Abstract:
Stresses or deformations can be applied to a specimen, by at least one carrier having a number of flexible fingers extending therefrom, the fingers being securable to the specimen at spaced-apart locations, the carrier typically being attached to a motion-control and load measuring system. The fingers are relatively rigid in tension (or compression if desired) but flexible with respect to motions in other in-plane directions. They may or may not be flexible in out-of-plane directions. Commonly, there may be two carriers on opposite sides of the specimen for applying substantially uniaxial stress, or four carriers at 90 degrees to each other around the specimen for applying substantially biaxial stress, though any number of carriers may be used.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to multiple point attachment to specimens, for biaxial or uniaxial loading, deformation or testing. 
   Situations exist where it desirable to induce specified loads, deformations or strain states in materials, specimens or objects or to restrain them against motions that would otherwise occur. These include situations where mechanical properties are determined by applying particular loads or combinations of loads in different directions or loading in one direction with or without restraint in another. Other situations include, but are not limited to, manufacturing processes, pre-conditioning of materials, use of stress or strain states to induce fiber alignment, crystallization in the material or situations where it is desired to otherwise regulate, alter or transform the material&#39;s properties or structure at the bulk, meso, micro or nano scale. 
   Attachment to the specimen is often of concern and, to produce specified internal strain or stress fields, careful design of the specimen and its loading system may be required, especially if uniform or other specified fields are desired or if large deformations are involved. 
   2. Description of the Prior Art 
   There are several methods known in the art used to load materials. One of the key difficulties is attachment of the load system to the specimen. 
   The three primary methods can be characterized as either structured specimen methods, substrate methods or attachment methods. 
   Structured Specimen Methods 
   Structured specimen methods are characterized as structuring the material into a geometry that facilitates loading and deformation control. A well-known version of this is the pressurized cylinder test, in which the material of interest either occurs as a cylindrical shape or is formed into one. The ends of the specimen are clamped to circular end plates. By controlling the pressure inside the hollow specimen and controlling the spacing between the end plates, through load or deformation, the stresses and strains in the hoop and axial directions can be controlled independently. Primary loading is in the local tangent plane to the specimen; i.e., the loading is “in-plane”. 
   This method has the advantage of allowing the in-plane conditions to be specified independently (subject to certain limitations). It works well for testing of tubes or blood vessels. 
   Substrate Methods 
   Substrate methods are characterized as affixing the material to be tested onto a substrate material. The substrate is then loaded by stretching it in a known manner, thus also stretching the material under test. 
   For example, Love et al. (U.S. Pat. No. 6,833,924 and others) teaches a method where the substrate is secured along its edges and is pressurized to form a dome having a height axis substantially perpendicular to its original plane. 
   In general with dome substrate methods, the induced stress or strain states are not uniform over the surface of the dome. This method has the advantage that it can perform multi-axial loading since the material is stretched biaxially. Subtracting the effects of the substrate is a source of error in the system. 
   Attachment Methods 
   The third method can be characterized as attachment methods. The invention is related to this classification of test method. The attachment method is generally characterized as taking a small section of the material (a specimen) to be tested and attaching to it in such a manner that the edge load or deformation can be specified in one or more axes. Attachment of the load to the material is known to be problematic in the art. Gripping, clamping, hooks and suture attachments are known. Substantially different experimental results can be obtained using different gripping methods on the very same specimen. Sun et al. (Journal of Biomechanical Engineering, August 2005, Vol. 127/709) teaches the importance of attachment geometry in planar biaxial testing. They conclude that suture based methods are a preferred attachment method for biaxial mechanical testing of biological materials. 
   Gripping or Clamping Methods. 
   One approach is to cut the specimen into a square and clamp along each edge as shown in  FIG. 1   a.    
   Another approach is cut the specimen into an “X” or “t” shape (also known as a cruciform method) and clamp on the arms as shown in  FIG. 1   b.    
   Attachment Point Methods 
   Still another approach is to have a number of attachment points along the edges of the specimen. For biaxial or uniaxial testing the attachments must be stiff in the direction in which they are pulling or pushing and flexible in the other in-plane direction. Sutures are attached through the specimen along the edge of the specimen, and such sutures are considered state of the art. They simultaneously satisfy the stiffness and flexibility requirements. Sutures are shown in  FIG. 1   c . Hooks attached to sutures have also been used as shown in  FIG. 1   d . This method reduces the artificial stiffening at the boundaries. 
   SUMMARY OF THE INVENTION 
   It is an object of the invention to provide an improved specimen attachment system for biaxial or uniaxial testing. 
   In the invention, a series of “fingers” are attached to a carrier, which is typically attached to a motion-control and load measuring system. The fingers are attached to the specimen at specific attachment sites. The fingers are relatively rigid in tension (or compression) but flexible with respect to motions in other in-plane directions. They may or may not be flexible in out-of-plane directions. 
   The invention provides an integrated specimen attachment system that can be engineered for specific loading conditions. It can be designed to have a different and controllable tension or compression in-axis, off-axis and out-of-plane. The attachment, connection and carrier elements are integrated into a single system facilitating handling and attachment and improving repeatability. A number of fingers can be used with consistent alignment to avoid unintentional variability. 
   Other features of the invention will be described or will become apparent in the following description of preferred and exemplary embodiments, taken together with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described in detail, with reference to the accompanying drawings of preferred and exemplary embodiments, in which: 
       FIG. 1   a  (prior art) is a schematic illustration of a prior art clamping method; 
       FIG. 1   b  (prior art) is a schematic illustration of a prior art cruciform method; 
       FIG. 1   c  (prior art) is a schematic illustration of a prior art suture method; 
       FIG. 1   d  (prior art) is a schematic illustration of a prior art hook method; 
       FIG. 2  is a schematic illustration of a complete multiple attachment point system for biaxial load mechanical testing, the invention being a part of this whole system; 
       FIG. 3   a  is a schematic illustration of a very simple embodiment of the invention, namely a multiple attachment point system for uniaxial or biaxial loading, hereinafter referred to as a uniaxial mount for simplicity; 
       FIG. 3   b  is a block diagram of a more complex variation of the  FIG. 2   a  embodiment, in which there are four of the fundamental building blocks shown in  FIG. 2   a;    
       FIG. 4   a  shows the invention under initial conditions; 
       FIG. 4   b  shows the invention under deformed conditions; 
       FIG. 4   c  shows the invention in a uniaxial configuration under deformed conditions; 
       FIG. 5  is a perspective view showing the creation and alignment of wire fingers and the attachment of the wire fingers to the carrier; 
       FIG. 6  a perspective view showing the termination creation of the wire fingers to form a simple cut end; 
       FIG. 7   a  shows the fingers being dipped in glue; 
       FIG. 7   b  shows the attachment of the fingers to a specimen; 
       FIG. 8   a  shows an example of the finger design being wire; 
       FIG. 8   b  shows an example of the finger design being foil; 
       FIG. 8   c  shows an example of the finger design having geometric variations from finger to finger to change stiffness; 
       FIG. 8   d  shows a pattern to allow close packing; 
       FIG. 8   e  shows and example of rigid out-of-plane fingers to facilitate attachment or loading or to provide through-thickness attachment; 
       FIG. 8   f  shows angled fingers to facilitate loading having a shear component or deformation that produces same; 
       FIG. 8   g  shows bi-directional corners; 
       FIG. 9   a  shows a variation where there is air between fingers (i.e. fingers fixed at carrier only); 
       FIG. 9   b  shows a variation where there is webbing (i.e. fingers having a secondary carrier between the primary carrier and the attachment to the specimen); 
       FIG. 10   a  shows an example of a simple finger termination, with attachment to the top surface of the specimen; 
       FIG. 10   b  shows a pad attachment to the top surface of the specimen; 
       FIG. 10   c  shows a hook or pin attachment by penetration through the specimen; 
       FIG. 10   d  shows a micro-grip attachment by gripping or friction with the top and bottom surfaces of the specimen; 
       FIG. 11  is a schematic illustration of a biaxial system with fiducial markers located on the finger carriers; 
       FIG. 12  is a schematic illustration of a subsystem located on each individual finger that provides individual force application, and finger linear displacement measurement; 
       FIG. 13   a  is a schematic illustration of a stress and displacement subsystem with individual sections that produce load and measure linear displacement; and 
       FIG. 13   b  is a schematic illustration of a further stress and displacement subsystem with individual sections that produce load and measure linear displacement. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 2  shows a complete biaxial load measurement system  10  schematically. A vision system  11  is used to measure the displacement of fiducial markers (not shown in  FIG. 2 ) on the specimen  16  during its initial state and during loading. Fiducial markers may be artificially placed on the test specimen  16  or they may be naturally occurring. A specimen can be almost anything but for this example only, consider the specimen to be a small amount of tissue from a human heart valve 4 mm square. Component  12   a  provides computer controlled linear motion causing stress [motion] in the X-axis and  12   b  provides similar stress [motion] in the Y-axis. Components  13   a  and  13   b  measure the load in the X and Y directions respectively. The measured stress is readable by the computer  15 . The vision system  11 , fiducial tracking methods, the loading system  12   a  and  12   b  and the load measurement system  13   a  and  13   b  are well known in the art and are not part of the invention. They are shown for clarification and understanding of the application and to increase the understanding of the problems that the invention are solving. The invention resides in the multiple attachment point system for uniaxial loading, further referred to herein as the “uniaxial mount” for simplicity. In this exemplary embodiment, there are four uniaxial mounts  20   a ,  20   b ,  20   c  and  20   d  according to the invention, forming the system defined by area  30  in  FIG. 2 . The uniaxial mount  20   a  is physically anchored to the X linear motion control  12   a  and the uniaxial mount  20   c  is physically anchored to the stationary X load measurement system  13   a . The uniaxial mounts  20   b  and  20   d  are likewise attached to the Y axis linear motion control  12   b  and Y load measurement system  13   b  respectively. The sections of the individual uniaxial mounts and the combined biaxial mount system in area  30  will be described in more detail below. 
   In a typical test, the specimen  16  is attached to uniaxial mounts  20   a ,  20   b ,  20   c  and  20   d , as described in detail below. The resulting biaxial mount system  30  is placed in the test system  10 . A computer  15  will control the amount of load placed in the X and Y-axes (by components  12   a  and  12   b  respectively) while measuring the displacement of the fiducials of the specimen  16  with the vision system  11 , while at the same time measuring the stresses in the X and Y axes (by components  13   a  and  13   b  respectively). These measurements taken together allow the system to determine mechanical properties of the specimen  16 . 
     FIG. 3   a  shows a uniaxial mount  20  in more detail. A series of “fingers”  21  are attached to a carrier  22 . The fingers  21  are attached to the specimen  16  at specific attachment sites  23 . 
     FIG. 3   b  shows a typical biaxial mount system  30  according to the invention in more detail. There are four uniaxial mounts  20   a ,  20   b ,  20   c  and  20   d  placed at 90 degree increments around the specimen  16 . Tabs  24   a ,  24   b ,  24   c  and  24   d  are located between the uniaxial mounts and act as an extension of the carrier  22 . These tabs are optional and can be used to simplify manufacturing or may be used to increase mounting options. The tabs have been shown in system  30  as being break-away in nature, but other attachment methods are possible, as known in the art. The uniaxial mounts  20   a  and  20   c  are free to move in the X direction and are attached to the loading subsystem  12   a  and  13   a . The uniaxial mounts  20   b  and  20   d  are free to move in the Y direction and are attached to the loading subsystem  12   b  and  13   b  respectively. 
   Referring again to  FIG. 2  and  FIG. 3   b , stress is applied in the X direction by the X load  12   a  when signaled by computer  15 . The fingers  21   a  are parallel to the X-axis and transfer the stress to the specimen  16  in the X direction. The X fingers  21   a  are rigid in the X direction and flexible in the Y direction. The fingers may or may not be flexible out-of-plane in the Z direction. The Y fingers  21   b  are parallel to the Y axis and they will bend freely in the X direction as stress is placed on X fingers  21   a.    
   Similarly, stress is applied in the Y direction by the Y load  12   b  when signaled by computer  15 . The fingers  21   b  are parallel to the Y-axis and transfer the stress to the specimen  16  in the Y direction. The Y fingers  21   b  are rigid in the Y direction and flexible in the X direction. The fingers may or may not be flexible out-of-plane in the Z direction. The X fingers  21   a  are parallel to the X-axis and they will bend freely in the Y direction as stress is placed on Y fingers  21   b.    
   When no load is present in the X and Y directions, the specimen  16  is undisturbed, as shown in  FIG. 4   a . When stress is applied in both the X and Y directions simultaneously, then the specimen  16  is deformed, as shown in  FIG. 4   b.    
     FIG. 4   c  shows a uniaxial configuration in which stress is applied in the X direction only, using two uniaxial mounts  20   e ,  20 . Note the deformation of the fingers  21  in the Y direction that occurs due the material properties of the test specimen. The uniaxial mount  20  device has been optimized for biaxial testing but  FIG. 4   c  shows that it is also very suitable as an improved multiple point attachment system for uniaxial testing. 
   Fingers  21  can be designed to have different tension or compression in the primary axis direction, off-axis direction and out-of-plane directions, as needed by the application. Specifically for X fingers  21   a  the primary axis direction is X, the off-axis direction is Y and the out-of-plane direction is Z. Similarly, for Y fingers  21   b  the primary axis direction is Y, the off-axis direction is X and the out-of-plane direction is Z. 
     FIGS. 5 ,  6 ,  7   a  and  7   b  illustrate the construction of one specific embodiment of the uniaxial mount  20 . In this embodiment, the fingers  21  are made from stainless steel wire  27 , 0.004 inches in diameter, and the carrier  22  is made from acrylic. The wire is wound on an alignment frame  25  as shown in  FIG. 5 . Tension is applied by turning the key  26  to ensure the wires  27  are parallel and placed as required in a repeatable and controlled manner. The wires  27  are placed touching the surface of the carrier  22 . A fixture can be used to ensure repeatable placement of the wire relative to the carrier  22 . The wires are glued to the carrier with cyanacrylate  28 .  FIG. 6  shows the wire being cut with a sharp chisel-like punch, forming a simple cut end. This entire process is repeated for the perpendicular direction and then the uniaxial mount  20  is complete. 
     FIGS. 7   a  and  7   b  show a variation of the method of attaching specimen  16  to the uniaxial mount  20 . In this variation, the fingers  21  are first placed onto the glue cyanacrylate  28  ( FIG. 7   a ). Then the fingers are placed onto specimen  16  at the specific attachment sites  23  ( FIG. 7   b ). The fingers  21  are still covered with glue, which will bond to the specimen  16 . 
     FIGS. 8   a - 8   g  show variations of the design of the fingers  21 . In general, parameters of the finger tension properties in-axis, off-axis and out-of-plane can be engineered by changing the geometry and/or the materials. Tension variance along the length of the finger  21  can be obtained by varying the cross sectional geometry and/or varying the material composition along the length. A composite material can be used. Other methods of construction are listed below. 
     FIG. 8   a  shows the use of wire, as in the above description. Wire is relatively easy to work with but does not have the advantage of configurable anisotropic behavior off-axis and out-of-plane. 
     FIG. 8   b  shows the use of foil, which has nearly ideal properties for many applications of this invention. It can be mass manufactured with various precise methods described below. By changing the dimensions of the material in width and thickness different tension can be obtained off-axis and out-of-plane as desired. 
     FIG. 8   c  shows the use of geometric variations. These geometric variations from finger to finger can change stiffness across the specimen as desired. 
     FIG. 8   d  shows the use of a pattern which allows close packing of attachment points  23  to the specimen  16 . 
     FIG. 8   e  shows the use of a rigid out-of-plane foil material, having a very thick finger relative to the width, causing the out-of-plane stress to be very rigid. 
     FIG. 8   f  shows a finger arrangement which is angled to facilitate loading having a shear component or deformation that produces same. 
     FIG. 8   g  shows the use of bi-directional corners which can be easily produced, where the corner fingers are attached to the same attachment site  23  of the specimen  16 . This facilitates alignment of four elements. 
   In addition, composite materials can be produced that have different properties in different axes. The composition can be also varied along the length of the finger  21 . 
     FIGS. 9   a  and  9   b  show variations of the space between fingers  21 . In  FIG. 9   a  there is no material between the fingers  21  in the space between the frame of the carrier  22  and the specimen  16 . In  FIG. 9   b  the fingers have a secondary carrier (webbing)  29  between the primary carrier  22  and attachment to specimen  16 . This webbing can be engineered to change the stress properties of the fingers off-axis as desired. 
   An engineered material with anisotropic mechanical properties can be used to fabricate a unified finger. 
     FIGS. 10   a - 10   d  show variations of the termination of fingers  21 . 
     FIG. 10   a  shows a simple termination, where the finger  21  terminates abruptly with a simple cut. Attachment of this type of finger is made to the top surface of the specimen  16  with glue  28 , for example. 
     FIG. 10   b  shows a pad, with the finger being deformed at the termination end to form a larger surface area for attachment to the top surface of the specimen  16  with glue  28 . 
   Hook or pin:  FIG. 10   c  shows a sharp mechanical structure  32 , i.e. a hook or pin, inserted through the specimen  16  and through two sides of the fork  31 . This is designed for attachment by penetration through the specimen  16 . 
     FIG. 10   d  shows a micro-grip, i.e. a set of jaws  33  designed for attachment by gripping the top and bottom surfaces of the specimen  16 . 
     FIG. 11  shows a method of measuring the finger carrier  22  position. Fiducial markers  40   a - 40   d  are placed on the finger carrier  22 . These markers are visible to a machine vision system  11  (see  FIG. 2 ). The markers may be simple dots, rings or patterns that are more complex. Generic Target patterns are shown in  FIG. 11 . 
   A single target such as  40   a  and  40   b  would allow the displacement of the carrier  22  to be measured. Two degrees of freedom can be measured from a single target constrained to planar motion. Typically, the motion is further constrained to a single axis. Fiducial targets  40   d  show two targets and  40   c  shows the use of three targets. The plurality of targets are used for determining higher degrees of freedom for the position and orientation of the carrier  22 , if such information is of interest, and if the carrier has freedom of movement. As is well known in the art, 5 degrees of freedom can be determined from the 3D position of 2 targets, and 6 degrees of freedom can be determined from the 3D position of 3 targets. The relative location of the targets is known. 
   Referring now to  FIGS. 12 ,  13   a  and  13   b , a load displacement subsystem  50  will be described. This subsystem is used on individual fingers  21  in conjunction with the full system  10  ( FIG. 2 ). It is not a requirement that each finger  21  be instrumented with subsystem  50 .  FIGS. 13   a  and  13   b  are two variants of the manner of implementation. 
   The following is common to both variants. The finger  21  is held in place by a plurality of beams or struts  51  of which two are shown in  FIGS. 13   a  and  13   b . The finger  21  is considered at position zero when the beams are in their minimum stress state. The beams are anchored to a frame  52 . The end of the finger  21  connected to the beams is constrained in motion to a single axis along the length of the finger  21 . The force  55  applied along finger  21  will cause the beams  51  to deflect. 
   The displacement of the finger  21  can be determined by the use of a plurality of capacitor plates  54   a ,  56   a  or other position sensor of which two are shown in  FIG. 13   a . and  FIG. 13   b . Plate  54   a  is attached to the finger and is free to move along one axis. Plate  56   a  is a stationary plate anchored to frame  52 . The capacitance varies linearly and directly proportional to the displacement of the finger  21 . As is known in the art the capacitance can be measured and the displacement is therefore determined from the capacitance. Likewise, the position of the anchor frame  52  can optionally be determined by using capacitor plates  54   b  and  56   b  as shown in  FIG. 13   b.    
   Methods other than variable capacitance are also known in the art for displacement measurement, including for example variable resistive measurements, optical targets, triangulation, etc. 
   The subsystem  50  has at least two variations of operation. In the first variant as shown in  FIG. 13   a , the beams  51  have a non-negligible stiffness and the anchor frame  52  is stationary. The force  55  upon the finger  21  is determined by measuring the displacement of the beams  51  using the variable capacitor  54   a ,  56   a  and knowing the mechanical properties of the beams  51 . This method of using beam  51  displacement equations for force measurement allows for the determination of extremely small forces that cannot be measured with state of the art force transducers. In this mode all subsystems  50   a  connected to the carrier  22  in  FIG. 12  are displaced in the same amount. It is then possible to measure the individual forces on each finger  21  that has been instrumented with subsystem  50   a.    
   In the second variant as shown in  FIG. 13   b , the beams  51  have a non-negligible stiffness and the anchor frame  52  is attached to a linear actuator  53 . The actuator  53  is stationary and can displace the anchor frame  52 . The force  55  upon the finger  21  is determined by measuring the displacement of the beams  51  using the variable capacitor  54   a ,  56   a  and knowing the mechanical properties of the beams  51 . In this variant all subsystems  50   b  can be displaced at an individual amount, thus allowing each finger  21  to have an individually applied load. It is then possible to both measure and control the individual forces on each finger  21  that has been instrumented with subsystem  50   b . This variation allows for interesting use of feedback from either the optional anchor displacement measurement capacitors  54   b  and  56   b  and the beam measurement capacitors  54   a  and  54   b . If the linear actuator  53  displacement is not sufficiently accurate, the anchor displacement measurement capacitor  54   b  and  56   b  can be used in closed loop. Insufficient open loop accuracy is very likely when using a miniature actuator that may have displacements dependant on the load seen by the actuator as well as the actuator control signal. 
   The force measurement provided by beam displacement measurement capacitor  54   a  and  56   a  can be used in a closed loop path controlling the displacement of linear actuator  53  such that a specified force  55  can be arbitrarily and accurately applied to finger  21 . 
   Subsystem  50  may be implemented using micro electronic machining (MEMs) technology. The linear actuator  52  can be implemented with a MEMs electrostatic device. 
   The uniaxial mount  20  can be secured to the specimen  16  by any suitable means, including the following: 
   a. Adhesion methods 
   b. Glue (eg., cyanacrylate) 
   c. Biological (eg., fibronectin) 
   d. Mechanical (eg., hook) 
   e. Surface treatment surface texture 
   f. Bioactive surface (eg., bioglass) 
   g. Freezing (e.g., as by pre-cooling of the fingers) 
   h. Heating (e.g., as by cauterizing) 
   The uniaxial mount  20  can be fabricated by any suitable method, including the following: 
   a. Micromachining (e.g., laser cutting) 
   b. Microinjection molding 
   c. Micro-assembly 
   d. Stamping 
   e. Microcircuit fabrication methods 
   f. Etching 
   g. Deposition 
   The uniaxial mount  20  can be made from any suitable material, including the following: 
   a. Metal 
   b. Glass 
   c. Polymer 
   d. Elastomer 
   e. Ceramic 
   f. Composite 
   g. Microcircuit material 
   Although only the wire finger embodiment is thoroughly described above, other variations as itemized above will be readily appreciated by those knowledgeable in the field of the invention. 
   It should also be readily understood that although most references above are to two or four carriers, any number of carriers could be used, and they could be offset from each other by any desired angle or angles, to apply stresses or deformation to a specimen in any desired directions. For example, there could be three carriers, likely but not necessarily offset from each other by 120 degrees. Or there could be 5, 6 or more carriers, in order to produce a corresponding number of different loading points on the specimen. 
   Similarly, it should be understood that although the vast majority of applications for the invention will have the fingers in tension, the invention may be readily adapted to apply compressive forces. Obviously, in such cases the fingers would have to have sufficient rigidity to prevent buckling thereof.