Real-Time Monitoring of Strain Processes With Large-Range and High-Spatial Resolution Using the Method of Weak Reflection FBG Measurement Based on OFDR

The real-time monitoring of strain processes that can simultaneously realize a wide range and high spatial resolution has always been a huge challenge and cannot be satisfied by traditional electrical and noncontact measurement methods. In this article, the optical frequency domain reflection (OFDR) collocated with weak reflection fiber bragg grating (FBG) sensors (WRFBGs) was proposed to actively carry out multiple experiments on the nonstandard reinforcement samples with complex strain processes in order to overcome this challenge. According to the result analysis, the complex strain variation law of the surface of the sample and the large span strain of adjacent locations during the whole process of tensile can be obtained by this method. The yield point (YP) can be accurately captured and positioned with millimeter-level horizontal precision. At the same time, real-time monitoring can be realized for the step process, when the strain of the point increases to more than 25000- $\mu \varepsilon $ instantly. The method can also realize the strain detection capability with a response speed of 38 Hz and a 1.28-mm high spatial resolution by monitoring the real-time high-density and large strain distribution.

tunnel cross section, and the analysis of track, pavement, and bridge load analysis, and so on [4], [5], [6], [7], [8].Taking RC structure as an example, rebar is widely used as an important engineering material.Studying its strain state under load can provide basic support for the study of the mechanical properties of RC structures [9], [10].The location of YPs is often random for nonstandard reinforcement samples.How to detect the positions of these YPs and the detection of the law of strain change near these positions are of great importance for the study of the mechanical properties of related materials [11], [12].According to the research, traditional detection methods generally measure the strain change in the tensile process of rebar by electrical strain gauge or by shooting with a high-definition camera or other methods [13], [14], [15].However, a traditional electrical strain gauge (ESG) can only realize point measurement.Moreover, only a few points can be measured on a rebar due to the large volume of strain gauges.As a result, the complete strain data of rebar tension cannot be monitored [16], [17].The measurement method by shooting with an HD camera can monitor the tensile deformation of the rebar by comparing the changes in the rebar position shot at different times, but it depends on the postprocessing algorithm and cannot ensure the test precision [18], [19].
Distributed optical fiber sensing technology has been widely applied in various engineering measurement fields [20], [21].As a contact-type signal measurement method, it is different from traditional point-type electrical signal measurement.A single flexible optical fiber can provide a large amount of strain measurement data for nonstandard materials.
Distributed fiber sensing technology is mainly realized by demodulation of Rayleigh scattering, Brillouin scattering, and Raman scattering, three different forms of scattered light signals inside the fiber [22].Rayleigh scattering signal is easier to detect.It belongs to elastic scattering and has higher energy than the other two scattering signals.The spatial resolution of Brillouin scattering spectrum demodulation of single-mode fiber exceeds 10 cm, and its system generally does not exceed 5000-µε strain signal detection from the literature survey [23], [24].The demodulation of Rayleigh scattering signals using optical frequency domain reflection (OFDR) technology can achieve millimeter-level high-spatial resolution and high-precision measurement.The detection of microstrains of several thousand levels can be achieved by demodulating traditional single-mode optical fibers [25], [26].Nevertheless, it remains a huge challenge for distributed fiber optic strain measurement technology with large ranges.The weak reflection fiber bragg grating sensors (WRFBGs) technology based on OFDR can measure tens of thousands µεlevel of strain and the sub-millimeter level of spatial resolution, realizing distributed, high resolution, and high precision strain sensing measurement [27].As a special distributed optical fiber strain sensor (DOFSS), WRFBG is attached to the surface of the rebar.The optical fiber is deformed with the rebar, when the rebar sample under test is deformed under tension.The strain distribution during rebar tension can be reflected simply by demodulating the spectral offset (SO) of WRFBG by the equipment of OFDR.
The research objective of this article is to provide a distributed strain measurement method that can simultaneously achieve high-spatial resolution and high-measurement range.The main contributions of this article are given as follows.
1) OFDR technology is used with WRFBGs, and these are interrogated using the OFDR principle to achieve the monitoring of high strain values and with high spatial resolution.2) Several verification experiments have been carried out for samples with typical strain processes.The complex strain process of the nonstandard steel bar samples can also be detected in real time.
3) It can be detected that the strain gradient near the yield point (YP) was large by analyzing the spectral information of WRFBGs.The maximum span value in the experimental process in this article was 18 000 pm, and the strain change at the YP was very rapid.The advantage of the distributed measurement can clearly monitor the complicated strain distribution in all positions.This article combines the respective advantages of OFDR technology and WRFBG sensor to obtain accurate position information and wide range strain threshold on the basis of millimeter spatial resolution.The experimental results show that the large yield strain distribution of rebar in the process of the abrupt increase to 25 000 µε in both the overall YP, and the single point yield position was detected in this experiment.Experimental data fully proved that this study provides a new idea for the application in the field of strain measurement technology.It is fully proved that this research provides a very effective detection method for the field of train measurement technology, especially for materials with complex laws of the strain variation.

A. Technical Principle of OFDR
The dynamic demodulator used in the experiment was mainly based on ODFR and combined with optical heterodyne detection technology in this article.The basic principle is shown in Fig. 1, the optical path was divided into two parts, the main optical path and the auxiliary interference optical path [28], [29].The linear swept light emitted by the laser was divided into two beams through Coupler 1.One beam entered into the main interferometer and the other entered into the auxiliary interferometer.The light entering into the main light path was divided into the detection light and the reference light through Coupler 2. The detection light entered through Port 1 of the three-port circulator and exited from Port 2 of the circulator and then entered into the sensing fiber.The Rayleigh scattering light produced in different positions of the sensing fiber was mixed with the reference light at Coupler 3 after passing Port 3 of the circulator.After passing the polarization beam splitter, the signal light and the reflected reference light passed the coupler, after which the coherent detection was performed through photoelectric detectors 1 and 2. OFDR can demodulate the frequency variation of optical signals inside the fiber optic, when the sensing fiber is subjected to strain, as shown in Fig. 2. The key technology of OFDR was a linear swept laser source.In order to compensate its nonlinearity, an auxiliary optical path composed of Michelson interferometer was added.The photocurrent detected by the photoelectric detector can be expressed as follows: where β is the factor of photoelectric conversion.The first three items (two are dc items and one is an HF item) in the expression above are filtered out with only the final beat frequency (BF) item left.ω S − ω L is the BF f b .The BF in   position z on the optical fiber can be expressed as where γ is the sweep velocity of the linear sweep light; τ is the delay inequality between the reference light and the detection light in Position z; n is the effective refractive index of the optical fiber; and ν is the speed of light in vacuum.According to the formula above, the physical position of each point in the optical fiber under test was in a linear relation with the BF of that point.Thus, the positioning along the optical fiber can be realized [30].The strength of the signal reflected from each point can be mapped to the reflectance of that point, thus the OFDR range-reflectance curve shown in Fig. 3 was formed.

B. WRFBGs Sensing Technology
WRFBGs engrafts the entire fiber with a 0.05% reflectance grating by using a drawing tower grating writing technique [31], [32].As shown in Fig. 4, the length of a single FBG was 9 mm with a 1-mm-length grid distance, which can achieve a single fiber to have high-spatial resolution measurement capability.As shown in Fig. 5, the grating had a clear characteristic reflection spectrum, so the Rayleigh scattering intensity of the entire fiber was 25 dB (approximately 1000 times higher) than traditional fibers.As shown in Fig. 6, the measurement accuracy of ±4 pm can still be guaranteed at a 1-mm-length gap.In the OFDR system, FFT-1 was performed on the electrical signal acquisition at the output end of the detector in Fig. 1 to obtain the scattered light intensity at each position of the sensing fiber.As shown in Fig. 5, the horizontal axis represents the length of the sensing fiber, and the vertical axis represents the scattered light intensity at each position of the fiber.A window data were intercepted at a certain position in Fig. 5(a), and the window width was the size of a single sensor, namely, gauge length.The signal in the frequency domain at this position can be obtained after fast Fourier transform (FFT) was performed again, namely, the Rayleigh scattering spectrum, as shown in Fig. 5(b).
From Fig. 6, the results were obtained by continuous sampling of a sensing point.Fig. 6(a) shows a continuous sampling of a sensing point of a common optical fiber, and it can be seen that the data repeatedly fluctuates between +4 and −24 pm.Fig. 6(b) shows the measurement results of the weak gate under the same condition.It can be seen that the data repeats in the band of −4 to +4 pm.Therefore, it can be determined that the repeatability of the weak gate is better than the traditional optical fiber in the case of high-value gauge length (such as 1-or 2-mm gauge length).
In this article, the OFDR laser with a sweeping range of 1540-1580 nm was used in order to improve the measurement range of the system.The measuring range of the system was −12 000 to +12 000 pm under normal conditions, when the ordinary single-mode optical fiber was used as the sensor.On the contrary, the initial central wavelength of 1542 nm was close to the initial scanning wavelength of 1540 nm, when WRFBG was used as the sensor.Therefore, the system theoretically can measure the scattering optical frequency shift at 38 nm, which corresponds to 30 000 µε when measuring the positive strain.It further satisfied the large strain measurement capability of OFDR.

C. Demodulation of Strain Sensing Information
Rayleigh scattering signals obtained in the reference state and measurement state were divided into multiple signal windows by the spatial resolution of sensing during the  strain sensing by the OFDR technology.The spectrum shift of each signal window was calculated by cross-correlation operation, as shown in Fig. 7(a).The strain change at this point was obtained in combination with the temperature or strain frequency shift coefficient [33].The average value (AV) of the time-domain waveform was generally obtained by multiple conversions and used as the credible signal since the reflected signal strength was weak.According to the Rayleigh scattering spectrogram of WRFBGs in Fig. 5, the reflectance spectrum had distinct characteristics and certain wavelength selectivity.The maximum value of a spectral signal can be directly found without multiple cross-correlation calculations.As shown in Fig. 7(b), the abscissa corresponding to the maximum of the spectrum can be directly found at the sensing point after calculating the spectrum.The Rayleigh scattering frequency shift at that point can be obtained by calculating the difference between this abscissa and the abscissa corresponding to the maximum of the initial reference spectrum.Since the amount of computation involved in cross-correlation operation was huge, when the cross correlation calculation was simplified, the demodulation speed of the system was substantially improved, thus satisfying the parameter requirements for 32-Hz demodulation speed.The 1.28-mm spatial resolution of the OFDR system can be realized in combination with the millimeter-level WRFBG writing size.

III. EXPERIMENTAL METHODS AND SETUP
Two measurement methods were adopted to realize method demonstration and comparison in this experiment, including the traditional ESG and the OFDR-based WRFBG measurement.Round rebars (total length 50 cm and diameter 12 mm) were used as the samples in this experiment, as shown in Fig. 8(a), a WRFBG was pasted to the front and rear sides of the sample with AB epoxy resin glue.The strain gauges were pasted in the corresponding position of the paste area on the front side of the sample and 50 mm left at both ends.The length of a single gauge was 7 mm, corresponding to five sampling points of WRFBG.Three samples were prepared by the above-mentioned method.Sample 1 has only one face with WRFBG, and its measuring area was 185-469 mm.The WRFBG areas on the front and rear sides of Sample 1 were 150-550 and 650-1050 mm, Sample 1 was 70-470 and 600-1000 mm, respectively.The oxidation protective coating on Sample 2 was polished off.
The WRFBG positions corresponding to strain gauges are listed in Table I.The whole experimental setup was shown in Fig. 8(a), including the dynamic high-precision DFOS  equipment and data acquisition system (DAQ) that realize the acquisition and demodulation of the spectral shift (SO) signal of WRFBG and strain measurement (SG) electrical signal from strain gauge, respectively.The strain sensing data in two real-time modes can be acquired from the upper computer.
The sample was clamped on the stretcher when the system was prepared, as shown in Fig. 9(b).The WRFBGs were connected to the DFOS device by a fiber optic jumper.The strain gauge was connected to the strain demodulator by the low-noise lines and then into the DAQ.The state without optical fiber loading is referred to as the zero strain value.Afterward, the speed of the stretcher was set to 0.2 mm/min to collect the strain values in each position in the whole process of rebar tension.

A. Characterization of Sensor Calibration and Overall Change in Stress
The spectrum of the WRFBG at t = 0 s was selected as the initial state.The uniform tension was performed on Sample 2 with a tensile machine.The strain changes on the front and rear surfaces of the sample 140.625 s later were shown in Fig. 8.The comparison of the two results showed that WRFBGs can directly read the time distribution events of the overall strain on the sample surface, while ESG only detects the single point measurement data corresponding to the collocation.When the strain reaches around 1500 µε, the WRFBG results distributed along the sample were basically consistent and basically coincide with the measured value of the strain gauge.The yield states described by the two correspond to each other, indicating that the strain changes of this sample in different positions in the tensile process were the same.In order to improve the scientificity of the experiment, the precision of the OFDR-based WRFBGs should be calibrated.As shown in Table II, the AV of strain gauge measurement data collected for many times is compared with the AV measured in the corresponding position for analysis.Since the value measured by strain gauges is the AV of strain in the grating area.Therefore, the test results of WRFBG are the AV of the five sensing points corresponding to strain gauges.
It can be clearly seen Fig. 10 that the trend of the numerical value of the measured value of SG and the SO of WRFBG at different times and multiple points is consistent, indicating that SO can be used to reflect the real strain distribution of samples.The analysis of the relative error between the two modes shows that, as shown in Fig. 11, compared with traditional ESG; the measurement errors of WRFBGs were all below 18%.
Calculating the relative error of the measurement data obtained by the two measurement methods of ESG and WRF-BGs, as shown in Fig. 11, which refers to the error relationship between the two measurement results.The different points have different errors in the first place (and why they behave differently in time), which can be explained by the following two reasons.On the one hand, the surface of the steel bars is rough and not smooth enough, resulting in insufficient straightness when laying optical fibers.Some areas do not fit well with the surface of the steel bars, and it can also lead to uneven application of glue.As the tension and strain increase, the impact of this fiber placement problem on the measurement results gradually weakens, thereby reducing the measurement error.On the other hand, because the strain gauge with centimeter-level size measured the average strain value of an area with a 2-D size, while the WRBGs have a diameter of a micrometer with an millimeter-level spatial resolution, its size is far smaller than the strain gauge.WRBGs measured the average strain of a 1-D size, which can be regarded as the strain value of a point relative to the strain gauge.Therefore, the results obtained by the two methods had certain errors that can be explained.The measurement results of the two methods have an obvious positive proportional linear relationship, and the strain calibration curve of WRFBG based on OFDR technology can be directly obtained through the fitting formula through direct analysis of the ESG value and the weak gate SO value, as shown in Fig. 12.Therefore, we will directly use the SO amount of the weak gate to represent the change process of the strain parameters on the sample surface in the following experiments.decrease at the same rate due to the elastic state of the test sample, and the tensile machine will stretch at the same rate at 200 s.Because of the distributed measurement characteristic, the fiber optic sensor can monitor the whole axial strain without a leakage point, as shown in Fig. 13.We can get the whole strain change of the surface during the tensile process by analyzing the weak-gate spectrum change.Also, the YPs were precisely captured.

B. Analysis of Surface Strain Variation
It can be seen that the SO of WRFBG had a good trend with the data of SG at the corresponding position, as shown in Fig. 14, especially for the tensile and rebound stages before the early yield stage, which has a good strain-tracking effect.The relationship between strain and SO can refer to the results in Fig. 12.The YP can not be detected because the SG was limited by the position of the distribution point, but three obvious YPs can be found by analyzing the YP from the data of WRFBG.Analysis of the process strain of YP 1 showed that the step increases after 1300 s, as shown in Fig. 15, which was the result of the yield of the sample.Sample 2 was further stretched.The WRFBG was not firmly pasted on the rear side and fell off since the oxidation protective coating on Sample 2 was not polished off.Therefore, the data of only 70-470 mm of WRFBG were selected for analysis.Fig. 16 Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.shows the strain change curve during the rebar tensile process.Sample 1 started at about 912 s as time went on.The SO value of the WRFBG at both ends increased rapidly, and then basically remained unchanged when exceeding 20 000 pm around.The large strain values spread from the two ends of Sample 2 to the middle.This process is the sample yield process.According to Fig. 16(b)-(d), the top yield position of Sample 2 goes down a little bit and then basically kept still, while the bottom yield position kept spreading upward, until the rebar fully yielded.
Sample 2 fully yielded.In this process after reaching 4258 s, the WRFBG measurement technology based on OFDR can realize the real-time detection of the whole process and can promptly and accurately capture and position the YPs that appear.Moreover, WRFBG can realize the large strain measurement of about 24 000 pm.Such capacity cannot be realized by traditional measurement methods.It can be known by analyzing the SO value of WRFBG that the step strain change occurred in adjacent positions during rebar tensile and yielding.Its maximum exceeded 18 000 pm.The strain change in the yield position was very rapid.As shown in Fig. 16(a), it can be obtained that its propagation velocity was about 327.68 mm/s with 1193.09375s as an example.
In the rebar tensile process, the overall strain got stable when it increased to about 2000 µε.Afterward, when the rebar began to yield at a certain point and then spread around, the strain change from 0 to 24 000 µε at the YP could be tested.
Besides, the position of the rebar YP could also be tested.The safety monitoring and positioning of the vulnerable area of the sample can be realized by presetting the risk threshold.This provides a good experience basis for the application and development of this technology.
2) Exploration of Strain Variation Law Based on OFDR Technology: As shown in Fig. 17, a tensile test was performed on Sample 3, the changes in the strain parameters on the surface of samples were monitored throughout the process by WRFBG measurement technology in the tensile loading process exceeding 6400 s.According to Fig. 17 It can be obviously seen by analyzing the SO curve of the WRFBG that the sample experiences three different stages in the tensile process from Fig. 18.At the first stage, the SO of the WRFBG symmetrically pasted on both sides of the sample has almost similar numerical values and the law of change.Meanwhile, the trend of the numerical value of the SG of strain gauges is consistent with that of the SO of WRFBG.As shown in Fig. 19, fitting the AV of the two measurement results can obtain the changing trend representing the process.
In the second stage, it can be obviously seen by comparing with Fig. 17(b), the strain on the surface of the sample showed a nonlinear increasing trend after 100 s of tension.At this time, the strain gauge has lost the strain detection ability at this point due to the fall-off.This was caused by the great impact of surface strain change on paste stability.
At the third stage, it can be seen by analyzing the trend of the SO of WRFBG that the strain on the sample surface shows a linear change trend about 1000 s later, as shown in Fig. 17(c).The second segment of WRFBG falls off the 2500 s later.The strain value reaches about 15 000 pm after 3600 s.The second segment of WRFBG fell off and the strain value returns 0. As shown in Fig. 17(d), the SO value of the first part of WRFBG increases with the tension.This part of WRFBG partly fell off when the AV of SO exceeds 25 000 pm.This is because its surface was pasted with WRFBGs and its visibility relaxed until it fell off when the sample yielded.The numerical value of SO in this stage was fit since the strain gauge had completely been inactivated in this process.
As shown in Fig. 20, Sample 3 has been in two linear yield stages at this point, after reaching the measurement limit of the strain gauge until falling off.
In the whole tensile process, the strain gauge detection technology based on OFDR can monitor the distribution of the overall strain change on the surface of the sample in real time.The exploration of this type of law, benefits from the high-spatial resolution and high-sensitivity measurement characteristics of the WRFBG measurement technology based on OFDR, especially the display of the irregular process of positive correlation.

C. Analysis of Repeatability Measurement of Large-Range and High-Density Strain Processes
In order to explore the strain measurement characteristics for the wide range and high-spatial resolution density of the WRFBG measurement technology based on OFDR, the local (339-379 mm) WRFBG on the surface of Sample 3 was pasted and reinforced for the large tensile experimental study.The tensile loading was further applied.The WRFBGs all fell off after a period of time, and it can be obtained by multiple demodulations of the SO value from WRFBG.As shown in Fig. 21, the WRFBG can still stably detect the 30 000-µε strain while maintaining the spatial resolution of 1.28 mm.This was Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.also the first time for the WRFBG measurement system based on OFDR technology to achieve the repeatability measurement of large-scale strain in the field of high-density measurement data.Further analysis of the measurement results of WRFBG from the three samples is shown in Fig. 22, and the AV and standard deviation (SD) of the data for all sensing points on WRFBG were 23572.36± 1817.11, 25463.03± 377.08, and 32377.98 ± 112.04 pm, respectively.The result fully proved that the proposed scheme can achieve a large-range and highdensity strain measurement capability.
It can be found by analyzing the SD value of the three measurements that there are certain differences among them.The SD of Sample 2 was relatively large.The possible reason may be that the surface of Sample 1 was not polished.As a result, the WRFBG might be unevenly pasted, which caused an uneven force on the optical fiber.Sample 3 was polished before pasting WRFBG, its SD value was stable.Sample 3-1 was made from Sample 3 by pasting it evenly again; thus, the measurement repetition accuracy of WRFBG was optimal.
Therefore, the pasting method between WRFBGs had a certain impact on the measurement results of the system.Attention must be paid to the installation method of WRFBGs in the process of later application.

V. CONCLUSION
The dynamic high-precision DFOS instrument based on the principle of OFDR with WRFBGs was used to make it have a large-range and high-spatial-resolution of the real-time measurement of strain parameters.A variety of system functional verification experiments have been actively carried out by taking the nonstandard reinforcement with a complex strain process as the experimental samples.First, the calibration curve of weak grid strain sensing based on OFDR was obtained.Meanwhile, the advantages of the weak grid in capturing and locating YPs were highlighted by comparing them with traditional SGs.Second, the step state that the surface strain of the sample increases more than 25 000 µε instantaneously during the tensile process was successfully captured, and the strain change law of the corresponding phase in the process was obtained.Finally, the high-density data with large strain distribution was obtained in the process of deep stretching the sample, and the strain detection capability up to AV + SD = 32377.98± 112.04 pm was achieved on the basis of 1.28-mm high-spatial resolution.
However, further research is needed in future work.On the one hand, noncontact measurement methods, such as digital image correlation (DIC) will be combined to conduct research on the sensing accuracy of the system based on specific measurement objects and other factors such as the installation method of WRFBGs.On the other hand, the detection technology proposed in this article for model validation will be introduced to achieve accurate prediction of difficultto-measure information, such as modeling ideas based on deep learning [34], [35].
In conclusion, this article fully proved that a new idea in the application field of large-range and high-density strain measurement technology was provided in this research, especially for materials with complex strain variation laws.

Fig. 4 .
Fig. 4. Schematic of the size and structure of WRFBGs.

Fig. 5 .
Fig. 5. Comparison between the Rayleigh scattering spectrum of ordinary optical fiber and spectrum characteristics of WRFBGs.(a) Comparison diagram of Rayleigh scattering light intensity.(b) Diagram of reflection spectrum.

Fig. 8 .
Fig. 8. Schematic of the test model.(a) Sample test diagram.(b) Test platform of tensile and samples.

Fig. 9 .
Fig. 9. Distribution curve of overall strain change process of Sample 2.

Fig. 10 .
Fig. 10.Comparison of measurement results of strain gauge and WRFBGs.
During the Tensile Process of Samples 1) Position Capture of YP and Measurement of Strain Parameters: Sample 1 was stretched at a rate of 0.2 mm/min and then compressed at the same rate.The surface strain will

Fig. 14 .
Fig. 14.Comparison of the strain data between SGs and WRFBG in corresponding position.(a) Comparison diagram of AV of five points on SG1 and WRFBG.(b) Comparison diagram of AV of five points on SG3 and WRFBG.(c) Comparison diagram of AV of five points on SG4 and WRFBG.(d) Comparison diagram of AV of five points on SG6 and WRFBG.
(a)-(d), the experimental results showed that this sample was different from Sample 2. The strain change occurred as a whole, until the sample reached a yield state.

Fig. 16 .
Fig. 16.Spectral curve of WRFBG characterizes the strain change curve of Sample 2 during stretching.(a) Spectra at both ends reached a stable value with time.(b) SO propagated from both ends to the center.(c) Sample reached the maximum at one end and continuing to propagate at the other end.(d) Surface strain of the whole sample reached a stable value.

Fig. 17 .
Fig. 17.SO value of the WRFBG varies with time in the tensile process of the Sample 3. (a) SO curve for 50-200-s stretching time.(b) SO curve for 300-1000-s stretching time.(c) SO curve for 1300-2500-s stretching time.(d) SO curve for 6100-6400-s stretching time.

Fig. 18 .
Fig. 18.Comparison curve between WRFBG and SG in the tensile process of high strain: the first stage represented as S1, the second stage represented as S2, and there are two situations in the third stage, represented as S3-1 and S3-2, respectively.

TABLE I CORRESPONDING
POSITION OF STRAIN GAUGE AND WRFBGS

TABLE II AV
OF STRAIN GAUGE MEASUREMENT DATA COLLECTED FOR MANY TIMES AND THE AV OF WEAK GRID MEASUREMENT AT CORRESPONDING POSITION Fig. 12. Calibration curve of WRFBG strain sensing based on OFDR.