Abstract:
A holding clamp for use in a testing apparatus for determining tensile and sheer strength characteristics of a reinforcement sheet. The holding clamp defines a channel having at least two adjacent bearing surfaces and an opening to an exterior surface. An elongate clamping bar conforming in cross-sectional shape at least relative to the pair of adjacent bearing surfaces is received within the channel with an end portion of the reinforcement sheet overwrapping the clamping bar and a portion of the reinforcement sheet extends outwardly through the opening for securing in the testing apparatus. A loading mechanism moves the clamp to load the reinforcement sheet, whereby the tensile or sheer strength characteristics can be determined. A method of clamping an end of a reinforcement sheet to be tested for tensile or sheer strength characteristics is disclosed for a testing apparatus.

Description:
TECHNICAL FIELD 
     The present invention relates to testing equipment for determining strength characteristics of sheets. More particularly, the present invention relates to clamps for testing equipment, which clamps uniformly grip end portions of sheets to be loaded for testing of tensile or shear strength characteristics. 
     BACKGROUND OF THE INVENTION 
     Elongate sheet materials are commonly used in mechanically stabilized earth retaining walls. The retaining walls are constructed with modular precast concrete members in the form of blocks or panels that stack on top of each other to create the vertical facing of the wall. The sheet materials extend laterally from connections with the blocks in the wall. The sheet materials are construction devices used to reinforce earthen slopes retained from slippage by the retaining wall, particularly where changes in elevations occur rapidly, for example, site developments with steeply rising embankments. These embankments must be secured against collapse or failure to protect persons and property from possible injury or damage caused by the slippage or sliding of the earthen slope. 
     Many designs for earth retaining walls exist today. Wall designs must account for lateral earth and water pressures, the weight of the wall, temperature and shrinkage effects, and earthquake loads. The design type known as mechanically stabilized earth retaining walls employ either metallic or polymeric tensile reinforcements in the soil mass. The tensile reinforcements extend laterally of the wall formed of facing units, typically precast concrete members, blocks, or panels stacked together. The tensile reinforcements connect the soil mass to the blocks that define the wall. The blocks create a visual vertical facing for the reinforced soil mass. The polymeric tensile reinforcements typically used are elongated lattice-like structures often referred to as grids. These are stiff polymeric extrusions. The grids have elongated ribs which connect to transversely aligned bars thereby forming elongated apertures between the ribs. Tensile reinforcements other than grids have been developed for use with mechanically stabilized earth retaining walls. These other tensile reinforcements are flexible reinforcement sheets, including large open-grid woven lattices and small aperture woven lattices, as well as woven textile sheets. 
     The specifications for earth retaining walls are based upon the strength of the interlocking components and the load created by the backfill. Once the desired wall height and type of ground conditions are known, the number of sheets, the vertical spacing between adjacent sheets, and lateral positioning of the sheets is determined, dependent upon the load capacity of the interlocking components. 
     To design an earth retaining wall, various strengths of the sheet material must be known in order to meet the specification for the site requirements. Sheet materials are tested to determine the tensile strength and also to test shear resistance to pullout. Tensile strength testing considers whether the sheet itself will fail by tearing. Pull-out resistance considers whether the sheet insufficiently engages the backfill material and thus slips laterally through the backfill. Testing of sheet materials is typically accomplished by independent labs. Design engineers use the test data to select the type and supplier of sheets for retaining wall projects. 
     During testing, at least one end of the sheet is secured by a clamp. There are a number of different types of clamps. Compression clamps secure the test sheet between two elongate members to which a compression loading is applied. Epoxy clamps use molding epoxy to form a build-up body around the test specimen. Bolts pass through the epoxy body for connecting to the test apparatus. Split wedge clamps apply a gripping force on the test sheet passing through a split wedge body. Roller grips wrap opposing portions of the test sheet around rollers. 
     For tensile strength testing, the opposing end of the sheet is secured by another clamp. Loading is then applied to the test sheet. A load cell measures the amount of force being applied to the test sheet. As the loading force continues to increase, the sheet ultimately fails. The tensile strength of the sheet is recorded. 
     Pullout testing examines the resistance of the sheet to pulling out from backfill material or from between the blocks in the wall. For backfill shear resistance testing, a portion of the test sheet is embedded with backfill material in a soil box. For normal load (block wall pullout), the sheet is placed between blocks used in constructing the wall. Loading is applied to move the clamp laterally. When the sheet pulls out from the soil box or from between the blocks, the shear resistance is recorded. 
     One drawback to some of these testing devices is that the loading is concentrated at the point of attachment. Accordingly, the failure of the sheet sometimes occurs at the point of attachment, rather than the failure occurring in an intermediate portion of the sheet. The testing therefore, does not provide a true measure of the tensile strength of the sheet material, but rather provides an indication of the strength at a concentrated point. 
     In some tests, the grip by the clamp on the test sheet slips. The higher the loading, the more likely the incidence of slippage in the clamp. This of course does not accurately test the sheet. 
     Also, clamps for some testing devices are time consuming to assemble and attach to the test sheet. Epoxy clamps particularly require a curing period, and the epoxy materials are single use only which increases costs. Split wedge clamps are awkward to assemble with the test sheet. 
     Accordingly, there is a need in the art for an improved clamp for use in testing apparatus for determining the tensile and pull-out resistance strengths of sheets. It is to such that the present invention is directed. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention meets the need in the art by providing a holding clamp for sheet testing apparatus, which holding clamp engages an end portion of a reinforcement sheet to be tested for tensile or pull-out strengths. The holding clamp defines a channel having at least two adjacent bearing surfaces and an opening between the bearing surfaces to an exterior surface. The channel receives an elongate clamping bar that conforms in cross-sectional shape at least relative to the pair of adjacent bearing surfaces. An end portion of the test sheet wraps around the clamping bar, which is received in the channel with the sheet extending laterally through the opening. Depending on the testing, the opposing end of the sheet is secured by another holding device or by the backfill material or blocks. A loading device applies an increasing load to the sheet through the clamp for determining the strength of the sheet. The clamping bar mechanically engages the bearing surfaces of the channel to distribute the tensile loading to the bearing surfaces of the block. 
     In another aspect, the present invention provides a method of securing a reinforcement sheet with a clamp for use in a testing apparatus for determining a tensile or pull-out strengths of the reinforcement sheet, comprising the steps of: 
     (a) providing a holding clamp that defines a channel extending between opposing sides and having at least two adjacent bearing surfaces and an opening to an exterior surface thereof between the bearing surfaces; 
     (b) sliding a clamping bar overwrapped with an end portion of a sheet to be tested for tensile or pull-out strength along the channel with a portion of the reinforcement sheet extending outwardly through the opening, the clamping bar conforming in cross-sectional shape at least relative to the pair of adjacent bearing surfaces defined in the channel; 
     (c) securing an opposing end of the reinforcement sheet; and 
     (d) loading the reinforcement sheet through the clamp with an increasing force, 
     whereby the clamping bar, being wrapped by the reinforcement sheet that is loaded with the increasing force, mechanically engages the two bearing surfaces of the channel such that the tensile loading on the reinforcement sheet is applied to the two bearing surfaces in the block for measuring the strength of the reinforcement sheet. 
     Objects, advantages and features of the present invention will become apparent from a reading of the following detailed description of the invention and claims in view of the appended drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a perspective cut-away view of a clamp according to the present invention useful in a sheet testing apparatus. 
     FIG. 2 illustrates in perspective view an embodiment of a clamping bar according to the present invention for use with the testing apparatus illustrate in FIG.  1 . 
     FIG. 3 illustrates in perspective view an alternate embodiment of the clamping bar illustrated in FIG.  2 . 
     FIG. 4 illustrates a design concept for the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now in more detail to the drawings in which like parts have like identifiers, FIG. 1 is a perspective view of a clamp  12  according to the present invention illustrated with a testing apparatus  10  for determining tensile strength of a reinforcement sheet  16  secured in the testing apparatus. The illustrated testing apparatus  10  uses a pair of opposed holding clamps  12 ,  14 . In this embodiment, the clamp  14  is similar to the clamp  12 . The reinforcement sheet  16  extends between the holding clamps  12 ,  14  for testing the tensile strength of the reinforcement sheet, as discussed below. The reinforcement sheet  16  is held in the clamp  12  by a clamping bar  18 . The reinforcement sheet  16  overwraps the clamping bar  18  which extends through a channel  34  in the holding clamp  12 . The reinforcement sheet  16  extends outwardly through an opening of the holding clamp  12 . The clamping bar  18  communicates the tensile loading on the reinforcement sheet  16  to the holding clamp  12 . 
     In the illustrated testing apparatus  10 , the holding clamps  12 ,  14  are each defined by a block body  20  adapted for engaging the reinforcement sheet  16 . The block body  20  is defined by opposing side walls  22 ,  24 , opposing front face  26  and back face  28 , and opposing top and bottom sides  30 ,  32 . The block body  20  defines a channel  34  extending between the opposing sides  22 ,  24 . In a preferred embodiment, the channel  34  defines a substantially triangular shape in cross-sectional view. In a preferred embodiment, the triangular channel  34  is substantially equilateral. The block body  20  defines an opening  36  in the face  26 . The edges of the opening to the face  26  are preferably radiused or tapered. The channel  34  defines a pair of bearing surfaces  38 ,  40 , for a purpose discussed below. The opening  36  is preferably between the two bearing surfaces  38 ,  40 . 
     The holding clamps  12 ,  14  are identical and can be connected to a base (platform) and a moving cross-head, respectively. Connections to a specific testing machine can be easily accomplished. Accordingly, one end of the sheet is fixed while the other end is secured to a movable holder. 
     In the illustrated embodiment, the holding clamp  14  is supported on a base  80 . The holding clamp  14  includes rollers  82 ,  84  on opposing sides of the block, two of which are illustrated. The rollers  82 ,  84  allow the holding clamp  14  to move between a first position towards the opposing holding clamp  12  to a second position away from the holding clamp  12 . The back face  28  of the holding clamp  14  attaches by a connector  86  to a cross-head of a testing machine (not illustrated), for moving the holding clamp  14  between the first and second positions. 
     FIG. 2 is a perspective view of an embodiment of the clamping bar  18  according to the present invention. The clamping bar  18  is received in the channel  34  of the block body  20 , as discussed below, for communicating the tensile loading from the reinforcement sheet  16  to the bearing surfaces  38 ,  40  of the block body. In cross-sectional view, the clamping bar  18  defines a substantially triangular shape for conformingly being received within the channel  34 . At least two surfaces  42 ,  44  conform to the bearing walls  38 ,  40 . In a preferred embodiment, the clamping bar  18  defines an equilateral triangle to facilitate installation in the channel  34 . The clamping bar  18  defines three apexes  46 ,  48 , and  50 . In the illustrated embodiment, the apexes  46 ,  48 , and  50  define radiused ends. For example, the clamping bar  18  in one embodiment has a length of twelve inches, and equilateral sides of approximately 1.5 inches reduced slightly to accommodate the apex radiuses of 0.1094 inches. In one embodiment, an exterior surface of the clamping bar  18  has texturing generally  52 , such as spaced-apart grooves and ridges, cross-hatching, roughened projections and recessed areas and the like, for a purpose discussed below. The clamping bar  18  is preferably formed of a high strength material, such as plastic or metal. 
     FIG. 3 is a perspective view of an alternate embodiment of a clamping bar  60 . In this embodiment, the clamping bar  60  defines a cavity  62  extending between opposing distal ends  64 ,  66  along a longitudinal axis. In the illustrated embodiment, the cavity  62  conforms in cross-sectional shape to the cross-sectional shape of the clamping bar  60 . 
     With reference to FIG. 1, the present invention provides a self-locking clamping bar  18  for securing laterally extending geosynthetic tie-back reinforcement sheet  16  to a clamp block  20  in a testing appartus  10 . In the preferred embodiment, the reinforcement sheet  16  extends laterally from the block body  20  on a cross-sectional transverse center line of the clamping bar  18 . With reference to FIG. 4, one of the apexes of the clamping bar  18  bearingly inserts into the opening  36  between the opposing bearing surfaces  38 ,  40 . A reinforcement sheet  16  that is not aligned with a center line  100  tends to cause the clamping bar  18  to twist, which is not preferred. It is preferred that the normal loading arising from the friction between the clamping bar  18  and the respective bearing surfaces of the channel are equal. 
     The clamp  14  is connected to the moving cross-head of a typical tensile test machine. The clamp  12  is connected to a platform of a typical tensile test machine. The reinforcement sheet (test specimen) is loaded when the cross-head moves away from the base. 
     With reference to FIG. 4, a design for the testing apparatus  10  may be described as follows, where: 
     P is the pull-out loading for the reinforcement sheet  16 , which equals the resisting force of the friction between the clamping bar  18  and the bearing surfaces  38 ,  40 . 
     N is the normal loading between the bearing surface  38 ,  40  and the clamping bar  18 . 
     α is the angle between the normal load N and a perpendicular line to the reinforcement sheet  16 . 
     φ is the friction angle at the planar interface between the reinforcement sheet  16  and the clamping bar  18 . This angle controls the self-locking attribute of the apparatus of the present invention. 
     The present invention is described by the following equation: 
     
       
           P= 2  N  sin α  (Eq.1) 
       
     
     The mobilized peak pull-out resistance is represented by the frictional load between the reinforcement sheet  16  and the bearing surfaces  38 ,  40  of the channel  34  and between the reinforcement sheet  16  and the clamping bar  18 . 
     The tensile loading on the reinforcement sheet  16  accordingly is resisted by four surfaces of frictional loading. This is described by the following equation: 
     
       
           P= 4  N  tan φ  (Eq.2) 
       
     
     Combining equations one and two shows: 
      2  N  sin α=4  N  tan φ  (Eq. 3) 
     which reduces to 
     
       
         sin α=2 tan φ  (Eq. 4) 
       
     
     Generally, higher values of the angle φ provide increased self-locking capability of the clamping bars  16 . 
     For example, assume that a equals 30°. In order to have a reinforcement sheet  16  fully locked in the block body  20  by the clamping bar  18 , 
     
       
         φ≧ arc tan (sin alpha/2), or arc tan (0.5/2). 
       
     
     Accordingly, φ≧14°. 
     It is noted that the friction angle φ between a clamping bar  18  and a reinforcement sheet  16  is likely greater than the computed 14°, thereby achieving the self-lock pull-out resistance of the present invention. In the event that sliding failure mode occurs, the angle of α can be reduced, and thus a smaller φ will meet the requirements for self-lock securing of the reinforcement sheet  16  to the block body  20  by the clamping bar  18 . 
     The clamp  14  is connected to the moving cross-head of a typical tensile test machine. The clamp  12  is connected to platform of a typical tensile test machine. The reinforcement sheet (test specimen) is loaded when the cross-head moves away from the base. 
     With reference to FIG. 1, the holding clamps  12 ,  14  are used in the testing apparatus  10 . Distally opposing ends of the reinforcement sheet  16  are gripped in the holding clamps  12 ,  14 . This is accomplished by first moving the holding clamp  14  towards the holding clamp  12  for placing the reinforcement sheet  16  in the testing apparatus  10 . An end portion of the reinforcement sheet  16  is wrapped around one of the clamping bars  18 . The clamping bar  18  with the wrapped reinforcement sheet  16  then is slidably inserted into the channel  34  of the holding clamp  12 . The lateral portion of the reinforcement sheet  16  is slidably moved through the opening  36  and extended to the second holding clamp  14 . The opposing distal end of the reinforcement sheet  16  wraps over another of the clamping bars  18 . This second clamping bar is then slidably received in the holding clamp  14 . As illustrated in FIG. 1, portions of the opposing ends of the holding sheets extend outwardly of the openings  36  in the holding clamps  12 ,  14 . The respective clamping bars  18  are then wedged in the opening  36  of the holding clamps  12 ,  14 . This is accomplished by grasping the extended portion and the main portion of the reinforcement sheet  16  and pulling towards the opposing holding clamp in order to wedge the clamping bar  18  against the bearing surfaces  38 ,  40  of the respective holding clamp. 
     Tensile loading is then applied to the reinforcement sheet  16 . In the illustrated embodiment, the second clamp  14  is moved in a direction away from the first holding clamp  12 . In the illustrated embodiment, the holding clamp  14  is moved by operation of a hydraulic cylinder. The rollers  82 ,  84  facilitate the travel of the holding clamp  14  between the first position towards the opposing holding clamp  12  and the second position away from the holding clamp. The moving-away of the holding clamp  14  applies increasing tensile load on the reinforcement sheet  16 . The tensile loading on the reinforcement sheet  16  impels the clamping bars  18  to wedgingly engage the respective openings  36  between the bearing surfaces  38 ,  40  of the channels  34 . The surfaces  42 ,  44  of the clamping bar  18  engage the bearing surfaces  38 ,  40 . This locks the reinforcement sheet  16  in place within the holding clamps  12 ,  14  together with the clamping bar  18  in the block body  20 . The tensile loading is monitored to determine the force required to cause the reinforcement sheet  16  to fail. It is noted that the illustrated embodiment of the testing apparatus  10  is horizontal, and that the clamps  12 ,  14  also work in the testing apparatus that is vertically oriented. 
     It is to be appreciated that one clamp  12  is gainfully used with a shear resistance testing apparatus (not illustrated), in which a portion of the test sheet  16  is embedded in a soil box of backfill material or between blocks for normal loading to test pull-out resistance. 
     It is thus seen that the present invention as disclosed herein provides a testing apparatus for determining the tensile strength of elongate reinforcement sheets particularly useful in constructing earth retaining walls. While this invention has been described in detail with particular reference to the preferred embodiments thereof, the principles and modes of operation of the present invention have been described in the foregoing specification. The invention is not to be construed as limited to the particular forms disclosed because these are regarded as illustrative rather than restrictive. Moreover, modifications, variations and changes may be made by those skilled in the art without departure from the spirit and scope of the invention as described by the following claims.