Abstract:
A device, assembly and method for clamping single fibers that utilizes snubbing surfaces simultaneously with magnetic gripping action are described. The device can be fabricated at miniature sizes with very low mass and volume, which are desirable requirements for testing of small fibers in highly sensitive force-transducing load frames. Advantageously, the combined use of snubbing surfaces and magnetic gripping greatly reduces the probability of slippage or fiber failure at the grip during testing.

Description:
BENEFIT OF PRIORITY 
       [0001]    Applicants hereby claim benefit of U.S. Provisional Application No. 61/450,309 titled “Single Fiber Snubbing Clamp Using Magnetic Gripping Action” filed on Mar. 8, 2011 which is hereby incorporated by reference herein. 
     
    
     GOVERNMENT INTEREST 
       [0002]    The invention described herein may be manufactured, used, and licensed by or for the United States Government. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    The ability to test the tensile strength and other mechanical properties of single fibers is of great importance for researchers, engineers, and manufacturers. Single fiber testing can be used to isolate trends and modes of failure for materials, such as for fundamental materials investigations or product-line quality control testing. During development of new fiber materials, the new fibers are often available in very small quantities due to complex processing or exotic materials. In these cases, multi-filament yarn-level testing is not practical. Instead, it is desired to generate mechanical properties of individual filaments. 
         [0004]    Developments in capacitive load cells, electromagnetic load cells, and stepper motor technology have led to the commercialization of compact load frames with extremely sensitive force (for example, nanoNewton (nN) sensitivity) and displacement (for example, nanometer (nm) sensitivity) measurement capabilities. These miniature load frames are ideally suited for testing single fibers. 
         [0005]    Standard testing procedures for single fibers [1]  call for directly gripping single fibers in clamping action grips, or gluing the fibers onto cardboard tabs that are then gripped in clamping action grips. The first technique, direct clamping, concentrates stresses at the grip point and often leads to failure at the fiber grips [2 ]. Grip failure is undesirable during fiber characterization, and these results are generally not accurate, and, thus, are not reportable. The second technique, bonding fibers onto cardboard tabs, relies on strong bonding between the fiber and a dollop of adhesive. For fibers that are difficult to bond to, such as aramid, polyethylene, or ultrahigh molecular weight polyethylene, it is common to observe fiber slippage from the adhesive. These results are also not accurate and, thus, are not reportable, and generally prevent loading the fiber to failure. In addition, the time and complication associated with utilizing curable adhesives adds considerable inconvenience, especially considering that many test repetitions are required to establish statistically significant data. Accordingly, there is a need for a method, a device and/or an assembly for clamping or otherwise securing a fiber in a tensile testing device. 
         [0006]    For testing yarns, it is common to use capstan grips (also called roller grips) or contour/horn grips (also called half-capstan grips) [2, 3] . In these designs, from the primary loading gage section of the yarn the yarn is passed over a series of “snubbing” surfaces which gradually reduce the tensile load on the yarn due to frictional interactions of the yarn with the snubbing surfaces. The snubbing surfaces are curved gently to induce normal forces on the yarn without creating stress concentrations that can fail the yarn. After passing over these snubbing surfaces, the yarn is gripped mechanically, typically using mechanical or pneumatic clamping action. Because the tensile loading on the yarn has been gradually reduced, the total stress on the yarn at the grip point is significantly less likely to lead to failure at the grip. 
         [0007]    The present invention improves upon previous single fiber techniques by providing a single fiber clamping device that can reliably grip single fibers and decrease slippage and/or failure at the grips. In addition, no adhesive is required, so that gripping is fast and uncomplicated. Furthermore, the device uses magnetic clamping action to grip the fibers, so that the total mass and volume of the clamping device are minimized. Maintaining low clamping device mass is desirable for operation in sensitive load frames, since the total load limits (including clamp weight) are typically very low for these miniature load frames (for example, 1 N maximum force capacity). For these applications, traditional clamping approaches such as mechanical action or pneumatics are undesirable because they are either impractical or cumbersome to implement. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention is a single fiber clamping device that utilizes capstan style snubbing surfaces to gradually reduce fiber tension as it approaches the clamping point, similar to the capstan-style grips used for testing of yarns. Magnetic forces are used to provide the clamping action on the fiber after the snubbing surfaces. The geometry of the device can be optimized for compactness, low mass, and manufacturability. 
         [0009]    In addition, a method for determining a mechanical property of a fiber or a yarn or other group of fibers is provided. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    A better understanding of the present invention will be had upon reference to the following detailed description when read in conjunction with the accompanying drawing, wherein like reference characters refer to like parts throughout the several views, and in which: 
           [0011]      FIG. 1A  is a plan view of an exemplary device of the present invention; 
           [0012]      FIG. 1B  is a cross-section view of the device of  FIG. 1A ; 
           [0013]      FIG. 2A  is a plan view of another exemplary device of the present invention; 
           [0014]      FIG. 2B  is a cross-section view of the device of  FIG. 2A ; 
           [0015]      FIG. 3A  is a plan view of an exemplary method of the present invention; 
           [0016]      FIG. 3B  is a cross-section view of the method of  FIG. 3A ; 
           [0017]      FIG. 4A  is a plan view of another exemplary method of the present invention; 
           [0018]      FIG. 4B  is a cross-section view of the method of  FIG. 4A ; 
           [0019]      FIG. 5A  is a plan view of yet another exemplary method of the present invention; 
           [0020]      FIG. 5B  is a cross-section view of the method of  FIG. 5A ; 
           [0021]      FIG. 6A  is a plan view of yet another exemplary method of the present invention; 
           [0022]      FIG. 6B  is a cross-section view of the method of  FIG. 6A ; 
           [0023]      FIG. 7A  is a plan view of yet another device and another exemplary method of the present invention; 
           [0024]      FIG. 7B  is a cross-section view of the method of  FIG. 7A ; 
           [0025]      FIG. 8A  illustrates an exemplary method of the present invention using a pair of devices illustrated in  FIGS. 1A and 1B ; 
           [0026]      FIG. 8B  illustrates another exemplary method of the present invention using a pair of devices of the present invention; 
           [0027]      FIG. 8C  illustrates yet another exemplary method of the present invention using a pair of devices of the present invention; and 
           [0028]      FIG. 8D  illustrates still another exemplary method of the present invention using a pair of devices of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0029]      FIG. 1A  illustrates a plan view and  FIG. 1B  illustrates a cross-sectional view of one exemplary embodiment of device of the present invention. The exemplary device is a device that can be used to grip an individual fiber and, thus, test the tensile strength of the single fiber on a testing system and device, for example the testing system described in U.S. Pat. No. 6,679,124 to MTS Systems which is hereby incorporated by reference herein. In certain embodiments, two devices of the present invention can be used to replace the upper specimen holder and lower specimen holder, items designated as  14 A and  14 B, respectively, in  FIG. 1  of the material testing system  10  illustrated in U.S. Pat. No. 6,679,124. 
         [0030]    An assembly of the present invention includes two devices, for example an upper specimen holder and a lower specimen holder, where each device includes a base  2 . A feature of base  2  is a toroidally contoured hole  1 . Hole  1  may be machined into base component  2 . Desirably, base  2  is composed of or otherwise includes a magnetizable alloy, for example an alloy or metal that will attract a magnet such as free-machining steel. The base may include only a surface portion that includes or is composed of a magnetizable alloy so that at least a portion of a surface of base  2  can be used to attract a magnet. A fore edge  8  of the base  2  device  10  is desirably machined to a rounded, semi-cylindrical contour. To grip a fiber  3 , a fiber from the gage direction of a testing system is passed over a portion of first face  9  of base  2  through the hole  1 . The fiber  3  is then passed over a second face  9 ′ of base  2  and routed back towards the gage direction. A smooth, low mass clamping magnet  4 , such as a polished rare earth permanent magnet, is then placed on top of the fiber  3 . In certain embodiments the magnet can be an electromagnet or a non-metallic ceramic magnet. The magnetic attraction of the clamping magnet  4  to a surface of the base component  2  provides clamping action. A threaded rod (not illustrated) is mounted to the aft edge  7  of the base component via a threaded hole  5 . The threaded rod can then be mated to the load frame using standard mechanical hardware. 
         [0031]      FIGS. 2A and 2B  illustrate a plan view, a top view and a cross-sectional view of an alternate embodiment of an assembly  20  of the present invention. The base  2  illustrated in  FIGS. 2A and 2B  includes a slot  6  that connects a side edge of base  2  to hole  1  so that a fiber  3  can be more easily and quickly threaded through hole  1 . To grip a fiber  3 , a fiber from the gage direction of a testing system is passed over a lower portion of first face  9  of base  2  through the hole  1  or though slot  2  into hole  1 . The fiber  3  is then passed over a lower portion of second face  9 ′ and routed back towards the gage direction. Although, the cross-sectional views in  FIGS. 1A and 1B  and  FIGS. 2A and 2B  illustrate both sides of the leading fore edge as rounded, it is sufficient for some embodiments that only one side of the leading edge is rounded. 
         [0032]      FIG. 3A  and cross-sectional view  3 B show a detailed view of an alternate fiber routing for the device  20  illustrated in  FIGS. 2A and 2B , thus, providing an alternate method of securing a fiber. The base component  2  and clamping magnet  4  are geometrically identical to the device  20  of  FIGS. 2A and 2B . However, in this embodiment fiber  3  is further routed along the snubbing surface by passing fiber  3  over a portion of surface  9 ′, through hole  1  over a portion of surface  9 , over fore edge  8 , and over a portion of surface  9 ′ again. Fiber  3  is then clamped with magnet  4 . As used herein, the term “snubbing surface” is a fixed surface around which a fiber is routed, so that when the fiber is loaded in tension there are forces generated between the fiber and the surface that restrain or otherwise suppress the movement of the fiber sufficiently so that the fiber can be tested for mechanical properties, for example a tensile test. The clamping magnet is positioned on edge to ensure that only the fully snubbed fiber is clamped. This embodiment, as compared to the embodiment of  FIGS. 1 and 3   a , provides a longer snubbing length and therefore is less likely to lead to fiber failure in the grip. 
         [0033]      FIG. 4A  and cross-sectional view  4 B show a detailed view of an alternate fiber routing for the device  10  illustrated in  FIGS. 1A and 1B , thus, providing yet another method of securing a fiber. The base component  2  and clamping magnet  4  are geometrically identical to the device  10  of  FIG. 1 . However, in this embodiment fiber  3  is routed along the snubbing surface by passing fiber  3  over the lower portion of surface  9 ′ through hole  1  over the lower portion of surface  9  and fore edge  8  and over surface  9 ′ again. Fiber  3  is then clamped with magnet  2 . The magnet  2  can be positioned on-edge to ensure that only the fully snubbed fiber is clamped. This method as compared to the method illustrated in  FIG. 1 , also provides a longer snubbing length, for example 360 degrees or one full wrap around a portion of the base  2 . 
         [0034]      FIG. 5A  and accompanying cross-sectional view  5 B shows yet another embodiment of a method of the present invention. The base component and clamping magnet are geometrically identical to the device  20  of  FIGS. 2A and 2B . However, in this case, the fiber  3  is routed along an even longer snubbing length than illustrated in the methods of  FIGS. 3A and 3  B, with the clamping magnet  4  used to clamp the fiber  3  above the threading hole  1 . To grip a fiber, a fiber  3  from the gage direction of a testing system is first passed over a lower portion of first face  9 ′, through hole  1  or slot  2  into hole  1 , passed over the lower portion of second face  9 , over fore edge  8 , over the lower portion of surface  9 ′ again and hole  1  and then over the upper portion of surface  9 ′. Fiber  3  is then clamped to the upper portion of surface  9 ′ with magnet  4 . This embodiment utilizes an even longer snubbing length, and is therefore even less likely to lead to fiber failure at the grip. 
         [0035]    Further embodiments of this design, assembly and/or method can be easily envisioned including, but not limited to, embodiments utilizing longer or more complex routing paths. 
         [0036]      FIG. 4  illustrate an embodiment of a device of the present invention that includes a slit  6 . In this illustrated embodiments, a lateral slit  6  is provided in the base component that extends from one side of the base and connects to the threading hole. This slit  6  does not affect the mechanical performance of the grip or snubbing surfaces, but can greatly simplify loading of a fiber  3  since the fiber end does not need to be passed end-first through the contoured hole  1 . Modifications such as rounding the slit or orienting the slit at a different angle are envisioned. 
         [0037]      FIG. 7A  and a cross-sectional view provided in  FIG. 7B  show yet another embodiment of the device. In this design, a machined cylindrical hole  17  is machined into the device so that the initial load path of the fiber  3  is more centered relative to the center of the threaded hole  5 . This design ensures that the load path of the fiber  3  is more parallel to the load path of the mounting hardware on the load frame of a testing system, such as the testing system described in U.S. Pat. No. 6,679,124. 
         [0038]    It may be desirable to polish the snubbing surfaces of the base component, as well as the faces of the magnet, to eliminate sharp edges or other stress concentrations that may prematurely lead to fiber failure. Rare earth magnets are preferred for the clamping magnet due to their extremely high magnetic forces relative to other magnet types. 
         [0039]    It is envisioned that the present gripping device, including clamping magnet, will weigh less than 1 kg and be used to test fibers of diameter 1 mm and smaller. More preferably, the device will weigh less than 100 g and will be used to test fibers of diameter 100 micrometers and smaller. In certain desirable embodiments, devices of the present invention can be used to hold and test fibers having an average diameter of less than 50 micrometers, or more preferably, having an average diameter less than 40 micrometers, or more preferably having an average diameter less than 30 micrometers or even more preferably having an average diameter of less than 25 micrometers. 
         [0040]      FIG. 8A  illustrates a pair of exemplary devices and clamping method illustrated in  FIGS. 1A and 1B  that can be used as upper and lower specimen holders in the testing system described in U.S. Pat. No. 6,679,124.  FIG. 8B  illustrates a pair of exemplary devices of  FIGS. 1A and 1B  using the clamping method illustrated in  FIGS. 4A and 4B  providing a longer snubbing length, for example one full wrap around a portion of the base  2  that is less likely to lead to fiber failure in the grip.  FIG. 8C  illustrates a pair of exemplary devices of  FIGS. 1A and 1B  using the clamping method illustrated in  FIGS. 5A and 5B  providing an even longer snubbing length, for example one full wrap around the lower portion of the base  2  and extending to the upper portion of base  2  that even is less likely to lead to fiber failure in the grip.  FIG. 8D  illustrates a pair of exemplary devices of  FIGS. 7A and 7B  that provides a load path for the fiber  3  that is more parallel to the load path of the mounting hardware on the load frame of the testing system (not illustrated). 
         [0041]    Having described our invention, many modifications thereto will become apparent to those skilled in the art to which it pertains without deviation from the spirit of the invention as defined by the scope of the appended claims. 
       REFERENCES 
       [0000]    
       
         1. ASTM Standard D 3822-07. “Standard Test Method for Tensile Properties of Single Textile Fibers.” ASTM International, West Conshohoken, Pa. 2007. 
         2. J. Scott Thornton, Sam R. Allen, and Shawn L. Arnett. “Effect of Gripping Technique on Tensile, Tensile Creep and Tensile Creep-Rupture Results for a High Tenacity Polyester Yarn.” In  Grips, Clamps, Clamping Techniques, and Strain Measurement for Testing of Geosynthetics.  P. E. Stevenson, Ed. ASTM STP 1379. ASTM International, West Conshohoken, Pa. p 47-67. 2000. 
         3. ASTM Standard D 885-04. “Standard Test Methods for Tire Cords, Tire Cord Fabrics, and Industrial Filament Yarns Made from Manufactured Organic-Base Fibers.” ASTM International, West Conshohoken, Pa. 2004.