Nondestructive testing method for anchoring performance of prestressed steel strand

Abstract

The application provides a prestressed steel strand anchoring performance nondestructive detection method, and belongs to the technical field of steel strand performance detection, and comprises the following steps: applying a pre-tension force to a steel strand to be detected, unloading to zero after load preservation, so as to eliminate initial disturbance; applying a preset axial tensile load to the steel strand to be detected, the axial tensile load enabling the steel strand to be in an elastic stress state; under the condition of maintaining the axial tensile load, measuring a torsion angle generated by the steel strand within a gauge length; according to the torsion angle, determining a characteristic parameter representing a tensile-torsion coupling characteristic of the steel strand; comparing the characteristic parameter with a preset qualified judgment threshold value, and judging whether the anchoring performance of the steel strand to be detected is qualified based on a comparison result. The prestressed steel strand anchoring performance nondestructive detection method provided by the application realizes nondestructive detection and root-by-root screening of the steel strand.

Description

Technical Field

[0001] This invention belongs to the field of steel strand performance testing technology, and more specifically, it relates to a non-destructive testing method for the anchorage performance of prestressed steel strands. Background Technology

[0002] In prestressed concrete structures, prestressed steel strands rely on anchorages for reliable anchorage, and their anchorage performance is typically measured by the anchorage efficiency coefficient. In existing technologies, the anchorage efficiency coefficient must be evaluated through a static load anchorage test. This method involves threading the steel strand through the matching anchorage and stretching it until failure, which is a destructive testing method. Once the sample is tested, it becomes invalid and cannot be reused. Furthermore, the test results only represent the quality level of the submitted batch of products, failing to achieve comprehensive quality screening of each strand before it leaves the factory. For major projects such as nuclear power plants and cross-sea bridges that require comprehensive quality verification, this sampling inspection method presents a significant blind spot in safety supervision. Summary of the Invention

[0003] The purpose of this invention is to provide a non-destructive testing method for the anchorage performance of prestressed steel strands, so as to solve the technical problem that existing destructive testing methods cannot achieve strand-by-strand screening.

[0004] To achieve the above objectives, the technical solution adopted by the present invention is: to provide a non-destructive testing method for the anchorage performance of prestressed steel strands, comprising the following steps: A pre-tension force is applied to the steel strand to be tested, and then unloaded to zero after holding the load to eliminate the initial disturbance. A preset axial tensile load is applied to the steel strand to be tested, so that the steel strand is in an elastic stress state; While maintaining the axial tensile load, measure the angle of torsion of the steel strand within the gauge length; Based on the stated torsion angle, characteristic parameters characterizing the tensile-torsional coupling properties of the steel strand are determined; The characteristic parameters are compared with a preset pass / fail threshold, and the anchoring performance of the steel strand under test is determined based on the comparison result.

[0005] In one possible implementation, the magnitude of the pre-tension force is 80% to 90% of the axial tensile load, and the holding time is 5 to 30 seconds.

[0006] In one possible implementation, the magnitude of the axial tensile load does not exceed one-tenth of the nominal breaking force of the steel strand under test.

[0007] In one possible implementation, the measurement of the torsion angle generated by the steel strand within the gauge length is performed using a non-contact optical measuring device. The measurement of the torsion angle generated by the steel strand within the gauge length includes: setting optical markers at both ends of the gauge length on the surface of the steel strand, and calculating the torsion angle by measuring the circumferential displacement change of the two markers before and after stretching, combined with the radius of the steel strand.

[0008] In one possible implementation, the characteristic parameter is the torsion angle under unit tensile force.

[0009] In one possible implementation, determining whether the anchoring performance of the steel strand under test is qualified further includes torsional relaxation behavior detection, which includes the following steps: While keeping the axial tensile load constant, the relaxation curve of the torsion angle as a function of time is continuously acquired; The relaxation feature is extracted from the relaxation curve, and the relaxation feature includes the relaxation time constant or the relaxation amplitude within a preset time. The relaxation characteristic quantity is compared with a preset relaxation threshold. If it deviates from the normal range, the anchoring performance is determined to be unqualified.

[0010] In one possible implementation, determining whether the anchoring performance of the steel strand under test is qualified further includes multi-section torsional uniformity testing, which includes the following steps: Multiple sections were selected along the axial direction of the steel strand to be tested, and the local torsional angle of each section under the axial tensile load was measured. Calculate the dispersion index of the local torsional angle of each cross section; When the dispersion index exceeds the preset limit, the anchoring performance is deemed unqualified.

[0011] In one possible implementation, the preset pass / fail threshold is determined through the following steps: Multiple steel strands of the same specifications and manufacturing process as the steel strand to be tested were selected as calibration samples. For each calibration sample, the following processes are performed in sequence: applying pre-tension force and unloading, applying axial tensile load, measuring torsion angle, and determining characteristic parameters. The values ​​of its calibration characteristic parameters are then recorded. Destructive static load anchorage tests were conducted on each calibrated sample after the aforementioned treatment, and its anchorage efficiency coefficient was measured. Based on the correspondence between the calibration characteristic parameter values ​​of each calibration sample and the anchoring efficiency coefficient, the distribution range of characteristic parameter values ​​corresponding to the calibration samples whose anchoring efficiency coefficient reaches the preset qualified index is determined as the qualified judgment threshold.

[0012] In one possible implementation, the number of calibration samples is no less than 30.

[0013] In one possible implementation, the preset qualification index for the anchoring efficiency coefficient is η≥95%.

[0014] The beneficial effects of the non-destructive testing method for the anchorage performance of prestressed steel strands provided by this invention are as follows: Compared with the prior art, the method of this invention first eliminates the random initial bending and uneven contact formed during the winding and transportation of the steel strand by pre-tensioning and unloading, establishing a standardized initial mechanical state for subsequent measurements. Then, a small axial load, strictly limited to the elastic range and not exceeding one-tenth of the breaking force, is applied to ensure that the steel strand is always in a non-destructive, purely elastic stage. Under this condition, the torsion angle within the gauge length is measured, and the tension-torsion coupling characteristic parameters are obtained. Since these characteristic parameters are directly derived from the mechanical response of the internal spiral twisting structure of the steel wires, they can sensitively characterize the inter-wire contact state and geometric regularity that determine the anchorage self-locking ability. Therefore, comparing them with the pre-calibrated qualified threshold can accurately determine the anchorage performance. The entire testing process does not cause any plastic deformation or performance damage to the steel strand, fundamentally realizing non-destructive screening of each product leaving the factory. This overcomes the shortcomings of traditional destructive anchorage tests, which can only perform random checks and have blind spots in safety supervision. At the same time, because the measurement state is stable and the calculation is simple, it can be seamlessly integrated into the production line to achieve rapid online judgment. Attached Figure Description

[0015] To more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0016] Figure 1 This is a flowchart illustrating a non-destructive testing method for the anchorage performance of prestressed steel strands, provided in an embodiment of the present invention. Detailed Implementation

[0017] To make the technical problems to be solved, the technical solutions, and the beneficial effects of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the present invention and are not intended to limit the present invention.

[0018] Prestressed steel strands are composed of multiple high-strength steel wires twisted together in a helical structure. Their load-bearing and anchoring mechanical properties rely on the frictional self-locking effect between the wires and between the strand and the anchor wedges. Steel strands with excellent anchoring performance exhibit stable helical geometry and uniform inter-wire contact stress, displaying regular torsional-tensile coupling behavior under axial tension. Conversely, if defects such as uneven twist pitch, localized looseness, or abnormal wire surface conditions exist, their torsional response will deviate from the normal range, and their frictional self-locking ability during anchoring will decrease. Extensive testing has confirmed a stable positive correlation between the tensile-torsional coupling characteristic parameters measured within the elastic range of the steel strand and the anchoring efficiency coefficient obtained from destructive testing. Based on this, this invention proposes first establishing a mapping relationship and acceptable threshold between characteristic parameters and the anchoring efficiency coefficient through a calibration stage. Then, during the testing stage, a small elastic tensile load is applied to the steel strand under test. By measuring its torsional response and extracting characteristic parameters, and finally comparing them with the acceptable threshold, a non-destructive, online, strand-by-strand determination of anchoring performance is achieved.

[0019] It needs further clarification that the physical essence of the aforementioned frictional self-locking effect originates from the balance between the radial compressive force and the axial frictional force between the wedge-shaped clamp and the surface of the steel strand. When the steel strand is subjected to axial tension, its cross-section undergoes elastic contraction, driving the clamp to slide deeper into the cone hole of the anchor plate, thereby generating a normal pressure at the contact interface. The product of this normal pressure and the coefficient of friction is the anchoring force resisting slippage. The helical twisting structure of the steel wire ensures that multiple steel wires can deform collaboratively, converting axial tension into normal contact stress between the steel wires. If there is local unevenness in the twisting pitch, the fluctuation of the helix angle will lead to inconsistent radial force components among the steel wires in the same cross-section, resulting in insufficient contact pressure for some steel wires. During the anchoring loading process, micro-slippage occurs first, which macroscopically manifests as a decrease in the anchoring efficiency coefficient. Therefore, measuring the torsional response of the steel strand under elastic tension is essentially an indirect detection of the geometric integrity and stress distribution uniformity of its internal helical structure, which has a clear physical basis.

[0020] Please see Figure 1 The present invention provides a non-destructive testing method for the anchorage performance of prestressed steel strands. The non-destructive testing method for the anchorage performance of prestressed steel strands includes the following steps: Step 1: Apply a pre-tension force to the steel strand to be tested, and then unload it to zero after holding the load to eliminate the initial disturbance.

[0021] The purpose of this step is to eliminate random initial bending, uneven surface residual stress, and unstable contact between wires that occur during the winding, transportation, and unwinding of the steel strand. This provides standardized and repeatable initial mechanical conditions for subsequent measurements, thereby ensuring the accuracy and repeatability of the torsion angle measurement. The pre-tension force is preferably set to 80%–90% of the axial tensile load used in subsequent formal measurements, with a holding time of 5–30 seconds. This magnitude is sufficient to fully realign the wire contact state while remaining far below the material's yield strength, thus avoiding any damage.

[0022] In practice, the application rate of the pre-tensioning force also needs to be controlled. A uniform loading rate of 0.2 times the nominal breaking force per minute is preferred to avoid localized indentations or micro-plastic deformation on the wire surface caused by impact loads. During the holding period, the edges of the wires inside the strand, which were initially raised due to winding bending, elastically approach and re-adhere under high stress, gradually homogenizing the previously uneven residual stress along the axial direction. The unloading process should maintain the same rate as loading to prevent reverse residual torsion due to elastic aftereffects. For steel strands that have undergone the above standardized pre-treatment, the initial torsion angle zero drift can be controlled within 0.05 degrees within the subsequent 30 minutes, meeting the requirements for high-precision measurement. For steel strands that have undergone pre-tensioning treatment, if a formal test is not performed within 30 minutes, step one must be repeated to ensure the reproducibility of the initial state.

[0023] Step 2: Apply a preset axial tensile load F to the steel strand to be tested after the treatment in Step 1.

[0024] The tensile load F should not exceed one-tenth of the nominal breaking force of the steel strand under test, ensuring that the steel strand remains within the range of pure elastic deformation throughout the entire testing process. After unloading, it can completely return to its original shape without any plastic deformation or permanent damage. This is the fundamental guarantee for achieving "non-destructive" testing. For example, for a steel strand with a nominal breaking force of 260kN, the measuring load should not exceed 26kN.

[0025] It is worth noting that the lower limit of the tensile load F is not necessarily better the smaller it is. If the load is too small, the amplitude of the torsion angle signal will approach the noise level of the measurement system, reducing the signal-to-noise ratio and affecting the repeatability accuracy of the characteristic parameters. Therefore, the preferred range for F is 5% to 10% of the nominal breaking force of the steel strand under test. Within this range, the axial strain of the steel strand is approximately 0.1‰ to 0.2‰, which is still far below its yield strain (usually greater than 7‰), and the material behavior strictly follows Hooke's Law, showing a good linear relationship between the torsion angle and the tensile load. Furthermore, for steel strands of different specifications, the value of F can be adjusted proportionally according to their nominal cross-sectional area to maintain the same stress level, thereby ensuring the consistency of the testing standards.

[0026] Step 3: While keeping the axial tensile load F constant, measure the torsion angle φ generated by the steel strand within the gauge length L.

[0027] The gauge length L is the axial distance between two pre-set measurement markers on the surface of the steel strand. To ensure that the measured torsion angle can fully reflect the torsional deformation of the wire within one helical cycle and to avoid boundary effects, the gauge length L is preferably set to an integer multiple of the nominal lay length of the steel strand, such as 1 or 2 times the lay length. When the gauge length includes the complete lay length, the measurement results have the lowest sensitivity to the starting position of the markers and the best data stability.

[0028] In this step, the measurement of the torsion angle φ is preferably performed using a non-contact optical method. The specific operation steps are as follows: set an optical mark at each end of the gauge length on the surface of the steel strand, and use an optical measuring device such as a high-speed camera or laser interferometer to record the circumferential position change Δs of the two mark points before stretching and after stretching stabilization. Combined with the nominal radius R of the steel strand, the torsion angle is calculated according to the formula φ=Δs / R.

[0029] In this step, a non-contact measurement method is used to measure the torsion angle, which avoids the additional constraints and damage to the surface of the steel strand caused by mechanical clamping. It has a fast response speed and is suitable for online inspection environments on production lines.

[0030] In actual production line environments, the surface of steel strands may be covered with oil, scale, or dust, which can interfere with the recognition accuracy of optical markers. To address this issue, the area to be measured can be wiped with anhydrous ethanol cotton balls before measurement, and a quick-drying diffuse reflection coating (such as magnesium oxide suspension or a special matte developer) can be sprayed on. This coating can be easily wiped off with a soft cloth after inspection, without affecting the subsequent use of the steel strand. The circumferential position change Δs of the markers can be extracted using a sub-pixel edge detection algorithm, improving the angular resolution to over 0.01 degrees. If a dual-laser interferometer scheme is used, the phase shift of the longitudinal texture (natural stripes formed by the gaps between steel wires) on the surface of the steel strand can be directly measured without manual marking, achieving fully automated unmanned measurement. In addition, low-frequency vibrations in the production line environment are the main source of error in torsion angle measurement. Mounting the optical measuring head on an independent vibration isolation bracket and introducing a reference point fixed to the frame for differential compensation in the signal processing algorithm can effectively suppress common-mode vibration interference.

[0031] Step 4: Based on the measured torsion angle φ, calculate the characteristic parameters that characterize the tensile-torsional coupling properties of the steel strand.

[0032] In this step, the characteristic parameter is the torsion angle under unit tensile force, i.e., φ / F. Here, φ is the measured torsion angle, and F is the axial tensile load, which physically represents the torsional deformation sensitivity induced by unit axial tension, with units of ° / kN or rad / kN. This parameter is simple to calculate, requires no material constant, and is suitable for rapid online determination of steel strand products of the same specification on the production line.

[0033] Considering the permissible deviation (e.g., ±0.4mm) in the nominal diameter of steel strands of the same specification, directly using φ / F as a characteristic parameter would introduce geometric variability. To further improve the accuracy of the judgment, a dimensionless characteristic parameter λ=(φ·D) / (F·L) can be introduced, where D is the measured outer diameter of the steel strand under test (which can be the average of three sections within the gauge length), and L is the gauge length. The physical meaning of this parameter λ is the shear strain induced by unit axial strain, eliminating the influence of diameter differences between different batches on the torsional response. For steel strands with ideal twisting structure, the value of λ should approach a constant; any phenomenon that significantly deviates from this constant directly points to an abnormality in the internal twisting parameters. During the calibration stage, the distribution of λ can be recorded simultaneously, and the λ range of qualified samples can be used as an auxiliary judgment basis.

[0034] Step 5: Compare the feature parameters obtained in Step 4 with the preset pass / fail threshold range. If the feature parameter value falls within the pass / fail threshold range, the anchoring performance of the steel strand under test is preliminarily determined to be qualified; if it falls outside the range, it is determined to be unqualified.

[0035] Furthermore, to improve the comprehensiveness and reliability of the testing, torsional relaxation behavior can be introduced as a supplementary testing dimension when determining anchoring performance. The specific steps are as follows: In step three, while keeping the tensile load F constant, the change in the torsion angle φ over time t is continuously recorded to obtain the relaxation curve φ(t). Relaxation characteristic quantities are extracted from this curve; these can be the relaxation time constant τ or the relaxation amplitude Δφ within a specified time window. The physical essence of torsional relaxation behavior is the frictional dissipation process accompanying the micro-slippage between the wires inside the steel strand. Its speed reflects the contact state and dissipation capacity between the wires. Excessive relaxation may indicate insufficient normal pressure between the wires or the presence of abnormal gaps; excessive relaxation may mean excessive interlocking between the wires or corrosion adhesion, both of which are detrimental to the stable self-locking of the interface between the wrench and the wire during anchoring. The extracted relaxation characteristic quantities are compared with a preset relaxation threshold; if they deviate from the normal range, the anchoring performance is deemed unqualified.

[0036] In practice, the recording time for the relaxation curve does not need to be too long; typically, recording continuously for 30–60 seconds after loading stabilizes is sufficient to extract stable feature quantities. A single exponential decay model φ(t) = φ can be used. ∞ +(φ0-φ∞ The curve is fitted with exp(-t / τ), where τ is the relaxation time constant. The physical meaning of τ is the decay of the torsion angle from the initial value φ0 to the remaining value φ. ∞ The characteristic scale of the required time. For steel strands with excellent anchoring performance, the value of τ usually falls between 15 and 50 seconds; if τ is less than 5 seconds, it indicates that there is almost no effective frictional constraint between the steel wires, which is a loose defect; if τ is greater than 120 seconds and the relaxation amplitude Δφ is less than 0.01 degrees, it may be a rigid contact between the steel wires caused by corrosion or abnormal compression, which should also be judged as unqualified.

[0037] Furthermore, to identify hidden defects caused by fluctuations in the local twisting process, multi-section torsional uniformity testing can be introduced. The specific steps are as follows: Multiple sections are uniformly selected along the axial direction of the steel strand to be tested, with no fewer than three sections. The local torsional angle φ of each section is measured under the same axial tensile load. i The dispersion index of the local torsion angle of each section is calculated, such as the coefficient of variation, which is the ratio of the standard deviation to the mean. When this dispersion index exceeds a preset limit, it indicates that the steel strand has significant local pitch unevenness, loose strands, or loose twists. Even if the overall characteristic parameters of the entire steel strand are qualified, the local weak link may fail first under anchoring stress, so the anchoring performance of the steel strand should be judged as unqualified. This "uniformity rejection" mechanism makes up for the insufficiency of the possibility that overall average measurement may mask local defects.

[0038] In practice, the selection of multiple cross-sections should avoid the 0.5-meter range at both ends of the steel strand, as irreversible loosening deformation may have occurred in this area during transportation and unwinding. The gauge length of each cross-section can be taken as one lay length, and the overlap rate between adjacent cross-sections is recommended to be controlled at around 50% to ensure continuous coverage of the entire length. Local twist angle φ i Measurements can be performed using a mobile optical probe or by arranging multiple fixed probes along the axial direction. The preset limit for the coefficient of variation should be determined based on the statistical distribution of qualified samples during the calibration phase, and should generally not exceed 5%. For example, if the local torsion angles of the three cross-sections are 0.30°, 0.31°, and 0.29°, respectively, the coefficient of variation is approximately 3.2%, which is considered acceptable for uniformity. However, if the local torsion angles of the three cross-sections are 0.35°, 0.28°, and 0.33%, respectively, the coefficient of variation is approximately 9.7%. Even if the average value of 0.32° falls within the threshold, it should still be judged as unacceptable due to excessive uniformity.

[0039] In practice, the preset pass / fail threshold, relaxation threshold, and uniformity limit used in the above detection steps are all predetermined through the following steps: The first step is to select calibration samples: randomly select several strands from a batch of products with the same specifications and production process as the steel strand to be tested as calibration samples. To meet the statistical confidence requirements, the number of samples should be no less than 30 strands to fully cover the dispersion of product quality within the normal production process fluctuation range.

[0040] The second step is to obtain the characteristic parameters of the calibration samples: For each calibration sample, the process is carried out in sequence according to steps one to four of the detection stage, that is, to perform the complete process of pre-tensioning to eliminate disturbance, applying elastic tensile load, measuring torsion angle, and determining characteristic parameters, and to record the characteristic parameter values ​​of each sample as calibration characteristic parameter values.

[0041] The third step is to obtain the anchoring efficiency coefficient through destructive testing: the same sample that has completed the above non-destructive testing is subjected to a static load anchoring test in accordance with relevant national standards. That is, the sample is inserted into the matching anchor and stretched until failure, and the measured anchoring efficiency coefficient η is recorded.

[0042] The fourth step is to establish a correspondence and determine the threshold: Compare and analyze the calibrated characteristic parameter values ​​of all calibrated samples with the measured anchoring efficiency coefficient η. Using the anchoring efficiency coefficient reaching the qualified standard as a screening condition, identify the distribution range of the characteristic parameter values ​​corresponding to all qualified samples, and take the lower and upper limits of this range as the qualified judgment threshold interval. For example, if η≥95% is the qualified standard, then the interval [K_min,K_max] jointly defined by the characteristic parameter values ​​corresponding to all samples with η≥95% is determined as the qualified judgment threshold. The relaxation threshold and uniformity limit can be determined using the same method, based on the statistical distribution of the relaxation characteristic quantity and the local torsional angle dispersion of the qualified samples, respectively.

[0043] In statistical processing, if the characteristic parameter values ​​of qualified samples approximately follow a normal distribution, the parameter estimation method can be used: calculate the mean μ and standard deviation σ, then the threshold interval can be set as [μ-kσ, μ+kσ], where k is the envelope coefficient. For general quality control, k=3 corresponds to a 99.7% confidence level; if the production line has high process capability, k=2.5 can be tightened to improve sensitivity. For the relaxation time constant τ, its distribution is usually right-skewed, and the percentile method is appropriate: take the 5th percentile of the qualified sample τ value as the lower threshold and the 95th percentile as the upper threshold. Samples exceeding this range are judged to have abnormal relaxation behavior. The uniformity limit (upper limit of the coefficient of variation) is taken as 1.2 times the maximum coefficient of variation among qualified samples as the engineering safety boundary.

[0044] The final determination result of this invention can be output to the production line control system to drive the automatic inkjet marking or sorting and rejection mechanism, so as to realize the online automatic grading of the anchoring performance of each steel strand leaving the factory.

[0045] Example 1 This embodiment uses a 1×7 prestressed steel strand with a nominal diameter of 15.2 mm and a nominal breaking force of 260 kN as the test object, and performs non-destructive testing of anchorage performance using the following steps: Step 1: Eliminate initial disturbances A pre-tension force is applied to the steel strand to be tested, and the magnitude of the pre-tension force is set to 85% of the subsequent axial tensile load. The subsequent axial tensile load is taken as 20kN (not exceeding 1 / 13 of the nominal breaking force), so the pre-tension force is 17kN. The load is applied at a constant rate to 17kN, held for 15 seconds, and then unloaded at a uniform speed to 0kN, and left to stand for 5 seconds.

[0046] Step 2: Apply an elastic axial tensile load An axial tensile load of 20 kN is applied to the steel strand after the disturbance has been eliminated, at a loading rate of 1 kN / s. The steel strand is in the stage of pure elastic deformation.

[0047] Step 3: Measure the torsion angle A gauge length L = 500 mm is set on the surface of the steel strand. Optical reflective markers are affixed to both ends of the gauge. The relative circumferential displacement Δs = 2.8 mm at both ends of the markers is measured using a dual CCD camera system. The nominal radius of the steel strand R = 7.6 mm. The torsion angle φ = 2.8 / 7.6 ≈ 0.3684 rad (approximately 21.11°) is calculated using the formula φ = Δs / R.

[0048] Step 4: Determine the characteristic parameters The torsion angle under unit tensile force is used as the characteristic parameter: φ / F=0.3684 rad / 20kN=0.01842rad / kN.

[0049] Step 5: Compare with the pass / fail threshold The pre-defined acceptable threshold range for this specification of steel strand is [0.0165 rad / kN, 0.0210 rad / kN]. The measured value of 0.01842 rad / kN falls within this range, therefore the anchoring performance of this steel strand is deemed acceptable.

[0050] Example 2 Based on Example 1, torsional relaxation behavior detection is added.

[0051] Once the axial tensile load of 20kN is kept stable, the change of the torsion angle φ with time t is continuously recorded, with a sampling interval of 0.5 seconds and a recording duration of 120 seconds.

[0052] The twist angle of the steel strand decays from 0.3684 rad to 0.3520 rad in the first 10 seconds, with a relaxation amplitude Δφ = 0.0164 rad; thereafter it changes slowly, and the relaxation time constant τ ≈ 22 seconds is obtained by single exponential fitting.

[0053] Preset relaxation threshold: A relaxation amplitude ≤ 0.025 rad and τ between 15-35 seconds are considered acceptable.

[0054] The measured values ​​of Δφ = 0.0164 rad and τ = 22 seconds both meet the requirements. Combined with the qualified characteristic parameters, the anchoring performance is finally determined to be qualified.

[0055] Example 3 Based on Example 1, a multi-section torsional uniformity test is added: Four sections were selected along the axial direction of the steel strand, at locations 0.5m, 1.0m, 1.5m, and 2.0m from the end.

[0056] Within the gauge length at each section (local gauge length is 100mm), the local torsional angles (angle values) were measured under the same 20kN load: φ1=4.2°, φ2=4.5°, φ3=4.1°, φ4=6.8°.

[0057] The average value μ is calculated as (4.2 + 4.5 + 4.1 + 6.8) / 4 = 4.9°.

[0058] Standard deviation σ = √[((4.2-4.9)] 2 +(4.5-4.9) 2 +(4.1-4.9) 2 +(6.8-4.9) 2 ) / 4]=√[(0.49+0.16+0.64+3.61) / 4]=√(4.9 / 4)=√1.225≈1.107°.

[0059] The coefficient of variation CV = σ / μ = 1.107 / 4.9 ≈ 0.226 = 22.6%.

[0060] The preset uniformity limit is a coefficient of variation ≤ 15%. The actual measurement showed that 22.6% exceeded the limit, indicating that there was local loose twist or abnormal pitch in the fourth section.

[0061] Although the overall characteristic parameters of the entire steel strand are qualified (the overall torsion angle of 21.1° corresponds to a characteristic parameter of 0.01842 rad / kN, which falls within the threshold), the anchoring performance is deemed unqualified due to local defects, based on the "uniformity rejection mechanism".

[0062] The present invention provides a non-destructive testing method for the anchorage performance of prestressed steel strands, which has the following significant advantages compared with the prior art: 1. It achieves true non-destructive testing and strand-by-strand screening. The load applied throughout the testing process is strictly limited to the elastic range. After testing, the steel strands do not undergo any plastic deformation or mechanical property degradation and can be used directly in the project as is. This fundamentally changes the limitation of destructive sampling inspection, which can only represent batches. It makes it possible to control the anchoring performance quality of every steel strand leaving the factory, completely eliminating blind spots in the safety supervision of major projects.

[0063] 2. The testing principle closely adheres to the mechanical essence of anchoring performance. Tensile-torsional coupling characteristic parameters directly reflect the integrity of the internal spiral structure of the steel strand and the self-locking ability of the inter-wire contact, which are the core factors determining whether the anchor wedges can effectively grip the steel strand. Therefore, there is an inherent physical correlation between the characteristic parameters and the anchoring efficiency coefficient, resulting in highly effective and reliable test results.

[0064] 3. The method is simple and suitable for online integration. The inspection station can be composed of a traction device, a tension application mechanism, and a non-contact optical measuring head. It has a compact structure, fast response speed, and can be seamlessly integrated into the steel strand production line before the winding process, realizing continuous online operation of inspection while production, without increasing the additional production cycle time, and with high inspection efficiency.

[0065] 4. By using auxiliary technologies such as pre-tensioning to eliminate disturbances, multi-section uniformity detection, and torsional relaxation behavior detection, the detection accuracy and robustness are systematically improved. This can effectively filter out interference caused by the randomness of the initial state, and sensitively identify local twisting defects and abnormal inter-filament contact states, thus fully ensuring the accuracy of the judgment results.

[0066] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A non-destructive testing method for the anchorage performance of prestressed steel strands, characterized in that, Includes the following steps: A pre-tension force is applied to the steel strand to be tested, and then unloaded to zero after holding the load to eliminate the initial disturbance. A preset axial tensile load is applied to the steel strand to be tested, so that the steel strand is in an elastic stress state; While maintaining the axial tensile load, measure the angle of torsion of the steel strand within the gauge length; Based on the stated torsion angle, characteristic parameters characterizing the tensile-torsional coupling properties of the steel strand are determined; The characteristic parameters are compared with a preset pass / fail threshold, and the anchoring performance of the steel strand under test is determined based on the comparison result.

2. The non-destructive testing method for the anchorage performance of prestressed steel strand as described in claim 1, characterized in that, The magnitude of the pre-tension force is 80% to 90% of the axial tensile load, and the holding time is 5 to 30 seconds.

3. The non-destructive testing method for the anchorage performance of prestressed steel strand as described in claim 1, characterized in that, The magnitude of the axial tensile load shall not exceed one-tenth of the nominal breaking force of the steel strand under test.

4. The non-destructive testing method for the anchorage performance of prestressed steel strand as described in claim 2, characterized in that, The torsion angle generated by the steel strand within the gauge length is measured using a non-contact optical measuring device. The measurement of the torsion angle generated by the steel strand within the gauge length includes: setting optical markers at both ends of the gauge length on the surface of the steel strand, measuring the circumferential displacement change of the two markers before and after stretching, and calculating the torsion angle in combination with the radius of the steel strand.

5. The non-destructive testing method for the anchorage performance of prestressed steel strand as described in claim 2, characterized in that, The characteristic parameter is the torsion angle under unit tensile force.

6. The non-destructive testing method for the anchorage performance of prestressed steel strand as described in claim 1, characterized in that, Determining whether the anchoring performance of the steel strand under test is qualified also includes torsional relaxation behavior testing, which includes the following steps: While keeping the axial tensile load constant, the relaxation curve of the torsion angle as a function of time is continuously acquired; The relaxation feature is extracted from the relaxation curve, and the relaxation feature includes the relaxation time constant or the relaxation amplitude within a preset time. The relaxation characteristic quantity is compared with a preset relaxation threshold. If it deviates from the normal range, the anchoring performance is determined to be unqualified.

7. The non-destructive testing method for the anchorage performance of prestressed steel strand as described in claim 1, characterized in that, Determining whether the anchoring performance of the steel strand under test is qualified also includes multi-section torsional uniformity testing, which includes the following steps: Multiple sections were selected along the axial direction of the steel strand to be tested, and the local torsional angle of each section under the axial tensile load was measured. Calculate the dispersion index of the local torsional angle of each cross section; When the dispersion index exceeds the preset limit, the anchoring performance is deemed unqualified.

8. The non-destructive testing method for the anchorage performance of prestressed steel strand as described in claim 1, characterized in that, The preset pass / fail threshold is determined through the following steps: Multiple steel strands of the same specifications and manufacturing process as the steel strand to be tested were selected as calibration samples. For each calibration sample, the following processes are performed in sequence: applying pre-tension force and unloading, applying axial tensile load, measuring torsion angle, and determining characteristic parameters. The values ​​of its calibration characteristic parameters are then recorded. Destructive static load anchorage tests were conducted on each calibrated sample after the aforementioned treatment, and its anchorage efficiency coefficient was measured. Based on the correspondence between the calibration characteristic parameter values ​​of each calibration sample and the anchoring efficiency coefficient, the distribution range of characteristic parameter values ​​corresponding to the calibration samples whose anchoring efficiency coefficient reaches the preset qualified index is determined as the qualified judgment threshold.

9. The non-destructive testing method for the anchorage performance of prestressed steel strand as described in claim 8, characterized in that, The number of calibration samples shall not be less than 30.

10. A non-destructive testing method for the anchorage performance of prestressed steel strands according to claim 8, characterized in that, The preset qualification index for the anchoring efficiency coefficient is η≥95%.

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