CA2254279A1 - Device for measuring material deformation and stress - Google Patents
Device for measuring material deformation and stress Download PDFInfo
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- CA2254279A1 CA2254279A1 CA 2254279 CA2254279A CA2254279A1 CA 2254279 A1 CA2254279 A1 CA 2254279A1 CA 2254279 CA2254279 CA 2254279 CA 2254279 A CA2254279 A CA 2254279A CA 2254279 A1 CA2254279 A1 CA 2254279A1
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N3/00—Investigating strength properties of solid materials by application of mechanical stress
- G01N3/40—Investigating hardness or rebound hardness
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
- G01N2203/0058—Kind of property studied
- G01N2203/0069—Fatigue, creep, strain-stress relations or elastic constants
- G01N2203/0071—Creep
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- Investigating Strength Of Materials By Application Of Mechanical Stress (AREA)
Description
UNIVERSAL MECHANICAL TESTING DEVICE
BACKGROUND OF THE INVENTION
(a) Field of the Invention s The invention relates to a universal testing device for determining certain material properties of a sample.
(b) Description of Prior Art Material testing refers to the evaluation of to mechanical properties of solid materials by simultaneously measuring material deformation (displacement) and stress (force). The technical area is mature and highly developed with respect to industrial sized objects with dimension of centimeters i5 or larger. When specimen dimensions encroach upon millimeters instrumentation and methods are less well developed, due to precision and control difficulties.
When materials are soft in addition to small, technical difficulties also arise in eliminating noise 2o from force signals.
Material testing systems are often developed with precise goals in mind. Thus some systems provide mechanical configurations appropriate for adhesion and tack tests (US 5,438,863) for extrusion of 2s thermoplastics in rheological testing (US 4,680,958) and others for hardness and bonding tests of pharmaceuticals (US 4,780,465, and US 5,140,861).
Common technical hurdles in these specific applications are precise control of displacement and 30 low noise acquisition of force and displacement. These problems are overcome to varying degrees but generally insufficiently so in modern instruments. Also in spite of the underlying commonality in all material testing which ,is control and acquisition of force and 35 displacement, instruments are often conceived and designed for the application of a limited number of tests where, for example, only one type of displacement is applied to obtain a certain force response upon which a particular analysis yields one s characteristic material parameter. The limited flexibility of such systems is evident since proper mechanical and electrical design combined with algorithmic computer control of tests can in principle provide a universal system capable of executing the to full range of material tests on small samples, as has been achieved for industrial sized objects. For example common measures of adhesion, tack, hardness, strength, modulus, viscoelasticity, plasticity etc.
can all be obtained by parametric control of a limited i5 number of fundamental tests such as ramp, stress relaxation, dynamic sinusoidal and creep tests.
In the biomedical domain of material testing, a particular need for testing samples in aqueous solutions under controlled environments of atmospheric 2o gas composition and humidity arises. In the absence of material testing needs, these environments are generally provided by cell or tissue culture incubators. In the past, the need to perform material tests under these controlled environments has been 2s addressed by developing testing chambers specific to the material testing apparatus to provide environmental control, since the material testing device is usually much too large to be placed in an incubator.
3o It would thus be highly desirable to be provided with a material testing instrument that would allow testing of these small specimens and that could be designed so as to fit inside a standard tissue culture incubator, thus adding to the universality of the device by including biomedical applications in their most standard format.
SZJN~lARY OF THE INVENTION
One aim of the present invention is to provide a precise and controlled material testing device for testing small specimens.
Another aim of the present invention is to provide a material testing device that could be io designed so as to fit inside a standard tissue culture incubator, thus adding to the universality of the device by including biomedical applications in their most standard format.
Another aim of the present invention is to i5 provide a material testing device for performing stress relaxation test, ramp test, Creep test, dynamic sinusoid measurements, long sinusoids, using an actuator for moving a sample at a constant speed, in which the actuator is so controlled as to mimic 2o sinusoidal displacement, when needed.
Another aim of the present invention is to provide a material testing device for testing for unconfined or confined compression test, indentation test, tension test, and bending test.
BRIEF DESCRIPTION OF THE DRAWINGS
Having thus generally described the nature of the invention, reference will now be made to the accompanying drawings, showing by way of illustration 3o a preferred embodiment thereof, and wherein:
Fig. 1 is a perspective view of a material testing device in accordance with a preferred embodiment of the invention;
BACKGROUND OF THE INVENTION
(a) Field of the Invention s The invention relates to a universal testing device for determining certain material properties of a sample.
(b) Description of Prior Art Material testing refers to the evaluation of to mechanical properties of solid materials by simultaneously measuring material deformation (displacement) and stress (force). The technical area is mature and highly developed with respect to industrial sized objects with dimension of centimeters i5 or larger. When specimen dimensions encroach upon millimeters instrumentation and methods are less well developed, due to precision and control difficulties.
When materials are soft in addition to small, technical difficulties also arise in eliminating noise 2o from force signals.
Material testing systems are often developed with precise goals in mind. Thus some systems provide mechanical configurations appropriate for adhesion and tack tests (US 5,438,863) for extrusion of 2s thermoplastics in rheological testing (US 4,680,958) and others for hardness and bonding tests of pharmaceuticals (US 4,780,465, and US 5,140,861).
Common technical hurdles in these specific applications are precise control of displacement and 30 low noise acquisition of force and displacement. These problems are overcome to varying degrees but generally insufficiently so in modern instruments. Also in spite of the underlying commonality in all material testing which ,is control and acquisition of force and 35 displacement, instruments are often conceived and designed for the application of a limited number of tests where, for example, only one type of displacement is applied to obtain a certain force response upon which a particular analysis yields one s characteristic material parameter. The limited flexibility of such systems is evident since proper mechanical and electrical design combined with algorithmic computer control of tests can in principle provide a universal system capable of executing the to full range of material tests on small samples, as has been achieved for industrial sized objects. For example common measures of adhesion, tack, hardness, strength, modulus, viscoelasticity, plasticity etc.
can all be obtained by parametric control of a limited i5 number of fundamental tests such as ramp, stress relaxation, dynamic sinusoidal and creep tests.
In the biomedical domain of material testing, a particular need for testing samples in aqueous solutions under controlled environments of atmospheric 2o gas composition and humidity arises. In the absence of material testing needs, these environments are generally provided by cell or tissue culture incubators. In the past, the need to perform material tests under these controlled environments has been 2s addressed by developing testing chambers specific to the material testing apparatus to provide environmental control, since the material testing device is usually much too large to be placed in an incubator.
3o It would thus be highly desirable to be provided with a material testing instrument that would allow testing of these small specimens and that could be designed so as to fit inside a standard tissue culture incubator, thus adding to the universality of the device by including biomedical applications in their most standard format.
SZJN~lARY OF THE INVENTION
One aim of the present invention is to provide a precise and controlled material testing device for testing small specimens.
Another aim of the present invention is to provide a material testing device that could be io designed so as to fit inside a standard tissue culture incubator, thus adding to the universality of the device by including biomedical applications in their most standard format.
Another aim of the present invention is to i5 provide a material testing device for performing stress relaxation test, ramp test, Creep test, dynamic sinusoid measurements, long sinusoids, using an actuator for moving a sample at a constant speed, in which the actuator is so controlled as to mimic 2o sinusoidal displacement, when needed.
Another aim of the present invention is to provide a material testing device for testing for unconfined or confined compression test, indentation test, tension test, and bending test.
BRIEF DESCRIPTION OF THE DRAWINGS
Having thus generally described the nature of the invention, reference will now be made to the accompanying drawings, showing by way of illustration 3o a preferred embodiment thereof, and wherein:
Fig. 1 is a perspective view of a material testing device in accordance with a preferred embodiment of the invention;
Fig. 2 is a side elevational view of a testing chamber of the material testing device of the present invention used for testing unconfined compression;
Fig. 3 is a side elevational view of another s testing chamber of the material testing device of the present invention used for testing confined compression;
Fig. 4 is a side elevational view of another testing chamber of the material testing device of the to present invention used for testing indentation;
Fig. 5 is a side elevational view of another testing chamber of the material testing device of the present invention used for tension test;
Fig. 6 is a side elevational view of another i5 testing chamber of the material testing device of the present invention used for bending test;
Fig. 7 is a side elevational view of another testing chamber of the material testing device of the present invention used for measuring variation of 2o electric potential;
Fig. 8 is a side elevational view of a humidifying testing chamber of the material testing device of the present invention;
Fig. 9A to 9C represent flow chart of a 2s sinusoidal algorithm developed for moving the actuator of the material testing device of the present invention;
Fig. 10 represents a flow chart developed for performing a creep test with the material testing 3o device of the present invention;
Fig. 11 represents a flow chart developed for correcting and synchronizing position and force measurements obtained with the material testing device of the present invention;
Fig. 3 is a side elevational view of another s testing chamber of the material testing device of the present invention used for testing confined compression;
Fig. 4 is a side elevational view of another testing chamber of the material testing device of the to present invention used for testing indentation;
Fig. 5 is a side elevational view of another testing chamber of the material testing device of the present invention used for tension test;
Fig. 6 is a side elevational view of another i5 testing chamber of the material testing device of the present invention used for bending test;
Fig. 7 is a side elevational view of another testing chamber of the material testing device of the present invention used for measuring variation of 2o electric potential;
Fig. 8 is a side elevational view of a humidifying testing chamber of the material testing device of the present invention;
Fig. 9A to 9C represent flow chart of a 2s sinusoidal algorithm developed for moving the actuator of the material testing device of the present invention;
Fig. 10 represents a flow chart developed for performing a creep test with the material testing 3o device of the present invention;
Fig. 11 represents a flow chart developed for correcting and synchronizing position and force measurements obtained with the material testing device of the present invention;
Fig. 12 represents a flow chart developed for setting up the material testing device of the present invention; and Figs. 13A to 13C illustrate the elements of a humidifying chamber, and its assembly.
DETAILED DESCRIPTION OF THE INVENTION
Fig. 1 illustrates a device in accordance with the present invention, which comprises a frame, an io actuator, a universal platform, a load sensing unit, a signal conditioning unit and a computerized algorithmic control for execution and analysis of tests. Precision of 1 part in 20,000 for load measurement (5mg) and 1 part in 1,000,000 for position control (25nm) is preferably used in the execution of stress relaxation, ramp, dynamic sinusoidal and creep tests.
The universal platform accepts test chambers for compression tests (Figs. 2 and 3), indentation zo tests (Fig. 4), tension tests (Fig. 5), and bending tests (Fig. 6). The frame including actuator and load sensing unit is designed to fit inside a tissue culture incubator for testing in controlled environments using, if needed, autoclave sterilized testing chambers. Attention is paid to ease of use and universality of all features and functions to provide a means for material testing of tissues, pharmaceuticals, adhesives, polymers and gels.
In the device of the present invention, the 3o cross-bar is designed such that it has a minimal mass and minimal vertical deflection, for not increasing system compliance (negligible deflection) while still exerting minimal resting force on an attached load cell in order to avoid damaging of the latter. A
DETAILED DESCRIPTION OF THE INVENTION
Fig. 1 illustrates a device in accordance with the present invention, which comprises a frame, an io actuator, a universal platform, a load sensing unit, a signal conditioning unit and a computerized algorithmic control for execution and analysis of tests. Precision of 1 part in 20,000 for load measurement (5mg) and 1 part in 1,000,000 for position control (25nm) is preferably used in the execution of stress relaxation, ramp, dynamic sinusoidal and creep tests.
The universal platform accepts test chambers for compression tests (Figs. 2 and 3), indentation zo tests (Fig. 4), tension tests (Fig. 5), and bending tests (Fig. 6). The frame including actuator and load sensing unit is designed to fit inside a tissue culture incubator for testing in controlled environments using, if needed, autoclave sterilized testing chambers. Attention is paid to ease of use and universality of all features and functions to provide a means for material testing of tissues, pharmaceuticals, adhesives, polymers and gels.
In the device of the present invention, the 3o cross-bar is designed such that it has a minimal mass and minimal vertical deflection, for not increasing system compliance (negligible deflection) while still exerting minimal resting force on an attached load cell in order to avoid damaging of the latter. A
preferred embodiment of the cross-bar is illustrated in Fig. 1 The means for fixing the vertically sliding cross-bar at a particular height could be any means s suitable for the functions of holding the cross-bar, such as butterfly bolts tightened and loosened by an operator, thus utilizing a variation of manual fasteners appropriate for this purpose.
In a particular embodiment of the invention, to for attaining vertical alignment with a specimen fixed to the actuator, an enlarged bore hole is provided through the cross-head, two rigid washers on each side of the bore hole and a bolt traversing the hole attaching to the force-sensing device and thereby the i5 sample. Alignment is achieved visually by sliding the bolt/washers system across the surface of the cross beam using the tolerance provided by the enlarged bore diameter.
The overall system dimensions and materials are 2o preferably compatible with placement in a standard cell or tissue culture incubator having for example interior dimension of 2 feet wide by 2 feet deep by 3 feet high). Atmospheric conditions in an incubator are typically 37°C, 5% CO2 and 95% relative humidity.
2s Many test chambers may alternately be attached to the device of the present invention so as to provide a single device for carrying out a multitude of tests. Accordingly, Fig. 2 illustrates a test chamber for unconfined compression of a sample 3o attached to the actuator and force sensing unit for use with the device of the present invention.
Fig. 3 illustrates a test chamber for confined compression of a sample attached to the actuator and force sensing unit also for use with the device of the 35 present invention.
_ 7 Fig. 4 illustrates another test chamber for indentation of a sample attached to the actuator and force sensing unit.
Fig. 5 illustrates another test chamber for s tension testing of a sample attached to the actuator and force sensing unit.
Fig. 6 illustrates a further test chamber for a bending test of a sample attached to the actuator and force sensing unit.
io The system of the present invention may further comprises micro-electrodes incorporated into testing chambers to detect electrical events occurring during testing. An example of such micro-electrodes is the system of electrodes incorporated into an unconfined i5 compression chamber to measure compression induced streaming potentials (Fig. 7).
In a particular embodiment of the invention, the chambers of the device of the present invention are designed to be autoclave sterilized and to accept 2o sterile specimens within an aseptic environment before transfer to a non-aseptic environment (tissue culture incubator) for testing in a controlled environment, while maintaining sterility. Accordingly, the chambers may be provided with covers to maintain 2s sterile condition. A hole is provided in the cover with a tolerance of 0.0005 for insertion of a rod mounted to the load cell.
Testing chambers may also be designed so as confine humidification of the sample environment to 3o the interior of the chamber via the inclusion of a humidifying section containing an aqueous solution separated from the bathing media of the sample. Such chambers avoid potential damaging effects of humidity on the electronic components of the actuator or other g _ parts of the system present inside the incubator (Fig 8 . ) .
A programmable digitizing amplifier situated in the vicinity of the load sensing unit is preferably s used to i) minimize noise in the load signal by providing a digital signal representing the load with a precision of 1 part in 20,000 ii) accommodate interchangeable load cells with specific energizing and gain parameters, and iii) provide a second digital io output indicating a user defined excess load (overload) condition on the load cell.
The digital signal indicating excess load is preferably coupled directly to the actuator controller to automatically execute a motor stop command in is response to excess load.
The actuator (providing a precision of 1-100 nm with a range of at least 0.25-5 million times that value) executing constant velocity motion is controlled with a computer program to execute stress 2o relaxation, ramp-release, dynamic sinusoidal and creep tests.
Stress relaxation refers to the application of a constant velocity displacement followed by a hold phase while measuring force.
2s The stress relaxation measurement routine allows to apply sequences of ramp-hold displacements, i.e. stress relaxation tests for a viscoelastic material. A sequence of ramp-hold displacements is specified by deciding on the number of ramps and the 3o amplitude and velocity of each ramp. There are two ways of determining the end of each relaxation profile before applying the next ramp. The first is to simply specify the time of acquisition of each profile. The second is to measure the slope of load vs. time and 35 end the profile when this slope is less than a criterion that you specify. With the latter technique a uniform estimation of equilibrium is made throughout the acquisition. The decision to end the relaxation profile using the slope calculation is affected not s only by the slope criterion but by the "Sample Time"
and the "Time for Measurement of the Slope" - small "Sample Time" and large "Time for Measurement of the Slope" allow the use of very strict criterion for equilibrium since the effect of noise on the slope to calculation is minimized.
Ramp-release refers to the application of a constant velocity displacement followed by the reverse of that displacement to obtain the initial position.
The Ramp-Release routine executes a sequence of i5 displacements (tension or compression) followed by a release each at the same constant velocity. This routine and the Long Sinusoid routine are particularly useful for mechanically stimulating specimens, especially in controlled environments such as cell or 2o tissue culture incubators. It is also useful for studying fatigue and related processes during prolonged loading periods. In addition to defining the amplitude and velocity of the displacement, two time parameters are defined - the time spent between the 2s displacement and release ("Rest Time at Peak") and the time following the release before the next displacement is executed ("Rest Time Between Cycles").
A sequence of identical displacements and releases is then executed to completion. Since loading times can 3o be extended and total data volume massive, there is an option of saving less than the entire data set by setting "Save Every? Cycles" to a number other than 1 and by "Reducing Acquisition Time After Each Cycle".
Creep test refers to the application of a constant force by feedback control of the actuator position.
The only difference between the creep and the s stress relaxation routine is the following. The stress Relaxation holds the position constant at the end of the ramp displacement, whereas the creep holds the load constant at the end of the ramp displacement.
Dynamic sinusoids refers to the application of to a displacement in the form of a sinusoidal wave of high precision and low total harmonic distortion.
The dynamic sinusoids measurement routine allows you to impose sinusoidal displacements (10-5 to 1 Hz) with amplitudes in the range 0.5~.m to lOmm. For is the larger amplitudes care must be taken that the extreme of the sinusoids are within the range of the actuator (26mm). The routine executes a sequence of sinusoidal displacements determined by the arrays "Amplitudes", "Frequencies" and "Cycles". The 2o execution order is the first amplitude with all the frequencies followed by the second amplitude with all of the frequencies etc. The "Number of Amplitudes" and the "Number of Frequencies" to be executed from each array are controlled by integer values. Each 2s "Frequency" has a number of "Cycles" to be executed and a number of "Cycles to wait before FFT". The latter refers to the number of cycles during which the transient response decays to negligible values, and after which Fourier analysis is applied to obtain 3o amplitudes and phases of fundamentals and harmonics of the position and load. It is also important to properly specify the "Time Between Sinusoids";
equilibrium should be attained before applying the next sinusoid. The execution time can be estimated 35 before starting.
The Long Sinusoid routine is used when sinusoidal displacements of a given frequency and amplitude is desired over an extended period of time, such as when mechanically stimulating specimens, s especially in controlled environments such as cell or tissue culture incubators. It is also useful for studying fatigue and related processes during prolonged loading periods. The amplitude, frequency and duration of loading are defined.
to The sinusoidal displacement of the actuator is achieved using a computer algorithm concatenating a sequence of constant velocity steps to achieve an optimal precision and minimal distortion of the sinusoidal displacement. Given the amplitude and is frequency of the sinusoidal displacement, the algorithm finds a sequence of constant velocity steps which best approximate the sine wave given the performance characteristics of the actuator (including step size and velocity range) (Figs 9A to 9C).
2o The creep test is executed by feedback control of the actuator to maintain a constant sensed force on the load cell. Another computer algorithm is used where an updated position is calculated to maintain a constant sensed force, based on the force and position 2s history and the performance characteristics of the actuator (including step size and velocity range) (Fig 10) .
The system of the present invention may also comprises an interpolation algorithm to temporally 3o align the position and force signals given known performance characteristics of the signal acquisition system including the delay time between acquisition of load and position (Fig 11), for signal conditioning and treatment.
Where initial contact of the sample and load cell is found using an automated computer controlled procedure, an algorithm applies a calculated displacement at a calculated velocity until a given s value of contact force is detected by the force sensing element (Fig. 12).
Figs. 13A to 13C illustrate the elements of a humidifying chamber, and its assembly. Stands are mounted on the cover with a double lock screw system.
to for mounting the rod through the cover, a lower rod is first inserted through the cover. A washer and a spacer are then inserted in the lower rod emerging from the other side of the cover. An upper rod, at one end thereof, is then screwed to the lower rod.
is The other end of the upper rod is to be screwed to the cell load. For doing so, the upper rod is centered with the load cell, and the upper rod is screwed thereto. Once the upper rod is screwed to the load cell, the spacer is then removed.
2o In use under sterile conditions, a sample is placed in the middle of the center ring with an appropriate testing media. The autoclaved outer ring is then filled with water and the chamber is closed with the cover.
25 The actuator and actuator controller can be any standard one but there are four important parameters to consider in the choosing it: the minimal and maximal velocities, the spatial resolution of the actuator, and the response time of the controller. All 30 of these parameters will be important in the sinusoidal and creep algorithms.
The load cell amplifier can be any standard load cell amplifier but there are two important parameters to consider in the choosing it. These are 35 the resolution of the amplifier and the possibility to be user programmable to allow it to be used with different load cells.
Four important parameters are to be considered in the choosing of a load cell. These are a minimal s deformation of the load cell, a minimal non-linearity, a minimal temperature effect and the maximal mV
output.
The cross-beam of Fig. 1 is designed with three major restrictions:
l0 1) A deformation, at its center, of less than 1 ~m for an applied force of 100 Newton so that this deformation is negligible in comparison of the deformation of the sample.
2 ) A mass of less than 1 kg to allow a user to is deposit the cross-beam on the load cell as a reference for the position of the actuator. The mass of 1 kg is chosen so as to be below the capacity of the load cell and thus to avoid an over load for a 1 kg load cell.
3) The cross-beam can easily move to allow 2o changes in the measurement configuration. (i.e.
Indentation, Electrodes, humidifying chamber, etc...) The materials used for the cross-beam, the frame and all the chamber except the chamber for the electrode are 316L Stainless Steel. For the electrode, 2s a DelrinT"' layer is added to have a non-conductive bath. Delrin is a commercial designation for a polyoxymethylene (POM) plastic.
This description demonstrates the best way to use our system and obtain best results in the case of 3o a compression test.
First, calibrate the load cell with the incorporated sub-routine. Deposit the cross-beam with the load cell and the testing rod on the testing chamber and set this position of the actuator as the 35 reference. Move down the actuator and put a sample in the testing chamber. Move back up the actuator with the "find contact" routine as illustrated in Fig. 12.
From the actual position of the actuator the thickness of the sample can be deduced. From this point any s available test can be performed, like a creep test, a stress relaxation test or a sinusoid test.
For calibration, the load on the "loaded" load cell is read. The load on the "unloaded" load cell is also read. Finally, the calibration factor is to calculated with the following formula:
Calibration factor - heavy load weight/(Read "loaded"-Read "unloaded").
While the invention has been described with particular reference to the illustrated embodiment, it is will be understood that numerous modifications thereto will appear to those skilled in the art. Accordingly, the above description and accompanying drawings should be taken as illustrative of the invention and not in a limiting sense.
In a particular embodiment of the invention, to for attaining vertical alignment with a specimen fixed to the actuator, an enlarged bore hole is provided through the cross-head, two rigid washers on each side of the bore hole and a bolt traversing the hole attaching to the force-sensing device and thereby the i5 sample. Alignment is achieved visually by sliding the bolt/washers system across the surface of the cross beam using the tolerance provided by the enlarged bore diameter.
The overall system dimensions and materials are 2o preferably compatible with placement in a standard cell or tissue culture incubator having for example interior dimension of 2 feet wide by 2 feet deep by 3 feet high). Atmospheric conditions in an incubator are typically 37°C, 5% CO2 and 95% relative humidity.
2s Many test chambers may alternately be attached to the device of the present invention so as to provide a single device for carrying out a multitude of tests. Accordingly, Fig. 2 illustrates a test chamber for unconfined compression of a sample 3o attached to the actuator and force sensing unit for use with the device of the present invention.
Fig. 3 illustrates a test chamber for confined compression of a sample attached to the actuator and force sensing unit also for use with the device of the 35 present invention.
_ 7 Fig. 4 illustrates another test chamber for indentation of a sample attached to the actuator and force sensing unit.
Fig. 5 illustrates another test chamber for s tension testing of a sample attached to the actuator and force sensing unit.
Fig. 6 illustrates a further test chamber for a bending test of a sample attached to the actuator and force sensing unit.
io The system of the present invention may further comprises micro-electrodes incorporated into testing chambers to detect electrical events occurring during testing. An example of such micro-electrodes is the system of electrodes incorporated into an unconfined i5 compression chamber to measure compression induced streaming potentials (Fig. 7).
In a particular embodiment of the invention, the chambers of the device of the present invention are designed to be autoclave sterilized and to accept 2o sterile specimens within an aseptic environment before transfer to a non-aseptic environment (tissue culture incubator) for testing in a controlled environment, while maintaining sterility. Accordingly, the chambers may be provided with covers to maintain 2s sterile condition. A hole is provided in the cover with a tolerance of 0.0005 for insertion of a rod mounted to the load cell.
Testing chambers may also be designed so as confine humidification of the sample environment to 3o the interior of the chamber via the inclusion of a humidifying section containing an aqueous solution separated from the bathing media of the sample. Such chambers avoid potential damaging effects of humidity on the electronic components of the actuator or other g _ parts of the system present inside the incubator (Fig 8 . ) .
A programmable digitizing amplifier situated in the vicinity of the load sensing unit is preferably s used to i) minimize noise in the load signal by providing a digital signal representing the load with a precision of 1 part in 20,000 ii) accommodate interchangeable load cells with specific energizing and gain parameters, and iii) provide a second digital io output indicating a user defined excess load (overload) condition on the load cell.
The digital signal indicating excess load is preferably coupled directly to the actuator controller to automatically execute a motor stop command in is response to excess load.
The actuator (providing a precision of 1-100 nm with a range of at least 0.25-5 million times that value) executing constant velocity motion is controlled with a computer program to execute stress 2o relaxation, ramp-release, dynamic sinusoidal and creep tests.
Stress relaxation refers to the application of a constant velocity displacement followed by a hold phase while measuring force.
2s The stress relaxation measurement routine allows to apply sequences of ramp-hold displacements, i.e. stress relaxation tests for a viscoelastic material. A sequence of ramp-hold displacements is specified by deciding on the number of ramps and the 3o amplitude and velocity of each ramp. There are two ways of determining the end of each relaxation profile before applying the next ramp. The first is to simply specify the time of acquisition of each profile. The second is to measure the slope of load vs. time and 35 end the profile when this slope is less than a criterion that you specify. With the latter technique a uniform estimation of equilibrium is made throughout the acquisition. The decision to end the relaxation profile using the slope calculation is affected not s only by the slope criterion but by the "Sample Time"
and the "Time for Measurement of the Slope" - small "Sample Time" and large "Time for Measurement of the Slope" allow the use of very strict criterion for equilibrium since the effect of noise on the slope to calculation is minimized.
Ramp-release refers to the application of a constant velocity displacement followed by the reverse of that displacement to obtain the initial position.
The Ramp-Release routine executes a sequence of i5 displacements (tension or compression) followed by a release each at the same constant velocity. This routine and the Long Sinusoid routine are particularly useful for mechanically stimulating specimens, especially in controlled environments such as cell or 2o tissue culture incubators. It is also useful for studying fatigue and related processes during prolonged loading periods. In addition to defining the amplitude and velocity of the displacement, two time parameters are defined - the time spent between the 2s displacement and release ("Rest Time at Peak") and the time following the release before the next displacement is executed ("Rest Time Between Cycles").
A sequence of identical displacements and releases is then executed to completion. Since loading times can 3o be extended and total data volume massive, there is an option of saving less than the entire data set by setting "Save Every? Cycles" to a number other than 1 and by "Reducing Acquisition Time After Each Cycle".
Creep test refers to the application of a constant force by feedback control of the actuator position.
The only difference between the creep and the s stress relaxation routine is the following. The stress Relaxation holds the position constant at the end of the ramp displacement, whereas the creep holds the load constant at the end of the ramp displacement.
Dynamic sinusoids refers to the application of to a displacement in the form of a sinusoidal wave of high precision and low total harmonic distortion.
The dynamic sinusoids measurement routine allows you to impose sinusoidal displacements (10-5 to 1 Hz) with amplitudes in the range 0.5~.m to lOmm. For is the larger amplitudes care must be taken that the extreme of the sinusoids are within the range of the actuator (26mm). The routine executes a sequence of sinusoidal displacements determined by the arrays "Amplitudes", "Frequencies" and "Cycles". The 2o execution order is the first amplitude with all the frequencies followed by the second amplitude with all of the frequencies etc. The "Number of Amplitudes" and the "Number of Frequencies" to be executed from each array are controlled by integer values. Each 2s "Frequency" has a number of "Cycles" to be executed and a number of "Cycles to wait before FFT". The latter refers to the number of cycles during which the transient response decays to negligible values, and after which Fourier analysis is applied to obtain 3o amplitudes and phases of fundamentals and harmonics of the position and load. It is also important to properly specify the "Time Between Sinusoids";
equilibrium should be attained before applying the next sinusoid. The execution time can be estimated 35 before starting.
The Long Sinusoid routine is used when sinusoidal displacements of a given frequency and amplitude is desired over an extended period of time, such as when mechanically stimulating specimens, s especially in controlled environments such as cell or tissue culture incubators. It is also useful for studying fatigue and related processes during prolonged loading periods. The amplitude, frequency and duration of loading are defined.
to The sinusoidal displacement of the actuator is achieved using a computer algorithm concatenating a sequence of constant velocity steps to achieve an optimal precision and minimal distortion of the sinusoidal displacement. Given the amplitude and is frequency of the sinusoidal displacement, the algorithm finds a sequence of constant velocity steps which best approximate the sine wave given the performance characteristics of the actuator (including step size and velocity range) (Figs 9A to 9C).
2o The creep test is executed by feedback control of the actuator to maintain a constant sensed force on the load cell. Another computer algorithm is used where an updated position is calculated to maintain a constant sensed force, based on the force and position 2s history and the performance characteristics of the actuator (including step size and velocity range) (Fig 10) .
The system of the present invention may also comprises an interpolation algorithm to temporally 3o align the position and force signals given known performance characteristics of the signal acquisition system including the delay time between acquisition of load and position (Fig 11), for signal conditioning and treatment.
Where initial contact of the sample and load cell is found using an automated computer controlled procedure, an algorithm applies a calculated displacement at a calculated velocity until a given s value of contact force is detected by the force sensing element (Fig. 12).
Figs. 13A to 13C illustrate the elements of a humidifying chamber, and its assembly. Stands are mounted on the cover with a double lock screw system.
to for mounting the rod through the cover, a lower rod is first inserted through the cover. A washer and a spacer are then inserted in the lower rod emerging from the other side of the cover. An upper rod, at one end thereof, is then screwed to the lower rod.
is The other end of the upper rod is to be screwed to the cell load. For doing so, the upper rod is centered with the load cell, and the upper rod is screwed thereto. Once the upper rod is screwed to the load cell, the spacer is then removed.
2o In use under sterile conditions, a sample is placed in the middle of the center ring with an appropriate testing media. The autoclaved outer ring is then filled with water and the chamber is closed with the cover.
25 The actuator and actuator controller can be any standard one but there are four important parameters to consider in the choosing it: the minimal and maximal velocities, the spatial resolution of the actuator, and the response time of the controller. All 30 of these parameters will be important in the sinusoidal and creep algorithms.
The load cell amplifier can be any standard load cell amplifier but there are two important parameters to consider in the choosing it. These are 35 the resolution of the amplifier and the possibility to be user programmable to allow it to be used with different load cells.
Four important parameters are to be considered in the choosing of a load cell. These are a minimal s deformation of the load cell, a minimal non-linearity, a minimal temperature effect and the maximal mV
output.
The cross-beam of Fig. 1 is designed with three major restrictions:
l0 1) A deformation, at its center, of less than 1 ~m for an applied force of 100 Newton so that this deformation is negligible in comparison of the deformation of the sample.
2 ) A mass of less than 1 kg to allow a user to is deposit the cross-beam on the load cell as a reference for the position of the actuator. The mass of 1 kg is chosen so as to be below the capacity of the load cell and thus to avoid an over load for a 1 kg load cell.
3) The cross-beam can easily move to allow 2o changes in the measurement configuration. (i.e.
Indentation, Electrodes, humidifying chamber, etc...) The materials used for the cross-beam, the frame and all the chamber except the chamber for the electrode are 316L Stainless Steel. For the electrode, 2s a DelrinT"' layer is added to have a non-conductive bath. Delrin is a commercial designation for a polyoxymethylene (POM) plastic.
This description demonstrates the best way to use our system and obtain best results in the case of 3o a compression test.
First, calibrate the load cell with the incorporated sub-routine. Deposit the cross-beam with the load cell and the testing rod on the testing chamber and set this position of the actuator as the 35 reference. Move down the actuator and put a sample in the testing chamber. Move back up the actuator with the "find contact" routine as illustrated in Fig. 12.
From the actual position of the actuator the thickness of the sample can be deduced. From this point any s available test can be performed, like a creep test, a stress relaxation test or a sinusoid test.
For calibration, the load on the "loaded" load cell is read. The load on the "unloaded" load cell is also read. Finally, the calibration factor is to calculated with the following formula:
Calibration factor - heavy load weight/(Read "loaded"-Read "unloaded").
While the invention has been described with particular reference to the illustrated embodiment, it is will be understood that numerous modifications thereto will appear to those skilled in the art. Accordingly, the above description and accompanying drawings should be taken as illustrative of the invention and not in a limiting sense.
Claims (21)
1. A universal material testing device comprising:
a) a frame, b) an actuator mounted on said frame for displacement of a sample to be tested;
c) a force sensing device, such as a load cell for detecting a force applied on said sample by said actuator;
d) a signal conditioning unit or a force sensing device amplifier for reducing input noise,;
e) an actuator controller connected to the actuator for same; and f) means for processing of said signals and for executing specific tests by coordination of displacement control and load signals.
a) a frame, b) an actuator mounted on said frame for displacement of a sample to be tested;
c) a force sensing device, such as a load cell for detecting a force applied on said sample by said actuator;
d) a signal conditioning unit or a force sensing device amplifier for reducing input noise,;
e) an actuator controller connected to the actuator for same; and f) means for processing of said signals and for executing specific tests by coordination of displacement control and load signals.
2. The device of claim 1, further comprising a cross-bar designed to have a minimal mass and minimal vertical deflection for not increasing device compliance (negligible deflection) while still exerting minimal resting force on an attached load cell, to avoid damaging of the latter.
3. The device of claim 1, wherein the cross-bar is capable of vertically sliding and is fixed at a given height with manual means fastened by one operator, thus utilizing a variation of manual fasteners appropriate for this purpose. One particular fastener is the butterfly bolt.
4. The device of claim 1, further comprising means for attaining vertical alignment with a specimen fixed to the actuator consisting of an enlarged bore hole through the cross-head, two rigid washers on each side of the bore hole and a bolt traversing the hole attaching to the force-sensing device and thereby the sample. Alignment is achieved visually by sliding the bolt/washers system across the surface of the cross beam using the tolerance provided by the enlarged bore diameter.
5. The device of claim 1, wherein the overall device dimensions and materials are compatible with placement in a standard cell or tissue culture incubator (interior - 2' x 2' x 3'). Atmospheric conditions in an incubator are typically 37°C, 5% CO2 and 95%
relative humidity.
relative humidity.
6. The device of claim 1, further comprising a test chamber for unconfined compression of a sample, said test chamber being attached to the actuator and force sensing unit.
7. The device of claim 1, further comprising a test chamber for confined compression of a sample, said test chamber being attached to the actuator and force sensing unit.
8. The device of claim 1, wherein a test chamber for indentation of a sample is attached to the actuator and force sensing unit.
9. The device of claim 1, wherein a test chamber for tension testing of a sample is attached to the actuator and force sensing unit.
10. The device of claim 1, wherein a test chamber for a bending test of a sample is attached to the actuator and force sensing unit.
11. The device of claims 6-10, wherein micro-electrodes are incorporated into testing chambers to detect electrical events occurring during testing to measure compression induced streaming potentials.
12. The device of claims 6-10, wherein the chamber is designed to be autoclave sterilized and to accept sterile specimens within an aseptic environment before transfer to a non-aseptic environment (tissue culture incubator) for testing in a controlled environment all the while maintaining sterility.
13. The device of claim 12, wherein humidification of the sample environment is confined to the interior of the chamber via the inclusion of a humidifying section containing an aqueous solution separated from the bathing media of the sample so as to avoid potential damaging effects of humidity on electronic components of the actuator or other parts of the device present inside the incubator.
14. The device of claim 1, further comprising a programmable digitizing amplifier situated in the vicinity of the load sensing unit to i) minimize noise in the load signal by providing a digital signal representing the load with a precision of 1 part in 20,000 ii) accommodate interchangeable load cells with specific energizing and gain parameters, and iii) provide a second digital output indicating a user defined excess load (overload) condition on the load cell.
15. The device of claim 14, where the digital signal indicating excess load is directly coupled to the actuator controller to automatically execute a motor stop command in response to excess load.
16. The device of claim 1, wherein an actuator, providing a precision of 1-100 nm with a range of at least 0.25-5 million times that value and executing a constant velocity motion, is controlled with a computer program to execute stress relaxation, ramp-release, dynamic sinusoidal and creep tests, said stress relaxation refers to the application of a constant velocity displacement followed by a hold phase while measuring force, said ramp -release refers to the application of a constant velocity displacement followed by the reverse of that displacement to obtain the initial position, said dynamic sinusoids refers to the application of a displacement in the form of a sinusoidal wave of high precision and low total harmonic distortion, and said creep refers to the application of a constant force by feedback control of the actuator position.
17. The device of claim 16, wherein a user-defined sequence of tests is applied.
18. The device of claim 1, wherein a sinusoidal displacement of the actuator is achieved using a computer algorithm concatenating a sequence of constant velocity steps to achieve an optimal precision and minimal distortion of the sinusoidal displacement, given the amplitude and frequency of the sinusoidal displacement, said algorithm finds a sequence of constant velocity steps which best approximate the sine wave given the performance characteristics of the actuator (including step size and velocity range).
19. The device of claim 1, wherein a creep test is executed by feedback control of the actuator to maintain a constant sensed force on the load cell, said computer algorithm is used where an updated position is calculated to maintain a constant sensed force, based on the force and position history and the performance characteristics of the actuator (including step size and velocity range).
20. The device of claim 1, wherein signal conditioning and treatment include an interpolation algorithm to temporally align the position and force signals given known performance characteristics of the signal acquisition system including the delay time between acquisition of load and position.
21. The device of claim 1, wherein initial contact of the sample and load cell is found using an automated computer controlled procedure where an algorithm applies a calculated displacement at a calculated velocity until a given value of contact force is detected by the force sensing element.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 2254279 CA2254279A1 (en) | 1998-11-16 | 1998-11-16 | Device for measuring material deformation and stress |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 2254279 CA2254279A1 (en) | 1998-11-16 | 1998-11-16 | Device for measuring material deformation and stress |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2254279A1 true CA2254279A1 (en) | 2000-05-16 |
Family
ID=29425670
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2254279 Abandoned CA2254279A1 (en) | 1998-11-16 | 1998-11-16 | Device for measuring material deformation and stress |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2254279A1 (en) |
Cited By (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN107314941A (en) * | 2017-07-23 | 2017-11-03 | 张广 | Magnetorheological glue creep and stress relaxation characteristics test device and method under double-shear pattern |
| CN109060102A (en) * | 2018-08-22 | 2018-12-21 | 河南理工大学 | Ultrasonic drilling amplitude detection device under simulated load state |
| CN114026404A (en) * | 2019-03-28 | 2022-02-08 | 伊利诺斯工具制品有限公司 | Collision mitigation device and material testing system with collision mitigation device |
| CN107678386B (en) * | 2017-10-11 | 2024-03-22 | 中建八局第二建设有限公司 | Overload-proof and inclination-proof discharging platform |
-
1998
- 1998-11-16 CA CA 2254279 patent/CA2254279A1/en not_active Abandoned
Cited By (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN107314941A (en) * | 2017-07-23 | 2017-11-03 | 张广 | Magnetorheological glue creep and stress relaxation characteristics test device and method under double-shear pattern |
| CN107678386B (en) * | 2017-10-11 | 2024-03-22 | 中建八局第二建设有限公司 | Overload-proof and inclination-proof discharging platform |
| CN109060102A (en) * | 2018-08-22 | 2018-12-21 | 河南理工大学 | Ultrasonic drilling amplitude detection device under simulated load state |
| CN109060102B (en) * | 2018-08-22 | 2024-04-16 | 河南理工大学 | Ultrasonic drilling amplitude detection device under simulated load state |
| CN114026404A (en) * | 2019-03-28 | 2022-02-08 | 伊利诺斯工具制品有限公司 | Collision mitigation device and material testing system with collision mitigation device |
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