069]    As will be appreciated, the effective length of the tube for purposes of calculating mechanical properties, e.g., under a bending stress condition, is the length between where the opposed clamps are in contact with the tube. One advantage of the design is that the length over which the clamp acts is not significant. Therefore, a greater percentage of the length of the tubing can correspond to the theoretical length of the tubing for purposes of calculating a flexural (EI) modulus or glass transition temperature (I g ). In one example the TS is 1″ in length having the above diameters and received in a fixture  100 / 200  that is mounted within the test chamber of the Q800 DMA, or the DMA described in Reed. An additional concern is avoiding a torque pre-load, especially for thin-walled tubes. 
         [0070]    Referring to  FIGS. 12-13  depicted are embodiments of the tube clamp  180 / 140  described earlier.  FIG. 12  shows a perspective view of the clamp  140 / 180  mounted on the upper plate  150 ′ associated with fixture  200  (described earlier). With reference to  FIGS. 12 and 13  depict one embodiment of clamping fingers  300 , namely fingers  304  disposed angularly about a center of the clamp, i.e., where the peg  154  is located. For instance, fingers  304  may be used to grip an extruded polymer hollow cylindrical tube having a 0.064″ outer diameter (0.021″ inner diameter) or an extruded polymer hollow cylindrical tube having a 0.136″ outer diameter (0.124″ outer diameter). 
         [0071]    As mentioned above, the upper end  11   b  of the TS is secured in the clamp  140  by placing the end  11   b  in the opening  186  such that the peg  154  passes into the bore of the TS and the walls of the TS are between bearing surfaces of the clamp  140  and the surfaces of the peg  154 . The bearing surfaces of the clamp are indicated as surfaces  309  in  FIG. 12 . They are surrounding the outer surface of the peg  154 . After the arm  185   a  has been secured to the plate  150 ′ via the fastener  184 , and with the pins or set screws  188  on the mating face portion of the arm  185   a  received in their matching holes  156  formed on the plate portion  155   b ′ (as discussed earlier), the end  11   b  is inserted into the opening  186  such that the walls of the end  11   b  are between the peg  154  and the bearing surfaces  309 . Once in position, the fastener  182  (not shown) is inserted into the hole  182   a , e.g., by inserting the tip of the fastener  182  first into the portion of opening  182  formed on arm  185   a . The tip of this fastener then engages the tapped hole  182   a  on the arm  185   a . When so engaged and the screw  182  turned clockwise the engaging threads cause the arm  185   b  to be pulled towards the arm  185   a  (since the arm  185   a  is fixed in place by the set screws, it does not move). The rate at which the arm  185   a  is brought towards the arm  185   b  may be controlled by the pitch on the threads of the fastener  182 . A higher or finer pitch can offer more control over the applied pressure as a function of the turning angle. A fastener having  32  threads per inch may be satisfactory to provide the desired amount of control over the clamp pressure and can provide an acceptable guarantee that the threads will not slip during testing, thereby possibly causing the TS to slip out of the clamp. A lockable fastener may be used to ensure the pressure applied to the TS during a test does not change. The desired amount of pressure to be applied to the TS may be controlled by using a tool in which the maximum torque applied to the fastener  182  can be controlled. 
         [0072]    The foregoing action of the fastener  182  and arm  185   b  may thus close the bearing surfaces  309  down upon the outer surface of the end  11   b  of the TS, thereby clamping the TS. The direction of motion in which the arm  185   a  is drawn to close the clamp is indicated in  FIGS. 12 and 13  by B and the structure described thereon. Referring briefly to  FIG. 13 , depicted are the locations of the set screws  188   a ,  188   b  relative to the hole  184   a  receiving the captive screw  184  and the tapped hole  182   a  that draws the arm  185   b  towards the arm  185   a  (thereby closing the clamp  140 ). From this drawing it will be appreciated that arm  185   a  is fixed in place (due to the set screws and captive screw) and as one observes the deflection of fingers (i.e., finger  304   a  through finger  304   f ) radially inward, it will be appreciated that the deflection inward increases as one moves clockwise in  FIGS. 13  from arm  185   a  towards arm  185   b . This aspect of the clamp  140  will be described in greater detail shortly. 
         [0073]    A peg  154  receivable in the TS&#39;s bore is preferred. Such an inner wall support can reduce chances of buckling of the piece when the surfaces  309  are brought down upon the TS, facilitate accurate mounting and allows greater pressure to be applied to the tube. The peg  154  may extend slightly forward of the ends of the bearing surfaces  309 , as shown, and the peg  154  may have a chamfered end to make it easier to place a tube on the peg  154 . In other embodiments the peg height may extend substantially further out from the ends of the surfaces  309  as this will further reduce instances of tube damage during assembly. Since such an embodiment may not be desirable for a transverse load test (since the extended length peg  154  can interfere with an intended deflection of the TS during testing). The extended peg embodiments may be preferred only for tensile load tests. 
         [0074]    Referring now to  FIG. 13  (a slice of the clamp  140 / 180  in a plane that is perpendicular to the longitudinal axis of the TS when received in the clamp), the clamp  140  is capable of applying a uniform radially inward pressure to the outer surface of the end  11   b  when received within the circle of bearing surfaces  309  depicted in  FIG. 13 . The clamp  140  structure is also capable of applying a pressure to the outer surface of a hollow cylindrical tube without a significant net torque applied to the tube as the arm  185   b  is pulled towards the arm  185   a . Such a requirement may be particularly important for an extruded polymer hollow tube of material of small dimensions, see e.g. examples of dimensions above, which are used to construct medical device because when there is a torsional preload (caused by the clamp) the resulting measured/computed mechanical properties (which typically assume no such preload is present in the material) will be inaccurate. For a solid tube the effects of torsion may not warrant a concern over a torsional preload applied by the clamp, as is known in the art. However, when a hollow tube with relatively thin walls is being tested, e.g., a stent tube, the effects of a torsional preload can be significant. 
         [0075]    There may be six, less than six, or more than six fingers  304  provided on the clamp  140 . The clamp  140  may exhibit the following stiffness properties, which will generally be described in terms of polar coordinates where the Z axis is out of plane, radial (r) and angular (A) are the in-plane radial and angular components of movement with origin at the center of the clamping area (i.e., where the end  11  of the TS is placed). The clamp is formed as an annular body  181  in which the arms  185  extend out from the open end (as discussed earlier). The clamp includes a ring  312  connecting the arm  185   a  to the arm  185   b . The ring  312  thickness is much less than the arm thickness  185   a  and thus the ring  312  is flexible in terms of radial deflection compared with the arms  185 . Located inside of the ring  312  are six fingers  304   a ,  304   b ,  304   c ,  304   d ,  304   e  and  304   f . Each finger includes a tip  308   a ,  308   b ,  308   c ,  308   d ,  308   e  and  308   f  that forms the bearing surfaces  309   a ,  309   b ,  309   c ,  309   d ,  309   e  and  309   f . Collectively, the bearing surfaces describe an arc length that extends approximate 360 degrees. Between each bearing surface  309  there is a spacing ta. When the clamp  140  is brought down upon the TS the space ta vanishes and the bearing surfaces  309  come together to form an essentially 360 degree contiguous bearing surface that retains the TS end in the clamp by the application of a 360 degree uniform pressure to the outer surface of the TS. 
         [0076]    The foregoing deformation of the clamp structure when clamped to the TS is such that a minimal net torque exists on the end  11  when the tips  308  come together. This result may be achieved by varying the thickness of the ring  312  sections, e.g., the section extending between portions  305   f  and  305   a  and having a thickness t 2 , the section extending between portions  305   f  and  305   a  and having a thickness t 3 , which is different from t 2 , etc. (see  FIG. 13 ). By varying the thickness, the stiffness of these ring sections can be increased or decreased relative to the each other, which produces a corresponding redistribution of the load about the clamp and applied to the TS through the bearing surfaces  309  of the fingers  304 . Thus, t 1  can be modified relative to t 2  (e.g., the average thickness of t 1  decreased relative to the average thickness of t 2 ) to effect the percentage of the total load applied by  308   e  or  308   f , t 2  can be modified relative to t 3  and t 1  to equalize the contributions to the total load originating from each of the fingers  304   a ,  304   e  and  304   f , etc. As will be understood in view of this disclosure, varying the stiffness of the ring  312  in this manner can produce a desired boundary loading on the TS during test, e.g., no net torque and/or each finger applying an equal amount of force to hold the TS in place during testing. It will also be understood that there are a variety of structural optimization methods/techniques known in the art which can be utilized in view of the disclosure for the purpose of producing a desired boundary condition for a hollow tube during testing. For example, a Finite Element Model (FEM) approach may used to determine the appropriate stiffness distribution for a test article and mounting system in view of this disclosure. 
         [0077]    While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications can be made without departing from this invention in its broader aspects. Therefore, the appended claims are to encompass within their scope all such changes and modifications as fall within the true spirit and scope of this invention.