Distributed Strain Measurement Research Articles

Analysis of the Strain Distribution and Texture Measurements in Asymmetric Rolling (AR) and Asymmetric Accumulative Roll Bonding (AARB)

Severe plastic deformation (SPD) with strong shear component is required to promote both grain refinement and texture randomization. When Asymmetric rolling (AR) is applied as asymmetric accumulative roll bonding (AARB), it enables the production of architectured microstructures and metallic composites. Finite element (FE) simulations of AR and AARB were employed to understand the influence of pass thickness reduction (PTR) on the through thickness variation of the velocity gradient. The influence of the PTR up to a total thickness reduction of 50% and the effect of a single 50% reduction step in a bi-layer bonding condition was analyzed. The influence of these process parameters on the strain and rigid body rotation components was compared with the experimental data obtained on an AA1050 aluminum. A better shear to compression ratio across the sheet thickness is achieved by PTRs lower than 30%; at a PTR of 50% the texture is dominated by the frictional shear generated at the roll-sheet interface and the process has a stronger compressive character. This indicates that simple ARB followed by AR with smaller PTRs should generate a better shear distribution than AARB alone.

Strain distribution measurement with digital image correlation method by using metallographic image under cyclic loading

Strain distribution measurement with digital image correlation method by using metallographic image under cyclic loading

The Measurement and Discrimination of Asymmetric Non-Uniform Strain Distribution Based on Spectrum Characterization of Fbg Sensor

The Measurement and Discrimination of Asymmetric Non-Uniform Strain Distribution Based on Spectrum Characterization of Fbg Sensor

Flight Test of Optical Fiber Strain Distribution Measurement System for Airframe Structure

Flight Test of Optical Fiber Strain Distribution Measurement System for Airframe Structure

Measuring crack width using a distributed fiber optic sensor based on optical frequency domain reflectometry

This study develops a method to measure crack width using a distributed fiber optic sensor. To this end, a customized specimen was designed to manipulate cracks under precise displacement control. An optical fiber was attached on the specimen and served as a distributed sensor which measured strain distributions along the fiber based on optical frequency domain reflectometry. An algorithm was presented to analyze crack width based on the measured strain distribution. Compared with the crack width measured by an extensometer, the crack width measured by the distributed sensor had an accuracy of 5.6 µm. Parametric studies were conducted and revealed that the coating thickness of optical fiber, the spatial resolution of strain distribution, and the spacing of cracks had significant effects on the measurement of cracks using the distributed sensor. This study is expected to enhance the capability of detecting, locating, and quantifying cracks by using distributed fiber optic sensors.

Prediction of the business jet Global 7500 wing deformed shape using fiber Bragg gratings and neural network

To develop a common deflection and twist measuring method applicable for both ground and flight tests, the left wing of a Global 7500 business jet was instrumented with fiber Bragg gratings and underwent durability and damage tolerance certification testing. This fiber optical sensing system allows distributed strain measurements on the wing. Fatigue and static tests were monitored. It was shown that the front and rear spar deflections could be analytically calculated with an acceptable accuracy. However, the twist could not be successfully calculated. A neural network was then used to address this issue. Even though the deflection predictions were less accurate than those analytically calculated, the wing twist was successfully predicted. This study shows the potential of using fiber Bragg grating as a shared ground and flight tests strain measuring method allowing to calculate the aircraft wing deflection and to determine the wing twist angle using a neural network.

Spatially resolved fibre cavity ring down spectroscopy

This paper presents a fibre cavity ring down spectroscopy probed by Rayleigh scattering optical frequency domain reflectometry (OFDR), which provides spatial location of stimuli and improved signal to noise ratio for distributed sensing measurements. A section of optical fibre was integrated into an active fibre ring cavity with optical gain and interrogated by the OFDR system for 11 cycles with a single laser scan. Through the cavity ring down configuration, root-mean-squared (RMS) noise of distributed temperature and strain measurements was reduced to 6.9 mK and less than 0.1 με, respectively for 1-cm spatially resolved measurements. Our work shows that the active fibre cavity configuration can be combined with distributed fibre sensing schemes to achieve both high spatial resolution and high sensitivity measurements.

Quantitative strain measurement and crack opening estimate in concrete structures based on OFDR technology

Abstract Concrete structures are easy to produce cracks which seriously affect the safety of the structures. Rayleigh based distributed optical fiber sensing technique has been used to measure the strain field evolution associated with crack initiation and growth in concrete structures. A reinforced concrete beam has been tested under four-point loading test with optical fibers both pasted on the surface and embedded as the distributed sensors. Experimental results indicate that cracks can be detected at sharp peaks of the measured strain distributions, which can be utilized to effectively capture the number of cracks and locate multiple cracks. The abilities of using optical frequency domain reflectometry (OFDR) technology to detect hairline cracks and distinguish neighboring cracks are also discussed. Single crack opening displacement is determined through fitting process which taken the influence of intermediate layers into consideration, the widening and propagation of cracks in the overall concrete beam were detected.

Quantitative amplitude-measuring Φ-OTDR with pε/√Hz sensitivity using a multi-frequency pulse train.

We report an amplitude-measuring Rayleigh-based sensor that uses a series of frequency-shifted pulses to extract quantitative distributed strain measurements. By using frequency multiplexing, we are able to inject a train of 10 pulses into the fiber at once. This allows us to use a higher average input power than standard phase-sensitive optical time domain reflectometry systems, improving the sensitivity. The sensor recovers the strain by tracking the time-dependent amplitude of the Rayleigh backscattered light from all 10 pulses. This approach enables a sensor with a noise floor of 1.5pε/√Hz over 10 km of fiber with 12 m spatial resolution, a 5 kHz bandwidth, and a dynamic range of 80 dB at 1 kHz. The sensor exhibits a high degree of linearity and is immune to interference fading.

A novel OFDR-based distributed optical fiber sensing tape: design, optimization, calibration and application

The significance of implementing structural health monitoring (SHM) system for civil engineering, especially infrastructure engineering, has been recognized. Distributed optical fiber sensor (DOFS) based on optical frequency domain reflectometer (OFDR) offers an attractive technique for in situ strain measurement due to its high resolution and high precision. However, a main challenge that naturally arises is how to guarantee sensors survival in harsh construction and service environments. In this paper, a new, but simple, packaging technique is proposed to solve this issue. The novel sensing tape is fabricated by embedding DOFS into two fiberglass tapes and its strain transfer mechanism is analyzed in detail to provide scientific criteria for the choice of the material parameters. Then, a calibration test is carried out to evaluate the performance of the sensing tape in distributed strain measurement. Finally, the tape sensor is applied to the surface strain monitoring of the reinforced concrete (RC) beam tested in three-point bending. Experimental results verify that the sensing tape works well on concrete materials and the strain measured has the capability to reveal and localize cracks. Both analytical and experimental results demonstrate that the proposed packaging technique is feasible and effective.

Shear Characteristics between FRP-Concrete Bonding Interface

At present, fiber reinforced composite materials (FRP) are widely used in the reinforcement of concrete structures. The bonding interface between FRP plates and concrete is the key part of the strengthening of concrete structures with FRP plates. The bonding performance directly determines the success or failure of structural reinforcement. Based on the self-developed test device, the development of FRP and concrete in direct shear debonding test specimens, with the aid of advanced digital image correlation DIC technology, accurate measurement of specimen strain distribution on the surface of the FRP plate, and the FRP plate surface strain along the plate long distribution rule and the bond strength of the specimens was well researched, it reveals that the stripping of FRP and concrete interface failure process, and provides technical guidance for the treatment of FRP-concrete bond interface in practical projects.

Comparison of different fiber coatings for distributed strain measurement in cementitious matrices

Abstract. Distributed fiber optic strain measurement based on Rayleigh scattering has recently become increasingly popular in automotive and mechanical engineering for strain monitoring and in the construction industry, especially structural health monitoring. This technology enables the monitoring of strain along the entire fiber length. This article addresses integrating optical fibers of different coatings into the concrete matrix to measure the shrinkage deformations. However, previous studies do not give a clear statement about the strain transfer losses of fiber optic sensors in this application. In this context, three different coating types were investigated regarding their strain transfer. The fibers were integrated into fine-grained concrete prisms, and the shrinkage strain was compared with a precise dial gauge. The analysis shows a high correlation between the reference method and the fiber measurement, especially with the ORMOCER® coating. The acrylate coating used is also consistent in the middle area of the specimen but requires a certain strain introduction length to indicate the actual strain. The main result of this study is a recommendation for fiber coatings for shrinkage measurement in fine-grain concretes using the distributed fiber optic strain measurement. In addition, the advantages and disadvantages of the measurement method are presented.

Measurement of Shear Strain Field in a Soft Material Using a Sensor System Consisting of Distributed Piezoelectric Polymer Film.

Measurement of the internal stress and strain distributions within soft materials is necessary in the field of skin contact safety. However, conventional interactive force sensors cannot efficiently obtain or estimate these distributions. Herein, a shear strain sensor system consisting of distributed built-in piezoelectric polyvinylidene fluoride (PVDF) polymer films was developed to measure the internal shear strain field of a soft material. A shear strain sensing model was mathematically established, based on the piezoelectricity and mechanical behavior of a bending cantilever beam, to explain the sensing principle. An experiment in three-dimensional measurement of the shear strain distribution within an artificial skin was designed and conducted to assess the sensitivity of the sensing model. This sensor system could visualize the shear strain field and was sensitive to different contact conditions. The measurement results agreed well with the results of numerical simulation of the substrate, based on contact mechanics. The proposed sensor system was confirmed to provide a new sensing method for the field of shape analysis. The sensor system can be applied to develop sufficiently sensitive electronic skin and can significantly contribute to skin damage analysis and skin contact safety assessment.

Optimized and validated prediction of plastic yielding supported by cruciform experiments and crystal plasticity

The predictability of strain distributions and the related prediction of hardening and failure plays a central role in tool and process design for any metal forming process. Studying yielding behaviour, it was discovered that for various well established yield loci no satisfying agreement between DIC (digital image correlation) measurement of strain distribution and simulation result could be obtained, even after optimization of parameters based on associated flow assumption. In parallel, crystal plasticity simulations were investigated with the objective to predict the relation between stress and strain ratios for a large number of load cases based on texture measurement. To include macroscopic data, a secondary strategy uses cruciform tension specimen as defined in ISO16842 to obtain strain and stress ratios. The resulting relations were then applied separately as input parameters for the plastic yield description. Both approaches reach high agreement between forming experiment and simulation. Against the previous assumption, the output can be obtained with either anisotropic or free shape yield loci and associated flow description. The methodology discussed provides an alternative and challenges to rethink the definition of yielding for aluminium alloys on the example of an AA6014-T4 aluminium alloy.

High-sensitivity, high-spatial-resolution distributed strain sensing based on a poly(methyl methacrylate) chirped fiber Bragg grating

In this study, a high-sensitivity, high-spatial-resolution distributed strain-sensing approach based on a poly(methyl methacrylate) chirped fiber Bragg grating (CFBG) is proposed and experimentally demonstrated. Linearly chirped FBGs in a polymer optical fiber provide an alternative to the silica fiber owing to the lower Young’s modulus, which can yield a higher stress sensitivity under the same external force. According to the spatial wavelength-encoded characteristic of the CFBG, a fully distributed strain measurement can be achieved by optical frequency-domain reflectometry. Through time-/space-resolved short-time Fourier transform, the applied force can be located by the beat frequency originated from the space-induced time delay and measured by the differential frequency offset originated from the strain-induced dispersion time delay. In a proof-of-concept experiment, a high spatial resolution of 1 mm over a gauge length of 40 mm and a strain resolution of 0.491 Hz/με were achieved.

Effect of laser temperature control on Brillouin optical correlation-domain reflectometry

We show that 1 °C drift of laser temperature yields a strain error of ∼8 με in Brillouin optical correlation-domain reflectometry. Under the assumption that the strain measurement accuracy is +/–20 με (severe requirement), +/–2.5 °C drift of the laser temperature can be permitted for proper system operation. We prove this prediction by performing distributed strain measurements at different laser temperatures. Our results indicate that, if we allow a strain measurement accuracy of approximately +/–100 με (or more), we can even remove the thermal control of the laser, leading to cost reduction and downsizing of the system.

Probabilistic model for structural mechanics of fiber bundle and its application to PBO fiber

The stress-strain relationship of single fiber and fiber bundle based on the different strain distribution functions of single fiber, including rectangular distribution, normal distribution and Weibull distribution, was studied. Strain distribution of Poly-p-phenylene benzobisoxazole (PBO) single fiber was measured, and the stress-strain curve of PBO fiber bundle can be deduced from the different strain distribution functions of single fiber. The results showed that normal distribution was closer to the measured strain distribution of PBO single fiber than rectangular distribution, and the stress and strain values of PBO fiber bundle estimated by the normal distribution were closer to the measured stress and strain values of PBO fiber bundle. In addition, the stress and strain values of PBO fiber bundle estimated by Weibull distribution were between those of rectangular distribution and normal distribution. In a word, the normal distribution can be effectively used to predict the fracture process of PBO fiber bundle.

An analysis of railway track behaviour based on distributed optical fibre acoustic sensing

Trackside monitoring of railways provides useful data for understanding the condition and mechanical behaviour of railway track, prior research has shown that railway track performance varies significantly along its length, primarily owing to changing support conditions. Understanding the changing performance along track offers the potential for improved track design and maintenance. Different technologies are used to investigate this. For example, inertial sensors, or high-speed filming with digital image correlation (DIC) for track deflection, and traditional strain gauges for loads. The latter usually rely on in-situ calibration. These techniques are suitable for measurements at discrete locations along the track length but are not suited to measuring performance variability even along a few hundred meters of a railway line. This paper investigates the use of a recently developed sensing system known as distributed acoustic sensor (DAS) that uses optical fibres. This method has the potential to be used over very long lengths of track; offers high sample rates; and has a gauge length and spatial resolution suitable for investigating the load-deflection behaviour of the track. This study presents DAS optical fibre strain measurements from a study site and presents novel methods for determining the rail deflection and the load per sleeper end. The DAS results are compared with point location measurements using a traditional strain gauge and deflections determined using imaging and DIC. The DAS system offers reliable distributed strain measurement that convert to estimates of track deflection and load with the potential for continuous spatial and temporal coverage over significant lengths of track.

Strain transfer characteristics of surface-attached FBGs in aircraft wing distributed deformation measurement

Dynamic deformations of aircraft wing can lead to changes in baseline length, which seriously affects the performance of ground height measurement and multi-phase central array imaging. It is very important to measure the deformation of aircraft wing with high accuracy in dynamic environment. The layout of sensors is not allowed to affect the safety of aircraft structure and the sensor packaging design directly affects the accuracy of aircraft wing deformation measurement. For these, the strain calibration methods used in recent years are analyzed first, and then a strain calibration method of the surface-attached FBGs applied to distributed strain measurement of aircraft wing is proposed. In this paper, two sets of Aluminium Alloy Test Plates with different sizes were designed as matrix materials for tensile test. The strain sensitivity of surface-attached FBGs on test substrates with different sizes was compared by several groups of tests. The test results show that the tensile test based on AATP-A is better than AATP-B. By comparing the measurement results of surface-attached FBGs and strain gauge on AATP-A, the high measurement accuracy of surface-attached FBGs is verified. The test results show that the strain measurement method based on the surface-attached FBGs is simple and reliable, and this method can be used to calibrate the surface-attached FBGs before use, and improve the accuracy and accuracy of the measurement of wing deformation.

The nondestructive measurement of strain distributions in air plasma sprayed thermal barrier coatings as a function of depth from entire Debye-Scherrer rings.

The residual strain distribution has been measured as a function of depth in both top coat and bond coat in as-received and heat-treated air plasma sprayed thermal barrier coating samples. High-energy synchrotron X-ray beams were used in transmission to produce full Debye-Scherrer rings whose non-circular aspect ratio gave the in-plane and out-of-plane strains far more efficiently than the sin2ψ method. The residual strain in the bond coat is found to be tensile and the strain in the β phase of the as-received sample was measured. The residual strains observed in the top coat were generally compressive (increasing towards the interface), with two kinds of nonlinear trend. These was a 'jump' feature near the interface, and in some cases there was another 'jump' feature near the surface. It is shown how these trend differences can be correlated to cracks in the coating.