Abstract:
A flexible membrane for use in test cells includes instrumentation within the thickness of the membrane to accurately measure a property of a material including stresses, strains, deformation, temperature, moisture potential and moisture content of the sample. A test sample is enclosed within the membrane to isolate the specimen from testing fluids in the test chamber. The instrumentation may measure axial strains or radial strains or both.

Description:
RELATED APPLICATIONS  
       [0001]     This application is related to U.S. application Ser. No. 09/715,371 to Crockford which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention relates to the field of testing equipment and, particularly, to test cells for soil samples.  
       BACKGROUND OF THE INVENTION  
       [0003]     Geotechnical and paving materials such as soil and asphalt are often tested to obtain their engineering properties in axisymmetric triaxial testing cells. The specimen shape is usually a solid right circular cylinder. In rare cases, a hollow cylinder is used. The triaxial cells allow simultaneous pressurizing of the specimen periphery and deviatoric stress loading in one or more directions, usually the axial direction. The pressurizing media include a range of fluids such as air, mineral oil, or water though other fluids may be employed.  
         [0004]     Since the test specimens are porous in nature, and the porosity is often structured such that the specimen is permeable to the pressurizing medium therefore, it is necessary to introduce an impermeable seal around the specimen to isolate the mechanical effect of confining stress. In order to allow the specimen to change shape during testing, this impermeable seal must be flexible, usually made of polymers such as latex, nitrile or silicone for testing paving materials and soils, while it may be a metal (e.g. copper) jacket for testing solid rock.  
         [0005]     Displacement measurement(s) are usually necessary for computing the desired engineering properties from the stresses and strains (engineering strain is related to displacement through a very simple equation). It is often impossible to use the traditional resistance strain gauge in this application because (a) the specimen sometimes cannot be instrumented with a strain gauge that relies on adhesives, (b) large strains are difficult to precisely measure with typical resistance strain gauges, and (c) the surface void texture causes the strain gauge to be inaccurate. The axis of the cylinder is usually vertical, so the deviatoric loading and the related strains are parallel to this axis. Therefore, the vertical displacement measurements are required for all but the most basic engineering properties e.g., simple material strength does not require the measurement of strain, it only requires measurement of stress. Vertical strains combined with horizontal strains can be used for determining Poisson&#39;s ratio and dilation parameters.  
         [0006]     The horizontal or radial strain may be measured at a number of points on the surface of the cylinder, or by one or more circumferential measurements which has the advantage of reducing the number of transducers and improving the signal to noise ratio of a given transducer, if it is an analog device, or by other means such as volume change or optical measurements.  
         [0007]     U.S. Pat. No. 5,025,668 issued to Sarda et al. has instrumentation which is externally referenced. Such an apparatus has limitations: (1) it is not immune from end effects, and (2) it does not uncouple the vertical from the horizontal displacement. End effects alter strain measurements from the true value because the specimen deforms more like a whiskey barrel than like a right circular cylinder. The end effects are worse (a) when the friction between the specimen end and the loading platen is high, as is the case with many soils and virtually all asphalt materials, (b) when the measurement is taken with a gauge length that spans the whole specimen height from end to end, (c) when the specimen is short, and (d) when the properties at the ends of the specimen are different from the true properties of the specimen in the middle portion of the specimen for example, molded specimens tend to have somewhat different densities and air void properties close to the ends. The end effects often affect the vertical strain measurements more than the radial measurements.  
         [0008]     If radial measurements are taken, they should be uncoupled from the vertical movement. Since the radial measurements are usually very small in relation to the vertical on materials having Poisson&#39;s ratio smaller than 0.5, friction at the end of the shaft in contact with the specimen can introduce bending and/or binding in the shaft, causing the measured radial deflection to be incorrect.  
         [0009]     U.S. Pat. No. 4,579,003 issued to Riley also illustrates instrumentation that is externally referenced. Riley discloses an improvement over the device disclosed by Sarda in that the instrumentation is internal to the triaxial cell, but it is still external to the specimen. At the very small displacements commonly measured with soils and asphalt, any interface between any material other than the material being tested and some other material ,such as a metal or polymer platen, can cause enough deformation under load to totally mask the correct strain measurement. Therefore, measuring between the stages as disclosed by Riley is likely to produce better measurements than Sarda&#39;s device.  
       SUMMARY OF THE INVENTION  
       [0010]     Disclosed is a flexible membrane for use in test cells to measure a property of a material including stress, strain, temperature, deformation, moisture content, etc. The test cells include instrumentation within the thickness of the membrane to accurately measure stresses causing deformation of the sample. A test specimen may be enclosed within the membrane to isolate the specimen from testing fluids in the test chamber. The instrumentation may measure axial stresses and strains or radial stresses and strains or a combination thereof.  
         [0011]     U.S. patent application Ser. No. 09/715,371 submitted by the Applicant and incorporated herein by reference, notes several instrumentation means. When mounting of the instrumentation is done by mechanically attaching to the membrane, moment analyses are useful as described in the application. While moment analyses are always useful, in the particular case of an application in which the vertical displacement measurement means is either (a) the only measurement means, or (b) is capable of being completely separated from the radial measurement means in a combined measurement configuration, it is possible to make an even simpler instrumentation means by making the instrumentation an integral part of the membrane.  
         [0012]     Therefore, it is an objective of this invention to provide a flexible membrane for intimate contact with the surface of the specimen to isolate the specimen from the testing fluids in the test cell and permit the specimen to deform in response to testing stresses.  
         [0013]     It is another objective of this invention to provide instrumentation integrally incorporated within the membrane to quantify and record the strains.  
         [0014]     It is yet another objective of this invention to provide an instrumented membrane to measure radial or axial strains, alone.  
         [0015]     It is a further objective of this invention to provide instrumentation to measure axial and radial stresses. The circumferential approach would be the most appropriate method for measuring radial properties using the instrumentation and one skilled in the field will be able to extend the teaching directed toward the vertical measurement presented herein to include both the vertical and the horizontal or only the horizontal in various embodiments.  
         [0016]     It is still another objective of this invention to provide instrumentation in the membrane to measure the temperature, moisture content and/or soil suction of a specimen during a test.  
         [0017]     Other objectives and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of this invention. The drawings constitute a part of this specification and include exemplary embodiments of the present invention and illustrate various objects and features thereof. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]      FIG. 1  is a perspective view of instrumented membrane for axial measurements only;  
         [0019]      FIG. 2  is a partial section view of instrumented membrane for axial measurements only;  
         [0020]      FIG. 3  is a perspective of the flexible cord;  
         [0021]      FIG. 4  is a perspective of the mounting hardware for independent circumferential measurement;  
         [0022]      FIG. 5  is a perspective of the mounting hardware for combination vertical and circumferential measurement;  
         [0023]      FIG. 6  is a perspective view of instrumented membrane for combination axial and circumferential measurements;  
         [0024]      FIG. 7  is a partial section view of instrumented membrane for combination axial and circumferential measurements;  
         [0025]      FIG. 8  is a perspective of an alternate embodiment of the membrane for combination measurements with a temperature probe and moisture content chamber;  
         [0026]      FIG. 9  is an exploded view of membrane manufacturing mold;  
         [0027]      FIG. 10  is a perspective of the circumferential measurement mounting hardware; and  
         [0028]      FIG. 11  is a perspective of an alternate embodiment of mold hardware for circumferential measurement capability. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0029]      FIG. 1  shows the straight tube portion  52 A of the membrane and  FIG. 2  shows a section view through the membrane and one of the vertical measurement instruments. In a constant thickness version of the membrane, the thickness of  52 A in  FIG. 1  is thicker than the cross section dimension of the displacement sensor, which requires its minimum thickness to be larger than that necessary to produce the pressure barrier alone. The thickness may vary in other embodiments, such as that produced by the fabrication mold assembly, discussed below in the manufacturing method portion of this application. The membrane section shown in  FIGS. 1 and 2  show a cavity  52 B that is molded into the membrane material, such as silicone or latex rubber, or other polymeric materials, and this cavity receives the LVDT (linear variable differential transformer) type displacement transducer  76 A,  76 B, and  76 C. The cavity  52 B is tubular in nature, but it is not centered within the wall thickness of the membrane. By offsetting the cavity toward the outside surface of the membrane, the large diameter portions of the cavity are actually open to the outside surface of the membrane by the slit  52 C in  FIG. 1 .  
         [0030]     The inside diameter of the membrane is, therefore, continuous so that no leakage of the pressurizing fluid occurs from one side of the membrane to the other. Although the outside slits are provided to make it possible to insert the parts of the transducer into the cavity after the membrane has been molded, it would be possible to mold the transducer into the rubber during manufacture. However, there are wires that come into an LVDT type device that are not shown at  76 A, and these wires may make membrane manufacture with the transducer in place during the manufacturing somewhat cumbersome. Further, individual molded-in transducers, even if they were wireless, would be difficult to replace without destroying the membrane. Of course, the instrumented membranes may be a single use type or capable of multiple tests. In the preferred embodiment, the LVDT type sensor would be placed in the cavity after the membrane has been manufactured.  
         [0031]     The temperature probe  66  may also be molded in the membrane, as well as, the screen  67 A of the moisture content chamber  67 B, shown in  FIG. 8 . The moisture content chamber  67 B may have an instrument  67 C therein which measures soil suction or soil potential. A source of these dielectric probes or tensiometers is Soilmoisture Equipment Corp in California.  
         [0032]     The screen  67 A may be a semipermeable membrane to maintain isolation of the sample from the testing fluid or in the event the walls of the moisture content chamber  67 B are made impervious the screen  67 A may be a sieve.  
         [0033]     The LVDT type sensor usually comprises three or more parts: a transformer body  76 A, a core and core rod extension  76 B and a piece of anchoring hardware  76 C used in conjunction with the core rod extension to establish the desired gauge length for the measurement. Since these three components can be separated, and since the membrane is usually a flexible elastomer such as a silicone or latex rubber or a cast urethane, it is possible to (a) insert the body of the LVDT into the upper or lower slit  52 C in the membrane and allow the wires to hang out of the slit, and (b) insert the other components into the other slit in the membrane. Once in place, the friction between the inside surface of the membrane and the outside diameter of the specimen under test will allow measurement of the vertical deflection of the specimen under load. The changing distance between  76 C and  76 A when the specimen experiences strain due to the axial load generates a measurable displacement signal.  
         [0034]     The preferred embodiment would use multiple sets of vertical LVDTs for example, three sets of parts  76 A,  76 B, and  76 C arranged in a pattern at 120 degree angle increments about the central axis of a cylindrical specimen, although fewer or more LVDTs would be possible. Four sets would be preferred, if the specimen were rectangular instead of cylindrical.  
         [0035]     In order to measure radial strain using a circumferential measurement on a cylindrical specimen, the preferred embodiment comprises a flexible cord or ribbon  79  shown in  FIG. 3 , mounting hardware  77  shown in  FIG. 4 , or  76 D shown in  FIG. 5 , and a spring-loaded LVDT  78 ,  80 , and/or  82 , shown in  FIG. 6 . Mounting hardware  77  would be mounted in an additional membrane cavity  84 . The flexible cord may be of any cross sectional shape. If the available clearance space is very small, flat or oblong shapes may be improvements over circular cross sections to reduce stress on the membrane.  
         [0036]     For a single radial measurement, the preferred embodiment would position the circumferential measurement components at the mid location, as shown by LVDT  80 . As shown in  FIG. 7 , this flexible cord should pass freely through a cavity  86  in the membrane that is closer to the inside surface of the membrane than the core rod  76 B. In  FIG. 4 , LVDT  80  would be mounted in the larger of the two holes  77 A in the mount  77 , one end of the flexible cord would be attached to the LVDT and the other end of the cord would be attached in the smaller hole  77 B using an adhesive. The two holes in the mounting hardware  77  should be fabricated at an angle that will allow the two ends of the flexible cord that are attached at the LVDT and at  77 B to maintain tangency to the circle they define in the membrane cavity as closely as possible.  
         [0037]     For two radial measurements, one embodiment would comprise two such component assemblies positioned above and below the mid-height at some distance that would give a representative picture of the radial deformation if it were to deform in a barrel shape instead of a perfect right circular cylinder. In the preferred embodiment, using either two or three radial measurements, the vertical anchor hardware  76 C could be modified to perform double duty as a combination radial LVDT holder and anchor for the vertical LVDT  76 D. Using the double duty configuration along with the mid-height mount enables three radial measurements to be taken that can be averaged or used independently to better quantify the overall shape during deformation. The combination unit  76 D, incorporates features additional to the basic features that comprise the standard anchor unit  76 C, including a mounting feature such as a hole  76 E to receive the spring-loaded radial LVDT, and a notch  76 F. The notch  76 F is used to anchor one end of the cord  79  for example, with an adhesive or by crimping, in the anchor that also has the LVDT mounted in it. At other locations around the 120 degree pattern, the notch is simply a clearance notch that allows the cord  79  to pass through unobstructed.  
         [0038]     While there are various sophisticated methods of forming polymers, a very simple method of manufacturing membranes with cavities is illustrated in  FIG. 8 . When assembled, the device illustrated in  FIG. 9  is filled with an appropriate quantity of raw membrane material, closed on the ends, and inserted into a device that can be rotated around the central axis, such as a lathe. The centrifugal force during rotation will generate an evenly distributed layer of material on the inside of the tube, simultaneously filling the areas around the LVDT cavities.  
         [0039]     Various flexible materials can be used, and a chemical mold release agent may or may not be necessary to enable release of the cured membrane material from the mold surfaces. Dow Corning&#39;s two part Silastic material has been found suitable for a membrane material. This material cures faster under heat which allows reduction of the time necessary to rotate the mold.  
         [0040]     In  FIG. 9 , the mold tube  200  has an inside diameter that is determined by the specimen diameter and the desired membrane thickness. The outside diameter of the tube is determined by the diameter of the measurement device. One or more flats  202  are fabricated on the outer diameter of the mold. A mold plate  204  will be attached to the mold flats with plate attachment screws  206  after the instrument cavity components  210 A,  2108 , and  210 C have been attached to the plate with instrument cavity mold screws  208 .  
         [0041]     The instrument cavity components  210 A,  210 B, and  210 C are dimensionally designed to receive the instrument components  76 A,  768 , and  76 C. Instrument cavity shaft  210 B must be larger in diameter than instrument shaft  76 B because the instrument shaft must be free to move in the cavity without friction. Leakage of the pressurizing medium through the slits  52 C allows for pressure relief/equalization in the shaft cavity so that it neither appreciably inflates nor collapses on the instrument shaft during pressurization. The lower and upper mold body components  210 A and  210 C are preferably designed with slightly smaller dimensions than the corresponding instrument parts  76 A and  76 C so that the instrument components will be tightly held in the cavity of the finished membrane. The instrument cavity components  210 A and  210 C have a flat  212 A fabricated on them and one or more threaded holes  212 B on the flat. Cavities for different gauge lengths can be attained by attaching to the different mounting holes as desired.  
         [0042]     For applications only requiring vertical (i.e. axial) measurements, features  214 ,  216 , and  218  are unnecessary. For a single circumferential measurement, and for applications in which radial measurements do not occupy the same horizontal planes occupied by the vertical cavity components  210 A and  210 C, a cavity forming wire  218  used to form the cavity  86 , and a support for the wire  216 ,  216 A (also shown in another view in  FIG. 10 ) are used.  
         [0043]     For applications only requiring vertical (i.e. axial) measurements, and for applications in which radial measurements do not occupy the same horizontal planes occupied by the cavity components  210 A and  210 C, those components can be of the same design as shown in  FIG. 9 .  
         [0044]     For multiple circumferential measurements that require the use of mounting hardware  76 D, in  FIG. 5 , the corresponding mold components require the additional support features  217 B and/or  217 C, in  FIG. 10 , of another embodiment. The extrusion or boss  217 C is required only at those positions that require a circumferential instrument for example,  78  and  82 , and a vertical anchor at the same location. The notch  217 B is required at all positions for which the wire  218  needs to be supported on a vertical mold body component. As an example, for the case of the lower wire  218  located in the plane occupied by component  210 A, one mold component located at  210 A would need both the notch  217 B and the boss  217 C.  
         [0045]     The configuration of the other two vertical mold body components located around the 120 degree pattern would only require the notch  217 B in order to ensure that the wire  218  remains in a level plane perpendicular to the axis of the cylinder during the molding process and would therefore not require the boss.  
         [0046]     A number of embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the invention is not to be limited by the specific illustrated embodiment but only by the scope of the appended claims.