Abstract:
A portable device for measuring the shear properties of a material specimen under applied dynamic circumferential forces against the specimen perpendicular to a longitudinal axis of said specimen. The device may further apply a dynamic force against an end of the specimen. The device includes clamps that hold the specimen at its proximal and distal end, wherein the circumferential force applied to the specimen is affectively applied between the two clamps. The temperature of the specimen may be held constant by controlling the temperature within an environmental chamber through the use of a closed loop PID control system. The force against the specimen is dynamically loaded by a dynamic load reaction frame consisting of a shear load actuator and load reaction structure. A microprocessor-based controller operates the dynamic load reaction frame under closed loop control. The microprocessor-based controller may be servo controlled, utilizing feedback from either a load transducer or either or both of two linear displacement transducers.

Description:
I. FIELD OF THE INVENTION  
         [0001]    This invention relates generally to devices used to test the shear response of a material, and more particularly relates to a portable apparatus and method for testing the viscoelastic response of a material specimen to an applied shear force under either monotonic or dynamic loading conditions. The device of the present invention may apply a circumferential force against the specimen perpendicular to a longitudinal axis of the specimen and may also apply a compression force against an end of the specimen.  
         II. BACKGROUND OF THE INVENTION  
         [0002]    Over the years, the Federal Highway Administration (FHWA) has been encouraging a modeling technique known as the SuperPave Asphalt Mix Design Model (hereinafter referred to as “superpave”) as a method of predicting the life expectancy of various paving mixes. Paving mixes are typically custom tailored to the unique requirements dictated by local traffic, climate, materials selection, and structural section at the pavement site. The superpave model is intended as a useful tool to help estimate the pavement&#39;s future long term performance in terms of its resistance to permanent deformation (rutting), fatigue cracking and low temperature cracking.  
           [0003]    The superpave modeling technique requires the input of mechanical properties associated with the particular asphalt mix to be modeled. In order to determine the required input properties of the asphalt mix, several tests are performed to determine the linear and non-linear elastic response, viscous behavior, and tertiary creep tendencies of the asphalt mix sample. These tests are characterized by the application of dynamic and monotonic loads or strains in shear and thereafter measuring the resulting strain or stress response. The resulting test data is then implemented in the superpave modeling technique to estimate the life expectancy of the sample.  
           [0004]    In order to most effectively estimate the life expectancy of a sample using the superpave technique, the test data should be obtained at the field level. Hence, a portable testing apparatus is desirable to perform the required tests on the sample in the field. To further increase the efficiency of obtaining the required test data, the sample material should not require substantial specimen preparation.  
           [0005]    Often times, the sample test data is obtained in a laboratory setting using cumbersome testing equipment known in art as a Superpave Shear Tester (hereinafter the “SST”). The SST includes a fixture that directs a shear load to a cylindrical specimen parallel with the ends of the specimen contained within the fixture. Proper use of the SST requires that the ends of a cylindrical specimen be cut square relative to each other and then glued to metal platens in a precision gluing jig prior to installation in the fixture. The “glued specimen” approach of the SST requires additional time and experience to properly glue and align the specimen. Further, in order to keep the ends of the specimen parallel, precise bearings are required to guide the specimen face as the shear load is applied. The use of bearings creates the possibility of backlash and misalignment. Hence, a need exists for a simple, easy to use, portable device for testing the shear strength of a material in response to an applied force.  
           [0006]    Various fixtures have been developed that direct a shear load to a specimen contained within a fixture. In this regard, reference is made to the disclosures of Iosipescu et al., U.S. Pat. No. 3,566,681, Jones, U.S. Pat. No. 5,245,876, Terry, U.S. Pat. No. 3,406,567, Hall et al. U.S. Pat. No. 4,445,387, Peres et al., U.S. Pat. No. 5,280,730, Thompson et al., U.S. Pat. No. 5,289,723, and Buzzard, U.S. Pat. No. 4,916,954. These disclosures generally describe fixtures for applying shear loads to a specimen, but do not describe a fixture suitable for applying a circumferential force against a specimen perpendicular to a longitudinal axis of the specimen.  
           [0007]    Iosipescu in U.S. Pat. No. 3,566,681 describes a method and apparatus for shear testing of rocks and other building materials. A rectangular block specimen is described, wherein a v-groove is formed in a middle, top and bottom portion of the block and channels, aligned with the grooves, are formed in the front and back of the block. The fixture described by Iosipescu applies a shear stress proximate the center of the v-groove and channel. A cylindrical specimen held in the fixture described by Iosipescu would tend to rotate within the fixture as the shear force is applied. Further, the clamping of the rectangular specimen by the fixture does not provide for through-zero loading. Also, the fixture described by Iosipescu does not use flexures for maintaining the distance between the two clamping assemblies.  
           [0008]    Jones in U.S. Pat. No. 5,245,875 describes a fixture whereby a shear stress is applied to specimen with rectangular beam geometry. Jones describes using the fixture to shear polymeric materials and does not describe an active split clamping system to provide through zero loading and to prevent the specimen from rotating within the fixture. Further, Jones does not describe a fixture that includes flexures for maintaining the distance between the two specimen attachments and there is no mention of measuring specimen strain.  
           [0009]    McRae in U.S. Pat. No. 5,911,164 discloses a compaction and pavement design testing machine and method for testing flexible pavement materials. The device described by McRae provides a rotational or gyratory shear testing. The compacting device described by McRae is not simple and portable and further does not apply a force that is perpendicular to the longitudinal axis of the specimen.  
           [0010]    Vilendrer in U.S. Pat. No. 5,712,431, describes a device for testing the shear response of a material in response to an applied force. The &#39;431 patent describes applying a shearing force to a cylindrical specimen along the longitudinal axis of the specimen. The specimen could potentially rotate within the fixture as the shear force is applied to the specimen. In contrast to the device described in the &#39;431 patent, the device of the present invention applies a circumferential force against the specimen perpendicular to a longitudinal axis of the specimen, which may be in combination with a compression force against the ends of the specimen.  
           [0011]    Thus, there is a need for a device, used for testing a response of a material specimen to shear forces applied to the material specimen, that applies the shear force against the specimen perpendicular to a longitudinal axis of the specimen, that may also apply a compression force against an end of the specimen, and which inhibits twisting or rotation of the specimen as the shear force is applied. The present invention meets these and other needs that will become apparent from a review of the description of the present invention.  
         SUMMARY OF THE INVENTION  
         [0012]    The purpose of the present invention is to provide a portable field shear tester for determining shear stress test data corresponding to various asphalt mixes that can subsequently be used in modeling methods to estimate the future pavement life. The present invention provides a shear tester, wherein the shear strengths of the specimen can be tested with minimal preparation at the field site and with great speed. Testing of the asphalt material is performed by obtaining a cylindrical sample and placing it into a shear fixture that can be subjected to monotonic or dynamic forces, including a circumferential force against the specimen perpendicular to a longitudinal axis of the specimen. The specimen may also be subjected to an additional compression force against an end of the specimen.  
           [0013]    The shear-testing device of the present invention includes a base, first and second clamps, and corresponding flexures. The first clamp is attached to the base and clamps about a perimeter of the specimen in proximity to a distal end of the specimen. The second clamp is attached to the first clamp via flexures and clamps about the perimeter of the specimen in proximity to a proximal end of the specimen. The clamps fasten about the perimeter of the specimen with enough force to inhibit rotation of the specimen within the clamps. Without limitation, in the preferred embodiment opposite halves of each clamp are forced together with hydraulics. The flexures have a proximal and distal end, wherein the proximal end of the flexures is attached to the second clamp at the proximal end of specimen, and the distal end of the flexures is attached to the first clamp at the distal end of the specimen. A downward force to the second clamp causes the proximal end of the flexures and the second clamp to move downward while the first clamp remains stationary, thereby applying a circumferential force against the specimen perpendicular to a longitudinal axis of the specimen.  
           [0014]    An additional actuator may be coupled to the first clamp to thereby apply a compression force against an end of the specimen. Front and back plates are affixed to the first and second clamps, whereby the plates engage the respective ends of the specimen. The additional actuator applies a force against the plate engaged with the distal end of the specimen. In order to measure the relative displacement between the first and second clamps, when the circumferential or compression force is applied to the specimen, linear displacement transducers may be utilized to measure the same.  
           [0015]    The shear fixture is coupled to a monitoring and control system that includes a microprocessor-based servo-controller, which controls, via closed loop feedback, the amplitude and frequency of the applied load or displacement to the shear fixture. A microprocessor-based temperature controller is also used to control the environmental control system temperature.  
           [0016]    Those skilled in the art will appreciate that, although the preferred method of testing is to apply a load and thereafter measure the resulting displacement, an alternative test method would be to displace the sample a predetermined distance and then measure the load required to displace the sample the predetermined distance. To provide more control over the material properties, the specimen temperature is held constant by enclosing the shear fixture in an environmental control chamber.  
           [0017]    Without limitation, in the preferred embodiment shear tests can be performed to stress levels of 1200KPA (with 700KPA supply pressure) and strains to 12% at frequencies from 0 to 10 Hz. The device can perform various tests including a frequency sweep, simple shear and repeated shear to obtain relevant data corresponding to each test, the data of which is required in the superpave modeling technique. The applied load, specimen dimensions, and measured displacement are then analyzed to determine the material stress/strain of the specimen associated with the required test data properties. These properties, along with the controlled temperature, may then be used in the superpave modeling technique to thereby estimate the material&#39;s long-term performance.  
           [0018]    Further, both dynamic (sinusoidal or pulsed) or static loading can be applied to the shear fixture. A servo pneumatic shear load actuator having a shaft coupled to the shear fixture is used to create the applied load. A servo valve, mounted near the shear load actuator, ports high-pressure supply air (typically 80-175 psi) to either side of an actuator piston. The resulting imbalance of air pressure creates the desired load or force in the desired direction. To energize the servo shear actuator separately from the air supply, an on/off solenoid valve is provided. Those skilled in the art will appreciate that although a servo pneumatic actuator is used to generate the loading, other known load generators could be used including, for example without limitation, a servo hydraulic, electrodynamic or electromechanical actuator.  
           [0019]    An environmental control chamber surrounding the shear fixture may be a box type configuration with a door for sealably enclosing the fixture and specimen. The chamber preferably has both hot and cold capability and features an electric heater assembly and liquid CO2/N2 injector for cooling. Those skilled in the art will appreciate that while a chamber enclosure is used, other heating and cooling means including heating and cooling the material retaining clamps directly could be used. The chamber may include a temperature sensor for temperature readout and control. A signal corresponding to the resulting temperature is transmitted to the microprocessor-based temperature controller for monitoring and control purposes and can be used to ensure that the test is being run at a specific temperature. In this manner, a shear tester is provided that may be used for testing asphalt specimens, for example, at the field site for the purpose of generating test data that can be used in the superpave modeling technique.  
           [0020]    These and other advantages of the present invention will become readily apparent to those skilled in the art from a review of the following detailed description of the preferred embodiment especially when considered in conjunction with the claims and accompanying drawings in which like numerals in the several views refer to corresponding parts. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0021]    [0021]FIG. 1 is a partial perspective fragmentary and block diagram view showing the field shear tester of the present invention removed from the environmental control chamber but coupled to a microprocessor-based control system and power supply;  
         [0022]    [0022]FIG. 2 is a fragmentary front elevational view of the shear tester of the type shown in FIG. 1 and enclosed in an environmental chamber;  
         [0023]    [0023]FIG. 3 is a fragmentary side elevational view of the environmental chamber of the type shown in FIG. 2 used to enclose the shear tester;  
         [0024]    [0024]FIG. 4 is a fragmentary front perspective view of the clamps of the shear tester of the present invention;  
         [0025]    [0025]FIG. 5 is a fragmentary front elevational view of the shear tester of the type shown in FIG. 1 with the front plate removed to expose the specimen to view;  
         [0026]    [0026]FIG. 6 is a fragmentary side elevational view of the shear tester of the type shown in FIG. 1;  
         [0027]    [0027]FIG. 7 is a fragmentary back elevational view of the shear tester of the type shown in FIG. 1;  
         [0028]    [0028]FIG. 8 is a fragmentary top plan view of the shear tester of the type shown in FIG. 1;  
         [0029]    [0029]FIG. 9 is a fragmentary partial cross-sectional view taken along line  9 - 9  of FIG. 8;  
         [0030]    [0030]FIG. 10 is a fragmentary partial cross-sectional view taken along line  10 - 10  of FIG. 8; and  
         [0031]    [0031]FIG. 11 is a fragmentary partial cross-sectional view taken along line  11 - 11  of FIG. 8. 
     
    
     DETAILED DESCRIPTION  
       [0032]    In conjunction with the several views of the Figures, details of representative embodiments of the present invention will next be presented. Referring first to FIG. 1, there is shown generally the portable shear tester  10  of the present invention electrically coupled to a microprocessor-based controller  12 , and an uninterruptible power supply  14  of known suitable construction. To provide a quick shut off capability, a quick stop switch  16  of suitable construction is electrically coupled to the portable shear tester  10  (see also FIG. 3).  
         [0033]    The microprocessor-based controller  12  has both RAM (random access memory), and ROM (read only memory) for storing programs and data, which allows for: determining the shear strength of the material specimen, controlling monotonic or dynamic forces applied to the specimen by controlling activation of the shear tester, controlling the activation and sequence of testing by the shear tester  10 , and predicting characteristics of the material specimen. The various modes of the controller  12  will be described below in greater detail. While the controller  12  may control the shear tester  10  as described below, those skilled in materials testing will appreciate that other modes may be utilized to measure or test the strength of the material specimen. The field tester  10  may be enclosed within a temperature-controlled chamber  18  (see FIGS. 2 and 3). The microprocessor-based controller  12  may further be utilized to monitor and control the temperature within the chamber  18 . The control of the temperature within the chamber  18  will be described below in greater detail.  
         [0034]    The Figures including FIG. 1 show the shear tester  10  as generally including a base  20 , first and second clamps  22  and  24 , actuators  26  and  28 , and displacement transducers  30  and  32 . A front restraint plate  34  (shown in FIG. 2) is attached to the side of the second clamp  24  with quick release bolts  36  of known suitable construction. In the preferred embodiment, the first and second clamps  22  and  24  each include an upper and lower half or upper and lower restraining member  40  and  42  respectively.  
         [0035]    [0035]FIGS. 2 and 3 show the shear fixture  10  contained within the environmental control chamber  18 . In order to reduce the influence of temperature variances on the resulting test data, the shear fixture  10  is placed within an environmental control chamber  18 , which surrounds the fixture  10 . The environmental chamber  18  has hot/cold capability to maintain the temperature at a fixed level throughout the test. The environmental chamber  18  maintains the specimen temperature at a predetermined setting and is capable of either increasing or decreasing the temperature within the chamber  18 . In this regard, the chamber has a temperature transducer, an electric heater assembly for heating and a liquid CO2/N2 injector for cooling.  
         [0036]    The temperature transducer and heating/cooling elements within the chamber  18  are coupled to a microprocessor-based temperature controller (not shown) or alternatively temperature control programming may be included in the controller  12  and coupled to the heater and cooler. A signal is sent from the temperature transducer to the temperature controller, indicating the temperature of the chamber air temperature. The microprocessor-based temperature controller uses a PID control algorithm, whereby the temperature transducer signal is compared to a desired setpoint value. The difference or “error” is then scaled using a proportional (P) calculation, integrated over time and scaled using an Integration (I) calculation and differentiated with respect to time and scaled using a Differentiation (D) calculation. The temperature P, I, and D values are then summed together and the resulting value is used to drive a solid-state relay using a Pulse Width Modulation (PWM) technique. When the temperature controller determines that the inside air temperature is below a preset level, the heating elements are activated until the inside air temperature rises to the desired temperature. When the temperature controller determines that the inside air temperature is above a preset level, a solenoid valve is opened thereby cycling CO2/N2 until the air temperature drops to the desired temperature.  
         [0037]    The servo pneumatic actuator  26 , of known suitable construction, is attached to the frame  12  (see FIGS. 7 and 9- 1   1 ) and includes a servo valve  50  (see FIG. 7) for porting air to either side of the shear actuator piston  52 . A solenoid valve  54  (see FIG. 3) mounted on the outside of the environmental chamber  18  has on/off capability for manual control of the actuator  26 , to thereby isolate the servo valve  50  from the air supply. Maximum applied pressure is set using a pressure regulator  56 . In the preferred embodiment, the shear servo actuator  26  is mounted to the bottom baseplate  58  of the frame or base  20  (see FIG. 9). One end of a shaft  60  is attached to the actuator piston  52 . The other end of the shaft  60  extends through the actuator endcap  62  and is attached to a universal flexure  64 .  
         [0038]    Back and front specimen restraints  70  and  34  respectively are used to inhibit the specimen  72  from expanding along its longitudinal axis during testing. The back restraint  70  is attached to a piston  74  of actuator  28 . Without limitation, in the preferred embodiment the pneumatic axial load actuator  28  is energized using a hand valve (not shown) and the applied axial load is set using a pressure regulator  76 . Additionally, the rear restraint can be locked in place using a clamp block  78  that is preloaded using a hydraulic cylinder  80  (see FIG. 8) of known suitable construction. Without limitation, in the preferred embodiment the hydraulic cylinder  80  has a 1500 pound capacity at 5000 psi. The hydraulic cylinder  80  is mounted to the actuator mount  82  using bracket  84 , plumbed through port  86  and actuated via hydraulic hand pump (not shown) mounted to the outside of the environmental chamber  18 . The clamp block  78  is mounted directly to the actuator mount  82 .  
         [0039]    Referring next to FIGS. 4 and 5, the inside surface of the upper and lower restraining members  40  and  42  include concave recesses. The concave recesses are aligned diametrically opposite one another to form a cylindrical pocket to accommodate the material specimen  72 , and front and back restraint plates  34  and  70 . Alignment of the upper and lower material retaining members  40  and  42  is accomplished using alignment pins  90 . The upper and lower material retaining members  40  and  42  are pressed against the specimen using hydraulic cylinders  92  (see FIGS. 10 and 11). The cylinder  92  bodies are mounted to the upper material retaining members  40  using mounting brackets  94 . The hydraulic cylinders  92  are plumbed in parallel using ports  96  (see FIG. 11) and actuated by hydraulic hand pump (not shown) mounted external to the environmental chamber  18 . When hydraulic pressure is eliminated, complete release of the clamp load applied by the upper and lower material retaining members  40  and  42  is facilitated by coil springs  98 . The unlocking spring force is adjusted using adjustment bolts  100 .  
         [0040]    Referring now to FIGS. 6 and 8- 11  the first and second material retaining clamp assemblies  22  and  24  are attached to upper and lower flexure assemblies  102  and  104  (see FIG. 9). A first end of the upper flexure  102  is attached to the upper retaining member  40  of the first clamp  22 . A second end of the upper flexure  102  is attached to the upper material retaining member  40  of the second clamp  24 . Likewise, a first end of the lower flexure  104  is attached to the lower retaining member  42  of the first clamp  22 ; and a second end of the lower flexure  104  is attached to the lower material retaining member  42  of the second clamp  24 . The ends of flexures  102  and  104  are clamped to the material retaining members using bolts and clamp blocks  106 -  110 . Clamp block  110  attaches directly to a lower cross-piece  112 . The upper and lower flexures  102  and  104  allow movement of the second material retaining clamp assembly  24  in the vertical direction creating a shear condition perpendicular to the specimen&#39;s longitudinal axis, while at the same time the flexures  102  and  104  maintain the specimen in fixed position between the material retaining clamp assemblies  22  and  24 .  
         [0041]    Referring again to FIGS. 2 and 9, the front restraint plate  34  is held in place on the front face of the lower retaining member  42  of the second clamp assembly  24  via shoulder bolts  36 . The plate is designed to be easily removed for installation of the cylindrical specimen  72  within the fixture  10 . The thickness and spacing of the material clamp assemblies  22  and  24  are constant while and the length of the specimen  72  may vary from 50 to 150 mm, for example without limitation. To accommodate the varied specimen lengths, specimen spacer plates  114  are provided for shorter specimen lengths. The spacer lengths can be fabricated in different thicknesses as needed to accommodate any specimen length.  
         [0042]    To prevent an axial load applied to the back specimen restraint plate  70  from creating an overturning moment in the first and second clamp assemblies  22  and  24 , brackets and a linkage assembly are used. Brackets  116  attach between the axial actuator mount  82  and the lower retaining member  42  of the first clamp  22  (see FIG. 6). The linkage  118  attached between the actuator body  28  and the upper retaining member  40  of the first clamp  22  and includes the actuator block  120 , rear clamp assembly upper clamp block  106 , links  122 , and link pins  124 .  
         [0043]    The back plate  70  for the specimen  72  may be utilized for a dual purpose. First, during testing it is used to apply either a constant axial stress or constant axial strain to the specimen. Then, after testing it is used to push the specimen  72  partially out of the fixture  10  so that it can be removed by hand.  
         [0044]    As described above the lower retaining member  42  of the second material retaining clamp assembly  24  is attached to the lower cross-piece  112 . The lower cross piece is also attached to a load transducer and actuator piston  52 . The lower retaining member  42  of the first material retaining clamp assembly  22  is rigidly attached through the reaction framework to the servo actuator body  26 . In the preferred embodiment and without limitation, to reduce excessive over travel and damage to the flexure assemblies in the event of a specimen failure, the actuator stroke is limited to ¼″ or travel. The short actuator stroke ensures that the material retaining clamp assemblies  22  and  24  can only move a small distance with respect to one another. Universal flexure  64  also reduces extraneous side loads and moments from being transmitted to the load transducer  126  and actuator piston  52 .  
         [0045]    The applied load or force to the fixture  10  is measured by load transducer  126  that has one end coupled to the lower half of the second clamp assembly  24  by means of a lower crosspiece  112 . The other end of the load cell  126  is coupled to universal flexure  64  that is in turn attached to the shaft of the shear load actuator  26 . The universal flexure accommodates small angular misalignments while transmitting the applied load to the shear fixture. The body or outer cylinder of the shear load actuator  26  is attached to the load reaction structure. The lower half  42  of the first material retaining clamp assembly  22  is attached directly to the load reaction structure. A signal corresponding to the applied load is transmitted to the microprocessor-based controller  12  which is coupled to the transducer for monitoring and control purposes and can be used to ensure that the test is being run at a specific load amplitude. Those skilled in the art will appreciate that although a load transducer is preferred, one could measure the applied cylinder pressure or motor current in the case where a linear motor is used as the actuation system.  
         [0046]    Displacement transducers  30  and  32  are coupled to shear fixture  10 . The transducers  30  and  32  are of suitable known construction for measuring the respective displacement of the front material retaining clamp assembly  24  with respect to the rear material retaining clamp assembly  22 . The displacement transducers  30  and  32  are electrically coupled to the microprocessor-based controller  12  to provide displacement feedback for closed loop servo control and monitoring. Those skilled in the art will appreciate that although a spring loaded LVDT type displacement transducer is preferred, other transducers used to measure the relative displacement of the first and second material retaining members could be used.  
         [0047]    As an upward or downward force is applied by the actuator  26 , the load transducer  126  may compress or stretch and the framework will deflect slightly. For this reason, an actuator displacement transducer cannot be used as a reliable indication of relative displacement between the material retaining clamp assemblies  22  and  24 . The material retaining clamp assemblies  22  and  24  relative displacement is measured via the two spring loaded displacement transducers  30  and  32  which have their bodies mounted to the upper material retaining member  40  affixed to clamp assembly  22  via bracket  128  (see FIG. 6) and the measuring ends of the transducers  30  and  32  are pressed against the upper material retaining member  40  of clamp assembly  24 . Signals corresponding to the measured displacements are transmitted to the microprocessor-based controller  12  for monitoring and control purposes and can be used to ensure that the test is being run at a specific displacement amplitude.  
         [0048]    The microprocessor-based controller  12  uses a PID control algorithm for controlling the servo pneumatic actuator  26 . The feedback signals from either the load transducer  126  or linear displacement transducers  30  and  32  are amplified and then converted to a digital value by means of an internal analog to digital converter. Over time the resulting digitized feedback value can be represented as a waveform. This waveform is then subtracted from a baseline or desired “command waveform”. The resulting waveform or “error signal” is typically sinusoidal with respect to time if the “command waveform” is sinusoidal. Although the control of the servo pneumatic actuator feedback signal may correspond to a load or pressure within the actuator, load control is presently preferred, wherein the displacement feedback is used to monitor the specimen response.  
         [0049]    After the error signal is computed, the microprocessor  12  then performs several mathematical operations on the error signal known as PID control. First, the error signal is multiplied by a scaler value K1 to obtain a proportional (P) value. The error signal is also integrated over time and multiplied by scaler value K2 to obtain an Integration (I) value. The error value is also differentiated with respect to time and multiplied by scaler value K3 to obtain a differentiation (D) value. The P, I, and D values are then summed together and converted to a proportional drive output voltage by means of a digital to analog converter built into the microprocessor-based controller  12 . This output voltage is the input signal for the servo valve  50 , which controls the force applied by the actuator. The PID control tends to reposition the applied load of the servo pneumatic actuator  26  to minimize the error signal.  
         [0050]    To further enhance the accuracy of the control loop and maintain the desired applied load, the peak end levels of the feedback signal from load transducer  126  is monitored by the microprocessor based controller  12 . If the peak end levels of the feedback signal vary from a predetermined peak level (ie: due to changing specimen compliance conditions, changes in supply pressure, etc.), the software automatically adjusts the command waveform used in the PID control until the desired feedback signal end level is achieved.  
         [0051]    Having described the constructional features of the present invention, the mode of use will now be discussed. In order to load a specimen  72  into the fixture  10 , the user retracts the back plate  70 . The user then removes the front plate  34  from the lower material retaining member  42  of the second clamp assembly  24 , by turning the front plate  34  slightly counter clockwise until the larger diameter holes fit over the shoulder bolts  36 . The user then removes any clamp pressure on the material retaining clamp assemblies  22  and  24  by releasing any pressure applied by the hydraulic pumps. After inserting the specimen  72  until it is flush against back restraint plate  70 , the user inserts any required spacer plates and then reattaches the front specimen restraint plate  34  by placing it over the shoulder bolts  36  and rotating it slightly. The operator then loads the back plate against the specimen  72  by applying a pneumatic pressure to the axial actuator. The operator then engages the clamps  22  and  24  against the specimen  72  by activating the corresponding hydraulics. Once the specimen  72  is clamped within the fixture  10 , the user can optionally lock the back restraint plate  70  in place using the corresponding hydraulics. With the specimen installed, the user selects the desired applied load profile using the microprocessor-based controller. As the desired load (frequency sweep, simple shear, and repeated shear) is applied, the microprocessor based controller  12  measures the applied load and resulting displacement as a function of time. Depending on the material characteristic to be determined, the microprocessor program performs the required analysis and data storage. Upon completion of the test, the user removes the applied clamp pressure using the release valve on the hydraulic pump, optionally unlocks the back specimen restraint locking mechanism, retracts the back specimen restraint plate  70 , removes the front specimen restraint plate  34 , extends the back specimen restraint plate  70 , and extracts the specimen  72  from the shear fixture  10 .  
         [0052]    This invention has been described herein in considerable detail in order to comply with the patent statutes and to provide those skilled in the art with the information needed to apply the novel principles and to construct and use such specialized components as are required. However, it is to be understood that the invention can be carried out by specifically different equipment and devices, and that various modifications, both as to the equipment and operating procedures, can be accomplished without departing from the scope of the invention itself.