Distributed Strain Measurement Research Articles

Optical time-domain reflectometry based on a Brillouin dynamic grating in an elliptical-core two-mode fiber.

Optical time-domain reflectometry based on a Brillouin dynamic grating (BDG-OTDR) in an elliptical-core two-mode fiber (e-core TMF) is proposed and experimentally demonstrated for the discriminative measurement of strain and temperature distributions. Acoustic gratings are generated by the spontaneous Brillouin scattering of a pump pulse, which is used to reflect a probe pulse. Two orthogonal polarizations of the LP01 mode are sequentially used as the pump, while single polarization of the LP11 mode is used as the probe for intermodal BDG operation. Distribution maps of intermodal and polarization birefringence are acquired by analyzing the two BDG spectra, which are used to separately measure the temperature and strain distributions. In experiments, distributed measurement of intermodal BDG spectra along a 95m e-core TMF is performed with a 2m spatial resolution, where the strain and temperature coefficients of the BDG spectral shift are measured to be -0.085 MHz/με, +7.6 MHz/°C for the pump-probe of the LP01y-LP11x mode, and -0.093 MHz/με, +4.3 MHz/°C for the LP01x-LP11x mode, respectively. Distribution maps of the strain and temperature variation along the TMF are separately obtained by matrix calculation using the four coefficients with an error of ±105 με in strain and ±1.6°C in temperature.

A High-Speed Distributed Ultra-Weak FBG Sensing System With High Resolution

A high-speed high-resolution distributed sensing system with ultra-weak fiber Bragg grating (FBG) array is presented. The scheme uses a narrow bandwidth filter to translate wavelength shift measurement to a simple intensity measurement. By introducing a reference channel into the system to compensate for the intensity fluctuation from the light source or optical fiber bending loss, we can realize high-resolution and high-speed distributed static and dynamic strain measurement using 342 ultra-weak FBGs with reflectivity of 0.01% and spatial resolution of 2.5 m along 855-m optical fiber. The wavelength variation of all gratings was measured as 0.22 pm at a sampling rate of 100 kHz. Experimental results show that the resolution of strain measurement can achieve 222 $\text{n}\varepsilon $ , and dynamic frequency of 2000 Hz is achieved.

Non-Local Effects in Brillouin Optical Time-Domain Analysis Sensors

Brillouin optical time-domain analysis (BOTDA) sensors have great potential to provide distributed measurements of temperature and strain over large structures with high spatial resolution and measurement precision. However, their performance ultimately depends on the amount of probe and pump pulse power that can be injected into the sensing fiber, which determines the signal-to-noise ratio of the detected measurement signal. The probe wave power is constrained by the generation of noise induced by spontaneous Brillouin scattering and at lower power by the so-called non-local effects. In this work, we focus on the latter. We review the physical origins of non-local effects and analyze the performance impairments that they bring. In addition, we discuss the different methods that have been proposed to counteract these effects comparing their relative merits and ultimate performance. Particularly, we focus on a technique that we have devised to compensate non-local effects which is based on introducing an optical frequency modulation or dithering to the probe wave. This method is shown to provide a comprehensive solution to most of the impairments associated with non-local effects and also to enable some side benefits, such as amplification of the pump pulses to compensate the attenuation of the fiber.

Single-end-access distributed strain sensing with wide dynamic range using higher-speed Brillouin optical correlation-domain reflectometry

Single-end-access real-time fiber-optic distributed strain sensing has recently been demonstrated using an ultrahigh-speed configuration of Brillouin optical correlation-domain reflectometry (BOCDR). Its extremely high sampling rate was, however, achieved at the cost of a limited strain dynamic range (<0.2%). Here, by employing a noise-floor compensation technique, we develop a new cost-effective higher-speed configuration of BOCDR with a wide strain dynamic range (up to 2.0%; evaluated by static strain measurement). This value is larger than that of any other BOCDR configuration. Using this configuration, we demonstrate some fundamental distributed strain measurements and breakage detection.

Tailored pump compensation for Brillouin optical time-domain analysis with distributed Brillouin amplification.

A Brillouin optical time domain analysis (BOTDA) system utilizing tailored compensation for the propagation loss of the pump pulse is demonstrated for long-range and high-resolution distributed sensing. A continuous pump wave for distributed Brillouin amplification (DBA pump) of the pump pulse co-propagates with the probe wave, where gradual variation of the spectral width is additionally introduced to the DBA pump to obtain a uniform Brillouin gain along the position. In the experimental confirmation, a distributed strain measurement along a 51.2 km fiber under test is presented with a spatial resolution of 20 cm, in which the measurement error (σ) of less than 1.45 MHz and the near-constant Brillouin gain of the probe wave are maintained throughout the fiber.

Two-dimensional Moiré phase analysis for accurate strain distribution measurement and application in crack prediction.

Aimed at the low accuracy problem of shear strain measurement in Moiré methods, a two-dimensional (2D) Moiré phase analysis method is proposed for full-field deformation measurement with high accuracy. A grid image is first processed by the spatial phase-shifting sampling Moiré technique to get the Moiré phases in two directions, which are then conjointly analyzed for measuring 2D displacement and strain distributions. The strain especially the shear strain measurement accuracy is remarkably improved, and dynamic deformation is measurable from automatic batch processing of single-shot grid images. As an application, the 2D microscale strain distributions of a titanium alloy were measured, and the crack occurrence location was successfully predicted from strain concentration.

A new method for predicting advanced yield criteria input parameters from mechanical properties

Many advanced yield criteria have been developed in the last twenty years, but their use in forming simulations is still limited. In most simulations, the traditional Hill’48 model is still applied and this is understandable given that it is easy to use and does not require extensive test data. However, the Hill’48 criterion is too rigid to describe all stress states accurately, for this a more advanced model is needed. The drawback of such models is that they usually require data from an extensive range of laboratory tests.In the sheet metal forming industry, the tensile test is the standard procedure with which the mechanical properties of sheet metals are measured. It is therefore desirable that the multiple parameters required for advanced yield criteria are also able to be derived from the mechanical properties from the tensile test, as for Hill’48.In the work presented here, a correlation has been established between measured stress factor input parameters and the mechanical properties derived from the tensile test. Leading from this correlation, a new method is proposed to generate stress factors directly from simple mechanical properties. With this method it is possible to predict the biaxial, plane strain and shear points accurately using only tensile test data, notably tensile strength, uniform elongation and the plastic anisotropy r-value measured at 0°, 45° and 90° to the rolling direction.Initially aimed at low carbon steels, the proposed method is shown to be applicable to stainless steel and aluminium alloys as well. The accuracy of the predicted stress factors has been verified by comparing simulation results with measurements of strain distribution, punch force, displacement and temperature. The comparisons of three strain paths on Nakazima specimens taken from a forming steel and a dual phase steel show a very good agreement between the measured and simulation results. Additionally, cross die tests on eleven materials, ranging from forming steels to Advanced High Strength Steels, have been used to assess the model. The results demonstrate that the proposed method of generating parameters for advanced yield criteria from tensile mechanical properties is highly accurate and very easy to use.

Pipeline internal corrosion monitoring based on distributed strain measurement technique

Pipeline corrosion is an important issue that threatens the pipeline safety operation; therefore, the corrosion monitoring is essential. Based on the optical frequency domain reflectometry technology, which features distributed strain measurement with high resolution and high precision, a new method of pipeline internal corrosion monitoring is proposed to simultaneously locate corrosion and to evaluate corrosion severity. To verify the effectiveness and accuracy of this method, a series of tests were conducted including uniform corrosion test and local corrosion tests. These test results demonstrate that the corrosion location and corrosion severity evaluation can be achieved via the proposed distributed strain measurement, providing a valuable approach for pipeline corrosion monitoring.

A Low Bandwidth DFB Laser-Based Interrogator for Terahertz-Range Fiber Bragg Grating Sensors

This letter reports an all-electronic, low bandwidth swept frequency laser used to interrogate terahertz-range fiber Bragg gratings (THz FBGs) for distributed strain sensing applications. A distributed feedback laser with current injection modulation was employed as the swept frequency laser source. Using the resulting narrow bandwidth (~110 GHz) laser frequency sweep, high accuracy distributed strain measurements were achieved. In order to experimentally investigate this concept, a strain test was conducted using the THz FBGs. During the test, the laser sweeping time was limited to less than 2 ms. Highly linear results were found (R 2 = 0.996), with an observed sensitivity of -0.142 GHz/μe and the standard deviation of 1.167 μe. A multiplexing test was also conducted, and no cross-talk found between sensor elements. These results demonstrate that this interrogation system holds substantial potential as a method of rapid distributed optical fiber frequency-domain sensing.

3D beam shape estimation based on distributed coaxial cable interferometric sensor

We present a coaxial cable interferometer based distributed sensing system for 3D beam shape estimation. By making a series of reflectors on a coaxial cable, multiple Fabry–Perot cavities are created on it. Two cables are mounted on the beam at proper locations, and a vector network analyzer (VNA) is connected to them to obtain the complex reflection signal, which is used to calculate the strain distribution of the beam in horizontal and vertical planes. With 6 GHz swept bandwidth on the VNA, the spatial resolution for distributed strain measurement is 0.1 m, and the sensitivity is 3.768 MHz mε−1 at the interferogram dip near 3.3 GHz. Using displacement-strain transformation, the shape of the beam is reconstructed. With only two modified cables and a VNA, this system is easy to implement and manage. Comparing to optical fiber based sensor systems, the coaxial cable sensors have the advantage of large strain and robustness, making this system suitable for structure health monitoring applications.

Slope-Assisted Brillouin Optical Correlation-Domain Reflectometry Using Polymer Optical Fibers With High Propagation Loss

We have recently developed slope-assisted Brillouin optical correlation-domain reflectometry (SA-BOCDR) for higher speed distributed measurement of strain and temperature. However, no reports have been published yet on the use of high-loss fibers. Here, we implement SA-BOCDR using a high-loss polymer optical fiber (POF) for the first time. Due to the gradual reduction in the transmitted power along the POF, the measurement sensitivities are found to depend on sensing position. This unique effect is investigated experimentally, and then a correct POF-based distributed measurement is performed by compensating this effect.

Measurement of Strain Distribution and Volume Ratio in Tensile Test of Circular Shaft Specimens of steel and Consideration of Stress-Strain Relation

Measurement of Strain Distribution and Volume Ratio in Tensile Test of Circular Shaft Specimens of steel and Consideration of Stress-Strain Relation

Comparison between different Fiber Optical Strain Measurement Systems Based on the Example of Reinforcing Bars

Fiber optical measurement systems come to the fore after having been intensively investigated during the last two decades. One example is Fiber-Bragg-Gratings (FBG). This technology allows a sufficient strain measurement at predefined points. Another new technique deals with Rayleigh backscatter to realize a distributed strain measurement over the total length of an optical fiber. Such measurement is recognized to provide a high potential in civil engineering, especially in reinforced concrete.This paper deals with some preliminary comparisons between Rayleigh backscatter and FBG based on tension test with reinforcing bars. Therefor optical fibers were placed in notches, milled into the reinforcing bars. Analysis of the test data showed a strong correlation between the optical measurement systems and theoretical results up to the yield point of the steel. Furthermore, the advantages of distributed strain measurement were uncovered. In a last step, the paper offers a prospect concerning future applications for the Rayleigh backscatter system.

Surface Crack Detection in Prestressed Concrete Cylinder Pipes Using BOTDA Strain Sensors

Structural deterioration after a period of service can induce the failure of prestressed concrete cylinder pipes (PCCPs), with microcracks in the coating leading to the corrosion of the prestressed wires. In this paper, we propose the use of Brillouin optical time-domain analysis (BOTDA) strain sensors for detecting the onset of microcracking in PCCP coating: the BOTDA strain sensors are mounted on the surface of the PCCP, and distributed strain measurements are employed to assess the cracks in the mortar coating and the structural state of the pipe. To validate the feasibility of the proposed approach, experimental investigations were conducted on a prototype PCCP segment, wherein the inner pressure was gradually increased to 1.6 MPa. Two types of BOTDA strain sensors—the steel wire packaged fiber optic sensor and the polyelastic packaged fiber optic sensor—were employed in the experiments. The experimental distributed measurements agreed well with the finite element computations, evidencing that the investigated strain sensors are sensitive to localized deterioration behaviors such as PCCP microcracking.

Evaluation of Stiffness Degradation on CFRP Laminate by Measuring Strain Distribution Using Speckle Interferometry

Evaluation of Stiffness Degradation on CFRP Laminate by Measuring Strain Distribution Using Speckle Interferometry

経カテーテル大動脈弁留置術における大動脈弁モデルに生じるひずみ分布の計測に関する研究

経カテーテル大動脈弁留置術における大動脈弁モデルに生じるひずみ分布の計測に関する研究

Metallographic Analysis of Nakajima Tests for the Evaluation of the Failure Developments

The material characterisation in sheet metal forming is significantly improved with the introduction of optical measurement systems. The optical measurement is based on the digital image correlation (DIC) technique, which provides accurate strain measurement of the local strain distribution on specimens. The changes of the stochastic distribution due to the forming are therefore used in order to measure displacements and deformations. An acknowledged application of the DIC technique is the evaluation of the forming limit curve (FLC) for sheet metals. Typically, the FLC is evaluated by conducting Nakajima tests. The Nakajima test is a stretch forming-based test and allows the analysis of stretching at different stress conditions. A previous investigation based on pattern recognition on Nakajima tests has shown that for a conventional deep drawing steel DC04 the pattern evolution on the strain distribution during the test can be examined in order to predict the material failure. However, even if the investigation shows promising results, the correspondence between pattern on the surface and changes in the material structure is missing. The aim of the present work is an analysis of the surface structure of Nakajima tests at different stress states and drawing depths. The correlation between surface modifications and failure behaviour is conducted on the deep drawing steel DC04. The scanning electron microscope (SEM) (Merlin Zeiss GmbH) is employed in order to investigate the evolution of the surface at different strain conditions. The results of the surface and the metallographic investigation are finally compared with the forming limit prediction of the FLC and the failure behaviour at different stress conditions is discussed.

Measurement of strain and temperature distributions on an aluminum plate by using fiber-optic sensors

Measurement of strain and temperature distributions on an aluminum plate by using fiber-optic sensors

Determination of the formability limits for Grade 1 titanium sheet by means of ALSAD method

Stamping is one of the most widely used sheet metal forming processes. Esthetics and durability of the deformed products are limited by defects in the form of strain localization, grooves or cracks. Forming limits, e. g. in form of forming limit curve (FLC), help to avoid above-mentioned defects in the sheet metal during forming. Besides the common steel sheets for deep drawing processes, e.g. DC04, components can also be made of metal with specified strength and chemical resistance properties. One of such materials is titanium sheet. In this paper, FLC for GR1 titanium sheet will be presented. It was obtained by means of a new author’s method named as ALSAD (Analysis of Laser Speckle Activity Differences). So far, applied methods of FLC determination have been mainly based on the measurement of the strain distribution in the area of the created groove, deformation gradient and deformation speed. For this reason, detection of strain localization has been dependent on the strain measurement method. The ALSAD, based on laser speckle phenomenon allows detection of defects initiation on the sample surface independently of the strain distribution measurement and it allows, for example, to relate a moment of defect initiation directly to the sample height or forces occurring in the process. In this paper strain values for strain localization, grooves and cracks for GR1 titanium sheet will be shown. Obtained results show good formability of this material, which was verified by means of 3D digital microscope. Such results allow tracing precisely the formation of the defects starting from strain localization up to the crack appearance, which would be helpful for advanced numerical simulations.

Novel Non-Contact Evaluation of Strain Distribution Using Digital Image Correlation with Laser Speckle Pattern of Low Carbon Steel Sheet

An uni-axial tensile test is one of the general methods to obtain plastic properties for estimating deformation behavior of materials in the simulation of practical metal forming process. Non-contact measurement is generally used because of determination of accurate plastic properties excepting for contact effects. In the conventional method, strain distribution of specimen sprayed with random pattern is measured by digital image correlation (DIC). However, sprayed treatments on the specimen surface is unsuitable under various testing conditions, for example an elevated temperature, micro scale, and large deformation. Thus, non-sprayed specimen is desirable under such test conditions. In this study, we propose perfectly non-contact strain distribution measurement by using DIC with laser speckle pattern generated on specimen surface by irradiation of laser source. True strain in the un-axial tensile test is measured by following methods (1) laser speckle pattern by DIC, and (2) conventional measurement method with tracking gage marks. As a result, strain level measured by DIC with laser speckle pattern has good agreement with that of conventional extensometer. Furthermore, the strain distribution can be measured successfully under uniform and non-uniform deformation during necking by proposed method. Thus, we can verify the validity of proposed measurement method for determination of plastic properties.