Distributed Strain Measurement Research Articles

Strain distribution measurement in the lamina cribrosa by stepwise pressurization

Strain distribution measurement in the lamina cribrosa by stepwise pressurization

Continuous Measurement of Surface Strain Distribution in Tensile Test of Circular Shaft Specimens Using Movie and Consideration of Stress-Strain Relation

Continuous Measurement of Surface Strain Distribution in Tensile Test of Circular Shaft Specimens Using Movie and Consideration of Stress-Strain Relation

Monitoring of Real-Time Complex Deformed Shapes of Thin-Walled Channel Beam Structures Subject to the Coupling Between Bi-Axial Bending and Warping Torsion

Structural health monitoring (SHM) is a research focus involving a large category of techniques performing in-situ identification of structural damage, stress, external loads, vibration signatures, etc. Among various SHM techniques, those able to monitoring structural deformed shapes are considered as an important category. A novel method of deformed shape reconstruction for thin-walled beam structures was recently proposed by Xu et al. [1], which is capable of decoupling complex beam deformations subject to the combination of different loading cases, including tension/compression, bending and warping torsion, and also able to reconstruct the full-field displacement distributions. However, this method was demonstrated only under a relatively simple loading coupling cases, involving uni-axial bending and warping torsion. The effectiveness of the method under more complex loading cases needs to be thoroughly investigated. In this study, more complex deformations under the coupling between bi-axial bending and warping torsion was decoupled using the method. The set of equations for deformation decoupling was established, and the reconstruction algorithm for bending and torsion deformation were utilized. The effectiveness and accuracy of the method was examined using a thin-walled channel beam, relying on analysis results of finite element analysis (FEA). In the analysis, the influence of the positions of the measurement of surface strain distributions on the reconstruction accuracy was discussed. Moreover, different levels of measurement noise were added to the axial strain values based on numerical method, and the noise resistance ability of the deformation reconstruction method was investigated systematically. According to the FEA results, the effectiveness and precision of the method in complex deformation decoupling and reconstruction were demonstrated. Moreover, the immunity of the method to measurement noise was proven to be considerably strong.

Interface-Governed Deformation of Nanobubbles and Nanotents Formed by Two-Dimensional Materials.

Nanoblisters such as nanobubbles and nanotents formed by two-dimensional (2D) materials have been extensively exploited for strain engineering purposes as they can produce self-sustained, nonuniform in-plane strains through out-of-plane deformation. However, deterministic measure and control of strain fields in these systems are challenging because of the atomic thinness and unconventional interface behaviors of 2D materials. Here, we experimentally characterize a simple and unified power law for the profiles of a variety of nanobubbles and nanotents formed by 2D materials such as graphene and MoS_{2} layers. Using membrane theory, we analytically unveil what sets the in-plane strains of these blisters regarding their shape and interface characteristics. Our analytical solutions are validated by Raman spectroscopy measured strain distributions in bulged graphene bubbles supported by strong and weak shear interfaces. We advocate that both the strain magnitudes and distributions can be tuned by 2D material-substrate interface adhesion and friction properties.

Fiber-optic simultaneous distributed monitoring of strain and temperature for an aircraft wing during flight.

We applied a fiber optic distributed simultaneous strain and temperature measurement technique to the structural monitoring of the main wing of a middle-sized passenger jet aircraft during flight. We used 40 10cm long fiber Bragg gratings (FBGs), inscribed in a highly birefringent polarization-maintaining fiber. The FBGs were interrogated by optical frequency domain reflectometry, which could measure Bragg wavelength distributions at a sampling rate of 151Hz. The simultaneous measurement technique could detect structural behaviors of the wing during flight under temperature-changing conditions. In addition, we discuss the effect of the polarization mode-coupling and the apparent position shift of the FBGs over time, which occurred during flight.

Mechanical performance of polystyrene foam (EPS): Experimental and numerical analysis

In this work, we perform a series of tests to characterize the mechanical properties of polystyrene foam (EPS), including uniaxial compression tests, uniaxial tension tests and three point bending tests. EPS exhibits a ductile to brittle transition when the loading mode changes from compression to tension. The elastic modulus in tension is nearly one order larger than that measured in compression. The digital image correlation (DIC) technique is used to measure the strain distribution. A homogenous deformation can be observed in uniaxial deformation tests, where EPS exhibits a near-zero Poisson's ratio. For the three point bending tests, DIC can capture the strain localization behavior of specimens with a prefabricated crack. Based on the experimental observations, a simple phenomenological model is developed and further implemented for finite element analysis. The simulation results show reasonable agreement with experimentally measured strain distribution in the bending tests.

Monitoring of Beams in an RC Building during a Load Test Using Distributed Sensors

AbstractThe understanding of the complex behavior of reinforced concrete elements in situ would be aided if distributed strain measurements were captured during load tests, potentially leading to m...

Comparison between different fiber coatings and adhesives on steel surfaces for distributed optical strain measurements based on Rayleigh backscattering

Abstract. Optical fiber measurement systems have recently gained popularity following a multitude of intensive investigations. A new technique has been developed for these measurement systems that uses Rayleigh backscatter to determine the distributed strain measurement over the total length of a fiber. These measurement systems have great potential in civil engineering and structural health monitoring. This paper addresses some preliminary comparisons between three different fiber coatings and six different adhesives on steel structures. The results are based on a bending test with specimens made of precision flat steel; optical fiber strain measurements were compared with photogrammetric strain measurements. Analysis of the test data showed a strong correlation between the optical measurement system's results and the theoretical results up to the yielding point of the steel. Furthermore, the results indicate that fibers with the Ormocer® and polyimide coatings have almost the same strain values as the reference measurement method. The main results of this investigation are a guideline describing how to attach optical fibers to steel surfaces for distributed fiber optical strain measurements and recommendations for coatings to obtain realistic strain values. Additionally, the advantages of distributed strain measurements were revealed, which illustrates the potential of Rayleigh backscattering applications.

Fiber Optic Sensing for Geomechanical Monitoring: (1)-Distributed Strain Measurements of Two Sandstones under Hydrostatic Confining and Pore Pressure Conditions

In this study distributed optic fiber has been used to measure both the Rayleigh and Brillouin frequency shift of two different sandstone core samples under controlled hydrostatic confining and pore pressure in the laboratory. The Berea sandstone core is relatively homogeneous, whereas the Tako sandstone core is visibly heterogeneous with a coarse-grain and a fine-grain region. Rayleigh frequency has been found to have a superior performance over Brillouin frequency in terms of better consistency (less scattering) in the tests carried out. The strain gauge readings reveal considerable anisotropy in the stiffness of the Berea core between perpendicular (vertical) and parallel to the bedding (hoop) directions. The strains converted from Rayleigh frequency shift measurements agree reasonably well with readings by one of the four hoop strain gauge channels under increasing confining/pore pressure. For the Tako sandstone core, the contrast in the grain-size, and thus rock elastic properties, is clearly reflected in the hoop strain measurement by both strain gauges and distributed optic fiber. The outcomes of the test have demonstrated successfully the use of a single optic fiber for measuring rock strain response at different regions of a heterogeneous core sample along a continuous trajectory.

In situ observation and strain distribution measurements of atmospheric plasma-sprayed mullite and Si multilayered coatings on SiC substrates

A mullite/Si multilayer coating was deposited on a SiC substrate using atmospheric plasma spraying. Mud cracking of the protective surface mullite layer was examined using a high-temperature observation system (HTOS). In-plane surface strain distributions were also measured using the digital image correlation (DIC) method. The experimental results showed that mud cracks nucleated in the mullite layer during heating, primarily due to shrinkage of the mullite during crystallization. The cracking event was detectable by mapping the maximum principal strain distribution. We clearly demonstrated that HTOS and DIC are effective tools for investigating the degradation mechanisms of thick coatings at high temperatures.

Distributed Strain Damage Identification Technique for Long-Span Bridges Under Ambient Excitation

Distributed strain measurement, such as long-gauge fiber bragger grating (FBG) techniques, has developed rapidly in the field of structural health monitoring. However, strategies of corresponding damage identification still need to be enhanced. The damage identification technique based on distributed strain measurement is proposed identifying the structural damage under ambient excitation. Damage indices, like the distributed strain energy difference (DSED) and the relative distributed strain energy (RDSE), are derived from the power spectral density of the frequency response function of distributed strain response and further employed by detecting damages in the structure. A numerical analysis is performed on a long-span cable-stayed bridge with several assumed damage scenarios at various degrees in the girder. Damage localization capability and robustness of the proposed damage indices are discussed. In addition, damage quantification utilizing the proposed indices is conducted. The results indicate that the proposed technique can accurately identify the locations and extent of the damage under ambient vibration excitation.

In-situ micro-tensile testing of AA2024-T3 fibre laser welds with digital image correlation as a function of welding speed

In this paper, the influence of welding speed on tensile properties of AA2024-T3 fibre laser welds was investigated by monitoring the deformation behaviour during tensile loading. In-situ micro-tensile testing combined with a high-resolution optical microscope and DIC was used to measure strain distribution in narrow weld regions showing characteristics of fibre laser beam welding with limited metallurgical modifications. A chemical etching technique was used to generate a micro-scale random speckle pattern by revealing the weld microstructure. Such pattern enabled a sufficient spatial resolution of strain while keeping the weld seam visible during deformation. The results of microstructural and mechanical properties determined under numerous welding speeds indicated that increasing the welding speed led to the transition of weld pool shape from circular to elliptical to teardrop with a greater fraction of equiaxed dendrites. The weaker strength of the weld, as measured by local lower micro-hardness values, constrained significant plasticity development locally within the weld. Tensile tests revealed that increasing the welding speed resulted in greater yield strength and ultimate tensile strength, whereas, total elongation to failure dropped. The tensile properties improved with increasing welding speed as the fraction of equiaxed dendrites increased.

A Running Reference Analysis Method to Greatly Improve Optical Backscatter Reflectometry Strain Data from the Inside of Hardening and Shrinking Materials

Due to the increasing ease of use and the superiority of the results, distributed strain measurements, utilizing Optical Backscatter Reflectometry (OBR), have become more important and widespread over the last few years. Strains are calculated from the difference between an actual optical Raleigh backscattering measurement and an initial reference value. However, under certain physical conditions, e.g., pinching or microbending of the optical fiber, no meaningful strain values are yielded by the commonly-used method to analyze OBR data. Such conditions were experienced in this study where the optical fiber was embedded into hardening epoxy for measuring shrinkage due to curing. In this work, it is shown that a new data analysis method called the “running reference analysis method” can overcome such obstacles and deliver meaningful strain values in circumstances in which the traditional method fails. In the new approach, each measurement is compared to the previous measurement, and the strain differences are added up to the absolute strain value. This method does not require a new experimental technique and will also work on old measurement files. It is also useful for other types of (OBR) strain measurements that contain many outliers and is not restricted to the investigation of cured epoxy.

Influence of atomic site-specific strain on catalytic activity of supported nanoparticles

Heterogeneous catalysis is an enabling technology that utilises transition metal nanoparticles (NPs) supported on oxides to promote chemical reactions. Structural mismatch at the NP–support interface generates lattice strain that could affect catalytic properties. However, detailed knowledge about strain in supported NPs remains elusive. We experimentally measure the strain at interfaces, surfaces and defects in Pt NPs supported on alumina and ceria with atomic resolution using high-precision scanning transmission electron microscopy. The largest strains are observed at the interfaces and are predominantly compressive. Atomic models of Pt NPs with experimentally measured strain distributions are used for first-principles kinetic Monte Carlo simulations of the CO oxidation reaction. The presence of only a fraction of strained surface atoms is found to affect the turnover frequency. These results provide a quantitative understanding of the relationship between strain and catalytic function and demonstrate that strain engineering can potentially be used for catalyst design.

Assessment of a steel model truss using distributed fibre optic strain sensing

Steel truss bridges are a critical part of the North American rail bridge inventory, with many being over a century old. Current assessment techniques are often based on visual inspections, which do not provide quantitative data to assess critical areas of the bridge with damage or deterioration. Distributed fibre optic strain sensors may be able to provide quantitative data to assess the effects of damaged members, different connection types, or deterioration. As a first step towards developing this technology for truss bridge assessment, a model truss bridge with three types of joint conditions and two types of simulated deterioration was monitored using two types of distributed fibre optic strain sensors while under static and cyclic loading. Although the impact of the changing joint conditions was clearly evident from displacement measurements, it was only possible to explain the difference in displacement measurements with the aid of the distributed strain data. At model scale, member continuity and joint movement played a significant role in the truss behaviour, which was evident from the strain data. Localized deterioration was detectable using dynamic distributed strain measurements although proximity to the deterioration and choice of sensing fibre influenced the accuracy of the measurements. Based on the results of the lab scale testing, distributed strain monitoring shows promise for steel railway truss bridge assessments although field trials of the monitoring technology are a required next step.

Measurement of Strain Distribution of Hallux Nail in Normal Gait

Measurement of Strain Distribution of Hallux Nail in Normal Gait

Distributed strain measurement and possible breakage detection of optical-fiber-embedded composite structure using slope-assisted Brillouin optical correlation-domain reflectometry

Slope-assisted Brillouin optical correlation-domain reflectometry (SA-BOCDR) is a recently developed structural health monitoring technique for measurements of strain, temperature, and loss distributions along optical fibers. Although the basic operational principle of this method has been clarified, no measurements using optical fibers embedded in actual structures have been reported. As a first step towards such practical applications, in this study, we present an example of an SA-BOCDR-based diagnosis using a composite structure with carbon fiber-reinforced plastics. The system’s output agrees well with the actual strain distributions. We were also able to detect the breakage of the embedded fiber, thus demonstrating the promise of SA-BOCDR for practical applications.

Optical full-field strain measurement method from wrapped sampling Moiré phase to minimize the influence of defects and its applications

For strain measurement in the case of complex situations such as when defects (including cracks, notches and stains) exist, the traditional phase unwrapping algorithm will bring great error in strain distributions around defects. In this study, a simple local phase unwrapping algorithm was proposed to minimize the influence of defects on full-field strain measurement. From the specimen grid images before and after deformation, the Moiré phases are first acquired by the sampling Moiré technique. The wrapped Moiré phase difference is then calculated to determine the strain distributions by only unwrapping the phase difference at the boundaries of the wrapped phase. In other words, the partial differentials of the Moiré phase difference are corrected by local phase compensation for strain calculation. The accuracy of the developed strain measurement method was verified from numerical simulations. As applications, this method was successfully used in microscale strain distribution measurements of an aluminum specimen with a prefabricated crack and several grid defects under tensile loading, and a titanium alloy specimen with a prefabricated notch and an emerged irregular crack under tensile-fatigue loading. The local phase unwrapping algorithm can also be integrated with geometric phase analysis for strain measurement.

Mapping Microscale Strain Fields Around a Crack Tip in Molybdenum Via Geometric Phase Analysis and Digital Image Correlation

Uniaxial tension tests were conducted on single-edge-notched tensile specimens of pure molybdenum with a mesh grid pattern in front of the notch. A series of images of crack initialization and propagation with a distorted mesh grid pattern were obtained by means of in situ scanning electron microscopy. Strain fields around the crack tip were mapped successively using geometric phase analysis and digital image correlation techniques, and then compared with the predictions obtained through linear elastic fracture mechanics (LEFM). The comparison shows that the measured strain distribution ahead of the crack tip is consistent with the LEFM predictions of up to 25 $$\upmu $$ m from the crack tip.

Anisotropic distribution of the micro residual stresses in lath martensite revealed by FIB ring-core milling technique

Lath martensite structures in medium-carbon steels incorporate a significant amount of residual stresses that are mostly induced by the martensitic transformation process. Although former studies could identify these stresses using diffraction techniques, it was not possible to correlate the micro-scale distribution of the stress fields with respect to the morphological and the crystallographic parameters of the martensitic structure. In this study, we employ the micro-scale focused ion beam (FIB) ring-core milling technique for the measurement of local residual strain and stress distributions in fully martensitic microstructures. The aim is to study the residual stresses occurring within individual lath martensite crystals, and within areas of lath martensite which incorporate a parent austenite grain boundary. The relaxation strains obtained by the micrometer-sized ring-core milling, which correspond to the residual stresses prior to milling, are shown to exhibit an anisotropic distribution for each martensite variant. High extension relaxation strains (i.e. compressive stresses) prevail in the direction of the transformation-induced crystal shape deformation direction. Contraction strains (i.e. tensile residual stresses) are measured normal to the extension strains. In an area containing a prior austenite grain boundary, the residual stresses appeared – altogether – lower than in single crystal martensite laths. The significant residual tensile stresses identified in the martensite structures may support the formation of martensite micro-cracks, either in the as-quenched state or during deformation.