CN113310424B - Fixed-point optical fiber strain sensor and using method thereof - Google Patents

Abstract

The invention discloses a fixed-point optical fiber strain sensor and a use method thereof. The sensor body includes a sensing inner core and a sensor outer sheath, wherein the sensing inner core is coupled with the deformation of the monitoring structure at the anchor point; the sensor outer sheath is set Outside the sensing inner core, several windowed areas and non-opened areas are set up on the outer sheath body of the sensor at intervals. Among them, the windowed area is a hollow structure, which allows the external structural glue to enter and form an anchor with the sensing inner core. ; The outer layer of the non-opening area is provided with an insulating material for isolating the structural adhesive; the fixed-point optical fiber strain sensor provided by this application is slim and compact, and the method of use is simple, which is convenient for long-distance large-scale deployment and application.

Description

A fixed-point optical fiber strain sensor and its application method

technical field

The invention relates to the technical field of intelligent sensor monitoring, in particular to a fixed-point optical fiber strain sensor and its use method.

Background technique

In the field of distributed optical fiber sensing, there are two main layout methods of distributed optical fiber strain sensors: full-scale pasting and fixed-point layout. Full paste means to anchor the fiber optic sensor and the structure under test through the length of the adhesive or pre-embedded method to form a comprehensive deformation coupling; the fixed-point layout method is to connect the sensor and the structure only at the anchor point through a series of discrete anchor points. area for deformation coupling. In practical applications, due to the comprehensive paste layout method, it is easy to cause complex initial strain distribution of the sensor, which will be affected by the spatial resolution factors of the demodulation equipment, resulting in a decrease in the accuracy of the distributed sensing test, and due to the complex strain conduction characteristics of the sensor This makes it difficult to accurately convert the measured distributed strain and structural displacement. The fixed-point layout method avoids the measurement uncertainty caused by the optical cable structure and adhesive materials to the greatest extent, and has the advantages of higher measurement accuracy and accurate strain-displacement conversion.

For some special application scenarios, such as structural crack monitoring, it may be necessary to use a shorter sensor gauge design, which leads to the need for a more dense anchor point design. At present, there are mainly two anchoring methods for the fixed-point deployment of distributed optical fiber strain sensors, namely the external fixed-point method and the internal fixed-point method. The external fixed point method is to form an anchor through the external anchorage and the strain sensing optical cable; while the internal fixed point is to form a fixed point inside the sensor when the sensor is produced. At present, most of the actual projects are installed in the form of external fixed points. Using the external fixed-point installation method requires a series of cumbersome operations such as on-site fixed point, installation of anchors, and fixed optical fibers when the sensor is actually deployed, which is extremely time-consuming and laborious. At the same time, the anchor itself is usually bulky and expensive, which increases the cost of the sensing system, which is not conducive to long-term operation. The distance is quickly laid out. In addition, optical fibers are usually exposed, easily damaged and less secure. The internal fixed-point sensors used in a small number usually have complex structures, difficult processing and production, and are difficult to produce and apply in large quantities. When the designed sensing gauge length is short, it is often affected by factors such as sensor processing and pre-tensioning during layout. , causing problems such as uneven initial strain inside the sensor, which affects the sensing accuracy, and is not suitable for the monitoring needs of large-scale distributed structural crack damage.

In view of the above problems, in the field of health monitoring of various civil engineering infrastructure structures, it is necessary to have long and short gauge optical fiber strain sensors with simple structure, convenient for large-scale manufacturing and rapid installation, easy to form dense distribution, and small initial strain fluctuations of the sensors. Distributed fixed-point sensors to realize low-cost, long-distance large-scale fixed-point deployment of optical fiber sensing.

Contents of the invention

The technical problem to be solved by the present invention is to provide a fixed-point optical fiber strain sensor and its use method in view of the above-mentioned deficiencies in the prior art.

In order to solve the problems of the technologies described above, the technical solution adopted in the present invention is:

A fixed-point optical fiber strain sensor, the sensor body includes a sensing inner core and a sensor outer sheath, wherein,

The sensing inner core is coupled with the deformation of the monitoring structure at the anchor point;

The outer sheath of the sensor is set outside the sensing inner core, and several windowed areas and non-opened areas are set up on the sensor outer sheath at intervals. Among them, the windowed area is a hollow structure, which can allow the external structural glue to enter and communicate The sensing inner core forms an anchor; the outer layer of the non-window area is provided with an insulating material to isolate the structural adhesive;

As a further preference of the present invention, the sensing inner core adopts a strain sensing optical cable;

As a further preference of the present invention, the inner diameter of the sensor outer sheath is larger than the outer diameter of the sensing inner core, and the sensor outer sheath is set outside the sensing inner core, so that the distance between the sensing inner core and the sensor outer sheath is There is sliding space between;

As a further preference of the present invention, the outer sheath of the sensor has a certain structural strength, and different materials and laminated structures can be used according to specific application scenarios, so as to effectively protect the sensing inner core;

As a further preference of the present invention, the sensing inner core forms adhesion with the structure through penetration of the structural glue in the window opening area of the outer sheath of the sensor, and then forms a fixed-point anchoring area after the colloid is solidified;

As a further preference of the present invention, the structural glue is in the form of a paste, and the structural glue can penetrate the windowed area of the outer sheath and enter the inner core of the sensor while not entering the non-fenestrated area;

As a further preference of the present invention, the optical fiber component in the aforementioned strain sensing optical cable is a matching optical fiber, and the optical fiber is sheathed with an optical fiber outer sheath;

The method of using a fixed-point optical fiber strain sensor suitable for the inner surface of a straight line structure or an arc structure is as follows:

Step 1. Comprehensively determine the length of the window area according to the characteristics of the structural adhesive used and the type of the sensor core; select the length of the non-window area according to the application scenario;

Step 2. Clean the contact surface between the monitoring structure and the sensor body. After the surface of the monitoring structure is dry, use temporary fixings to fix the sensor body on the surface of the structure to be tested according to the design;

Step 3, anchoring the sensing inner core protruding from one end of the outer sheath of the sensor to the monitoring structure;

Step 4. After the anchoring takes effect, apply a pre-tightening force to the sensing inner core protruding from the other end of the outer sheath of the sensor, and anchor the sensing inner core at the force-applying end to the monitoring structure in time after the pre-tightening force is applied in place;

Step 5: The structural glue covers the sensor body. During the glue brushing process, the structural glue penetrates into the sensing inner core through the window area of the outer sheath of the sensor, so that the sensing inner core forms adhesion with the monitoring structure in the anchoring area, and then the colloid After solidification, fixed-point anchors are formed, fully coupling it to the monitoring structure.

The method of using the fixed-point optical fiber strain sensor suitable for long-distance deployment is as follows:

Step 1. Comprehensively determine the length of the window area according to the characteristics of the structural adhesive used and the type of the sensor core; select the length of the non-window area according to the application scenario;

Step 2. Before the sensor body is installed in the monitoring structure, the sensor core and the sensor outer sheath are anchored at both ends of the outer sheath, that is, the sensor outer sheath and the sensor core are only deformed and coupled at both ends of the sensor body. , while the middle part is still free to produce relative sliding;

Step 3. Clean the contact surface between the monitoring structure and the sensor body. After the surface of the monitoring structure is dry, anchor one end of the sensor body to the monitoring structure through a temporary fixture;

Step 4. After the anchoring takes effect, tension is carried out at the anchor point at the other end of the sensor body, so that the inner core of the sensor and the outer sheath are stretched synchronously to generate a pre-tightening force, and after the pre-tightening force is applied in place, the force-applying end is connected with the monitoring structure in time. to anchor;

Step 5: The structural glue covers the sensor body. During the glue brushing process, the structural glue penetrates into the sensing inner core through the window area of the outer sheath of the sensor, so that the sensing inner core forms adhesion with the monitoring structure in the anchoring area, and then the colloid After solidification, fixed-point anchors are formed, fully coupling it to the monitoring structure.

As a further preference of the present invention, in step 1, the length of the non-window area is selected according to the application scenario, and the application scenario is divided into use as a long gauge sensor and a short gauge sensor,

Among them, if it is used as a long gauge sensor, the length range of the non-window area is 10cm-10m,

If used as a short-gauge sensor, the length of the non-window area ranges from 2cm to 200cm.

The present invention has following beneficial effect:

1. The fixed-point optical fiber strain sensor proposed by the present invention adopts the form of a window-opening sheath, and the window-opening anchoring section is formed in the production stage of the sensor body, thereby avoiding the need for anchorage precision at the construction site for the usual external fixed-point sensor anchors. Positioning and installation of complex defects greatly simplifies and accelerates the positioning and installation of distributed fiber optic sensors.

2. The fixed-point anchoring of the sensor is naturally formed while fixing the sensor body through the process of colloid infiltration, so that the process of sensor body fixing, anchor point positioning, anchor point forming, and sensor body protection is simplified to only brushing or pouring glue. The process is completed, which greatly simplifies the on-site installation process of the sensor, and is conducive to the large-scale and rapid deployment of the sensor.

3. After the colloid is solidified, a hard protective layer is formed outside the sensor body, which can provide good protection for the optical fiber sensor and greatly improve the stability, durability and long-term safety of the sensing system; The outer sheath of the sensor can slide freely, and the initial strain will not change suddenly due to various reasons during the installation process, so it is of great significance for the high precision of the Brillouin sensing system and the data mining of local crack information.

To sum up, the fixed-point optical fiber sensor proposed in the present invention has a cable-like structure, is slim and compact, and is convenient for long-distance large-scale deployment and application.

Description of drawings

Fig. 1 is a structural representation of a fixed-point optical fiber strain sensor of the present invention;

Fig. 2 is a schematic diagram of the installation method of the first method of use of a fixed-point optical fiber strain sensor of the present invention;

Fig. 3 is a schematic diagram of the installation method of the first method of use of a fixed-point optical fiber strain sensor of the present invention;

Fig. 4 is a schematic diagram of the installation method of the first method of use of a fixed-point optical fiber strain sensor of the present invention;

Fig. 5 is a schematic diagram of the installation method of the second use method of a fixed-point optical fiber strain sensor of the present invention;

Fig. 6 is a schematic diagram of the installation method of the second use method of a fixed-point optical fiber strain sensor of the present invention;

Fig. 7 is a schematic diagram of the installation method of the second use method of a fixed-point optical fiber strain sensor of the present invention;

Fig. 8 is a schematic diagram of the installation method of the second method of using a fixed-point optical fiber strain sensor of the present invention;

Fig. 9a-Fig. 9b are the application display diagrams of the present invention in the subway shield tunnel structure;

Including:

1. Sensor outer sheath; 2. Sensing inner core; 3. Optical fiber; 4. Optical fiber outer sheath; 5. Anchoring area; 1. Temporary fixing parts; 10. Length of anchoring area; 11. Sensing gauge length; 12. Permanent fixing parts; 13. Anchoring end.

Detailed ways

The present invention will be further described in detail below in conjunction with the accompanying drawings and specific preferred embodiments.

In the description of the present invention, it should be understood that the orientations or positional relationships indicated by the terms "left side", "right side", "upper", "lower" are based on the orientations or positional relationships shown in the accompanying drawings, and are only For the purpose of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operate in a specific orientation, "first", "second" and the like do not represent components importance, and therefore should not be construed as limiting the invention. The specific dimensions used in this embodiment are only for illustrating the technical solution, and do not limit the protection scope of the present invention.

As shown in Figures 1 and 2, a fixed-point optical fiber strain sensor is composed of two parts: the sensing inner core 2 and the sensor outer sheath 1, wherein the sensing inner core 2 is only connected to the monitoring area at a series of discrete anchoring areas 5 The structure 7 is deformed and coupled, and the outer sheath 1 of the sensor can protect the inner core 2 of the sensor and at the same time automatically form the anchoring area 5 during the installation process of the sensor. The sensor body of the present invention uses various distributed optical fiber demodulation devices based on the principle of Brillouin scattering to read signals, and the types of instruments include but are not limited to BOTDA, BOTDR, BOFDA, and BOFDR.

For the sensing core 2, strain sensing optical cables with different packaging structures can be selected according to specific applications. For example, we prefer to adopt a tight-buffered strain sensing optical cable. The optical fiber component in the optical cable adopts the optical fiber 3 (such as G652D single-mode optical fiber) corresponding to the Brillouin demodulation equipment. The core and cladding of the optical fiber are respectively 9.5 μm and 125 μm in diameter. The coating layer and the tight sleeve fiber outer sheath 4 are preferably made of acrylic and nylon respectively.

The special sensor outer sheath 1 is one of the core components of the fixed-point optical fiber strain sensor, and it is also one of the characteristics of the patent of the present invention that is different from the prior art method. The outer diameter is slightly larger, and the sensing inner core 2 can slide freely relative to the outer sheath. Preferably, the outer surface of the sensing core and the inner surface of the outer sheath of the sensor should be as smooth as possible to reduce the frictional resistance between them. The outer sheath of the sensor has specially-made windowed areas 6 at certain intervals, through which the external structural adhesive 8 can penetrate into the interior of the sensor to form adhesion with the sensing inner core 2 . The outermost layer of the non-window area of the outer sheath of the sensor is made of an insulating material, and the external adhesive cannot penetrate. The length of the window opening area is also called the length of the anchoring area 10, which needs to be comprehensively determined according to the characteristics of the structural adhesive used and the type of the sensing core 2, etc., and is generally about 2-5 cm long. The length of the non-window area is called the sensing gauge length 11, which also needs to be selected according to the specific application scenario. For example, if it is used as a long-gauge sensor, the sensing gauge 11 can be set to a range ranging from tens of centimeters to several meters. Here, the applicant has given a constraint, which is 10cm-10m; if it is used as a short-gauge sensor, such as For quantitative monitoring of crack damage, etc., the sensing gauge distance 11 can be set in a range ranging from a few centimeters to tens of centimeters. Similarly, the applicant has given a constraint, that is, 2cm-200cm.

The window part of the outer sheath 1 of the sensor has a certain hollow structure, which can make the window 6 part of the outer sheath have a certain structural strength, thereby connecting the sections of the outer sheath without windows into a whole. For example, the hollow structure of the sensor example shown in Figure 1 adopts a sparse helical structure. The so-called sparse helical structure means that the number of helical turns is small and the distance between adjacent helical bands is relatively large. Preferably, the outer sheath of the sensor in the example shown is composed of an inner stainless steel sparse spiral tube and a segmented outer rubber-insulating plastic sleeve. The stainless steel open spiral tube provides the necessary strength to the sensor sheath against potential mechanical shock and contributes to the high durability of the sensor. According to specific application scenarios, the outer sheath of the sensor should have a certain structural strength, and adopt different materials and laminated structures, so as to effectively protect the sensing inner core 2 .

During implementation, the above-mentioned design of the sensor can greatly simplify the installation and layout of the sensor. During installation, it can be fixed on the surface or inside of the structure under test by comprehensive glue brushing or glue pouring process, and at the same time, fixed-point anchoring is formed inside the sensor. Choose the appropriate layout method (method 1 and method 2) according to the layout scenario and requirements. The following takes installation on the surface of the structure as an example to illustrate the specific installation and layout method:

As shown in Figure 2-4, when the layout scene is the inner surface of a linear structure or a curved structure, take method 1: first clean the surface of the structure to be tested with alcohol, water and other detergents, and use temporary fixing parts 9 (quick) after the surface is dry. Dry glue or adhesive tape) to fix the sensor on the surface of the structure to be tested according to the design. The fixed areas of the temporary fixing parts 9 (quick-drying glue or adhesive tape) should be sparsely distributed in the non-windowed area of the sensor to avoid contact with the windowed area. Subsequently, the sensing inner core 2 at one end of the sensor is anchored to the structure to be tested by quick-drying glue or other permanent fast anchoring methods (such as extrusion anchors, etc.). After the anchoring takes effect, apply a pre-tightening force to the sensing inner core 2 at the other end of the sensor, and after the pre-tightening force is applied in place, use quick-drying glue or other anchoring methods to anchor the sensing inner core 2 at the force-applying end to the structure in time . This method of pre-tensioning the strain sensing optical cable on site can minimize the initial strain non-uniformity of the sensing inner core, especially reduce the local strain fluctuation, which is useful for some special Deformation sensing applications are an essential prerequisite.

The application of a preload enables the sensor to monitor the compressive deformation of the structure 7 . The size of the prestress should be determined according to the specific application scenario and the type of the sensing core. Preferably, for general deformation tests, the sensing core is usually pre-stretched to about 3000με. During routing, care should be taken to prevent dust and dirt from entering the sensor. This can be achieved by providing some coverage or protection of the sensor prior to pretensioning.

After the tensioning is completed, the sensor is fully covered with a special paste-like structural glue 8 to fully couple it with the structure under test. During the glue brushing process, the structural glue 8 penetrates into the sensor through the window area of the outer sheath 1 of the sensor, so that the sensing inner core 2 forms adhesion with the structure in the designed anchorage area 5, and then forms a fixed-point anchor after the glue solidifies.

As shown in Figure 5-8, the second layout method can be adopted according to the specific usage scenario, the sensing length of a single layout, and the requirements for the uniformity of the pre-strain distribution of the sensing inner core. The second method is compared with the first This method is more suitable for long-distance layout, which can make the sensing inner core obtain a more uniform initial strain distribution. Applicable layout scenarios include linear layout, curved structure outer surface layout, guide hole layout and other scenarios.

According to the characteristics of the structural adhesive used and the type of the sensing core 2, the length of the window area is comprehensively determined; the length of the non-window area is selected according to the application scenario; before the sensor is installed in the structure, the sensor core and the outer sheath of the sensor are placed outside The two ends of the sheath form anchoring ends, that is, the outer sheath and the sensing inner core are only deformed and coupled at both ends of the sensor, while the middle part can still freely generate relative sliding; clean the contact surface between the monitoring structure 7 and the sensor, and prevent dust, dirt, etc. Subsequent sensor installation; anchor one end of the sensor to the structure to be tested. The anchor can use quick-drying glue or other permanent anchoring methods; The sleeve is stretched synchronously to generate a pre-tightening force, and after the pre-tightening force is applied in place, the force-applying end is anchored to the monitoring structure 7 in time; the structural adhesive 8 covers the sensor body, and the structural adhesive passes through the opening of the outer sheath of the sensor during the glue brushing process. The window area penetrates into the sensing inner core, so that the sensing inner core forms adhesion with the monitoring structure in the anchoring area, and then forms a fixed-point anchor after the colloid solidifies, completely coupling it with the monitoring structure.

The structural adhesive 8 is specially developed through a series of laboratory tests, with optimized viscosity and working time, so that the structural adhesive 8 can penetrate the window area of the sensor outer sheath 1 and penetrate into the sensor without invading The non-opening area, so as to achieve the effect of stably and firmly coupling the sensor and the structure, and at the same time precisely forming the design anchoring area. After the gel has hardened, the structural adhesive provides another effective layer of protection for the sensor against potentially harmful environmental influences.

Figure 9 is an example of the application of this sensor in a shield subway tunnel operated in a city in East China, illustrating its actual application effect. The sensors of this project are deployed along the circumference of the tunnel to evaluate the long-term structural performance evolution of the tunnel lining structure. Sensor installation takes place during the time window after all trains are out of service. The actual installation of this project proves that the installation of the sensor in the tunnel is convenient, fast and efficient.

The preferred embodiments of the present invention have been described in detail above, but the present invention is not limited to the specific details in the above embodiments. Within the scope of the technical concept of the present invention, various equivalent transformations can be carried out to the technical solutions of the present invention. These equivalent transformations All belong to the protection scope of the present invention.

Claims (9)

1. A method for using a fixed-point optical fiber strain sensor, characterized in that the sensor body includes a sensing inner core and a sensor outer sheath, and the sensing inner core is coupled with the deformation of the monitoring structure at the anchor point; the sensor outer sheath is set Outside the sensing inner core, a number of windowed areas and non-windowed areas are set up on the outer sheath body of the sensor at intervals, wherein the windowed area is a hollow structure, allowing the external structural glue to enter and form an anchor with the sensing inner core; The outer layer of the non-window area is provided with an insulating material to isolate the structural adhesive; It is suitable for the inner surface of straight line structure or arc structure, and the steps are as follows: Step 1. Comprehensively determine the length of the window area according to the characteristics of the structural adhesive used and the type of the sensor core; select the length of the non-window area according to the application scenario; Step 2. Clean the contact surface between the monitoring structure and the sensor body. After the surface of the monitoring structure is dry, use temporary fixings to fix the sensor body on the surface of the structure to be tested according to the design; Step 3, anchoring the sensing inner core protruding from one end of the outer sheath of the sensor to the monitoring structure; Step 4. After the anchoring takes effect, apply a pre-tightening force to the sensing inner core protruding from the other end of the outer sheath of the sensor, and anchor the sensing inner core at the force-applying end to the monitoring structure in time after the pre-tightening force is applied in place; Step 5: The structural glue covers the sensor body. During the glue brushing process, the structural glue penetrates into the sensing inner core through the window area of the outer sheath of the sensor, so that the sensing inner core forms adhesion with the monitoring structure in the anchoring area, and then the colloid After solidification, fixed-point anchors are formed, fully coupling it to the monitoring structure. 2. A method for using a fixed-point optical fiber strain sensor, characterized in that the sensor body includes a sensing inner core and a sensor outer sheath, and the sensing inner core is coupled with the deformation of the monitoring structure at the anchor point; the sensor outer sheath is set Outside the sensing inner core, a number of windowed areas and non-windowed areas are set up on the outer sheath body of the sensor at intervals, wherein the windowed area is a hollow structure, allowing the external structural glue to enter and form an anchor with the sensing inner core; The outer layer of the non-window area is provided with an insulating material to isolate the structural adhesive; It is suitable for long-distance deployment, and the steps are as follows: Step 1. Comprehensively determine the length of the window area according to the characteristics of the structural adhesive used and the type of the sensor core; select the length of the non-window area according to the application scenario; Step 2. Before the sensor body is installed in the monitoring structure, the sensor core and the sensor outer sheath are anchored at both ends of the outer sheath, that is, the sensor outer sheath and the sensor core are only deformed and coupled at both ends of the sensor body. , while the middle part is still free to produce relative sliding; Step 3. Clean the contact surface between the monitoring structure and the sensor body. After the surface of the monitoring structure is dry, anchor one end of the sensor body to the monitoring structure through a temporary fixture; Step 4. After the anchoring takes effect, tension is carried out at the anchor point at the other end of the sensor body, so that the inner core of the sensor and the outer sheath are stretched synchronously to generate a pre-tightening force, and after the pre-tightening force is applied in place, the force-applying end is connected with the monitoring structure in time. to anchor; Step 5: The structural glue covers the sensor body. During the glue brushing process, the structural glue penetrates into the sensing inner core through the window area of the outer sheath of the sensor, so that the sensing inner core forms adhesion with the monitoring structure in the anchoring area, and then the colloid After solidification, fixed-point anchors are formed, fully coupling it to the monitoring structure. 3. according to the use method of the arbitrary described fixed-point optical fiber strain sensor of claim 1 or 2, it is characterized in that, in step 1, select the length of non-window area according to application scene, application scene is divided into as long standard range sensor and as a short gauge sensor, Among them, if it is used as a long gauge sensor, the length range of the non-window area is 10cm-100cm, If used as a short-gauge sensor, the length of the non-window area ranges from 2cm to 200cm. 4. The method of using the fixed-point optical fiber strain sensor according to any one of claims 1 or 2, characterized in that: the sensing inner core is a strain sensing optical cable. 5. The method for using the fixed-point optical fiber strain sensor according to any one of claims 1 and 2, wherein the inner diameter of the sensor outer sheath is greater than the outer diameter of the sensing inner core, and the sensor outer sheath covers It is arranged outside the sensing inner core so that there is a sliding space between the sensing inner core and the outer sheath of the sensor. 6. The method of using the fixed-point optical fiber strain sensor according to any one of claims 1 or 2, characterized in that: the outer sheath of the sensor has a certain structural strength, and different materials and laminations can be used according to specific application scenarios structure, thus forming an effective protection for the sensing inner core. 7. The method of using the fixed-point optical fiber strain sensor according to any one of claims 1 and 2, characterized in that: the sensing inner core forms adhesion with the structure through the penetration of structural glue in the window area of the outer sheath of the sensor, Then the fixed-point anchoring area is formed after the colloid is solidified. 8. The method of using the fixed-point optical fiber strain sensor according to any one of claims 1 or 2, characterized in that: the structural adhesive is in the form of a paste, and the structural adhesive can penetrate the window area of the outer sheath and enter the sensor The core will not enter the non-fenestration area at the same time. 9. The method of using the fixed-point optical fiber strain sensor according to claim 4, characterized in that: the optical fiber element in the strain sensing optical cable is a matching optical fiber, and the optical fiber is sheathed with an optical fiber outer sheath.

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