Accurate Strain Measurements Research Articles

Calibration and correction of lens distortion for two-dimensional digital speckle correlation measurement

Lens distortion practically presents in optical imaging system using in two-dimensional digital speckle correlation measurement (2D-DSCM) system, and gives rise to additional errors in the displacement and strain measurement. Camera calibration procedure is performed to obtain the coefficients of radial distortion and tangential distortion. The corrected displacement fields can be calculated using the distortion coefficient. The influence of distortion on displacement and strain measurement errors in experiment is further discussed. The three-point bending test result shows that the camera lens calibration method can effectively eliminate the effect of lens distortion and improve displacement and strain measurement accuracy.

Cyclic pulse coding for fast BOTDA fiber sensors

A cyclic pulse coding technique is proposed and experimentally demonstrated for fast implementation of long-range Brillouin optical time-domain analysis (BOTDA). The proposed technique allows for accurate temperature and strain measurements with meter-scale spatial resolution over kilometers of standard single-mode fiber, with subsecond measurement times.

A Sampling Moiré Method to Measure the Dynamic Shape and Strain of Rotating Tires

ABSTRACT There has long been a strong desire to know the deformation and surface shape of tires rotating at high speeds, both in order to verify finite element modeling analysis results and for the design of tire construction and materials. We used the sampling Moiré method to measure sidewall deformation and surface strain of rotating tires when cornering or braking. This method takes photos simultaneously with two cameras placed at a distance from a tire sidewall with a grid sheet laid over it, allowing precision 3D rendering through analysis of these images. This enables a practical and accurate chronological measurement of deformation and surface strain in tires rotating at high speeds, something that was difficult to do until now. But owing to hardware limitations, this had only previously been done in the static state. By combining the sampling Moiré method with high-speed, high-definition cameras, it is possible to measure the time-based shape and surface strain of tires rotating at high speeds, with practical accuracy. In this paper we have made continuous, time-based measurements of surface strain when a sinusoidal slip angle is applied to the tires and measurements of changes in surface strain when enveloping a cleat.

Fiber Bragg grating-based plane strain monitoring of aerostat envelope structures

A theoretical analysis of fiber Bragg grating (FBG)-based plane strain monitoring of aerostat envelope structures is presented. Plane strain analysis of FBG-based aerostat envelope structures is much more complex than the case along the axis of the optical fiber because the effect of transverse stress on the FBG should be taken into consideration. To achieve accurate strain measurement of the aerostat envelope, a theoretical model is set up by using two perpendicular fibers in the monitoring. An analytical formula that evaluates the relationship between the strain measured by FBG sensors and the real one in the aerostat envelope is established. On the other hand, the real strain of aerostat envelope strain is affected by two unknown parameters, axial transfer rate K(L) and the radial transfer rate K(R). An equation is derived to calculate the axial transfer rate K(L). Then, the finite element method results show that K(R) is a very small value, but it cannot be ignored in accurate measurement. This paper would lay a theoretical groundwork for the research and design of FBG sensors in the structural health monitoring of aerostat envelope structures.

An In-Reflection Strain Sensing Head Based on a Hi-Bi Photonic Crystal Fiber

A photonic crystal fiber-based sensing head is proposed for strain measurements. The sensor comprises a Hi-Bi PCF sensing head to measure interferometric signals in-reflection. An experimental background study of the sensing head is conducted through an optical backscatter reflectometer confirming the theoretical predictions, also included. A cost effective setup is proposed where a laser is used as illumination source, which allows accurate high precision strain measurements. Thus, a sensitivity of ∼7.96 dB/ms was achieved in a linear region of 1,200 με.

Dynamic Micro-Strain Analysis of Ultrafine-Grained Aluminum Magnesium Alloy Using Digital Image Correlation

Tensile tests were performed in situ on an ultrafine-grained (UFG) Al-Mg alloy using a micro-tensile module in a scanning electron microscope. The micro-strain evolution was tracked and measured using digital image correlation (DIC). A fine random speckle pattern was required to achieve high resolution and accuracy of strain measurement using DIC. To produce the speckle pattern, a patterning method was developed using electron beam lithography to deposit a gold speckle pattern. The nanoscale feature size of this gold pattern (45 nm) was useful for identifying the micro-strain among individual grains of the UFG Al-Mg alloy. Microstructural aspects of the UFG Al-Mg alloy were revealed by analysis of electron backscattered diffraction (EBSD) patterns. Finally, the effect of the UFG Al-Mg alloy microstructure on the nanoscale deformation mechanism was investigated by combining EBSD and DIC data in a contour map. This combined technique provides a method for direct measurement of micro-strain and is potentially useful for deformation studies of a wide range of nanostructured materials.

Validation of three-dimensional strain tracking by volumetric ultrasound image correlation in a pubovisceral muscle model

Little is understood about the biomechanical changes leading to pelvic floor disorders such as stress urinary incontinence. In order to measure regional biomechanical properties of the pelvic floor muscles in vivo, a 3D strain tracking technique employing correlation of volumetric ultrasound images has been implemented. In this technique, local 3D displacements are determined as a function of applied stress and then converted to strain maps. To validate this approach, an in vitro model of the pubovisceral muscle, with a hemispherical indenter emulating the downward stress caused by intra-abdominal pressure, was constructed. Volumetric B-scan images were recorded as a function of indenter displacement while muscle strain was measured independently by a sonomicrometry system (Sonometrics). Local strains were computed by ultrasound image correlation and compared with sonomicrometry-measured strains to assess strain tracking accuracy. Image correlation by maximizing an exponential likelihood function was found more reliable than the Pearson correlation coefficient. Strain accuracy was dependent on sizes of the subvolumes used for image correlation, relative to characteristic speckle length scales of the images. Decorrelation of echo signals was mapped as a function of indenter displacement and local tissue orientation. Strain measurement accuracy was weakly related to local echo decorrelation.

Note: Reducing polarization induced sidebands in Rayleigh backscattering spectra for accurate distributed strain measurement using optical frequency-domain reflectometry

We describe a phenomenon called polarization-induced sidebands (PIS) in Rayleigh backscatter spectra (RBS) and discuss its deteriorating effects on the distributed strain measurement using an optical frequency-domain reflectometry. We propose using a special polarization diversity detection scheme to remove PIS and successfully demonstrate accurate distributed strain measurement in the range of 0.75 με-225 με in a 50 m standard single mode fiber, with a good linearity between the strain and the spectra shift in RBS.

Displacement tracking in single human trabecula with metal-plated micro-spheres using X-ray radiography imaging

This study presents an improved radiographic method for strain measurement in very small samples of a single trabeculae. X-ray micro-radiography was used to track the deformation behaviour of individual trabecula during mechanical loading. As the X-ray micro-radiography images of a single trabecula show no significant features applicable for digital image correlation (DIC) a random pattern of markers was created on the surfaces of the samples to improve the accuracy of tracking. Metal plated borosilicate glassmicro-spheres (mean diameter 10 μm) were used as the markers for trabecular displacement tracking.Two different X-ray imaging setups were used for this purpose. The specimens of isolated trabeculae were loaded by a micro-mechanical testing device developed with respect to radiographical observation. This compact device enables a high precision three-point bending measurement. The specimen was continuously irradiated during the loading procedure by a micro-focus X-ray source. The radiographs were acquired by a single-photon counting silicon pixel detector and s flat panel sensor with CsI flipped scintillator plate.Circular Hough transform was used to locate positions of the spherical markers in the sequence of acquired radiographs and to calculate the strain in the loaded sample. The gold-coated micro-spheres provide clearly visible features in the sequence of radiographs after beam hardening correction, which in conjunction with pattern recognition algorithm enables to substantially improve the accuracy of strain measurements.

Evaluation of Stress and Strain Measurement Accuracy in Hydraulic Bulge Test with the Aid of Finite-element Analysis

Numerical simulations of the hydraulic bulge test are carried out by the implicit static finite-element method. The sheet specimen is characterized as a rate-independent elastoplastic material with a power-law hardening rule. The stress and strain relationship of the specimen is evaluated from the internal pressure and nodal coordinates obtained in the finite-element simulation of the hydraulic bulge test. Varying the gauge lengths of the spherometer and extensometer and the ratio of the initial thickness to the diameter of the specimen, their influences on the estimated stress and strain are investigated. By comparing the estimated stress-strain relationship with that of exact input data, the stress and strain measurement accuracy is assessed. Furthermore, the stress state at the apex is examined for orthotropic specimens and is found to deviate by 1–5% from the equi-biaxial stress state.

806 高温における一方向CFRPの非主軸メカニカルラチェット変形の2次元画像相関全視野計測(OS8 オーガナイズドセッション《複合材料の変形と破壊およびマルチスケール計算技術》)

Two-dimensional off-axis ratcheting behavior of unidirectional CFRP laminates at high temperature has been examined for different fiber orientations on the basis of strain field measured by means of a digital image correlation (DIC) method with two CCD cameras. The accuracy of strain measurement with DIC is first evaluated by comparing the stress-strain curves identified with DIC and strain gauge, respectively, indicating that in-plane strains and their distributions can adequately be measured with DIC. Then, accumulation of in-plane strains in off-axis specimens at high temperature under an asymmetric cyclic loading at a constant stress ratio is observed for different fiber orientations. Non-uniform strain field in off-axis specimen during ratcheting that depends on fiber orientation is successfully visualized by DIC. It is shown that two-dimensional off-axis ratcheting behavior depends on fiber orientation, and the magnitude of ratchet strain tends to become larger than that of creep strain at the constant stress with a magnitude equal to the maximum stress of cyclic loading. It is also revealed that the time dependence of off-axis ratcheting behavior is similar to that of off-axis creep behavior.

An Experimental and Numerical Hybrid Technique for Analysis the Fracture Behaviors in Natural Fiber Reinforced Composites

Natural fiber reinforced composites have become excellent candidates for automotive applications and civil engineering facilities. The design procedures will require precisely evaluating the toughness of the materials, which demand accurate measurements of strain or displacement that are difficult to obtain for ductile materials. The primary objectives of this paper were to develop reliable experimental approaches which can effectively evaluate the fracture behaviors of random short natural fiber reinforced polypropylene (PP) composites. The digital image correlation method (DICM) was applied to compute the deformation fields around the crack tip of a compact-tension specimen in a process of loading. An experimental and numerical hybrid method was used to describe the fracture behaviors of the composites. The relationship between the micro-structure of random fibers and the macroscopic mechanical properties of the composites can also be understood.

Integrated Optical Fiber Sensing System by Combing Large-Scale Distributed BOTDA/R and Localized FBGs

Structural health monitoring (SHM) has been regarded as a significant tool for the safety of civil infrastructures. Local fiber Bragg grating (FBG) sensor and distributed Brillouin optical fiber sensor have been successfully applied in civil engineering fields. Unfortunately, neither single FBG nor single Brillouin sensing technique can satisfy the requirements of simultaneously positioning full-scale structural damages and accurate local damage details. So it still matters to establish balance between localized high-precision measurement and large-scale distributed measurement with lower accuracy. This paper introduces an integration system by combining distributed Brillouin and local FBG sensors on one single-mode optical fiber. The temperature self-compensation and the data processing method of the integration system have been investigated. The integrated FBG/Brillouin sensor has been further packaged by glass fiber-reinforced polymer to be used as a robust and smart structural component. Finally the proposed integrated sensing system has been applied in three case studies, including investigation of reinforced concrete beam, smart cable, and smart steel strand to verify the feasibility. The test results illustrate that the integrated system can simultaneously provide accurate strain measurement at critical points by FBGs and the rough distributed strain measurement by Brillouin sensor, which can be further applied to various applications.

Digital Image Correlation with Self-Adaptive Gaussian Windows

A novel subpixel registration algorithm with Gaussian windows is put forward for accurate deformation measurement in digital image correlation technique. Based on speckle image quality and potential deformation states, this algorithm can automatically minimize the influence of subset sizes by self-adaptively tuning the Gaussian window shapes with the aid of a so-called weighted sum-of-squared difference correlation criterion. Numerical results of synthetic speckle images undergoing in-plane sinusoidal displacement fields demonstrate that the proposed algorithm can significantly improve displacement and strain measurement accuracy especially in the case with relatively large deformation.

Fiber-Optic Strain Sensor System With Temperature Compensation for Arch Bridge Condition Monitoring

This paper presents an innovative sensor system, created specifically for new civil engineering structural monitoring applications, allowing specially packaged fiber grating-based sensors to be used in harsh, in-the-field measurement conditions for accurate strain measurement with full temperature compensation. The sensor consists of two fiber Bragg gratings that are protected within a polypropylene package, with one of the fiber gratings isolated from the influence of strain and thus responding only to temperature variations, while the other is sensitive to both strain and temperature. To achieve this, the temperature-monitoring fiber grating is slightly bent and enclosed in a metal envelope to isolate it effectively from the strain. Through an appropriate calibration process, both the strain and temperature coefficients of each individual grating component when incorporated in the sensor system can be thus obtained. By using these calibrated coefficients in the operation of the sensor, both strain and temperature can be accurately determined. The specific application for which these sensors have been designed is seen when installed on an innovative small-scale flexi-arch bridge where they are used for real-time strain measurements during the critical installation stage (lifting) and loading. These sensors have demonstrated enhanced resilience when embedded in or surface-mounted on such concrete structures, providing accurate and consistent strain measurements not only during installation but subsequently during use. This offers an inexpensive and highly effective monitoring system tailored for the new, rapid method of the installation of small-scale bridges for a variety of civil engineering applications.

Influence of mechanical and geometrical properties of embedded long-gauge strain sensors on the accuracy of strain measurement

In many civil and geotechnical applications it is of interest to monitor the strain deep inside the structure; consequently, it is necessary to embed the sensors into the structure's material. Construction and geotechnical materials, such as concrete and soil, can be affected by local defects, e.g. cracks, air pockets and inclusions. To monitor these materials at a structural level it is necessary to use long-gauge sensors. As the sensor has to be embedded in the host material, its presence causes perturbation of the strain field and influences the accuracy of the strain measurement. The aim of this research was to identify the critical parameters that influence the accuracy of the strain measurement, to study how these parameters affect the accuracy, and to give recommendations for sensor users. The study was based on finite element analysis and all involved materials were assumed to have the Möhr–Coulomb elastic, perfectly plastic behavior. A suitability of the numerical model for the analysis was verified using the experimental results of two cases reported in the literature and one on-site application. The study revealed that the most important parameters that influence the accuracy of the strain measurement are the goodness of interaction (strain transfer) between the host material and the anchor pieces of the sensor, the ratio between equivalent Young's modulus of the sensor and the Young's modulus of the host material, the radius of the anchor piece and the gauge length. The numerical model and parametric study are presented in detail along with practical recommendations.

Noninvasive high resolution mechanical strain maps of the spine intervertebral disc using nonrigid registration of magnetic resonance images

High resolution strain measurements are of particular interest in load bearing tissues such as the intervertebral disc (IVD), permitting characterization of biomechanical conditions which could lead to injury and degenerative outcomes. Magnetic resonance (MR) imaging produces excellent image contrast in cartilaginous tissues, allowing for image-based strain determination. Nonrigid registration (NRR) of MR images has previously demonstrated sub-voxel registration accuracy although its accuracy and precision in determining strain has not been evaluated. Accuracy and precision of NRR-derived strain measurements were evaluated using computer generated deformations applied to both computer generated images and MR images. Two different measures of registration similarity—the cost function which drives the registration algorithm—were compared: Mutual Information (MI) and Least Squares (LS). Strain error was evaluated with respect to signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), and strain heterogeneity. Additionally, the creep strain response from an in vitro loaded porcine IVD is shown and comparisons between similarity measures are presented. MI showed a decrease in strain precision with increasing CNR and decreasing SNR while LS was insensitive to both. Both similarity measures showed a decrease in strain precision with increasing strain heterogeneity. When computer generated heterogeneous strains were applied to MR images of the IVD, LS showed substantially lower strain error in comparison to MI. Results suggest that LS-driven NRR provides a more accurate image-based method for mapping large and heterogeneous strain fields and this method can be applied to studies of the IVD and, potentially, other soft tissues which present sufficient image texture.

Orientation and deformation of mineral crystals in tooth surfaces

Tooth enamel is the hardest material in the human body, and it is mainly composed of hydroxyapatite (HAp)-like mineral particles. As HAp has a hexagonal crystal structure, X-ray diffraction methods can be used to analyze the crystal structure of HAp in teeth. Here, the X-ray diffraction method was applied to the surface of tooth enamel to measure the orientation and strain of the HAp crystals. The c-axis of the hexagonal crystal structure of HAp was oriented to the surface perpendicular to the tooth enamel covering the tooth surface. Thus, the strain of HAp at the surface of teeth was measured by X-ray diffraction from the (004) lattice planes aligned along the c-axis. The X-ray strain measurements were conducted on tooth specimens with intact surfaces under loading. Highly accurate strain measurements of the surface of tooth specimens were performed by precise positioning of the X-ray irradiation area during loading. The strains of the (004) lattice plane were measured at several positions on the surface of the specimens under compression along the tooth axis. The strains were obtained as tensile strains at the labial side of incisor tooth specimens. In posterior teeth, the strains were different at different measurement positions, varying from tensile to compressive types.

Strain measurements on a cantilever beam with fiber bragg grating sensors using a pair of collimators

Fiber bragg grating (FBG) sensors are currently in use in various fields. However, it is difficult to use these sensors to obtain strain measurements at locations where it is not physically possible to connect the fibers to each other. In the present study, we performed strain measurements using many FBG sensors connected to a single optical fiber and a pair of collimators. The single optical fiber connected to the FBG sensors was installed on a cantilevered beam with another optical fiber line connected to a stationary optical interrogator. The two optical fiber lines were connected by a pair of collimators making use of the fact that light travels in a straight line through space. Our experiment demonstrated that the wavelength of the light changed linearly as it passed between the two collimators, and that this change increased with the applied strain. Accurate strain measurements could be obtained by applying a proposed collimator collection factor.

Crack Generating Detection Technique of Reinforced Concrete Structure Using Three-Dimensional Measurement Based on Stereo Camera System

It is extremely important to measure the strain distribution of reinforced concrete structures and to detect the generated cracks location. Currently, the digital image correlation (DIC) method is applied for measuring technique. However, the DIC method has several problems concerning accuracy; an out-of-plane deformation of structures has an adverse effect on the strain measurement accuracy. For improving the strain measurement accuracy of the DIC method, it is essential to compensate the equivalent strain owing to the out-of-plane deformation of the measurement objects so that it is necessary to measure the three-dimensional surface geometrization using the stereo camera system. This paper discusses the crack generating detection technique of reinforced concrete structure using the three-dimensional method measurement based on the stereo camera system. In particular, the previously our proposed compensation method for the equivalent strain is used. As a result, it is found that the three-dimensional measurement method based on stereo camera system with our compensation method is superior to existing both the strain distribution and the crack generating detection technique, on a basis of a series of experimental examination using the large-scale four-point bending test.