Distributed Strain Measurement Research Articles

Detection of matrix cracks in composite laminates using embedded fiber-optic distributed strain sensing

We used an optical frequency-domain reflectometry fiber Bragg grating (FBG) distributed strain measurement system to detect matrix cracks in carbon fiber-reinforced plastics. Load/unload tests were performed on cross-ply laminates under ambient-, high-, and low-temperature conditions using embedded 100mm gauge FBG sensors. We measured the strain distributed along the gauge and examined the crack initiation using optical microscopy and soft X-ray inspection. A peak appeared in the distributed strain corresponding to the occurrence of cracks. The strain distribution during crack initiation was calculated in each temperature range via finite element analysis and compared with the measurement results. We determined that the number and location of cracks can be determined by extracting the peaks from the distributed strain. However, the peaks in the distributed strain do not correspond to the occurrence of cracks that are close to each other, warranting a measurement method with a higher strain resolution.

Effect of delignification on shrinking and swelling of poplar wood assessed using digital image correlation technique.

This study investigates lignin's influence on the hygroscopic behavior of poplar wood. Delignification was achieved using an acidic NaClO2 solution, and digital image correlation (DIC) was employed to measure strain distribution during shrinking and swelling across relative humidity (RH) ranging of 0 % to 97 %. Results showed that lignin removal increased equilibrium moisture content (EMC) by up to 3.6 % in samples with 27 % lignin reduction. Strain analysis revealed significant radial strain differences between earlywood and latewood, with tangential strain suppressed by wood rays. Increased porosity and cellulose slippage were observed following lignin removal, and samples with the highest lignin removal rate exhibited reduction exhibited over 58 % greater εxx and over 43 % greater εyy compared to untreated wood. These effects were most pronounced in latewood and regions distant from wood rays, where stress concentrations occurred. Delignification-induced cell wall thinning amplified the wood rays' inhibitory effect on anisotropic deformation during shrinking，enhancing the contrast between isotropic deformation in latewood and anisotropic deformation in earlywood during swelling. These findings provide insights into lignin's role in wood-water interactions, supporting the optimization of wood modification techniques.

Understanding of additively manufactured material cyclic behavior at the grain scale by neutron diffraction and crystal plasticity modeling

Additive manufacturing (AM) offers distinct advantages in terms of complexity, reduced material waste, and shorter production times. However, the microstructures of AM alloys differ significantly from conventionally manufactured ones, and their impact on grain behavior remains uncertain. This study employs neutron diffraction, coupled with Crystal Plasticity Finite Element (CPFE) modeling, to investigate microstructural effects on cyclic behavior in 316L LPBF alloys. Neutron diffraction provides high-resolution strain and stress data at the polycrystalline level during loading, offering valuable insights into grain behavior. The CPFE model is initially validated at the grain scale using neutron diffraction, demonstrating its ability to predict local mechanical behavior in additively manufactured materials. This methodology holds promise for broader applications in various AM alloys, particularly when macroscopic identification of mechanical behavior is insufficient. Furthermore, the comparison between neutron diffraction and CPFE predictions allows to identify the influence of dendrites and dislocation substructures on the mechanical behavior at the grain scale. Notably, the micromechanical anisotropy resulting from dislocation organization along dendritic structures with specific orientations is highlighted. This study shows that despite the observed anisotropy, experimental measurement of intragranular strain distribution are correctly captured with a quantification of the deviation observed on some specific orientations due to the above cited microstructure sub-structures.

Analisis Karakteristik Material Baja dengan Metode Digital Image Correlation (DIC)

The use of steel materials in building construction opens new opportunities for sustainable development, as steel exhibits corrosion resistance, durability, and reliability in terms of strength and ductility. Digital Image Correlation (DIC) is non-contact technique in which digital images of the surface of a test object are captured using high-resolution cameras. This study conducted measurements of strain distribution on the specimen's surface using the DIC method throughout the entire tensile testing process. The study particularly focuses on examining changes in strain distribution during the melting phase and the local deformation phase leading to fracture. In this research, a comparison will be made between the load-displacement curves obtained from experimental laboratory testing and the results analyzed using the DIC method for SS400-grade steel material. Based on the results of the tensile test and DIC analysis that have been conducted, conclusions have been drawn in the research. The tensile test results of SS400 steel material with a thickness of 6 mm, 8 mm, 10 mm, and 12 mm meet the quality requirements in the tested specification standards, and the results of the force-displacement curve between the experimental test results and the DIC method obtained a minimum deviation with a value below 10%,. Therefore, it can be concluded that the DIC method exhibits a reasonably good level of accuracy, making it suitable for validating the results of experimental tests.

Measuring strain distributions in an asphalt pavement using fibre optic sensors under static loading by test vehicles

Understanding the in-situ structural behaviour of pavements plays an important role in road engineering. Hence, various measurement methods were developed in the past to get an insight into the response of roads to traffic loading. In the present study, distributed fibre optic strain sensors were embedded into a hot mix asphalt pavement. The sensors were used for strain measurements with high spatial resolution in order to assess its feasibility to gain the continuous strain distribution in the pavement under short-term static loading. In addition, the loading process was measured with high temporal resolution, indicating that – using shorter sensor lengths – vehicle passages could also be studied. The strain distributions gathered with this type of sensor were compared to the results of a simple finite element model and the effect of applying various sensor cables. In addition, new opportunities provided by distributed fibre optic sensors in road engineering are discussed.

Experimental research on strain distribution measurement of PC beams based on weak grating array sensing technology.

The strain and deformation of bridges are important parameters to reflect the state of bridge structures. The traditional measurement methods are to obtain the partial strain of a bridge by point strain sensor or to obtain the deformation of some special bridge positions by static level and other methods. These methods can not realize the continuous distributed measurement of bridge stress state. The grating array is a new sensing technology with the characteristics of large capacity, long distance, distribution, and high precision. In this paper, the grating array strain optical cable is installed on PC (pre-stressed concrete) beams to conduct loading and unloading experiments under various working conditions compared with other measurement methods. The experimental results show that the grating array strain sensing technology can realize the distributed measurement of bridge strain, and offer a what we believe is a novel approach to bridge health monitoring.

Simultaneous measurement of the distributed longitudinal strain and velocity field for a cantilevered cylinder exposed to turbulent cross flow

The structural response of a cantilevered cylinder under free-stream conditions both with low and high turbulence intensity generated by turbulence-generating grids is analysed with concurrent measurements of the velocity field and the distributed strain. The thin-walled cylinder, with a diameter (D) of 50 mm, is mounted in a water flume as a cantilevered beam supported at one end, with 95% of its body submerged and exposed to cross flow yielding a Reynolds number of Re = 25000. We employ a novel combination of simultaneous particle image velocimetry and distributed strain measurements using Rayleigh backscattering fibre optic sensors. These sensors are embedded onto the surface of the cylinder to measure the experienced strain (ε\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\varepsilon $$\\end{document}) of the structure along its spanwise direction, covering both the windward and leeward faces of the cylinder. The sensors fine spatial resolution allows us to discern the influence of the flow on the structural response of the cylinder in two distinct regions of the structure: upstream and downstream of the mean separation location. This differentiation allows us to isolate the local effects introduced by the free-stream conditions on the loading events over the body from the global force generated by the vortex shedding and other coherent motions present within the flow. Distinguishing between these direct and indirect effects helps determine which is more relevant for fatigue-life cycle analysis. The cross power spectral density between the fluctuating velocity field and the strain reveals that the load is dominated by the vortex shedding, and this relationship is intensified with the introduction of free-stream turbulence. It also helps to discern the different dynamics imposed by the two free-stream turbulence conditions.

The Impact of Liquids and Saturated Salt Solutions on Polymer-Coated Fiber Optic Sensors for Distributed Strain and Temperature Measurement.

The application of distributed fiber optic strain and temperature measurement can be utilized to address a multitude of measurement tasks across a diverse range of fields, particularly in the context of structural health monitoring in the domains of building construction, civil engineering, and special foundation engineering. However, a comprehensive understanding of the influences on the measurement method and the sensors is essential to prevent misinterpretations or measurement deviations. In this context, this study investigated the effects of moisture exposure, including various salt solutions and a high pH value, on a distributed strain measurement using Rayleigh backscattering. Three fiber optic sensors with different coating materials and one uncoated fiber were exposed to five different solutions for 24 h. The study revealed significant discrepancies (∼38%) in deformation between the three coating types depending on the surrounding solution. Furthermore, in contrast to the prevailing literature, which predominantly describes swelling effects, a negative deformation (∼-47 με) was observed in a magnesium chloride solution. The findings of this study indicate that corresponding effects can impact the precision of measurement, potentially leading to misinterpretations. Conversely, these effects could be used to conduct large-scale monitoring of chemical components using distributed fiber optic sensing.

Tensile modulus measurement of composite materials based on distributed optical fiber strain monitoring technology

In this paper, a method was proposed to measure the elastic modulus of each layer-material in multi-layer cylindrical structures by OFDR in a large temperature range. Distributed measurement technology was used to carry out accurate measurements of strain distribution and numerical value on specimens. The accurate force-strain relationship can be obtained if the specimen is small or there is inevitable non-uniformity on the specimen. In this paper, a tensile test platform was built, and the tensile tests of optical fiber (−51.6 °C to 82.15 °C), polymer double-layer specimen (−54 °C to 63 °C), and composite three-layer specimen (−46 °C to 62 °C) were carried out. The elastic modulus of the three kinds of material and their relationships with temperature were obtained. By comparing with the standard test results, the accuracy and reliability of the test were verified.

Enhanced cryogenic measurement: Rayleigh-Scattering measurements with dual optical fibers for precise decoupling of temperature and strain

Assessing the operational dynamics of high-temperature superconducting coils during cooling and excitation is essential for their reliable performance and longevity. Rayleigh-scattering-based optical frequency-domain reflectometry distributed optical-fiber sensors (DOFSs) offer a promising solution to real-time monitoring in extreme conditions such as in the presence of intense electromagnetic fields and at cryogenic temperatures; these devices boast immunity to electromagnetic interference and high spatial resolution, and they can be embedded in superconducting coils without compromising their integrity. However, they typically face difficulties distinguishing between thermal and mechanical strains in environments above the temperature of liquid nitrogen. To counter this, we embedded a pair of single-mode Rayleigh-scattering DOFSs side by side—one encased in miniscule capillary tubing to exclusively measure temperature, and the other to measure both strain and temperature. Across a wide temperature range from 77 K to room temperature, we systematically calibrated the temperature- and strain-sensitivity coefficients and demonstrated the technique’s accuracy through comparison against conventional sensors by applying nonuniform tensile and thermal stresses. This dual-sensor approach provides a robust and non-disruptive strategy for the precise measurement of distributed temperature and strain in cryogenic environments.

A preliminary investigation of the inverse Finite Element Method applied to bridge structures

Deflection monitoring plays a key role in bridge Structural Health Monitoring, as deflection data may offer valuable insights into potential structural issues, such as cracks and corrosion. However, monitoring bridge deflections under operative loads presents challenges, particularly in scenarios where establishing a fixed reference point for displacement sensors is impractical, such as when a bridge spans a waterway or the sensors may be vulnerable to vandalism. In recent years, the inverse Finite Element Method (iFEM) has emerged as a possible solution. This method provides a load- and material-independent means of computing structural displacements from strain measurements, in real time at a minimal computational cost. Additionally, the iFEM can serve as a virtual sensing approach to monitor strains at locations lacking sensors, enabling the assessment of the fatigue life consumption at virtually any point in the structure. Despite its potential, applications of the iFEM in the civil engineering field are very limited, with no validation on full scale bridges has been performed to date. This study addresses this gap by investingating the application of the iFEM for bridge deflection monitoring through distributed strain measurements. The proposed approach is tested on experimental case studies, involving cross-validatiion of reference-free deflections inferred by the iFEM with those obtained from displacement sensors. This work contributes to the advancements of structural health monitoring methodologies in civil engineering, demonstrating the practicality and reliability of iFEM in bridge applications.

Distributed Strain Measurements for a 5T Split NbTi Superconducting Magnet Based on Rayleigh Scattering Distributed Fiber

Distributed Strain Measurements for a 5T Split NbTi Superconducting Magnet Based on Rayleigh Scattering Distributed Fiber

High Spatial Resolution OFDR System Based on Independent Component Analysis Algorithm for Long-Range Distributed Strain Measurement

High Spatial Resolution OFDR System Based on Independent Component Analysis Algorithm for Long-Range Distributed Strain Measurement

Nonlinear bond-slip model for fiber-reinforced polymer laminates externally bonded to thermally damaged concrete

This paper presents a novel nonlinear local bond-slip model for fiber-reinforced polymer (FRP) laminates externally bonded to thermally damaged concrete substrates. The proposed model is an extension of an existing two-parameter bond-slip model and incorporates two key parameters including interfacial fracture energy ([Formula: see text]) and interfacial brittleness index ([Formula: see text]). To study the variations of [Formula: see text] and [Formula: see text] with different thermal damage levels of the concrete substrate, an extensive experimental database of shear tests on FRP-to-thermally damaged concrete bonded joints was collected from the existing literature. The [Formula: see text] values were calculated from the peak pull loads with proper consideration of the bond length and width effects, while the [Formula: see text] values were obtained by least-squares regression analysis using experimental load-displacement curves or measured strain distributions in the FRP laminates. The results have indicated that the [Formula: see text] values initially exhibit a slight increase accompanied by mild thermal damage of the concrete substrate after exposure to moderately high temperatures; however, these values significantly decrease when the exposure temperature exceeds 300°C. The [Formula: see text] values initially decrease with high-temperature exposure and stabilize at around 50% of the initial values when the temperatures reach around 400°C. Despite the inherent variability in the test database, the proposed temperature-dependent bond-slip model has demonstrated its accuracy, as demonstrated by the comparisons between the theoretical predictions generated by the model and the corresponding shear test results. This interfacial bond-slip model is expected to serve as a constitutive law to characterize the bond behavior between externally bonded FRP laminates and thermally damaged concrete substrate, thus facilitating the practical application of high-performance FRP composites in the repair and strengthening of thermally or fire-damaged RC members.

Dispersion-induced Delay Deviation and its Influence on Distributed Strain Measurement using OFDR

Dispersion-induced Delay Deviation and its Influence on Distributed Strain Measurement using OFDR

Hysteresis Stress, Strain and Penetration Analysis of Spur Gear Assembly for Various Sustainable Design

This paper considers and compares the hysteresis stress and strain and the penetration property of spur gear assemblies based on three unique designs. Spur gear plays an important part in mechanical structures, and any mechanical setup should consider the execution of such a mechanical component under distinct designs to improve its mechanical productivity and sustainability. To explore the ways in which the mechanical behaviour of the designs varies with the design configurations, we integrate simulation analysis with an experimental study. The outcomes of this paper indicate considerable differences in both hysteresis stress, strain distribution, and penetration behavior measurements between three designs. The paper explains the stated disparities by the unique geometric layouts and material characteristics of each design. Furthermore, it emphasizes that some of the examined designs have lower hysteresis losses and favourable stress and strain distributions, which positively affects the long-term performance of gear systems. Other designs, however, exhibit severe penetration and stress concentrations leading to rapid gear wear and likely premature failure. In distinguishing these events, the present study offers a valuable approach to the parameters that influence the performance of gear systems and aids in the improvement of the design methodology.

Designing a Distributed Sensing Network for Structural Health Monitoring of Concrete Tunnels: A Case Study

Structural health monitoring is essential for the lifecycle maintenance of tunnel infrastructure. Distributed fiber‐optic sensor (DFOS) technology, which is capable of distributed strain measurement and long‐range sensing, is an ideal nondestructive testing (NDT) approach for monitoring linear infrastructures. This research aims to develop a distributed sensing network utilizing DFOS for structural integrity assessment of concrete immersed tunnels. The primary innovations of this study lie in the development of a general flowchart for establishing a sensing network and obtaining reliable field data, as well as its subsequent validation through a detailed case study. Concentrated joint deformations in typical immersed tunnels, detectable by the DFOS, are key indicators of structural integrity. This study addresses crucial elements of field monitoring system design, including the selection of appropriate optical fibers or cables and the determination of vital interrogator system parameters. It also covers sensor parameter determination, installation techniques, field data collection, and postanalysis. Furthermore, this research is exemplified by a case study that illustrates the successful implementation of a distributed sensing network in an operational immersed tunnel, and monitoring data reveals cyclic structural deformations under impacts of daily tide and seasonal temperature variations. The data obtained from this network play a significant role in subsequent condition assessments of tunnel structures. The research findings contribute to the assessment of large‐scale infrastructure health conditions through the application of DFOS monitoring.

A novel damage detection method for carbon fibre reinforced polymer structures based on distributed strain measurements with fibre optical sensor

This paper presents a novel approach for damage detection in carbon fibre reinforced polymer (CFRP) structures based on local strain feature matching. The proposed approach utilizes distributed in-plane strain measurements for plate structures under load (in-service application) to define a local point autocorrelation coefficient based on Delaunay triangulation and Hausdorff distance. This coefficient effectively identifies the discontinuities in distributed in-plate strain, enabling accurate capture of damage information through the damage index which is defined as the integral of this coefficient. Notably, the proposed approach requires no prior knowledge of the non-damaged strain distribution and yields excellent detection results for both Barely Visible Impact Damage (BVID) and Visible Impact Damage (VID). Leveraging the advantages of distributed strain sensing, coupled with image processing technology for the purpose of in-service damage detection of CFRP structures, the proposed novel method is a promising technology. The efficacy of this technology has been demonstrated in this paper through theoretical analysis and experimental investigations, offering valuable insights and reference for future research in this domain.

High-Spatial-Resolution Dynamic Strain Measurement Based on Brillouin Optical Correlation-Domain Sensors

Brillouin-scattering-based sensors have been widely applied in distributed temperature or strain measurement in recent 20 years. Brillouin optical correlation-domain technology has extensive development and application prospects because of its millimeter-level spatial resolution, distribution measurement, and high accuracy. Traditional Brillouin-scattering-based sensors, requiring a time-consuming frequency-sweep process, struggle to achieve dynamic strain measurement. In this article, Brillouin optical correlation-domain analysis and reflectometry based on fast-sweep frequency and slope-assisted methods will be reviewed. The main merits, drawbacks, and performances of these schemes are compared, and the avenues for future research and development of these two technologies are also explored.

Horizontal Deformation Monitoring of Concrete Pile with FRP-Packaged Distributed Optical-Fibre Sensors

Horizontal deformation is a key parameter in the structural assessment of concrete piles, especially in landslide cases. However, the existing deformation-monitoring methods cannot satisfy the demands of long-term monitoring. Therefore, a new method based on distributed optical-fibre sensing technology is proposed for the long-term monitoring of the horizontal deformation of concrete piles. First, a distributed long-gauge optical-fibre sensor is embedded into a fibre-reinforced polymer (FRP) for the excellent distributed strain measurement of the concrete piles in damage cases, such as concrete cracking and reinforcement yielding. Second, based on the typical Winkler beam model, a calculation theory can be constructed for the horizontal deformation of the concrete piles with the input of the strain measurement. Lastly, the proposed method is verified via finite element simulation and static experiments in a laboratory, and the results show good accuracy. Before the case of reinforcement yielding, the largest measurement error of deformation is about 1 mm. It can be up to several millimetres after reinforcement yielding due to the large gap between the calculation model and the actual structure, while the relative measurement error is only about 10%. Due to the distributed strain measurement, the inside horizontal deformation distribution of the concrete piles can be monitored online with the proposed method to implement a detailed assessment of the pile health. Additionally, considering the excellent long-term performance of FRPs and optical-fibre sensors, the proposed method can be applied for the long-term deformation monitoring of concrete piles.