CN114112769B - Triaxial tester pressure chamber piston friction force calibration method and device - Google Patents
Triaxial tester pressure chamber piston friction force calibration method and device Download PDFInfo
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- CN114112769B CN114112769B CN202111403565.0A CN202111403565A CN114112769B CN 114112769 B CN114112769 B CN 114112769B CN 202111403565 A CN202111403565 A CN 202111403565A CN 114112769 B CN114112769 B CN 114112769B
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Abstract
The invention discloses a method and a device for calibrating the friction force of a pressure chamber piston of a triaxial tester, wherein the calibration method comprises the following steps: obtaining the friction force of the piston in the pressure chamber based on the change value of the axial load when the piston is driven by exciting forces with different frequencies and/or amplitudes and rises or falls without load in the pressure chamber; and (4) performing calibration according to the obtained friction force of the pressure chamber piston. The invention can accurately calibrate the friction force of the pressure chamber piston of the triaxial tester.
Description
Technical Field
The invention relates to a method for calibrating the friction force of a pressure chamber piston of a triaxial tester.
Background
The triaxial tester can apply axial load and confining pressure to the sample, and measure the pore pressure and volume change of the sample, thereby simulating the performance of the sample under various actual working conditions under different pressures.
The axial loading frame comprises four upright posts, an oil cylinder piston rod, a pressure chamber piston rod, a sample cap, a servo motor, a test force sensor and the like, the accuracy of stability control of axial loading, the accuracy of measurement and the simplicity of operation can be guaranteed, a shaft pressure system can realize force control, displacement control and the like, and a confining pressure system of the tester comprises a triaxial pressure chamber, a confining pressure pressurizing device, a confining pressure sensor and the like. The pore pressure measuring system and the body variable measuring system respectively comprise a pore pressure sensor, a body variable sensor and the like.
In the prior art, the confining pressure loading is usually carried out by adopting a hydraulic pressure or air pressure loading mode, so that a sufficient sealing effect is required in a pressure chamber. The pressure chamber piston rod and the cylinder piston rod are connected to the outside of the pressure chamber, and an O-ring is required to seal the piston rod to ensure basic sealing performance, so that friction force between the sealing ring and the pressure chamber piston is generated.
In the existing loading and testing technology, because the dynamic load test is applied according to the target value, the force sensor measures, and the influence of the friction force in a closed-loop control system is undoubted. For a soft soil body under a low confining pressure condition, due to the influence of friction force, the application frequency is ensured, and a dynamic load test is usually carried out at the cost of sacrificing precision. Due to the lack of a method for verifying the friction force of the pressure chamber piston at present, the influence of the friction force of the pressure chamber piston on the test precision cannot be quantitatively determined. The magnitude of the friction is closely related to the magnitude of the load, the loading rate and the degree of equipment aging (especially sealing rings).
Disclosure of Invention
In view of the defects in the prior art, the invention aims to provide a method and a device for accurately calibrating the friction force of a pressure chamber piston of a triaxial tester.
The technical scheme of the invention is as follows:
a triaxial tester pressure chamber piston friction force calibration method comprises the following steps:
obtaining the piston friction force of a pressure chamber of the triaxial tester through a piston friction force calculation model;
calibrating the triaxial tester according to the obtained friction force of the pressure chamber piston;
the piston friction force calculation model obtains the pressure chamber piston friction force based on the axial load change value of the piston driven by exciting forces with different frequencies and/or amplitudes when a no-load sample in a pressure chamber rises or falls.
According to some preferred embodiments of the invention, the excitation frequency is 0.1 to 2 Hz.
According to some preferred embodiments of the invention, the excitation is a sine wave excitation.
According to some preferred embodiments of the present invention, the piston friction force calculation model is as follows:
when the piston descends, ff is P + G-ma (1),
when the piston rises, ff is P-G + ma (2),
a＝-4π2f2Asin(2πft) (3)，
wherein, ff represents the friction force of the pressure chamber piston, a represents the motion acceleration of the piston, f represents the motion frequency of the piston, A represents the displacement amplitude of the piston, P represents the axial load actually measured by the sensor on the piston, G represents the self weight of the piston rod, and m represents the mass of the piston rod.
According to some preferred embodiments of the invention, the calibration method specifically comprises:
(1) fixedly connecting the piston with a force sensor on the piston under the condition that no sample is arranged in the pressure chamber;
(2) under the control of deformation or displacement, the vibration exciter selects sine waves with a plurality of frequencies and amplitudes within 0.1-2Hz respectively to drive the piston to ascend or descend, and records the maximum and minimum force values displayed by the force sensor;
(3) obtaining a friction force change process of the piston through a piston friction force calculation model;
(4) taking the peak value of the piston friction force change process as the piston friction force of the pressure chamber.
According to some preferred embodiments of the present invention, the plurality in step (2) is 3.
The invention further provides a calibration device capable of realizing the calibration method, which comprises a vibration exciter for providing the exciting force and a force sensor for obtaining the axial load value.
The invention can effectively and quantitatively determine the friction force of the pressure chamber piston in a certain loading frequency and amplitude range and clearly determine the influence of the friction force on the test control precision. The invention can ensure the measurement precision under different test conditions and improve the test efficiency; and a measuring and calculating basis is provided for debugging and improving equipment.
Drawings
FIG. 1 is a schematic view of a triaxial tester.
FIG. 2 is a graph illustrating the friction test of the pressure chamber piston in the embodiment.
Wherein:
1-an oil cylinder piston rod; 2-a force sensor; 3-a pressure chamber piston rod; 4-O type seal ring; 5, covering the upper cover; 6-a triaxial chamber pressure inlet; 7-locking the pull rod; 8-triaxial chamber; 9-lower sample cap; 10-a bottom cover; 11-sample bottom port; 12-a lock nut; 13-upper sample cap; 14-permeable stone; 15-sample top port; 16-sample membrane; 17-soil sample.
Detailed Description
The present invention is described in detail below with reference to the following embodiments and the attached drawings, but it should be understood that the embodiments and the attached drawings are only used for the illustrative description of the present invention and do not limit the protection scope of the present invention in any way. All reasonable variations and combinations included within the spirit of the invention are within the scope of the invention.
According to the technical scheme of the invention, the specific piston friction force calibration method comprises the following steps:
(1) fixedly connecting the piston with a force sensor on the piston under the condition that no sample is arranged in the pressure chamber;
(2) under the control of deformation or displacement, the vibration exciter selects 3 sine waves with frequency and amplitude within 0.1-2Hz respectively to drive the piston to ascend or descend, and records the maximum and minimum force values displayed by the force sensor;
(3) obtaining the variation process of the piston friction force through the following piston friction force calculation model:
when the piston descends, ff is P + G-ma (1),
when the piston rises, ff-G + ma (2),
a＝-4π2f2Asin(2πft) (3)，
wherein ff represents piston friction, kN; a represents the acceleration of piston motion, m/s2(ii) a f represents the frequency of piston movement, Hz; a represents the piston displacement amplitude, m; p represents the sensor reading, kN; g represents the dead weight of the piston rod, kN; m represents the piston rod mass, t;
(4) taking the peak value of the piston friction force change process as the piston friction force of the pressure chamber.
Example 1
According to the calibration method provided by the specific embodiment, the friction force of the pressure chamber piston of the triaxial tester is calibrated, and the data obtained in the step is recorded and finally calculated according to the following recording table:
in this example, the obtained data includes the force sensor test value, and the maximum frictional force at a loading frequency of 2Hz and a vibration amplitude of 5.0% L is 50N according to equations (1) to (2), which indicates that the frictional force is 13% of the confining pressure under the low confining pressure condition (200kPa) for a sample having a sample diameter of 50mm and a height of 120 mm; at high ambient pressures (above 500 kPa) this is only 5%. Therefore, in order to perform the test under the condition of low confining pressure, lubricating oil needs to be added to reduce the friction force of the piston in the pressure chamber; under the condition of high confining pressure, the test result is not obviously influenced.
The above examples are merely preferred embodiments of the present invention, and the scope of the present invention is not limited to the above examples. All technical schemes belonging to the idea of the invention belong to the protection scope of the invention. It should be noted that modifications and adaptations to those skilled in the art without departing from the principles of the present invention should also be considered as within the scope of the present invention.
Claims (6)
1. A friction force calibration method for a pressure chamber piston of a triaxial tester is characterized by comprising the following steps:
obtaining the friction force of a pressure chamber piston of the triaxial tester through a piston friction force calculation model;
calibrating the triaxial tester according to the obtained friction force of the pressure chamber piston;
the piston friction force calculation model obtains the piston friction force of the pressure chamber on the basis of the change value of the axial load of the piston driven by exciting forces with different frequencies and/or amplitudes when a no-load sample in the pressure chamber rises or falls;
the piston friction force calculation model is as follows:
when the piston descends, ff is P + G-ma (1),
when the piston rises, ff-G + ma (2),
a＝-4π2f2Asin(2πft) (3)，
wherein, ff represents the piston friction force of the pressure chamber, a represents the piston motion acceleration, f represents the piston motion frequency, A represents the piston displacement amplitude, P represents the axial load of the piston measured by the sensor, G represents the dead weight of the piston rod, and m represents the mass of the piston rod.
2. The calibration method according to claim 1, wherein the excitation frequency is 0.1 to 2 Hz.
3. The calibration method according to claim 1, wherein the excitations are sine wave excitations.
4. The calibration method according to claim 1, characterized in that it comprises in particular:
(1) fixedly connecting the piston with a force sensor on the piston under the condition that no sample is arranged in the pressure chamber;
(2) under the control of deformation or displacement, the vibration exciter selects sine waves with a plurality of frequencies and amplitudes within 0.1-2Hz respectively to drive the piston to ascend or descend, and records the maximum and minimum force values displayed by the force sensor;
(3) obtaining a friction force change process of the piston through a piston friction force calculation model;
(4) taking the peak value of the piston friction force change process as the piston friction force of the pressure chamber.
5. The calibration method according to claim 4, wherein the plurality in step (2) is 3.
6. Calibration device for carrying out the calibration method according to any one of claims 1 to 5, characterized in that it comprises a vibration exciter for providing said exciting force and a force sensor for obtaining said axial load value.
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