CN206818160U - Fiber Bragg Grating Aperture Deformation Sensor for Long-term Measurement of Surrounding Rock Stress - Google Patents

Abstract

The utility model discloses a kind of fiber grating aperture deformation-sensor of long-term measurement surrouding rock stress, including：Rigid substrate, to be circumferentially arranged in drilling, corresponding deformation can occur with the deformation in country rock aperture；Rigid dynamometry end, is arranged on the rigid substrate, directly to contact borehole sidewall to measure the radial deformation of the drilling；First fiber grating, the surface of the rigid substrate is fixed on, its grating wavelength changes with the deformation of the rigid substrate.The corresponding relation of the deflection and first optic fiber grating wavelength at the rigid substrate dynamometry end is obtained according to displacement calibrating, by measuring the wavelength case of first fiber grating, to determine the deflection at the dynamometry end, and then measures the deformation in country rock aperture.

Description

Fiber Bragg Grating Aperture Deformation Sensor for Long-term Measurement of Surrounding Rock Stress

technical field

The utility model belongs to the technical field of underground engineering stress testing, in particular to an optical fiber grating aperture deformation sensor for long-term measurement of surrounding rock stress.

Background technique

With the development and utilization of underground space, underground engineering has become an indispensable part of human life. During the construction of underground engineering, the surrounding rock stress field directly affects the stress and deformation of the structure, and is the basic data for underground engineering design; accurate measurement of the initial ground stress and disturbance stress of surrounding rock is an important guarantee for engineering safety.

Aperture deformation method is a surrounding rock stress testing method with the longest development time and relatively mature technology. It has wide applicability and high reliability. The traditional borehole deformation method calculates the stress state in a plane perpendicular to the borehole axis by measuring the deformation of the borehole diameter, and determines the three-dimensional stress state of a point through the measurement of three mutually unequal boreholes.

However, the traditional aperture deformation method is to fix the sensor by filling the gap between the sensor and the surrounding rock with colloid. Due to the strong rheological properties of the consolidated colloid, after a general sensor is loaded for one day, the colloid will produce a large rheological deformation, as shown in Figure 8, so the traditional aperture deformation method can only achieve short-term testing of ground stress. , but the long-term monitoring of ground stress cannot be realized.

Utility model content

In view of the above-mentioned defects or deficiencies in the prior art, it is desired to provide a fiber grating aperture deformation sensor for long-term measurement of surrounding rock stress, so as to solve the problem that the above-mentioned aperture deformation measurement can only be monitored in a short period of time, so as to realize long-term monitoring of aperture deformation measurement.

A fiber grating aperture deformation sensor for long-term measurement of surrounding rock stress, including:

Rigid base material, which is used to be arranged in the borehole along the circumferential direction, and can be deformed correspondingly with the deformation of the surrounding rock aperture;

Rigid force-measuring end, set on the rigid base material, used to directly contact the side wall of the borehole to measure the radial deformation of the borehole;

The first optical fiber grating is fixed on the surface of the rigid substrate, and its grating wavelength changes with the deformation of the rigid substrate. According to the displacement calibration, the corresponding relationship between the deformation of the force-measuring end of the rigid base material and the wavelength of the first fiber Bragg grating is obtained, and the deformation of the force-measuring end is measured by measuring the wavelength of the first fiber Bragg grating, and then the deformation of the surrounding rock aperture is measured.

The rigid base material is a steel ring, and the rigid base material, the first fiber grating and the force-measuring end together constitute the measuring part of the aperture measurement sensor. Angles are set so that the stress state in a plane perpendicular to the borehole axis can be measured.

The steel ring is made of 65si2mnwa spring steel.

The radius of curvature of the steel ring is 4.0-5.0mm. The first fiber grating adopts anti-bending fiber grating, which can adapt to the curvature of the steel ring after deformation. The length of the first fiber grating is 2-4mm, which greatly improves the survival of the fiber grating. The reflectivity is not less than 90%, and the 3dB bandwidth of the first fiber grating is 0.2-0.4nm.

Welding is used between the first fiber grating and the steel ring.

The aperture measurement sensor is equipped with four measuring parts, and the staggered angles of the force measuring ends of each measuring part are evenly distributed along the circumferential direction, which can measure the magnitude and direction of the principal stress in the plane perpendicular to the axis of the borehole, and measure the stress state more accurately.

The force measuring end is two force transmission caps fixed on the outer surface of the steel ring. The two force transmission caps are radially opposed to the steel ring. The front end of the force transmission cap is a hemisphere for direct contact with the surrounding rock. The end is a cylinder in direct contact with the steel ring. The force transmission cap is made of hard metal, such as stainless steel. The structure of the force transmission cap is simple. As a protruding force measuring end, it can directly contact the side wall of the surrounding rock. Since the force transmission cap and the rigid base material are both hard, their rheology can be ignored. Therefore, the force transmission cap measured The displacement is the displacement of the surrounding rock, which can realize the measurement of the long-term strain of the surrounding rock.

The fixed position of the first fiber grating has an included angle of 90° with respect to the center of the steel ring and the force transmission cap, and is arranged on the outer surface of the steel ring. In this position, the wavelength change of the first fiber grating is the most sensitive when the force transmission cap is displaced, which can be measured more accurately.

The measuring part also includes a housing, the housing has a cylindrical part, the rigid base material is fixed inside the round part of the housing, and the housing is provided with a through hole for the force transmission cap to pass through, so that the force transmission cap protrudes from the shell The outer surface of the body and directly contacts the sidewall of the surrounding rock. The arrangement of the housing can ensure the service life of the sensor.

The shell is made of stainless steel, the front end of the shell is hemispherical, the middle part of the shell is a cylindrical part, and the rear part of the shell has an expansion part with a gradually increasing section for clamping in the drilled hole. The hemispherical front end plays a guiding role when drilling into the hole, and the middle and rear ends have cylindrical cavities. The middle cavity is mainly used for arranging measurement settings; the conical rear end is used to clamp the rock wall to fix the sensor.

The steel ring is fixed inside the shell through the fixed support to ensure the fixing effect.

The fixed support has a thick-walled cylindrical structure and is fixed inside the housing. The fixed support has a limit groove for limiting the inclination of the steel ring. The central part of the fixed support serves as an optical fiber channel. The fixed support has a simple structure, low cost, and can achieve a good fixing effect at the same time.

The sensor is also provided with a second fiber grating in a free deformation state as a temperature compensation grating, each of the first fiber grating and the second fiber grating is welded to the armored optical cable at the rear end of the housing. The influence of temperature change on wavelength change can be eliminated by setting the temperature-compensated grating, and the measurement accuracy can be improved.

Using a temperature-compensated grating to eliminate the influence of temperature change on the wavelength change of the first fiber grating is a conventional technical means in the art, and will not be repeated here.

According to the technical solution provided by the embodiment of the present application, the first fiber grating sensor is arranged on the rigid substrate, and the corresponding relationship between the deformation amount of the force-measuring end of the rigid substrate and the wavelength of the first fiber Bragg grating is obtained according to the displacement calibration. The wavelength of the instrument is used to measure the deformation of the force-measuring end, and then measure the deformation of the surrounding rock aperture. Due to the design and application of this structure and corresponding principles, the rigid base material is directly in contact with the surrounding rock of the borehole through the rigid force-measuring end, omitting the colloid as a connecting material, which greatly reduces the rheological impact of the colloidal connecting material during the long-term monitoring process. The impact of monitoring accuracy and long-term stability ensures the stability of sensor monitoring results.

It can be seen that the technical solution provided by the present application has the beneficial effect of being able to realize long-term monitoring of aperture deformation. Therefore, the technical problem of the long-term monitoring of the three-dimensional stress of the underground engineering surrounding rock in the prior art is solved, and the technical gap in this field is filled in the industry.

In addition, according to some embodiments of the present application, the steel ring is high-performance spring steel, which has good elastic properties and fatigue resistance. At the same time, the first fiber grating adopts anti-bending grating, which can adapt to the deformation requirements of small-diameter steel rings, solves the problem of grating chirping when drilling large deformations in surrounding rocks, and effectively improves the survival rate of the sensor. The welding between the first fiber grating and the steel ring greatly reduces the influence of the rheology of the substrate and connecting materials on the monitoring accuracy and long-term stability during the long-term monitoring process, ensuring the stability of the sensor monitoring results.

At the same time, according to some embodiments of the present application, the first fiber grating has the characteristics of anti-electromagnetic interference and corrosion resistance, which overcomes the problems of poor anti-interference and easy corrosion of the traditional resistive surrounding rock stress sensor; it is used together with the temperature compensation grating, It has the characteristics of high precision and good temperature stability, and solves the technical problems of temperature zero point temperature drift and sensitivity temperature drift when the sensing resistance aperture strain gauge is used for long-term monitoring.

Description of drawings

Other characteristics, objects and advantages of the present application will become more apparent by reading the detailed description of non-limiting embodiments made with reference to the following drawings:

Figure 1 is a top view of the sensor structure;

Figure 2 is a front view of the sensor structure;

Figure 3 is a cross-sectional view of steel ring and grating layout;

Fig. 4 is temperature and temperature compensation grating test curve and mathematical relationship;

Fig. 5 is the test curve and the mathematical relationship between temperature and No. 1 first fiber grating;

Fig. 6 is the test curve and the mathematical relationship between the deformation of the steel ring and the No. 1 first fiber grating;

Fig. 7 is the experiment curve of No. 1 first fiber Bragg grating wavelength changing with time under the condition of 0.6mm deformation;

Fig. 8 is the experimental curve of the deformation of the ordinary sensor with time under the condition of constant stress.

Among them, 1-stainless steel shell, 2-fixed support, 3-steel ring, 4-force transmission cap, 5-first fiber grating, 6-temperature compensation grating, 7-armored optical cable.

detailed description

The application will be further described in detail below in conjunction with the accompanying drawings and embodiments. It can be understood that the specific embodiments described here are only used to explain the relevant utility model, rather than to limit the utility model. It should also be noted that, for the convenience of description, only the parts related to the utility model are shown in the drawings.

It should be noted that, in the case of no conflict, the embodiments in the present application and the features in the embodiments can be combined with each other. The present application will be described in detail below with reference to the accompanying drawings and embodiments.

Please refer to Figures 1-3, a fiber grating aperture deformation sensor for long-term measurement of surrounding rock stress, mainly including a steel ring 3, a force transmission cap 4, and a first fiber grating 5 as the first fiber grating.

The steel ring 3 is made of 65si2mnwa spring steel, with an outer diameter of 24mm, a thickness of 1mm, and a width of 6mm. The force transmission cap 4 is made of stainless steel, the front end is in contact with the hole wall, and is hemispherical with a diameter of 5 mm; the rear end and the steel ring 3 contact end is cylindrical, with a diameter of 5 mm and a length of 6.2 mm. The first fiber grating 5 and the temperature-compensated grating 6 adopt bending-resistant fiber gratings, which can be used for bending with a radius of curvature of 4.6 mm, the length of the grid area is 3 mm, the reflectivity is not less than 90%, and the 3dB bandwidth is 0.3 nm.

A casing is provided outside the steel ring, preferably, a stainless steel casing 1 is used. The stainless steel shell 1 is made of stainless steel and is divided into three sections along the axis. The front section is hemispherical with a radius of 3cm. Take 4mm; the tapered section at the rear end has a wall thickness of 4mm and a length of 10cm. The tapered rear end is used to clamp the rock wall to fix the sensor. A cylindrical cavity is opened in the middle and the rear end, and the middle part is mainly used for arranging measurement settings.

In addition, the shell can also adopt a simpler structure, as long as it can fix and protect the steel ring, such as a simple cylindrical structure, and a fixing piece for fixing is provided on the outside of the cylindrical structure.

Preferably, the steel ring is fixed in the casing through a fixed support, and the fixed support is fixed in the cavity of the stainless steel casing through bolts. The fixed support 2 is made of PVC plastic material, which is a thick-walled cylindrical structure with an outer diameter of 24 mm, and is fixed on the stainless steel shell 1 by bolts. The thick-walled here refers to a wall thickness of 5 to 8 mm; The 3 position is provided with an annular groove for fixing the steel ring. The number of steel rings can be one or more, such as four steel rings, as shown in Figure 1 and Figure 2, in order to avoid failure of a certain steel ring due to failure. The four steel rings are arranged in parallel along the axis of the stainless steel shell 1, evenly arranged in the vertical axis plane (0°, 45°, 90°, 135°), the displacement of the steel ring 3 is limited by the fixed support 2, and there are 3 steel rings Radial deformation can occur but cannot rotate.

It is worth noting that the steel ring can also be fixed in the housing by other methods, such as fixing the steel ring at an obtuse angle with the force transmission cap by a fastener similar to a snap ring, so as to ensure that the steel ring can undergo radial deformation without Offset rotation occurs.

The first fiber grating 5 is connected to the steel ring 3 by welding. As shown in FIG. 3 , the grating is located outside the steel ring, arranged along the ring line of the steel ring, and forms an angle of 90 degrees with the force transmission cap 4 . The temperature-compensated grating 6 is located at the rear end of the stainless steel casing 1 and is in a state of free deformation. Four first optical fiber gratings and one temperature-compensated grating communicate with the outside world after being welded to the armored optical cable 7 at the rear end of the sensor.

The calibration methods of the fiber grating aperture deformation sensor include:

(1) Temperature calibration of the grating: place the sensor in an adjustable incubator, then adjust the temperature to the lowest calibrated temperature value, and connect it to the fiber optic demodulator. After the temperature is constant, record the temperature of the constant temperature box, the wavelength of the first fiber grating 5 and the temperature compensation grating at this time; then increase the temperature of the constant temperature box, and after the temperature is stable, record the corresponding temperature and the wavelength of each grating again; use the same method to continuously increase The temperature of the incubator is reached until the highest calibration temperature is reached. Thereby obtain the relational curve λ _T- T (Fig. 4) of temperature-compensated grating wavelength and temperature T, and the relational curve of wavelength and temperature of each first fiber grating 5 (i=1,2,3,4). (the No. 1 first fiber grating is given in Fig. 5 Relationship lines). The expression of the relationship between wavelength and temperature of temperature-compensated grating obtained by mathematical fitting

f(T)=λ _T ＝0.01054 _T +1550.192 (Fig. 4), and the relationship expression between the wavelength and temperature of each first fiber grating As shown in Figure 5, the relational expression of No. 1 first fiber grating is:

(2) Displacement calibration of the grating: place the sensor on the displacement calibration frame and connect it to the optical fiber demodulator. Read Temperature Compensated Grating Wavelength obtained in step (1)

f(T)=λ _T =0.01054T-1550.192, and the calibration temperature T ₀ =12.8°C is calculated. Then according to the obtained in step (1) Calculate the initial wavelengths of the four first fiber gratings at this temperature For example, the initial wavelength of the first fiber grating is

According to the estimated maximum stress level and rock conditions on site, the maximum deformation of the aperture does not exceed 0.6mm. Therefore, the maximum displacement in this displacement calibration is 0.6mm. Select No. 1 steel ring, apply displacement to a pair of force transmission caps at both ends of it through the displacement calibration frame, and the applied displacement increases linearly from zero to 0.6mm, and then decreases from the maximum value to zero; record the applied displacement U and the first optical fiber The relationship curve of the corresponding wavelength λ ₁ of the grating is shown in FIG. 6 . Subtract the initial wavelength from the wavelength λ ₁ of the first fiber grating The relationship curve of wavelength change and displacement caused only by displacement is obtained A relational expression Δλ ₁ =D ¹ (U)=6.0686U−0.01314 between the wavelength change of the first fiber grating caused by the displacement and the displacement is obtained through mathematical fitting. The other three steel rings are calibrated by the same method, and the corresponding relational expression D ⁱ (U)=Δλ _i is obtained.

(3) Sensor accuracy check: Select a plexiglass block with uniform material and isotropy, the size is 30cm*30cm*30cm, and a through hole with a diameter of 36mm is opened in the center of one side. The sensor is placed in the round hole. Since the diameter of the round hole is smaller than the distance between the force transmission caps at both ends of the steel ring, the steel ring has a certain amount of initial compression after being placed in the round hole. Connect the sensor to the demodulator, and record the wavelength of each grating through the computer. The plexiglass block is placed on a rigid servo press, and pressures P ₁ =10 MPa and P ₂ =0 MPa are respectively applied in two directions perpendicular to the axis of the circular hole (the stress levels of the two directions can be combined in different ways).

After the measurement is completed, according to the wavelength of the temperature-compensated grating recorded during the test From the f( _T )=λT obtained in step (1), determine the ambient temperature T ₁ =12.24°C (the calibration temperature is the same as the test temperature). According to ambient temperature T1 and step ( ₁ ) obtained in Calculate the initial wavelength of each first fiber grating under the test temperature condition For example, the initial wave of the first grating is: According to the wavelength _λi of each first fiber grating recorded during the test, the wavelength change caused only by external force is obtained The wavelength change of No. 1 grating is Δλ ₁ =2.012nm. Then, the displacement U _i between the force transmission caps of the four steel rings is obtained from D ⁱ (U)=Δλ _i obtained in step (2). For example, the displacement of both ends of the No. 1 grating is U ₁ =0.334mm.

The compression test of the plexiglass was carried out with a rigid servo press, and the elastic modulus of the plexiglass was 2.7GPa, and the Poisson's ratio was 0.3.

According to the displacement of the force transmission cap and the elastic parameters of the plexiglass, and using the knowledge of elastic mechanics, the two principal stresses σ ₁ =9.2MPa and σ ₂ =0.0MPa and their directions in the plane are calculated. Compared with the test applied P ₁ =10MPa and P ₂ =0MPa and direction, the relative error of the test sensor is 4.7%, which meets the accuracy requirement.

(4) Sensor long-term stability check

Apply a displacement of 0.6mm to both ends of the steel ring and stabilize it, and detect the long-term stability of the sensor through the change of wavelength. It can be seen from Figure 7 that the wavelength of the grating has almost no change during the 192 hours of application, which proves that the sensor has good long-term stability and can meet the requirements of long-term monitoring. Figure 8 is the force deformation diagram of the traditional colloid-filled sensor after one day of loading. It can be seen that the traditional colloid-filled sensor will produce a large rheology after one day of loading, which is not suitable for long-term surrounding rock stress measurement.

The above description is only a preferred embodiment of the present application and an illustration of the applied technical principles. Those skilled in the art should understand that the scope of the utility model involved in this application is not limited to the technical solution formed by the specific combination of the above-mentioned technical features, and should also cover the above-mentioned utility models without departing from the concept of the utility model. Other technical solutions formed by any combination of technical features or equivalent features. For example, a technical solution formed by replacing the above-mentioned features with technical features with similar functions disclosed in (but not limited to) this application.

Claims (10)

1. A kind of 1. fiber grating aperture deformation-sensor of long-term measurement surrouding rock stress, it is characterised in that including：

   Rigid substrate, to be circumferentially arranged in drilling, corresponding deformation can occur with the deformation in country rock aperture；

   Rigid dynamometry end, is arranged on the rigid substrate, directly to contact borehole sidewall to measure the radial direction of the drilling Deformation；

   First fiber grating, be fixed on the surface of the rigid substrate, its grating wavelength with the deformation of the rigid substrate and Change.
2. 2. a kind of fiber grating aperture deformation-sensor of long-term measurement surrouding rock stress according to claim 1, its feature It is：The rigid substrate is steel loop, and the rigid substrate, the first fiber grating and dynamometry end collectively form inside diameter measurement sensing The measuring part of device, the inside diameter measurement sensor are provided with multiple measuring parts be arrangeding in parallel, the different measurements The dynamometry end of part is staggered set angle.
3. 3. a kind of fiber grating aperture deformation-sensor of long-term measurement surrouding rock stress according to claim 2, its feature It is：The steel loop uses 65si2mnwa spring steel materials, and the radius of curvature of the steel loop is 4.0-5.0mm, first light Fine grating uses bend insensitive optical fiber grating, can adapt to the curvature of steel loop after deformation, and the grid region length of first fiber grating is 2-4mm, reflectivity are not less than 90%, three dB bandwidth 0.2-0.4nm, using weldering between first fiber grating and the steel loop Connect.
4. 4. a kind of fiber grating aperture deformation-sensor of long-term measurement surrouding rock stress according to Claims 2 or 3, it is special Sign is：The aperture deformation-sensor is provided with four measuring parts, and the dynamometry end of each measuring part is staggered Angle is circumferentially uniformly distributed, and the dynamometry end is two power transmission caps for being fixed at the steel loop outer surface.
5. 5. a kind of fiber grating aperture deformation-sensor of long-term measurement surrouding rock stress according to claim 4, its feature It is：The diametrically contraposition of steel loop described in two power transmission shades, the front end of the power transmission cap is directly to be contacted with country rock Hemisphere, rear end is the cylinder that is directly contacted with the steel loop, and the power transmission cap is hard metal material.
6. 6. a kind of fiber grating aperture deformation-sensor of long-term measurement surrouding rock stress according to claim 4, its feature It is：The fixed position of first fiber grating has 90 ° of angles relative to the center of the steel loop with the power transmission cap, and It is laid in the outer surface of the steel loop.
7. 7. a kind of fiber grating aperture deformation-sensor of long-term measurement surrouding rock stress according to claim 4, its feature It is：The measuring part also includes a housing, and the housing has cylindrical portions, and the rigid substrate is fixed on the housing Cylinder portion inside, the housing is provided with the via worn to the power transmission cap so that the power transmission cap stretch out in it is described The outer surface of housing simultaneously directly contacts country rock side wall.
8. 8. a kind of fiber grating aperture deformation-sensor of long-term measurement surrouding rock stress according to claim 7, its feature It is：The housing uses stainless steel, and the front end of the housing is hemispherical, and the middle part of the housing is the cylindrical portion Point, the afterbody of the housing has the enlarged portion to clamping in the borehole that section gradually increases.
9. 9. a kind of fiber grating aperture deformation-sensor of long-term measurement surrouding rock stress according to claim 7, its feature It is：The steel loop is fixed on the inside of the housing by hold-down support, and the hold-down support is thick cyclinder shape structure, Gu Due to the inside of the housing, the hold-down support has to limit the inclined stopper slot of the steel loop, the hold-down support Centre as optical-fibre channel.
10. 10. a kind of fiber grating aperture deformation-sensor of long-term measurement surrouding rock stress according to claim 7, its feature It is：Be additionally provided with the sensor in free deformation to the second fiber grating as temperature compensation grating, it is each First fiber grating and second fiber grating are in the rear end of the housing and armored optical cable welding.