Vibrating wire type stress meter and application thereof in tunnel surrounding rock stress monitoring

Abstract

The invention relates to the field of geotechnical engineering monitoring. The invention provides a vibrating wire type stress meter and application thereof in tunnel surrounding rock stress monitoring. The device comprises a shaft sleeve, a steel bar, a fixing part, a steel string and an excitation sensing module, the end of the steel bar is located in the shaft sleeve, and the fixing part and the steel string are axially arranged in the shaft sleeve. Fixing parts are arranged at the two ends of the steel string, and the ends, away from the steel string, of the fixing parts are connected with the steel bars. The excitation sensing module is arranged above the steel string and installed outside the shaft sleeve. Wherein the middle part of the steel string is a twisted section, and two ends of the steel string are linear sections. According to the invention, the steel string preferentially releases geometric deformation under the action of axial tensile stress through the pre-arranged twisting section in the middle of the steel string, thereby effectively enlarging the amplitude change caused by micro stress, enhancing the response sensitivity of the steel string, breaking through the structural limitation that the stability and the sensitivity of a traditional stress meter are difficult to achieve at the same time, and improving the reliability of the stress meter. The method is suitable for high-reliability and high-precision application scenes in tunnel surrounding rock stress monitoring.

Description

Technical Field

[0001] The present application relates to the field of geotechnical engineering monitoring, and in particular to a vibrating wire stress gauge and its application in tunnel surrounding rock stress monitoring. Background Art

[0002] As the primary load-bearing element in underground structures, the long-term stability of tunnel surrounding rock is directly related to the safe operation of tunnel projects. To monitor the stress state of the surrounding rock in real time and provide timely warnings before subtle structural deformation or potential instability occurs, vibrating wire stress gauges are often embedded for monitoring. By capturing changes in axial stress within the surrounding rock mass or supporting structures, they enable long-term tracking of project status.

[0003] Vibrating-wire strain gauges monitor changes in the natural frequency of their internal steel wires and infer the stress state. They offer advantages such as intuitive response, strong anti-interference capabilities, and suitability for long-term burial. The sensitivity of the frequency response to stress changes is a key indicator of monitoring accuracy, directly impacting its ability to identify early anomalies such as crack initiation and structural loosening. To improve sensitivity, existing technologies generally use thin-diameter steel wires to reduce their mass and stiffness, making them more sensitive to tension changes. However, in the complex underground environments of tunnel projects, thin steel wire structures generally face severe durability and stability challenges. Their large surface area and fragile structure make them susceptible to corrosion from moisture, water vapor penetration, and underground chemical components, leading to gradual fatigue or rust weakening of the wires, posing a risk of sudden breakage. Furthermore, thin steel wires are highly sensitive to changes in boundary conditions and installation errors, often exhibiting frequency drift and unstable responses, which seriously impact the accuracy and consistency of long-term monitoring results. In contrast, while increasing the diameter of the steel wire improves structural stability and service life, the increased stiffness weakens the natural frequency's response to tension changes, significantly reducing sensitivity and making it difficult to identify subtle stress changes in the surrounding rock. Therefore, existing vibrating-wire strain gauges struggle to achieve both high sensitivity and high stability in practical applications. This structural contradiction limits the foresight and effectiveness of monitoring systems, particularly in long-term monitoring of tunnel surrounding rock.

[0004] To solve the above problems, it is urgent to propose a new vibrating wire stress gauge structure that can improve the frequency response sensitivity while ensuring the service stability of the steel wire, thereby breaking through the trade-off limitation between stability and sensitivity of the existing structure and meeting the dual requirements of reliability and high precision for tunnel surrounding rock stress monitoring. Summary of the Invention

[0005] The purpose of the present invention is to address the deficiencies in the above-mentioned prior art and provide a vibrating wire stress gauge and its application in tunnel surrounding rock stress monitoring, so as to solve the problem in the prior art that the steel wire is difficult to achieve both high sensitivity and high stability at the same time.

[0006] To achieve the above object, the technical solution adopted by the present invention is as follows: The present application provides a vibrating wire strain gauge comprising a sleeve, a steel bar, a fixing portion, a steel string, and an excitation sensing module. The ends of the steel bar are positioned within the sleeve, and the fixing portion and the steel string are axially disposed within the sleeve. The steel string is centrally located within the sleeve, with fixing portions disposed at both ends. The fixing portions are connected to the steel bar at the ends distal to the steel string. The excitation sensing module is disposed above the steel string and mounted on the exterior of the sleeve. The steel string has a twisted middle section and straight sections at both ends.

[0007] This application introduces a pre-set twisting structure in the middle of the steel chord, causing the chord to preferentially undergo a localized geometric response under axial stress. This is manifested as an increase in pitch or a decrease in amplitude in the twisted section, thereby causing fine-tuning of the geometry of that section. Unlike the straight, symmetrical chord structures commonly used in the prior art to achieve modal stability and linear response, this application actively introduces asymmetric, nonlinear geometric perturbations. This enhances the device's sensitivity to the initial stress evolution of the tunnel's surrounding rock without weakening the overall structural strength. Specifically, the geometric changes in the central twisted section under the influence of small stress perturbations affect the chord's natural frequency, increasing the slope of the chord's frequency change in low-stress sections and thereby amplifying its ability to sense weak stresses. This design helps enhance the device's ability to identify hidden deformation or potential signs of instability in the tunnel's surrounding rock. Furthermore, to ensure structural stability and measurement repeatability, the ends of the chord are retained as straight segments and connected to external steel bars via fixings, forming clear boundary constraints. This boundary structure not only effectively avoids modal drift and boundary interference, but also stabilizes the excitation response area, concentrating the vibration energy in the central twisted section, improving signal strength and recognition accuracy. Therefore, the structural solution proposed in this application not only improves the ability to identify small stress disturbances, but also effectively ensures the structural reliability and frequency response stability of the vibrating string device in complex environments. It is suitable for application scenarios such as tunnel surrounding rock, which require high long-term monitoring stability and early warning capabilities.

[0008] Furthermore, a rotational coupling structure is provided between the fixed part and the steel string, with the two ends of the rotational coupling structure connecting the fixed part and the steel string. The rotational coupling structure can realize the energy conversion from axial load to torsional deformation, improve the coordination of the torsion section release process and the controllability of the structural response. Furthermore, the rotational coupling structure includes a guide pin and a housing, with a guide slot disposed within the housing. The guide pin and the guide slot within the housing cooperate to guide the conversion of axial tension into rotational deformation, thereby improving the stability and response consistency of the twisting section release process.

[0009] Furthermore, the twisted section in the middle of the steel string is spiral or wavy. The spiral or wavy structure can generate a flexible response under axial loading, which helps to enhance the frequency sensitivity of the steel string to small stress changes.

[0010] Furthermore, the cross-section of the steel string is circular or elliptical, which facilitates uniform modal excitation and smooth stress distribution, and helps improve the stability and recognition accuracy of the frequency signal.

[0011] Furthermore, the diameter of the steel string is 0.8-1.6 mm. This range is set to enhance the overall structural strength of the steel string after the twisting section is set, which helps to improve the service stability and reliability of the device under long-term monitoring conditions.

[0012] Furthermore, the outer surface of the middle part of the steel string is provided with a plurality of micro-grooves spaced apart along the axial direction, which are used to control the local stress concentration area, thereby enhancing the excitation response amplitude and improving the recognition sensitivity of the vibration frequency.

[0013] Furthermore, the excitation and sensing module includes an excitation coil and an induction coil, which are used to drive the steel string vibration through electromagnetic excitation and sense the steel string's vibration frequency, respectively. This enables non-contact driving and sensing, reduces mechanical coupling interference, and improves frequency signal accuracy and the system's anti-interference capability.

[0014] Furthermore, the steel string has a non-uniform cross-section structure, with a smaller diameter in the middle than at both ends. This makes the middle the primary vibration and stress response area, concentrating modal energy and improving signal resolution and detection sensitivity.

[0015] Furthermore, the elastic modulus of the material at both ends of the steel string is high, and the elastic modulus of the material in the middle section is low, forming a rigid-flexible composite structure, which is conducive to the unification of boundary stability and middle deformation capacity, and enhances the comprehensive service performance of the device.

[0016] This application also proposes the application of a vibrating string stress gauge in tunnel surrounding rock stress monitoring. The vibrating string stress gauge is installed at the tunnel surrounding rock face, arch, side or fault zone area, and obtains the vibration frequency of the steel string through the excitation sensing module to reflect the stress change state inside the tunnel surrounding rock structure, realize dynamic monitoring of key stress-bearing parts, and help to identify local anomalies or potential instability risks at an early stage.

[0017] Furthermore, the vibrating wire strain gauge is drilled and embedded into the tunnel's surrounding rock, with the center of the steel wire in a free vibration state. This ensures that the stress transfer direction is consistent with the vibrating wire strain gauge and reduces boundary interference, improving the accuracy of the frequency response and the credibility of the monitoring data.

[0018] Compared with the prior art, the present invention has the following beneficial effects: (1) This application provides a preset twisting structure in the middle of the steel string, so that the steel string no longer undergoes direct linear elongation under axial tensile stress, but instead preferentially releases its geometric deformation. This nonlinear slow-release mechanism effectively expands the amplitude of the equivalent tension change caused by small stresses, thereby enhancing the response sensitivity of the vibration frequency to stress disturbances. This enables the thick steel string, which originally only has the advantage of structural strength, to have a high response capability to weak stress changes, breaking through the structural limitations of traditional strain gauges that are difficult to achieve both stability and sensitivity.

[0019] (2) In the design of the steel string structure, the present application retains the straight segments at both ends for rigid fixation and sets the middle portion as a free response area. Through clear boundary constraints and deformation guidance paths, the vibration modal distribution is optimized and the output purity and repeatability of the frequency signal are improved. Compared with the problems of strain distribution being easily affected by boundary interference and unstable vibration area in traditional structures, this structural combination helps to maintain a stable natural frequency response during long-term monitoring and enhance the reliability and consistency of the frequency signal. It is particularly suitable for tunnel surrounding rock stress monitoring projects with high requirements for continuity and reliability. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 A schematic structural diagram of a vibrating wire strain gauge provided by the present invention; Figure 2 A schematic diagram of the steel string structure of another vibrating wire strain gauge provided by the present invention; Figure 3 A schematic diagram of the steel string structure of another vibrating wire strain gauge provided by the present invention.

[0021] Icon: 1-sleeve; 2-steel bar; 3-fixed part; 4-steel string; 5-excitation sensing module; 6-rotational coupling structure; 41-twisted segment; 42-straight segment. DETAILED DESCRIPTION

[0022] In order to make the implementation process of the present invention clearer, it will be described in detail below with reference to the accompanying drawings.

[0023] Example 1: The present invention provides a vibrating wire strain gauge, such as Figure 1 As shown, the device comprises a sleeve 1, a steel bar 2, a fixing portion 3, a steel chord 4, and an excitation and sensing module 5. The ends of the steel bar 2 are positioned within the sleeve 1, within which the fixing portion 3 and the steel chord 4 are arranged axially. The steel chord 4 is centrally located within the sleeve 1, with fixing portions 3 provided at both ends. The end of the fixing portion 3, facing away from the steel chord 4, is connected to the steel bar 2. The excitation and sensing module 5 is positioned above the steel chord 4 and mounted on the exterior of the sleeve 1. The steel chord 4 has a twisted section 41 in the middle and straight sections 42 at both ends.

[0024] The sleeve 1 is an elongated tubular structure, preferably made of high-strength engineering plastic or stainless steel, offering excellent rigidity and corrosion resistance. An internal axial channel extends through it, accommodating the steel chord 4 and the fixing portion 3, providing physical isolation and installation guidance. A bonded structure forms between the inner wall of the sleeve 1 and the connection between the fixing portion 3 and the rebar 2, preventing axial slippage during stress transfer. A limited free vibration space is reserved for the steel chord 4, maintaining a non-contact relationship between the outer wall of the steel chord 4 and the inner wall of the sleeve 1 to ensure modal stability and frequency response during operation. The rebar 2 is a type of hot-rolled ribbed steel bar widely used in conventional construction projects, exhibiting excellent axial stiffness and anchoring properties, making it suitable for long-term service in underground structures. In this device, the rebar 2 serves as the primary transmission path for external stresses in the tunnel surrounding rock. Its ends are inserted into the sleeve 1 and rigidly connected to the fixing portion 3, effectively and stably transmitting external axial loads to both ends of the steel chord 4, establishing a clear input boundary condition. This structural design helps maintain the boundary constraint stability of the vibration mode and provides effective support for the nonlinear response and high-sensitivity vibration detection in the middle of the steel string 4. The fixing part 3 is a metal anchor symmetrically arranged at both ends of the steel string 4. One end is rigidly connected to the steel string 4, and the other end is reliably connected to the end of the steel bar 2. The fixing part 3 is connected to the inner wall of the sleeve 1 by a mechanical locking method, preferably by screwing or expanding ring limiting structure to achieve rigid fixation, thereby preventing slippage or loosening during stress transfer and ensuring structural stability and response reliability under axial stress. This structure not only ensures the rigidity of the boundary conditions at both ends of the steel string 4, but also provides effective constraints during force transfer, helping to maintain the stability of the vibration mode and the consistency of the frequency response, thereby enhancing the nonlinear amplification characteristics of the twisted section 41 in the middle of the steel string 4.

[0025] The steel string 4 is axially arranged at the center of the sleeve 1. Its center is defined by a pre-set twisted segment 41, connected by straight segments 42 at both ends. The straight segments 42 are constrained and connected by a fixed portion 3, ensuring a clear force path and stable boundary conditions for the steel string 4 when subjected to stress. The twisted segment 41 can be designed in a helical or wavy configuration. Different geometric forms guide the steel string 4 to produce differentiated local deformation behavior under axial stress, thereby achieving a nonlinear conversion of stress into a frequency signal. Specifically, the helical structure absorbs axial deformation through pitch variations, offering excellent strain tolerance and frequency regulation linearity, making it suitable for monitoring continuous, slowly varying loads. The wavy structure, on the other hand, flexibly releases the axial deformation through amplitude variations, providing faster transient response and suitable for identifying early-stage abrupt changes. Both configurations enhance the steel string 4's ability to detect small disturbances and enhance the overall device's dynamic recognition capabilities in low-stress regimes. The twisted segment 41 can be formed during the string manufacturing process through precision winding, heat setting, or shape memory treatment, resulting in a releasable, pre-set deformation. High-strength spring steel or martensitic stainless steel is preferred for steel string 4, as it offers excellent rigidity, tensile strength, and fatigue life, making it suitable for high-frequency monitoring under long-term tunnel service conditions. In conditions subject to complex loads or repeated deformation, nickel-titanium alloys with shape memory and flexible deformation capabilities can also be used to enhance the response stability and structural resilience of steel string 4 during repeated straightening and rebounding.

[0026] In this embodiment, the steel string 4 is constructed from a metal wire with a circular or elliptical cross-section, exhibiting excellent structural symmetry and uniform stress distribution. This facilitates the formation of stable vibration modes and reduces the impact of boundary perturbations on frequency response. Its diameter is set within the range of 0.8-1.6 mm, with a relatively coarse gauge to ensure sufficient tensile strength and fatigue resistance under long-term service conditions in the tunnel surrounding rock. This also provides a stable mechanical foundation for the nonlinear twisting structure in the center. Preferably, the twisted section 41 in the center of the steel string 4 is helical, introducing a controllable geometric nonlinear response region. The length of this twisted section 41 accounts for 50%-70% of the total length of the steel string 4, concentrating deformation release at a location that does not affect boundary fixation. To achieve good deformation response and structural balance, the pitch is set to 1 / 4-1 / 2 of the length of the twisted section 41, resulting in 3-6 evenly distributed helical segments. The rotationally symmetrical arrangement of the structure helps reduce stress eccentricity and submodal interference. The outer diameter of the helix is ​​controlled to be between 1%-5% of the total length of the steel string 4 to avoid ineffective geometric perturbations that are too small, and to prevent frequency distortion caused by excessive length. When the steel string 4 is subjected to axial tension, the twisted section 41 structure can preferentially undergo geometric changes such as slow expansion or amplitude reduction, thereby producing a nonlinear tension enhancement effect, thereby amplifying the frequency deviation under weak stress disturbance and effectively improving the sensitivity of the frequency response to micro-stress changes.

[0027] A rotational coupling structure 6 is also provided between the fixed portion 3 and the end of the steel string 4, thereby achieving energy conversion from axial load to torsional deformation, improving the coordination of the release process of the twisted segment 41 and the controllability of the structural response. The rotational coupling structure 6 includes a guide pin and a housing. The housing is provided with a guide groove. The guide pin cooperates with the guide groove in the housing to achieve guided conversion of axial tensile force into rotational deformation. The guide pin connects to the straight segment 42 at the end of the steel string 4. The housing is securely mounted in the inner cavity of the fixed portion 3. The housing is provided with a spiral guide groove. The guide pin on the end of the steel string 4 slides along this guide groove. The groove surface of the guide groove adopts a variable pitch structure, which enables the guide pin to produce a rotational response with adjustable angular velocity during the sliding process. The rotation direction is consistent with the spiral direction of the guide groove. The overall rotation angle can be effectively controlled according to the groove line design, thereby forming an adjustable path. The pitch of the guide groove gradually decreases along the guide pin's direction of motion, ensuring smooth rotation during the initial sliding phase, preventing false triggering or inoperative responses. Subsequent, slight axial tension allows the guide pin to respond more quickly, converting into rotational displacement. This amplifies the release amplitude of the mid-section spiral structure and improves the sensitivity of the vibration response to minor stress disturbances. When a slight axial stress change occurs in the external structure (such as the tunnel surrounding rock), the stress is transmitted through the steel bar 2 to the fixed portion 3, which then acts on the rotational coupling structure 6. As the guide pin slides along the spiral groove under load, the ends of the steel string 4 undergo rotational displacement, causing the central, helically twisted section 41 to slowly expand or deepen. During this process, the steel string 4 does not elongate overall, but instead responds primarily through its geometric structure, nonlinearly increasing the tension increase in the midsection. This ultimately makes the natural vibration frequency more sensitive to minor stress changes. This structural design effectively amplifies frequency shifts caused by subtle perturbations, improving the strain gauge's ability to detect initial deformation and latent instability signals.

[0028] To prevent excessive deformation of the rotating coupling structure 6 during long-term service or under sudden loads, which could cause structural instability or response distortion in the steel string 4, a limiting structure is also installed within the housing of the rotating coupling structure 6. This limiting structure includes limit stops at each end of the guide groove and a damping element at the end of the guide pin. The limit stops, which can take the form of metal bosses, elastic retaining rings, or embedded ceramic blocks, are located at the beginning and end of the spiral guide groove to limit the maximum sliding travel of the guide pin, thereby limiting the range of rotational angles that can be generated at the end of the steel string 4. The damping element absorbs some of the kinetic energy when the guide pin approaches the limit stops, increasing the buffering force and preventing the guide pin from impacting the inner wall of the housing, thereby avoiding local damage or noise interference. This limiting structure not only prevents structural overload during the torsional response process but also helps control the dynamic rotation rhythm of the torsional segment 41, making the vibration process of the steel string 4 more controllable and repeatable, thereby ensuring the long-term stability and measurement accuracy of the strain gauge under complex load conditions. Friction plates or magnetic resistance devices can also be installed in the rotating contact area of ​​the guide pin or housing to adjust the rotation release speed and response angle during the guide pin sliding process. By constructing a stable energy dissipation path, the structural impact and response fluctuations caused by excessive rotation can be suppressed, and the response consistency and dynamic adaptability of the system under different stress change rates can be enhanced.

[0029] In this embodiment, a plurality of axially spaced microgrooves are provided on the outer surface of the central portion of the steel string 4 to regulate the local stress field distribution and vibration response characteristics in this area. The microgrooves can be created by laser etching or mechanical engraving, with common forms including spiral and linear grooves. The groove dimensions range from 10-50 μm in width and 5-20 μm in depth, with the groove spacing optimized based on the target frequency range. The microgrooves are distributed throughout the axial central region of the steel string 4 and range in length from 10-30 mm. The introduction of the microgrooves creates periodic perturbation structures on the surface of the steel string 4. These microstructures induce local stress concentrations under load, altering the equivalent stiffness distribution in this region and making the steel string 4 more susceptible to local vibration response under external excitation. Compared to smooth structures, the grooved region is more susceptible to strain perturbations under axial stress, thereby improving the coupling efficiency of the steel string 4 to electromagnetic or mechanical excitation. The stress concentration effect induced by the micro-grooving enhances the absorption and conversion efficiency of the excitation energy in this region, thereby increasing the amplitude and stability of the initial oscillation response, facilitating the rapid establishment and accurate identification of the frequency signal. Furthermore, the groove structure, in conjunction with the pre-defined twisted section 41 in the middle of the steel string 4, further enhances the nonlinear mechanical response of the central region, allowing the steel string 4 to produce a measurable frequency shift under the influence of tiny stress perturbations. This improves the strain gauge's sensitivity to early subtle strain evolution and provides a structural foundation for microstress monitoring under complex operating conditions.

[0030] The excitation and sensing module 5 is mounted on a pre-set mounting platform outside the sleeve 1. Its structural position corresponds to the area above the free vibration section of the steel string 4. It is rigidly connected to the sleeve 1 shell via a positioning bracket, snap-on, or clamping structure, ensuring good structural stability and spatial positioning accuracy during service. The excitation and sensing module 5 is an integrated package structure, integrating an excitation coil and an induction coil, which are used to excite the vibration of the steel string 4 and collect vibration response signals, respectively. The two coils are symmetrically mounted above the vibration section of the steel string 4 and maintain a fixed non-contact distance with the steel string 4, thereby achieving stable electromagnetic coupling without interfering with its free vibration state. The internal wires of the coils are arranged through a unified lead-out channel and are covered with shell partitions, cable troughs, or casing to ensure electromagnetic stability and signal transmission integrity in long-term service environments.

[0031] When the excitation sensing module 5 is operating, the excitation coil is connected to an external drive circuit, which periodically inputs pulsed current, generating an alternating magnetic field that excites the steel string 4 into micro-vibrations. The natural vibration frequency of the steel string 4 under axial tension is received by the induction coil and converted into an electrical signal for output. This signal is then connected to the external data acquisition and signal processing module via a shielded lead. Because the natural frequency of the steel string 4 changes with tension, minute stress perturbations in the tunnel's surrounding rock are transmitted to the steel string 4 via the rebar 2 and the boundary connection structure, causing a slight shift in its vibration frequency. The frequency changes detected by the induction coil, combined with the calibrated relationship between frequency and stress, can be used to infer and identify the stress changes in the tunnel's surrounding rock, enabling high-sensitivity, high-resolution, long-term structural health monitoring.

[0032] In this embodiment, Figure 2As shown, the steel string 4 can also be configured as a hollow structure, with its inner cavity filled with a flexible material. This hollow structure reduces the mass and overall stiffness of the steel string 4, allowing it to generate greater axial strain under the same tensile stress, resulting in a more significant frequency shift and improved response sensitivity to small stress perturbations. The flexible material has viscoelastic properties, providing local buffering and energy coupling during vibration, helping to enhance the excitation response amplitude and improve the recognition of weak signals. The steel string 4 preferably has a circular cross-section with an outer diameter of 0.8-1.6 mm and an inner cavity diameter of 30%-50% of the outer diameter, extending axially along the steel string 4. The flexible material can be a low-modulus, high-energy polymer such as silicone, polyurethane, or thermoplastic elastomer (TPE). These materials possess a moderate elastic modulus and dimensional stability, are capable of absorbing local excitation energy, and suppress high-order modal responses. The hollow steel string 4 can be manufactured by rolling a metal tube or cold-working a hollow wire. Filling with the flexible material can be achieved by low-pressure injection, vacuum infusion, or liquid injection followed by heat curing. After filling, the steel string 4 is sealed at both ends. This sealing structure can be achieved using mechanical capping, epoxy resin sealing, or welding to prevent leakage or performance degradation of the flexible material during long-term service. This structure forms a stable internal energy dissipation path under vibration excitation, which not only helps to reduce high-frequency interference caused by boundary reflections or installation errors, but also optimizes the mass distribution and vibration modes of the steel string 4, improving the main modal stability and frequency response consistency. This makes it suitable for long-term monitoring scenarios in tunnels with significant surrounding rock disturbances and strong ambient vibration.

[0033] In actual operation, when the tunnel surrounding rock generates minute axial tensile stress due to geological deformation or support structure instability, this stress first acts on the exposed rebar 2 and is transmitted longitudinally along the rebar 2 to the fixing portion 3 inside the sleeve 1. The fixing portion 3 acts as a rigid force transmission node, stably introducing the external stress into the internal structure and then transmitting it to the connected rotary coupling structure 6. Under the action of axial tension, the guide pins within the rotary coupling structure 6 slide along the guide groove, guiding the ends of the steel string 4 to undergo subtle rotational displacement. This in turn drives the twisted segment 41 in the middle of the steel string 4 to undergo geometric expansion or morphological changes, resulting in a primary nonlinear stress release. At this point, the steel string 4 as a whole does not experience significant elongation; only the central twisted structure responds flexibly, generating a corresponding tension increment, thereby enabling sensitive frequency perturbation sensing under low-stress conditions. If the external stress continues or exceeds the release capacity of the twisted segment 41, the steel string 4 as a whole gradually enters a state of axial tension, generating a linear tension increment, transitioning the frequency response from the nonlinear to the linear range. This expands the measurement range and ensures the continuity and discernibility of frequency changes. As the tension of steel string 4 changes, its natural frequency drifts slightly. The excitation coil in the excitation sensing module 5, located directly above the steel string 4, periodically applies a pulsed magnetic field, causing the steel string 4 to vibrate slightly. The induction coil synchronously senses the frequency response and outputs an electrical signal, which is transmitted to the back-end data acquisition system for processing and recording. The system compares the offset between the current vibration frequency and the initial calibration frequency and, based on a preset mapping between frequency and stress, calculates the corresponding axial tensile stress, thereby quantitatively determining the stress changes in the tunnel surrounding rock.

[0034] In summary, this embodiment, by incorporating a controllable geometrically deformable twisting structure in the middle of the steel chord 4 and cooperating with a rotational coupling structure 6, achieves nonlinear tension response and frequency perturbation amplification under minimal axial stress, thereby enhancing the device's sensitivity to subtle stress changes. This structure effectively overcomes the trade-off between sensitivity and operational stability in traditional structures, making it suitable for high-resolution stress monitoring and hazard identification in complex working conditions, such as tunnel surrounding rock.

[0035] Example 2: Based on the basic structure described in Example 1, this embodiment performs differentiated optimization design on the geometric configuration of the steel string 4, such as Figure 3Specifically, the steel string 4 is designed with a variable cross-section structure that is unevenly distributed along the axial direction, with the central cross-section diameter being smaller than the cross-section diameters at the ends, to enhance the vibration response and frequency modulation amplitude of the central region. The uneven configuration of the steel string 4 is preferably a symmetrical, tapered arrangement, transitioning from a thick, straight segment 42 at each end to a thinner, twisted segment 41 in the middle, which then converges symmetrically to the center. The minimum diameter in the middle is preferably 0.6-1.0 mm, and the diameters of the straight segments 42 at the ends are 1.2-1.5 mm. The straight segments 42 at both ends converge continuously to the central twisted segment 41 through a transition segment 15%-25% of the total length of the steel string. The cross-sectional curvature radius of the transition segment is no less than 2 mm, and the profile is rounded to smoothly transition to avoid stress concentration, thereby ensuring the fatigue life of the steel string 4 under cyclic loading. This design not only facilitates the convergence of tension in the central region but also suppresses the dissipation of boundary modal energy, thereby improving the response sensitivity and modal stability of the central section of the steel string 4.

[0036] The structural steel chord 4 can be manufactured by integral turning or cold drawing, achieving the desired contour shape by controlling the outer diameter variation curve. Alternatively, two sections of different-diameter metal can be welded and then heat-treated to achieve a rigid connection at the transition section. The thick straight segments 42 at both ends of the chord 4 are plugged into and matched with the rotational coupling structure 6. The thin twisted segment 41 in the middle is located in a non-contact area directly below the excitation sensing module 5, ensuring the same free vibration space as in Example 1. During stress loading, the smaller cross-section in the middle produces a local strain amplification effect under the same tension, increasing the natural frequency offset of the twisted segment 41. This helps convert low-amplitude stress signals into high-amplitude frequency responses, improving the ability to perceive weak disturbances. The thick straight segments 42 at both ends enhance boundary constraint stiffness, suppressing the dissipation of vibration energy toward the support ends. The symmetrically tapered transition section maintains the axial tension gradient distribution, prompting vibration modes to converge toward the center, thereby converting weak tunnel surrounding rock stress changes into frequency response signals.

[0037] This non-uniform cross-section design is particularly suitable for long-term monitoring of tunnel surrounding rocks where microstress monitoring is required, environmental noise is strong, or modal control accuracy is required.

[0038] Example 3: Building on the basic structure described in Example 1, the steel string 4 in this embodiment is configured as a zoned elastic modulus composite structure. Specifically, the material at the ends of the steel string 4 has a high elastic modulus, while the material in the middle has a low elastic modulus. This dissimilar material connection creates a rigid-flexible structure, which simultaneously achieves boundary rigidity constraint and central sensitive response. The straight segments 42 at the ends of the steel string 4 are preferably made of a metal material with an elastic modulus in the range of 150-200 GPa, such as high-strength spring steel or martensitic stainless steel, which exhibits strong shape retention and stress relaxation resistance. The central twisted segment 41 is made of a flexible metal material with an elastic modulus in the range of 60-90 GPa, such as copper-zinc alloy, nickel-titanium alloy, or other flexible alloys, to enhance the strain amplitude and deformation reversibility of the central section under low stress conditions. In this structure, the rigid segments at the ends of the steel string 4 act as boundary constraints, maintaining modal stability under excitation. The flexible segment in the middle deforms under tensile stress, causing a change in the overall tension of the steel string 4, thereby inducing a measurable natural frequency shift.

[0039] The above-mentioned different material segments can be connected by means of crimping, welding or limiting glue. The connection area can be covered with a metal sleeve and a radial pressing force can be applied to form a stable shell crimping structure, which is suitable for high-strength connection of segments of the same or similar materials; butt welding can also be used to achieve integrated connection of segments of different materials through precision welding technology, and combined with subsequent heat treatment to improve the structural consistency and mechanical properties of the weld area; for different diameters or nested structures, a preliminary connection can be formed by interference fitting between the two segments, supplemented by limiting glue filling or mechanical locking ring reinforcement sealing to ensure the anti-loosening ability and environmental adaptability of the connection position. In order to ensure effective axial force transmission between different modulus segments in the connection area, the length of the connection segment is preferably 3-5mm, and local stress concentration should be avoided as much as possible. The structural steel chord 4 can improve the response sensitivity of the vibration frequency to low-amplitude stress disturbances while maintaining structural strength and service stability.

[0040] This partitioned elastic modulus combination structure is suitable for tunnel engineering environments with non-uniform deformation of the tunnel surrounding rock, significant long-term temperature fluctuations, or frequent structural stress fluctuations. It can improve the response resolution and frequency consistency of the vibrating wire stress gauge under dynamic loads.

[0041] Example 4: The present invention provides an application for a vibrating-wire stress gauge in tunnel surrounding rock stress monitoring, enabling dynamic monitoring of minute stress disturbances in the rock mass. The vibrating-wire stress gauge is installed in key stress-bearing areas of the tunnel surrounding rock, such as the tunnel face, vault, side, or fault zone. The gauge uses an excitation sensing module 5 to obtain the vibration frequency of the steel string 4, reflecting the stress changes within the tunnel surrounding rock structure. Preferably, the vibrating-wire stress gauge is structurally coupled to the tunnel surrounding rock using a drilled embedded method, with the middle portion of the steel string 4 in a free vibration state, ensuring modal stability and frequency response accuracy.

[0042] The placement of vibrating-wire stress gauges can be flexibly adjusted based on the geological conditions of the tunnel's surrounding rock and the monitoring objectives. While suitable for local single-point deployment, they also support array-based multi-point deployment to improve spatial resolution and data integrity. Priority should be given to locations where stress concentration in the tunnel's surrounding rock or significant deformation risk exists. Typical locations include the tunnel face, vault, side walls, and near fault fracture zones. These areas are significantly affected by tunnel excavation, and existing ground stresses are prone to redistribution. Localized areas are more susceptible to crack expansion, structural loosening, or abnormal deformation, necessitating focused monitoring of their stress evolution.

[0043] Select monitoring points on the surface of the tunnel surrounding rock and drill positioning holes in the predetermined direction for installation. The positioning hole diameter ranges from 45-60mm, and the hole depth is slightly larger than the vibrating wire stress gauge body. The hole axis should be consistent with the principal stress direction of the tunnel surrounding rock, with a deviation of no more than 10° to ensure that the stress is effectively transmitted to the middle response section of the steel string 4. A high-strength base or rigid gasket can be laid at the bottom of the hole to form a pressure-bearing support surface to enhance boundary stability. When inserting the vibrating wire stress gauge, the operation should be slow to ensure that the axis of the steel string 4 is aligned with the hole axis to prevent the torsion section 41 from affecting the preset modal response due to interference deformation. The two ends of the vibrating wire stress gauge are fixed to the inner side of the hole wall by bolts, locking rings and other structures. The clamping position must be firm and reliable to prevent loosening later and causing frequency fluctuations. The signal lead of the vibrating wire stress gauge is led out to the data acquisition terminal through the insulating sleeve to form a complete electrical path. After installation, the borehole is backfilled with a low-expansion, high-adhesion cementitious grouting material. During this operation, the grouting speed and pressure should be controlled to ensure that the grout does not enter the free section of steel string 4 to prevent interference with frequency identification or modal drift. After the grouting has fully cured (24-48 hours), the first excitation test is performed to obtain the initial vibration frequency of steel string 4, which will serve as the baseline for subsequent data.

[0044] During operation, the excitation coil excites the steel string 4 with a periodic pulsed magnetic field, while the induction coil collects the vibration frequency and outputs it as an electrical signal. Tension fluctuations caused by minute stress changes are converted into slight frequency drifts. The system performs real-time inversion based on a preset frequency-stress calibration curve, quantifying the current stress state of the tunnel surrounding rock and dynamically tracking its changing trends. To enhance long-term monitoring stability, a temperature sensor can be installed near the strain gauge to correct for thermally induced frequency drift and improve data reliability.

[0045] In summary, the application approach described in this embodiment is applicable to vibrating-wire stress gauges with different structural characteristics in Examples 1-3, enabling high-precision dynamic monitoring of small stress disturbances without damaging the tunnel's surrounding rock structure. Depending on the tunnel's surrounding rock type, stress distribution characteristics, and environmental interference conditions, the corresponding vibrating-wire stress gauge structure can be flexibly selected, improving the system's adaptability to complex operating conditions and data reliability, thus offering excellent engineering feasibility and field application value.

[0046] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

Claims

1. A vibrating wire strain gauge, comprising a sleeve, a steel bar, a fixing portion, a steel chord, and an excitation sensing module, wherein the end of the steel bar is located within the sleeve, the fixing portion and the steel chord are axially arranged within the sleeve, the steel chord is centrally located within the sleeve, the fixing portions are provided at both ends of the steel chord, the fixing portion is connected to the steel bar at one end away from the steel chord, the excitation sensing module is disposed above the steel chord and mounted on the outside of the sleeve, and is characterized in that: The middle portion of the steel string is a twisted segment, and both ends of the steel string are straight segments.

2. The vibrating wire strain gauge according to claim 1, wherein: A rotation coupling structure is further provided between the fixing portion and the steel string, and two ends of the rotation coupling structure connect the fixing portion and the steel string.

3. The vibrating wire strain gauge according to claim 2, wherein: The rotary coupling structure includes a guide pin and a shell, and a guide groove is provided inside the shell.

4. The vibrating wire strain gauge according to claim 3, wherein: The twisted section in the middle of the steel string is spiral or wavy.

5. The vibrating wire strain gauge according to claim 4, wherein: The cross section of the steel string is circular or elliptical.

6. The vibrating wire strain gauge according to claim 5, wherein: The diameter of the steel string is 0.8-1.6 mm.

7. The vibrating wire strain gauge according to claim 6, wherein: The outer surface of the middle portion of the steel string is provided with a plurality of micro grooves distributed at intervals along the axial direction.

8. The vibrating wire strain gauge according to claim 7, wherein: The excitation sensing module includes an excitation coil and an induction coil, which are respectively used to drive the steel string to vibrate through electromagnetic excitation and to sense the vibration frequency of the steel string.

9. The vibrating wire strain gauge according to claim 8, wherein: The steel chord has a non-uniform cross-section structure, and the diameter of the middle portion is smaller than the diameters of the two ends.

10. The vibrating wire strain gauge according to claim 9, wherein: The elastic modulus of the materials at both ends of the steel string is high, and the elastic modulus of the material in the middle portion is low.

11. An application of a vibrating wire stress gauge in monitoring tunnel surrounding rock stress, characterized by: The vibrating wire stress gauge is installed on the tunnel face, vault, side or fault zone of the tunnel surrounding rock, and the vibration frequency of the steel string is obtained through the excitation sensing module to reflect the stress change state inside the tunnel surrounding rock structure.

12. Application of the vibrating wire stress gauge according to claim 11 in monitoring stress in tunnel surrounding rock, characterized in that: The vibrating wire stress gauge is coupled to the tunnel surrounding rock in a drilled embedding manner, and the middle portion of the steel wire is in a free vibration state.

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