Strain Measurements In Structures Research Articles

A Mechanized Air-Atomized Spray Coating Method for Fabricating Flexible Nanocomposite Piezoresistive Strain Sensors

Nanocomposite piezoresistive strain sensors have offered new avenues for strain measurement of structures with significant deformations, large geometries or complex contours. The affordable and efficient spray coating technique, especially the air-atomized spray coating method, has considerable potential for batch production of such sensors. However, one of the most popular ways to fabricate sensors is manual spray coating, which not only has a low transfer efficiency, resulting in material waste and environmental pollution, but also makes it challenging to precisely control the homogeneity of sensor coatings, leading to inconsistent quality of existing sprayed sensors. Therefore, a mechanized air-atomized spray coating method is proposed to actualize the stable batch production of sprayed sensors, and spraying parameters are then determined to establish proper process conditions. Carbon black (CB) nanoparticle reinforced polyvinyl pyrrolidone (PVP) nanocomposites are prepared to fabricate CB/PVP strain sensors to validate the proposed method. Compared with the manual spray coating, the proposed method can manufacture CB/PVP coatings with uniform thickness distribution and micro-morphology. Additionally, the mechanical and electrical properties of CB/PVP nanocomposites are subsequently investigated to verify the consistency of coating quality using nanoindentation and four-probe techniques, respectively. Furthermore, the superior sensitivity and large strain range of CB/PVP strain sensors are evaluated by uniaxial tensile testing. In conclusion, the mechanized air-atomized spray coating method is not only time-saving, inexpensive and straightforward to implement, but also features wide applicability due to minor requirements for nanocomposites, thus laying a foundation for batch production and popularization of nanocomposite piezoresistive strain sensors.

Patch-Antenna-Based Structural Strain Measurement Using Optimized Energy Detection Algorithm Applied on USRP

A new structural health monitoring (SHM) program using spectrum sensing and radio-frequency identification technology is presented to measure structural strain. The proposed program involving universal software radio peripheral (USRP) provides an economical alternative to these type of measurements as compared to high-end equipment while this method can achieve more flexible data processing. The overall system for strain detection consists of three main parts: 1) a patch antenna as the sensor for strain; 2) an USRP as a measuring instrument; 3) a computer for data processing. The patch antenna can be used to measure structural strain by interrogating resonant frequency shift due to changes in antenna length. In this article we use the optimized energy detection algorithm applied on USRP to detect the spectrum of the patch antenna, the final measurement results show that the patch antenna sensor has a strain sensitivity 1.7678 kHz/ $\boldsymbol{\mu } \boldsymbol{\varepsilon }$ while the error between the measured value and the true value of the stress is 2.443% after corrected by machine learning.

Non-contact strain measurement for laterally loaded steel plate using LiDAR point cloud displacement data

Strain measurement in structures provides key information for structural health monitoring (SHM); therefore, researchers have developed various sensing methods for measuring strain in structures. Recently, light detection and ranging (LiDAR) has been studied as a non-contact strain measurement method (NCSMM). LiDAR is a technology for remotely acquiring high precision three-dimensional (3D) coordinate information for a terrain or a structure using a 3D laser scanning system. In recent years, LiDAR applications have expanded to include systems used to monitor structural behavior. Using this technology, structural behavior can be monitored without directly attaching sensors to the surfaces of target structures. It is advantageous to use non-contact 3D laser scanning measuring methods to obtain the 3D coordinates of a specific region or the shape of the target structures. LiDAR technology does not have some of the limitations associated with existing types of measuring sensors used for SHM. Therefore, in this study, we will present a method for estimating deformation and strains for a whole structure by measuring discretized 3D coordinate data of the target structure obtained from the LiDAR. The 3D coordinate data was revised using a regression analysis, and the strain was estimated by developing a finite element model based on the corrected 3D coordinate information. The experimental structural model is a steel plate, which is the primary material used for the outer wall construction of LNG storage tanks. The estimated strains will be compared to the measured values from real strain gauges, and the applicability of the proposed strain measurement method to structural behavior monitoring will be verified.

Advancement of long-gauge carbon fiber line sensors for strain measurements in structures

This article proposes a new technique that advances long-gauge carbon fiber line sensor technology, with and without post-tensioning of the sensor, to measure changes in strain levels in structural areas. Carbon fiber line sensors were fabricated to produce a slim high-strength sensor with a diameter of less than 1.4 mm using a carbon fiber tow with a width of 6 mm. A theoretical analysis of these sensors as well as several series of experiments was conducted to investigate the effect of fiber arrangement on the error compensation of the carbon fiber line sensors. The results revealed that using two sets of carbon fiber line sensors, one as an active sensor and the other to compensate the errors of the first, is an effective method when both sensors have a convergent fiber arrangement and change in resistance. A post-tensioning method was implemented to enhance the overall behavior of the sensor. The results showed that the post-tensioning method yields significant improvement in the linearity and cyclic ability up to 6000 microstrains and reduces the fluctuation errors in the change in resistance from ±0.031% to ±0.007%. Finally, the possibility of repairing damaged carbon fiber line sensors is also discussed.

Distributed Strain Measurement along a Concrete Beam via Stimulated Brillouin Scattering in Optical Fibers

The structural strain measurement of tension and compression in a 4 m long concrete beam was demonstrated with a distributed fiber-optic sensor portable system based on Brillouin scattering. Strain measurements provided by the fiber-optic sensor permitted to detect the formation of a crack in the beam resulting from the external applied load. The sensor system is valuable for structural monitoring applications, enabling the long-term performance and health of structures to be efficiently monitored.

Design of an optical fibre sensor patch for longitudinal strain measurement in structures

The purpose of this study is to design a composite sensor plate, based on a FBG sensor, to be used as alternative to the conventional electric extensometers for structural health monitoring applications. Two different cases are studied: composite patches made from carbon and glass woven reinforcement. Special care is devoted to the embedding process of the optical fibre sensor in the composite plate in order to avoid significant alteration of the optical power spectrum of the light reflected back by the fibre Bragg grating. The strain response of the composite sensor patch to axial loading is compared to electrical strain gage’s. A three-dimensional finite element analysis is also proposed to understand the relation between the strain measured by the FBG sensor and the strain imposed to a plate and the consequence, of the application of such composite sensor plate, on the behaviour of a specimen submitted to tensile stress.

Development and validation of a strain-based Structural Health Monitoring system

An innovative Structural Health Monitoring (SHM) methodology, based on structural strain measurements, which are processed by a back-propagation feed-forward Artificial Neural Network (ANN), is proposed. The demonstration of the SHM methodology and the identification of its capabilities and drawbacks are performed by applying the method in the prediction of fatigue damage states of a typical aircraft cracked lap-joint structure. An ANN of suitable architecture is developed and trained by numerically generated strain data sets, which have been preprocessed by Fast Fourier Transformation (FFT) for the extraction of the Fourier Descriptors (FDs). The Finite Element (FE) substructuring technique is implemented in the stress and strain analysis of the lap-joint structure, due to its efficiency in the calculation of the numerous strain data, which are necessary for the ANN training. The trained network is successfully validated, as it is proven capable to accurately predict crack positions and lengths of a lap-joint structure, which is damaged by fatigue cracks of unknown location and extent. The proposed methodology is applicable to the identification of more complex types of damage or to other critical structural locations, as its basic concept is generic.

Tensile and compressive strain measurement in the lab and field with the distributed Brillouin scattering sensor

The structural strain measurement of tension and compression in the steel beam was demonstrated with a distributed fiber-optic sensor system based on Brillouin scattering. The experiments were conducted both in the laboratory and outdoors. When it is in the outdoor environment, the temperature compensation has been taken into account for the strain measurement due to sunlight radiation. The compressive strain has been measured, without needing pretension on the fiber with a Brillouin scattering-based distributed sensor system, when the fiber is glued to the steel beam at every point. The dynamic range in the strain measurement has been increased, due to the elimination of the pretension requirement. The spatial resolution of the strain measurement is 0.5 m. The strain measurement accuracy is /spl plusmn/10 /spl mu//spl epsi/(/spl mu/m/m) in the laboratory environment with nonuniform-distributed strain. With uniform strain distribution, the strain accuracy for this system can be. /spl sim//spl plusmn/5 /spl mu//spl epsi/. These results were achieved with the introductions of a computer-controlled polarization controller, a fast digitizer-signal averager, a pulse duration control, and the electrical optical modulator bias setting in the software.

A digital signal processing algorithm for structural strain measurement by a 3 × 3 passive demodulated fiber optic interferometric sensor

A digital signal processing algorithm, the moving reference line crossing count method (MRLCCM), was developed to implement the structural strain measurement by a passive demodulated fiber optic Michelson interferometric sensor. The fiber optic Michelson sensor, which is constructed of a 3 × 3 fiber optic coupler, can give information about the magnitude and direction of the strain of structures. The beating, drifting and noise, which are caused by the longitudinal strain and the lateral strain of the fiber, bring about the counting error of the phase differences which are directly converted to the structural strain. The developed algorithm is based on the reference line crossing count method and resets the reference line during the presence of the signal drifting. The accuracy of the strain calculation was confirmed by the various simulated fiber optic signals with signal beating, drifting and noise. A passive demodulated 3 × 3 fiber optic Michelson interferometric sensor was bonded on the cantilevered aluminum beam for experimental strain sensing. The capability of real-time processing was verified by the real fiber optic signals.

A fiber optic microbend sensor for distributed sensing application in the structural strain monitoring

A fiber optic microbend sensor with an elastic, arched sensing diaphragm has been developed for structural strain measurement. The combination of multiple microbend sensors can form a sensor array for the quasi-distributed sensing application in the monitoring of local strain or deformation along structures, and the optical time domain reflectometry can be conveniently used for interrogation of each sensor unit. The sensor sensitivity can be set at a specific value according to the requirements of the measurement condition. Connected with multiplexed sensing processing schemes, the sensor array may find an application in the real-time monitoring and damage detection of large and critical engineering structures.

Three-Dimensional Structural Strain Measurement with the Use of Fiber-Optic Sensors

Three-dimensional strain sensing inside a structure is not feasible with conventional strain sensing techniques such as electrical strain gauges, which are limited to surface measurements. Three-dimensional strain measurement inside a structure would find uses in a variety of new applications: enhanced understanding and detection of composite failure modes, such as delamination; sensing for adaptive structural control; intelligent vehicle highway systems; and structural health monitoring systems for civil structures. The latter application could involve remotely monitoring structural integrity during and after an earthquake, for example. A fiber-optic strain sensor array (FOSSA) in a planar, patch-like configuration was developed, and accurate measurement of the three principal strains inside a simple structure was demonstrated. The planar configuration was chosen to avoid the difficulty and structural degradation of embedding optical sensors in three planes. Two extrinsic Fabry-Perot interferometric (EFPI) sensors and one polari-metric sensor form the planar sensor array. The two EFPI sensors were placed perpendicular to each other in the sensor plane to extract the two normal strain components along the x and y axes. The polarimetric sensor embedded in the plane was used to extract the third normal strain acting on the z axis. The sensor array was embedded in an epoxy resin cube and loaded to 454 kg (1,000 1b) with a loading machine. The strains that were measured correlated well with the external strains measured with surface-bonded electrical strain gauges. The variation in measured strain between the two sensor systems was less than 4 percent for all three principal axes.

Strain measurement of structures with curved surface by means of personal computer-based picture processing

An optical strain measurement system was developed that includes computer processing of stereoscopic picture data of marks painted on the specimen. This system allows us to measure large strains in specimens with curved surfaces as well as plane surfaces even at elevated temperatures. The results of a verification test performed on a bellows specimen at room temperature have shown that the strain error of this measurement system is allowable considering the limitation of the apparatus.