Device and method for testing the shear response of a material in response to an applied force

A portable device for measuring the shear properties of compacted asphalt mixes under applied dynamic loading conditions. The device includes first and second material retaining members which form a material receiving pocket for containing the compacted asphalt mix. The first and second material retaining members are connected via flexure assemblies which allow movement of the first and second material retaining members only along the longitudinal axis of the compacted asphalt mix. A dynamic load is applied to the flexures, thereby creating a shear condition parallel to the longitudinal axis of the compacted mix. The temperature of the asphalt mix may be held constant by controlling the temperature within an environmental chamber through the use of a closed loop PID control system. The specimen is dynamically loaded by a dynamic loading frame consisting of a servo pneumatic actuator and load reaction structure. A microprocessor-based controller operates the dynamic loading frame under closed loop control. The microprocessor-based controller may be servo controlled, utilizing feedback from either a load transducer or either of two linear displacement transducers.

BACKGROUND OF THE INVENTION 
I. Field of the Invention 
This invention relates generally to the field of asphalt characterization 
devices, and more particularly 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. 
II. Discussion of the Related Art 
Over the past several 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 retirements dictated by local 
traffic, climate, materials selection, and structural section at the 
pavement site. The model is intended as a useful tool to help estimate the 
pavement's future long term performance in terms of it's resistance to 
permanent deformation (rutting), fatigue cracking and low temperature 
cracking. 
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. 
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 necessary 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. 
Currently, 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, portable device for testing the shear strength 
of a material in response to an applied force. 
Various fixtures have been developed that direct a shear load to a specimen 
contained within the 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 all describe 
fixtures for applying shear loads to a specimen. However, the disclosed 
fixtures do not apply a shear stress to the specimen wherein the applied 
force is both perpendicular to the outer cylindrical surface and along the 
longitudinal axis of the specimen. McRae in U.S. Pat. No. 5,036,709 
discloses a method of compacting a specimen, prior to shear testing, the 
disclosure of which is incorporated herein by reference, however the 
compacting device is not simple and portable and further does not apply a 
force that is both perpendicular to the outer cylindrical surface and 
along the longitudinal axis of the specimen. Furthermore, the referenced 
devices do not disclose guides that inhibit twisting of the fixture as the 
shear load is applied. The present invention overcomes these and other 
disadvantages of the prior art. 
SUMMARY OF THE INVENTION 
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. Testing of the asphalt material is 
performed by compacting it into a cylindrical sample and placing it into a 
shear fixture that can be subjected to monotonic or dynamic forces. The 
shear fixture includes first and second material retaining members each 
having a concave arcuate surface. The arcuate surface of each retaining 
member is aligned diametrically opposite one another, thereby defining a 
material receiving pocket adapted for containing the cylindrical specimen. 
An applied force to the shear fixture, that is both perpendicular to the 
outer cylindrical surface and along the longitudinal axis of the specimen, 
causes a certain response in the cylindrical sample which can be measured 
from the displacement of the first and second material retaining members. 
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. 
Shear tests can be performed to stress levels of 2500KPA and strains to 5% 
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 are 
then used in the superpave modeling technique to thereby estimate the 
material's long term performance. 
The shear tester generally includes a shear fixture with a displacement 
measuring transducer, loading frame with load transducer, an actuator with 
a displacement transducer, an environmental control chamber with 
temperature measurement transducer, and heating/cooling system. The shear 
fixture is coupled to a monitoring and control system which includes a 
microprocessor-based servocontroller, 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 chamber control temperature. 
The shear fixture includes first and second material retaining members. 
Front and back restraint plates mounted to each retaining member prevent 
the specimen from expanding along it's longitudinal axis during the 
duration of the test. The back plates are bolted directly to the material 
retaining members. Without any limitation intended, the front plates are 
attached to the material retaining members via thumb screws. The thickness 
of the material retaining members is constant and the specimen lengths may 
vary from 50 to 150 mm. To accommodate the varied specimen lengths, 
specimen spacer plates are provided for shorter specimen lengths. The 
spacer lengths can be fabricated in different thicknesses as needed to 
accommodate any specimen length. 
The two material retaining members are maintained in alignment with respect 
to one another by two flexures. The flexures allow movement of the 
retaining members in a direction that creates shearing of the specimen in 
a plane that is parallel to the specimen's longitudinal axis, while the 
specimen is maintained in a clamped position between the material 
retaining members. 
Installation and removal of the specimen is facilitated by a threaded 
handle that temporarily increases the diameter of the cylindrical pocket. 
This creates a looser fit between the first and second material retaining 
members, thus allowing the specimen to be installed and removed easily. 
The applied load or force to the fixture is measured by a load transducer 
that has one end coupled to an upper cross-piece of the shear fixture and 
the other end to a secondary flexure. The secondary flexure is attached to 
a shaft of the actuator. The secondary flexure or universal flexure 
accommodates small angular misalignments while transmitting the applied 
load to the shear fixture. A lower cross-piece and universal flexure is 
used to couple the lower cross-piece of the shear fixture to the 
cross-brace of the loading frame. A signal corresponding to the applied 
load is transmitted to a microprocessor-based controller which is coupled 
to the transducers for monitoring and control purposes and can be used to 
ensure that the test is being run at a specific load amplitude. 
The relative movement of the first and second material retaining members is 
measured via a spring loaded displacement transducer which has it's body 
mounted to the second material retaining member and the measuring end 
pressed against the first material retaining member. A signal 
corresponding to the resulting displacement is transmitted to the 
microprocessor-based controller for monitoring and control purposes and 
can be used to ensure that the test is being run at a specific 
displacement amplitude. 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. 
Both dynamic (sinusoidal or pulsed) or static loading can be applied to the 
shear fixture. A servo pneumatic actuator having a shaft coupled to the 
shear fixture is used to create the applied load. A servo valve mounted to 
the 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 actuator separately from the air supply, an on/off solenoid 
valve is provided. The servo actuator is attached to the upper cross-brace 
of the loading frame and is attached via an actuator shaft to a 
cross-piece of the shear fixture. The lower part of the shear fixture 
attaches directly to a cross-brace of the loading frame. 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 a servo hydraulic, electrodynamic or electromechanical 
generator. 
The environmental control chamber surrounding the shear fixture is a box 
type configuration with a door for sealably enclosing the fixture and 
specimen. The chamber 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 other methods of heating/cooling 
could be used, however electric heat and liquid cooling are preferred. The 
chamber uses 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. 
OBJECTS 
It is accordingly a principal object of the present invention to provide a 
shear tester for testing asphalt specimens at the field site for the 
purpose of generating test data that can be used in the superpave modeling 
technique. 
Another object of the present invention is to provide a shear tester, 
wherein the specimen can be tested with minimal preparation at the field 
site. 
Still another object of the present invention is to provide a means of 
shearing a cylindrical specimen along its longitudinal axis, wherein the 
shearing force is applied perpendicular to the cylindrical surface. 
These and other objects, as well as these and other features and 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 in conjunction with the accompanying drawings and 
claims, in which like numerals in the several views refer to corresponding 
parts.

DETAILED DESCRIPTION 
In conjunction with the several views of the figures, details of 
representative embodiments will next be presented. Referring first to FIG. 
1, there is shown a shear fixture 10 connected to a frame 12, a 
microprocessor-based controller 14, an uninterruptable power supply 16, 
and an environmental control chamber 18. To provide a quick shutdown 
capability, a quick stop button 20 of suitable construction is 
electrically coupled to a solenoid valve 28. 
Attached to the frame 12 is a servo pneumatic actuator 22 of known 
construction, with servo valve 24 for porting air to either side of the 
actuator piston 26. A solenoid valve 28 has on/off capability for manual 
control of the actuator 22, to thereby isolate the servo valve 24 from the 
air supply. In the preferred embodiment, the bottom 30 of the servo 
actuator 22 serves both as a bottom to the actuator 22 and a top 
cross-brace of the loading frame 12. One end of a shaft 32 is attached to 
the actuator piston 26, and the other end extends through the bottom 30 
and is attached to a universal flexure 34. 
The first end of a load transducer 36 is attached to the flexure 34 and the 
second end is attached to an upper cross-piece 38 of the shear fixture 10 
(see FIGS. 1 and 3-7). A lower cross-piece 40 of the frame 12 is attached 
to a lower universal flexure 42 of known construction. The universal 
flexure 42 is further attached to a bottom cross-brace 44 of the frame 12. 
Two spaced vertical support columns 46, 48 are attached to and support the 
top and bottom cross-braces 30, 44. 
Displacement transducer 50 is coupled to the actuator 22 and displacement 
transducer 52 is coupled to shear fixture 10. The transducers 50, 52 are 
of suitable known construction for measuring the respective displacement 
of the actuator 22 and fixture 10 and are electrically coupled to the 
microprocessor-based controller 14 to provide displacement feedback for 
closed loop servo control and monitoring. 
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 (see FIGS. 1 and 2). 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 CO.sub.2 /N.sub.2 
injector for cooling. The temperature transducer and heating/cooling 
elements within the chamber 18 are coupled to a microprocessor-based 
temperature controller (not shown). A signal is sent from the temperature 
transducer to the temperature controller, indicating the temperature of 
the chamber air temperature. 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 the preset level, a solenoid valve is 
opened thereby cycling CO.sub.2 /N.sub.2 until the air temperature drops 
to the desired 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. The 
solid state relay activates the heating elements or injection solenoids, 
when necessary, to maintain the fluid temperature at the desired 
temperature setpoint. 
Referring now to FIGS. 3-7, the shear fixture 10 is comprised generally of 
upper and lower cross-pieces 38, 40, upper and lower flexure assemblies 
(which include flexures 60, 62 and spacers 64-70), first and second 
material retaining members 54, 56, each having a concave arcuate surface 
aligned diametrically opposite one another to form a cylindrical pocket to 
accommodate the material specimen 58, and restraint plates 72, 74. 
Alignment of the first and second material retaining members 54, 56 is 
accomplished using upper and lower flexure assemblies. A proximal end of 
the upper flexure 60 is attached between the proximal end of the upper 
cross-piece 38 and the first material retaining member 54. An upper spacer 
64 separates the upper cross-piece 38 and upper flexure 60. Attached to 
the distal end of the flexure 60 are spacer members 68 and 70 having a 
combined thickness that is less than the combined thickness of spacers 64 
and 66. The lower flexure 62 is attached between the lower cross-piece 40 
and second material retaining member 56 in a similar manner wherein the 
distal end of the lower flexure 62 is fixedly attached between the lower 
cross-piece 40 and second material retaining member 56. The upper and 
lower flexures 60 and 62 allow movement of the material retaining members 
54, 56 in the vertical direction creating a shear condition along the 
specimen's longitudinal axis, while at the same time the flexures 60, 62 
maintain the specimen in fixed position between the material retaining 
members 54, 56. 
Front and back restraint plates 72, 74 are attached to each material 
retaining members 54, 56, thereby inhibiting the specimen 58 from 
expanding along it's longitudinal axis during the test. The back plates 74 
are bolted directly to the material retaining members 54, 56 while the 
front plates 72 are attached to the material retaining members 54, 56 via 
thumb screws 76. Those skilled in the art will appreciate that other 
methods of attachment could be used. Although the thickness of the 
material retaining members 54, 56 is fixed and the specimen lengths may 
vary from 50 to 150 mm. To accommodate for various specimen lengths, 
spacer plates 78 are provided for shorter specimens. 
Installation and removal of the specimen is facilitated by a stud 80 and 
threaded handle assembly 82 that couples the proximal end of the lower 
flexure 62 to the bottom of the first material retaining member 54. 
Tightening the stud 80 and handle 82 against the first material retaining 
member 54 increases the distance between the lower portion of the material 
retaining members 54, 56. This creates a looser fit between the material 
retaining members 54, 56, thus allowing specimen 58 to be installed and 
removed easily. 
As described above the upper cross-piece 38 is attached to the load 
transducer 36 and the lower cross-piece is attached to universal flexure 
42. To reduce excessive over travel and damage to the flexure assemblies 
in the event of a specimen failure, shoulder bolts or over travel stops 84 
are used. Likewise, spacers 68, 70 inhibit over travel in the event a 
compression failure. The shoulder bolts 84 and spacers 68, 70 ensures that 
the material retaining members 54, 56 can only move a small distance with 
respect to one another. Universal flexures 34 and 42 also reduce 
extraneous side loads and moments from being transmitted to the load 
transducer 36 and fixture 10. 
As an upward or downward force is applied by the actuator 22, the load 
transducer may stretch or compress slightly. For this reason, the actuator 
displacement transducer 50 cannot be used as a reliable indication of 
relative displacement between the material retaining members 54, 56. The 
material retaining members 54, 56 relative displacement is measured via a 
spring loaded displacement transducer 52 which has it's body mounted to 
the second material retaining member 56 via bracket 88 and the measuring 
end of the transducer 52 is pressed against bracket 86 attached to the 
first material retaining member 54. A signal corresponding to the measured 
displacement is transmitted to the microprocessor-based controller 14 for 
monitoring and control purposes and can be used to ensure that the test is 
being run at a specific displacement amplitude. 
The microprocessor-based controller 14 uses a PID control algorithm for 
controlling the servo pneumatic actuator 22. The feedback signals from 
either the load transducer 36 or linear displacement transducers 50 and 52 
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. 
After the error signal is computed, the microprocessor 14 then performs 
several mathematical operations on the error signal known as PID control. 
First, the error signal is multiplied by a scaler value K.sub.1 to obtain 
a proportional (P) value. The error signal is also integrated over time 
and multiplied by scaler value K.sub.2 to obtain an Integration (I) value. 
The error value is also differentiated with respect to time and multiplied 
by scaler value K.sub.3 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 14. This output voltage is the input 
signal for the servo valve 24 which controls the force applied by the 
actuator. The PID control tends to reposition the applied load of the 
servo pneumatic actuator 22 to minimize the error signal. 
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 36 is monitored by the microprocessor based controller 14. 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. 
Having described the constructional features of the present invention, the 
mode of use will now be discussed. The user loosens threaded handle 82 of 
the shear fixture 10 so that the material retaining members 54, 56 move 
apart. The user then positions servo actuator 22 via microprocessor-based 
controller 14 until cylindrical specimen 58 fits between the two saddles. 
After inserting the specimen 58 until it is flush against back restraining 
plates 74, the user turns threaded handle 82 until the material retaining 
members 54 is tight against the lower flexure 62. The operator then 
attaches front restraining plates 72, tightening the plate 72 against the 
specimen 58 using hand screws 76. 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 14 
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 front retraining plates 72 
and extracts the specimen 58 from the shear fixture 10. 
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 details and operating procedures, can be accomplished without 
departing from the scope of the invention itself.