CN109187194B - OFDR-based soil body tension mechanical property optical fiber monitoring and testing method and device - Google Patents
OFDR-based soil body tension mechanical property optical fiber monitoring and testing method and device Download PDFInfo
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- CN109187194B CN109187194B CN201811256745.9A CN201811256745A CN109187194B CN 109187194 B CN109187194 B CN 109187194B CN 201811256745 A CN201811256745 A CN 201811256745A CN 109187194 B CN109187194 B CN 109187194B
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- G—PHYSICS
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- 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
- G01N3/08—Investigating strength properties of solid materials by application of mechanical stress by applying steady tensile or compressive forces
Abstract
The invention relates to an OFDR-based soil body tension mechanical property optical fiber monitoring and testing method and device, wherein a strain sensing optical fiber is paved in a soil beam along a horizontal direction; the tester mainly comprises a counter-force support, a loading system, an OFDR signal demodulation and processing module and a digital image acquisition and analysis device, wherein the OFDR signal demodulation and processing module is connected with strain sensing optical fibers in the soil beam, strain distribution data inside the soil beam are acquired in real time, the light wave quantity, the soil beam strength and the like are displayed, the digital image acquisition and analysis device tracks the position change of the surface of the soil beam, and a strain field and a displacement field after the soil beam is stressed and deformed are obtained and the data obtained by optical fibers are checked mutually. The invention can monitor the cracking deformation information of the surface and the inside of the soil beam and the time-space evolution rule of the tensile stress strain in the four-point bending test process in real time, measure the tensile strength of the soil under the conditions of different water contents, dry density and the like, and master the elastoplastic stress-strain constitutive relation of the soil after being pulled.
Description
Technical Field
The invention relates to the technical field of rock-soil body stress deformation and strength test and distributed optical fiber monitoring engineering, in particular to an optical fiber monitoring and testing method and device for the tension mechanical characteristics of a soil body based on OFDR.
Background
The tensile mechanical property is one of the basic mechanical properties of the rock-soil medium, plays a very important role in the deformation and destruction process of the rock-soil body, and therefore, a plurality of geotechnical engineering problems related to the rock-soil body are also generated. The tensile strength, compressive strength, shear strength and other indexes of the soil body are the same, and are important parameters for measuring the mechanical properties of the soil body. In the forming process of the soil body, the integrity and the integrity of the original rock are damaged to different degrees, the mechanical property is shown to have certain compressive strength and shear strength, but the strength value is greatly reduced, and the tensile strength is mostly or almost completely lost.
In the past engineering practice, the mechanical properties or the anti-damage capability under load of the soil body are mainly measured based on the compression resistance or the shear strength of the soil body, because the tensile strength is much smaller in value than the compression resistance and the shear strength, and is difficult to accurately measure, and the strength index is generally selected to be ignored in the engineering. This neglect, which in most cases is manifested as a 0 matrix suction and 0 tensile stress load, is a relatively conservative estimate of soil body strength, and there is a need for improvement in contemporary geotechnical engineering designs.
In addition, the soil body damage mode encountered in the actual engineering is mainly represented by shear damage, such as landslide, foundation soil instability and the like. However, phenomena that soil body is stretch-broken and cracks appear under the action of tensile stress are also common, such as stretch-broken cracks at the rear edge of a side slope, stretch-broken damage of a core wall of a earth-rock dam under the action of a soil arch, cracking phenomena of the soil body in a dry environment, development of ground cracks and the like, and peripheral soil body stretch-broken easily occurs to some power transmission line towers and wind towers under the action of horizontal load. The existence of cracks can greatly damage the structural integrity of soil, weaken mechanical properties, reduce stability, increase permeability, exacerbate evaporation, aggravate slope water-soil and efflorescence and the like, and bring a series of negative effects to geotechnical engineering and environmental geotechnical engineering. The soil body has tension cracks because the tension stress exceeds the tensile strength of the soil body. Therefore, the stress and strain change rule in the tensioning and cracking process of the soil body is monitored, the tensile strength is measured, and the system is important in engineering significance in grasping the tensioning and breaking mechanism of the soil body and preventing the soil body from cracking on the basis of the tensile strength, so that the control level of related geological disasters can be effectively improved, and a large amount of manpower and material resources are saved.
As the tensile mechanical property of the soil body is not paid attention to in the field of geotechnical engineering in the past, the research reports are relatively few, and the soil body tensile strength testing method is more fresh in China, and most of existing soil body tensile strength testing methods reference the field of other materials such as rock, concrete and the like. The test method of the tensile strength of the soil body can be divided into two major types, namely a direct method and an indirect method, wherein the direct method is divided into a uniaxial tension test and a triaxial tension test, and the indirect method mainly comprises 4 modes of a soil beam bending test, an axial fracturing test, a radial fracturing test and an air pressure fracturing test. The uniaxial tension and the triaxial tension tests apply tensile stress to the test sample under the condition of no lateral limit and triaxial stress respectively, and the peak tensile stress is directly measured to obtain the tensile strength. The indirect method is based on certain theoretical assumption, and is tested by means of fracturing, bending and the like, and finally the tensile strength is calculated through a corresponding theoretical formula. Compared with the problem of stress concentration in the three-point bending test, the bending moment of the soil body is constant between load acting points in the four-point bending test of the soil beam, so that the tensile stress distribution is relatively uniform, and the four-point bending test is an ideal test method. In these tests, tensile stress can be obtained by a certain means, but obtaining soil strain information faces many challenges. Because of the uneven distribution of strain along the tensile direction and the uncertainty, the tensile strain is concentrated near the fracture surface, but the conventional displacement monitoring method can only calculate the average strain of the sample, which is far from the actual situation. If the resistance strain gauge commonly used in civil engineering is selected, the problems of difficult installation, large disturbance to in-situ soil mass and the like are also existed. Due to the blank of the soil strain monitoring technology, the understanding of the soil tensile stress-strain constitutive relation is unclear, and the theoretical research and engineering practice in the field are severely restricted.
Distributed fiber monitoring (DFOS) technology has evolved rapidly in recent years and has found some successful applications in detecting cracking in concrete, asphalt, and the like. By means of monitoring technologies such as quasi-distributed Fiber Bragg Gratings (FBGs), full-distributed Brillouin Optical Time Domain Reflectometry (BOTDRs), brillouin Optical Time Domain Analysis (BOTDAs) and the like, distribution conditions of monitoring information such as strain, temperature and the like along the length direction of the whole optical fiber can be automatically obtained. But is limited to monitoring accuracy (typically tens of microstrain), spatial resolution (typically in meters) and sampling time (typically tens of minutes to tens of minutes are required), this technique has not been well utilized in soil cracking monitoring. While OFDR (Optical Frequency Domain Reflectometer) technology is a tip sensing technology with millimeter-scale spatial resolution and 1 micro-strain accuracy that has begun to rise in recent years. Compared with other monitoring methods, the OFDR has the advantages of large data acquisition amount, high signal-to-noise ratio, small sampling interval, high accuracy of the obtained result, suitability for long-distance monitoring and high-frequency acquisition and the like, and therefore has wide application prospect in the field of soil tension crack measurement.
Recently, some researchers at home and abroad try to embed the strain sensing optical fiber into the soil body to be monitored, analyze the deformation characteristics of the soil body based on the optical fiber sensing data, or monitor whether the soil body is dry-shrunk and cracked. The research can not control the whole test process and boundary conditions because special, standardized and integrated test equipment is not adopted, so that only qualitative conclusions can be obtained, and the research has little significance on engineering reference. Because of long test time, the optical fiber reading is also affected by ambient temperature, humidity and the like, and the reliability of analysis results is poor. In addition, the direct burial method is convenient in construction, but the interaction mechanism and the coordination deformation problem between the soil body and the strain sensing optical fiber cannot be guaranteed, and meanwhile, no scientific basis is provided for the selection of the strain sensing optical fiber and the setting of an anchoring point, so that whether the optical fiber strain monitoring result is effective or not has great uncertainty, and the popularization and the application of the technology in engineering are greatly restricted. Based on the OFDR technology, the micro-deformation can be monitored with high precision and high spatial resolution in the soil body cracking process, and the interface deformation coordination characteristics between the soil body and the strain sensing optical fiber are subjected to fine analysis, so that the sensor layout process is further optimized, and the monitoring reliability is improved.
Disclosure of Invention
Aiming at the defects of the prior art, the invention aims to provide an OFDR-based soil body tension mechanical property optical fiber monitoring and testing method and device.
The invention adopts the following technical scheme: an OFDR-based soil body tension mechanical property optical fiber monitoring and testing device comprises a test soil beam, a tester and a strain sensing optical fiber; the tester comprises a chassis shell, a counter-force support, a loading plate, a speed regulation driving device, an OFDR signal demodulation and processing module and a digital image acquisition and analysis device, wherein the speed regulation driving device is arranged inside the chassis shell and is connected with a dynamometer and the loading plate, the loading plate can move up and down along the vertical direction of the inner wall of the chassis shell, a test soil beam is placed between the loading plate and the counter-force support, a strain sensing optical fiber penetrates through the test soil beam along the horizontal direction, the strain sensing optical fiber is communicated with the OFDR signal demodulation and processing module through a signal transmission optical fiber, and a digital image acquisition window of the digital image acquisition and analysis device corresponds to a test observation surface of the test soil beam.
The speed regulation driving device comprises a stepping motor and a gearbox.
The digital image acquisition and analysis device comprises a high-speed camera and a computer.
And rollers are arranged at two ends of the loading plate.
A method for monitoring and testing a device by adopting an OFDR-based soil body tension mechanical property optical fiber comprises the following steps:
firstly, preparing a test soil beam: layering and pressing the soil beam in the soil beam pressing mould according to a given dry density, when the soil beam is pressed to the layout position of the strain sensing optical fibers, sequentially arranging the strain sensing optical fibers in the soil body through the optical fiber penetrating holes on the side surface of the box body of the soil beam pressing mould, properly hanging a weight to enable the weight to be in a slight tension state, and then continuing filling and pressing according to the given dry density;
secondly, marking a soil sample: taking out the pressed soil beam, densely pricking pinholes on a test observation surface, taking the pinholes as the surface texture of the soil beam, and naturally air-drying or drying the soil beam until the required water content is reached, and coating a film;
thirdly, connecting an OFDR signal demodulation and processing module: uncovering the soil beam coating film, horizontally arranging the soil beam coating film on a loading plate of a tester, connecting all the strain sensing optical fibers to an interface of an OFDR signal demodulation and processing module after adopting a parallel or serial mode to connect the strain sensing optical fibers with each other;
fourth, start the test: the method comprises the steps of sequentially opening an OFDR signal demodulation and processing module, a digital image acquisition and analysis device and a switch of a speed regulation driving device, wherein the speed regulation driving device pushes a loading plate to move downwards at a set speed, and a test soil beam is bent at four points along with the loading plate; the OFDR signal demodulation and processing module acquires and presents the strain distribution state in the soil beam in real time; the digital image acquisition and analysis device tracks texture changes of the surface of the soil beam in real time, and obtains a strain field and a displacement field of the soil beam after stress deformation;
fifth, data processing: based on the measured data, establishing a soil body stretching stress-strain constitutive relation, and obtaining tensile strength and cracking strain value parameters of the soil body.
The digital image acquisition and analysis device acquires and analyzes based on a digital image phase method or a particle image velocimetry.
The OFDR signal demodulation and processing module can customize the resolution in the range of 1mm-10cm according to the requirements of test precision, denoising and the like.
The soil beam pressing die consists of 4 side plates fixed on a bottom plate, wherein two side plates at two ends are provided with optical fiber penetrating holes.
The strain sensing optical fiber is used for carrying out threading treatment on the sheath through electrode carving and electric discharge machining technology.
The strain sensing optical fiber is fixed in the soil body by adopting a tubular or plate type anchoring device.
The beneficial effects are that: by adopting the method and the device for monitoring and testing the tension mechanical characteristics of the soil body based on the OFDR, the fracture deformation information of the surface and the interior of the soil beam and the time-space evolution rule of the tensile stress in the four-point bending test process can be monitored in real time, the tensile strength of the soil body under the conditions of different water contents, dry density and the like can be measured, and the elastoplastic stress-strain constitutive relation of the tensioned soil body can be mastered on the basis, so that the method and the device have the advantages of economy, reliability, accuracy in testing, high automation degree and the like.
Drawings
FIG. 1 is a schematic diagram of an apparatus for monitoring and testing soil body tension mechanical characteristics in an optical fiber according to a preferred embodiment of the present invention.
The method comprises the following steps: the test device comprises an OFDR signal demodulation and processing module, a signal transmission optical fiber, a strain sensing optical fiber, a test soil sample, a loading plate, a gearbox, a stepping motor, a camera, a computer, a soil beam pressing die, an optical fiber penetrating hole, a chassis housing, a counter-force bracket, a tester, a digital image acquisition and analysis device and a dynamometer.
Fig. 2 is a schematic structural diagram of a digital image acquisition and analysis device according to an embodiment of the invention.
Fig. 3 is a schematic view of a soil beam compacting die.
FIG. 4 is a graph showing the lengthwise strain distribution of a strained fiber using an embodiment of the present invention.
Fig. 5 is a graph comparing fiber-measured strain and PIV-measured strain.
FIG. 6 shows PIV treatment results of the bottom of a soil beam during four-point bending using an embodiment of the present invention.
FIG. 7 is a graph showing the results of several exemplary cracking conditions of a soil body using an embodiment of the present invention.
FIG. 8 is a graph showing the change of tensile strength of soil under different conditions of water content and dry density measured by the method of using an embodiment of the present invention.
Detailed Description
The invention will be described in more detail below with reference to the drawings and the preferred embodiments.
An OFDR-based soil body tension mechanical property optical fiber monitoring and testing method comprises the following steps:
first, preparing a test soil beam. And pressing the soil beam in a soil beam pressing mould according to a given dry density layer by layer, and when the soil beam is pressed to the arrangement position of the strain sensing optical fibers, arranging the strain sensing optical fibers in the soil body by sequentially penetrating through the optical fiber penetrating holes on the side surface of the box body of the soil beam pressing mould, and properly suspending the heavy objects to enable the soil beam to be in a slight tension state.
And secondly, marking the soil sample. Taking out the pressed soil beam, densely pricking pinholes on a test observation surface by adopting a steel needle with the diameter of 0.1mm, and taking the steel needle as the surface texture of the soil beam. Then naturally air-drying (or drying) to the required water content, and coating the film;
and thirdly, uncovering the soil beam coating film, transversely placing the soil beam coating film on a loading plate of the tester, connecting all the strain sensing optical fibers with each other in a parallel or serial mode, and then connecting the strain sensing optical fibers to an interface of an OFDR signal demodulation and processing module.
Fourth, the test was started. The OFDR signal demodulation and processing module, the digital image acquisition and analysis device, the stepping motor and the switch of the gearbox are sequentially turned on, the stepping motor pushes the loading plate to move downwards at a specific speed, and the soil beam is bent at four points; the OFDR signal demodulation and processing module acquires and presents the strain distribution state in the soil beam in real time; the digital image acquisition and analysis device tracks texture changes of the surface of the soil beam in real time, and obtains a strain field and a displacement field of the soil beam after stress deformation.
Fifthly, based on the measured data, establishing a soil body tensile stress-strain constitutive relation, and obtaining tensile strength and cracking strain values of the soil body.
The digital image processing system is based on a digital image phase dry method or a particle image velocimetry.
The OFDR signal demodulation and processing module comprises an optical fiber demodulator and a terminal calculation processing and visualization system. The OFDR signal demodulation and processing module can customize the resolution in the range of 1mm-10cm according to the requirements of test precision, denoising and the like.
The device used in the soil body tension mechanical property optical fiber monitoring and testing method comprises a soil beam pressing die 10, a tester 14 and a strain sensing optical fiber 3; the soil beam pressing die 10 is formed by mutually embedding and assembling 5 steel plates; the 4 plates on the side face are connected by bolts and are integrally embedded in the notch of the bottom plate and are connected with the bottom plate by bolts, wherein the two side plates are provided with small round holes 11; the strain sensing optical fiber 3 passes through the soil beam pressing die 10 along the horizontal direction and is paved in the soil beam 4; the tester 14 mainly comprises a counter-force bracket 13, a case shell 12, a loading plate 5, a stepping motor 7, a gearbox 6, an OFDR signal demodulation and processing module 1 and a digital image acquisition and analysis device 15. The stepping motor 7 regulated by the gearbox 6 is connected with the dynamometer (16) and pushes the loading plate 5 with balls at two ends to move downwards along the inner wall of the chassis shell 12. The OFDR signal demodulation and processing module 1 is connected with the strain sensing optical fiber 3 in the soil beam 4 to collect strain data in the soil beam 4 in real time and display parameters such as light wave quantity and soil beam strength in real time, the digital image collection and analysis device 15 is arranged in front of and behind the tester 14, and the strain field and displacement field after the soil beam is stressed and deformed are obtained by recognizing the texture of the soil beam 4 and tracking the position change of the surface of the soil beam 4, and the strain data of the optical fiber is calibrated or verified. The digital image acquisition and analysis device comprises a high-pixel digital camera and a computer. The loading system comprises a stepping motor, a gearbox and a loading plate; the strain sensing optical fiber carries out thread treatment on the sheath of the strain sensing optical fiber through electrode carving and electric discharge machining technology; the strain sensing optical fiber is anchored in the soil body in a heat shrinkage tube and small wafer mode so as to ensure the deformation coordination of the strain sensing optical fiber and the soil body.
As a further optimization of the scheme, the rigid side plates forming the soil beam pressing mold 10 are provided with optical fiber penetrating holes 11 for penetrating the optical fibers which are transversely arranged, and the strain sensing optical fibers 3 are subjected to threading treatment on the sheath of the strain sensing optical fibers through electrode carving and electric discharge machining technology; the OFDR signal demodulation and processing module can customize the resolution in the range of 1mm-10cm according to the requirements of test precision, denoising and the like.
Further, the digital image acquisition and analysis device 15 further includes:
(1) A test soil beam 4 marked with texture; densely pricking pinholes on a test observation surface by adopting a steel needle with the diameter of 0.1mm, and taking the steel needle as the surface texture of the soil beam 4;
(2) A high pixel camera 8; the high-pixel camera is arranged in front of the test soil beam (4), the position change of the surface of the soil beam (4) is tracked by identifying the texture of the soil beam (4), the strain field and the displacement field of the soil beam after the stress deformation are obtained, and the fiber strain data are calibrated or verified. The method comprises the steps of carrying out a first treatment on the surface of the
(3) Digital image processing software; the digital image processing software is based on a digital image phase dry method (Digital Image Correlation, abbreviated as DIC) or a particle image velocimetry (Particle Image Velocimetry, abbreviated as PIV) or the like.
Examples
As shown in fig. 1 and 2, an OFDR-based soil body tension mechanical property optical fiber monitoring and testing device comprises a soil beam pressing die 10, a tester 14 and a strain sensing optical fiber 3; the soil beam pressing die 10 is formed by mutually embedding and assembling 5 steel plates; the 4 plates on the side face are connected by bolts and are integrally embedded in the notch of the bottom plate and are connected with the bottom plate by bolts, wherein the two side plates are provided with small round holes 11; the strain sensing optical fiber 3 passes through the soil beam pressing die 10 along the horizontal direction and is paved in the soil beam 4; the tester 14 mainly comprises a counter-force bracket 13, a case housing 12, a loading plate 5, a stepping motor 7, a gearbox 6, an OFDR signal demodulation and processing module 1 and a digital image acquisition and analysis device 15, wherein the OFDR signal demodulation and processing module 1 is connected with a strain sensing optical fiber 3 in a soil beam 4 to acquire strain data in the soil beam 4 in real time and display parameters such as light wave quantity, soil beam strength and the like in real time, the digital image acquisition and analysis device 15 is arranged in front of and behind the tester 14, and tracks the position change of the surface of the soil beam 4 by identifying the texture of the soil beam 4 to obtain a strain field and a displacement field after the stress deformation of the soil beam, and calibrates or verifies the strain data of the optical fiber.
The rigid side plates of the soil beam forming pressing mold 10 are provided with optical fiber penetrating holes 11 for passing through the optical fibers which are transversely arranged; the strain sensing optical fiber 3 carries out thread treatment on the sheath of the strain sensing optical fiber through electrode carving and electric discharge machining technology; the OFDR signal demodulation and processing module can customize the resolution in the range of 1mm-10cm according to the requirements of test precision, denoising and the like. The digital image acquisition and analysis device further comprises: (1) marking a textured test soil beam; densely pricking pinholes on a test observation surface by adopting a steel needle with the diameter of 0.1mm, and taking the steel needle as the surface texture of the soil beam; (2) a high pixel camera; the high-pixel camera is arranged in front of the test soil beam, the position change of the surface of the soil beam is tracked by identifying soil Liang Wenli, a strain field and a displacement field after the stress deformation of the soil beam are obtained, and the fiber strain data are calibrated or verified; (3) digital image processing software; the digital image processing software is based on a digital image phase dry method (Digital Image Correlation, abbreviated as DIC) or a particle image velocimetry (Particle Image Velocimetry, abbreviated as PIV) or the like.
The test method of the optical fiber monitoring and testing device for the tension mechanical property of the soil body provided by the embodiment comprises the following steps:
1) Test soil beams 4 were prepared. The soil beam is pressed in layers according to a given dry density in the soil beam pressing mold 10, and when the soil beam is pressed to the arrangement position of the strain sensing optical fibers 3, the strain sensing optical fibers are sequentially arranged in soil bodies through the optical fiber penetrating holes 11 on the side surface of the box body of the soil beam pressing mold 10, and weights are suspended appropriately to enable the soil beams to be in a slightly tensioned state.
2) And marking the soil sample. Taking out the pressed soil beam, densely pricking pinholes on a test observation surface by adopting a steel needle with the diameter of 0.1mm, and taking the steel needle as the surface texture of the soil beam. Then naturally air-drying (or drying) to the required water content, and coating the film;
3) The soil beam coating film is uncovered, the soil beam coating film is transversely arranged on a loading plate 5 of the tester, and all the strain sensing optical fibers (3) are connected with each other in a parallel or serial mode and then connected to an interface of the OFDR signal demodulation and processing module 1.
4) The test was started. The OFDR signal demodulation and processing module 1, the digital image acquisition and analysis device 15, the stepping motor 7 and the switch of the gearbox 6 are sequentially turned on, the stepping motor 7 pushes the loading plate 5 to move downwards at a specific speed, and the soil beam 4 is bent at four points; the OFDR signal demodulation and processing module 1 acquires and presents the strain distribution state in the soil beam 4 in real time; the digital image acquisition and analysis device 15 tracks texture changes of the surface of the soil beam in real time, and obtains a strain field and a displacement field after the soil beam is stressed and deformed.
5) Based on the measured data, a soil body tensile stress-strain constitutive relation is established, and the tensile strength and the cracking strain value of the soil body are obtained. Specifically, the fracture strain is derived from fiber monitoring data, and the tensile strength can be derived in combination with load cell data: f-t curve is made according to the readings of the dynamometer, and a soil body stretching stress value F which is finally close to stable is determined 0 Correspondingly obtain the bending moment M 0 By the following constitutionCalculated tensile strength sigma t Wherein I is the moment of inertia of the soil beam, h is the height of the soil beam, and t is the time.
When the optical fiber monitoring and testing device for the tension mechanical property of the soil body of the embodiment is specifically used, firstly, soft kaolin with the water content of 32% is prepared, and then the soft kaolin is compacted in layers (average density is 1.87 g/cm) by using a soil beam compacting die 10 (the dimensions of length, width and height are 50cm multiplied by 15 cm) 3 ). In the pressing process, 6 strain sensing optical fibers (3) (OF 1-1, OF1-2, OF2-1, OF 3-2) are horizontally arranged in the soil beam in three layers and two rows, the heights OF the optical fibers from the bottom surface OF the soil beam are respectively 3cm (OF 1-1, OF 1-2), 6cm (OF 2-1 ) and 9cm (OF 3-1, OF 3-2), and the transverse spacing is 5cm. Taking out the pressed soil beam, densely pricking pinholes on a test observation surface by adopting a steel needle with the diameter of 0.1mm, and taking the steel needle as the surface texture of the soil beam. The prepared soil beam is placed on the corresponding position of the loading plate 5, then 6 strain sensing optical fibers 3 are respectively connected to the interfaces of the OFDR signal demodulation and processing module 1, and the OFDR signal demodulation and processing module 1, the digital image acquisition and analysis device 15, the stepping motor 7 and the switch of the gearbox 6 are sequentially opened. The stepping motor 7 pushes the loading plate 5 to push the loading plate 5 to move downwards at the speed of 0.28mm/min, the soil beam starts to bend at four points, and the OFDR signal demodulation and processing module 1 acquires and presents the strain distribution state inside the soil beam in real time; the digital image acquisition and analysis device 15 tracks the texture changes of the earth beam surface in real time,the strain field and the displacement field after the soil beam is stressed and deformed are obtained through built-in PIV digital image processing software; meanwhile, the OFDR signal demodulation and processing module 1 also monitors and obtains strain time-course curves of soil bodies at different positions of the sample in the four-point bending process.
It should be noted that, in addition to the above embodiments, other embodiments of the present invention are also possible. All technical schemes formed by equivalent substitution or equivalent transformation fall within the protection scope of the patent claims of the invention.
Claims (5)
1. The method for monitoring and testing the device by adopting the soil body tension mechanical property optical fiber based on the OFDR is characterized by comprising the following steps:
firstly, preparing a test soil beam: layering and pressing the soil beam in the soil beam pressing mould according to a given dry density, when the soil beam is pressed to the layout position of the strain sensing optical fibers, sequentially arranging the strain sensing optical fibers in the soil body through the optical fiber penetrating holes on the side surface of the box body of the soil beam pressing mould, properly hanging a weight to enable the weight to be in a slight tension state, and then continuing filling and pressing according to the given dry density;
secondly, marking a soil sample: taking out the pressed soil beam, densely pricking pinholes on a test observation surface, taking the pinholes as the surface texture of the soil beam, and naturally air-drying or drying the soil beam until the required water content is reached, and coating a film;
thirdly, connecting an OFDR signal demodulation and processing module: uncovering the soil beam coating film, horizontally arranging the soil beam coating film on a loading plate of a tester, connecting all the strain sensing optical fibers to an interface of an OFDR signal demodulation and processing module after adopting a parallel or serial mode to connect the strain sensing optical fibers with each other;
fourth, start the test: the method comprises the steps of sequentially opening an OFDR signal demodulation and processing module, a digital image acquisition and analysis device and a switch of a speed regulation driving device, wherein the speed regulation driving device pushes a loading plate to move downwards at a set speed, and a test soil beam is bent at four points along with the loading plate; the OFDR signal demodulation and processing module acquires and presents the strain distribution state in the soil beam in real time; the digital image acquisition and analysis device tracks texture changes of the surface of the soil beam in real time, and obtains a strain field and a displacement field of the soil beam after stress deformation;
fifth, data processing: based on the measured data, establishing a soil body tensile stress-strain constitutive relation, and acquiring tensile strength and cracking strain value parameters of the soil body;
the optical fiber monitoring and testing device for the tension mechanical properties of the soil body based on the OFDR comprises a test soil beam (4), a tester (14) and a strain sensing optical fiber (3); the tester (14) comprises a case shell (12), a counter-force bracket (13), a loading plate (5), a speed regulation driving device, an OFDR signal demodulation and processing module (1) and a digital image acquisition and analysis device (15), wherein the speed regulation driving device is arranged in the case shell (12), the speed regulation driving device is connected with a dynamometer (16) and the loading plate (5), the loading plate (5) can move up and down along the vertical direction of the inner wall of the case shell (12), a test soil beam (4) is placed between the dynamometer (16) and the counter-force bracket (13), a strain sensing optical fiber (3) passes through the test soil beam (4) along the horizontal direction, the strain sensing optical fiber (3) is communicated with the OFDR signal demodulation and processing module (1) through a signal transmission optical fiber (2), and a digital image acquisition window of the digital image acquisition and analysis device (15) corresponds to a test observation surface of the test soil beam (4); the speed regulation driving device comprises a stepping motor (7) and a gearbox (6); the digital image acquisition and analysis device (15) comprises a high-speed camera (8) and a computer (9); rollers are arranged at two ends of the loading plate (5); the signal demodulation and processing module comprises an optical fiber demodulator and a terminal calculation processing and visualization system.
2. The method for monitoring and testing the soil body tension mechanical property optical fiber based on the OFDR according to claim 1, wherein the digital image acquisition and analysis device is used for acquisition and analysis based on a digital image phase dry method or a particle image velocimetry.
3. The method for monitoring and testing the soil body tension mechanical property optical fiber based on the OFDR according to claim 1 is characterized in that the soil beam pressing mould (10) consists of 4 side plates fixed on a bottom plate, wherein two side plates at two ends are provided with optical fiber penetrating holes (11).
4. The method for monitoring and testing the soil body tension mechanical property optical fiber based on the OFDR according to claim 1, wherein the strain sensing optical fiber is used for carrying out threading treatment on the sheath through electrode carving and electric discharge machining technology, and the strain sensing optical fiber is fixed in the soil body by adopting a tubular or plate type anchoring device.
5. The method for monitoring and testing the soil body tension mechanical property optical fiber based on the OFDR according to claim 1 is characterized in that the OFDR signal demodulation and processing module is self-defined in resolution in the range of 1mm-10cm according to the testing precision and the denoising requirement.
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