Accurate Strain Measurements Research Articles

Integrated Assessment of Left Ventricular Electrical Activation and Myocardial Strain Mapping in Heart Failure Patients: A Holistic Diagnostic Approach for Endocardial Cardiac Resynchronization Therapy, Ablation of Ventricular Tachycardia, and Biological Therapy.

This study sought to test the accuracy of strain measurements based on anatomo-electromechanical mapping (AEMM) measurements compared with magnetic resonance imaging (MRI) tagging, to evaluate the diagnostic value of AEMM-based strain measurements in the assessment of myocardial viability, and the additional value of AEMM over peak-to-peak local voltages. The invivo identification of viable tissue, evaluation of mechanical contraction, and simultaneous left ventricular activation is currently achieved using multiple complementary techniques. In 33 patients, AEMM maps (NOGA XP, Biologic Delivery Systems, Division of Biosense Webster, a Johnson & Johnson Company, Irwindale, California) and MRI images (Siemens 3T, Siemens Healthcare, Erlangen, Germany) were obtained within 1 month. MRI tagging was used to determine circumferential strain (Ecc) and delayed enhancement to obtain local scar extent (%). Custom software was used to measure Ecc and local area strain (LAS) from the motion field of the AEMM catheter tip. Intertechnique agreement for Ecc was good (R2= 0.80), with nonsignificant bias (0.01 strain units) and narrow limits of agreement (-0.03 to 0.06). Scar segments showed lower absolute strain amplitudes compared with nonscar segments: Ecc (median [first to third quartile]: nonscar -0.10 [-0.15 to -0.06] vs. scar -0.04 [-0.06 to -0.02]) and LAS (-0.20 [-0.27 to -0.14] vs. -0.09 [-0.14 to -0.06]). AEMM strains accurately discriminated between scar and nonscar segments, in particular LAS (area under the curve: 0.84, accuracy= 0.76), which was superior to peak-to-peak voltages (nonscar 9.5 [6.5 to 13.3] mV vs. scar 5.6 [3.4 to 8.3] mV; area under the curve: 0.75). Combination of LAS and peak-to-peak voltages resulted in 86% accuracy. An integrated AEMM approach can accurately determine local deformation and correlates with the scar extent. This approach has potential immediate application in the diagnosis, delivery of intracardiac therapies, and theirintraprocedural evaluation.

Accuracy of surface strain measurements from transmission electron microscopy images of nanoparticles

Strain analysis from high-resolution transmission electron microscopy (HRTEM) images offers a convenient tool for measuring strain in materials at the atomic scale. In this paper we present a theoretical study of the precision and accuracy of surface strain measurements directly from aberration-corrected HRTEM images. We examine the influence of defocus, crystal tilt and noise, and find that absolute errors of at least 1–2% strain should be expected. The model structures include surface relaxations determined using molecular dynamics, and we show that this is important for correctly evaluating the errors introduced by image aberrations.

Mesoscale experimental procedure and finite element analysis for an indirect tensile test of asphalt concrete

This paper describes the experimental procedures used and finite element analyses performed at the mesoscale for an indirect tensile (IDT) strength test of an Asphalt Concrete-25 Marshall specimen. The mesoscale test specimen was a three-phase composite material composed of aggregate, asphalt mastic, and void. The test results demonstrate that digital image technology combined with a finite element numerical analysis model can be used to simulate the mesoscopic damage process of asphalt concrete in an IDT test. A contactless optical system with a strain measurement accuracy of 50 was used and was a suitable method for experimentally verifying the mesoscopic strain measurements. Although the optical method provided results that were always slightly higher than those of the mesoscopic finite element method (FEM) analysis, the optical microstructure test results showed reasonably good agreement with those obtained from the FEM. Both the experimental and FEM results demonstrate that the mastic binder and interface are locations of weakness during the growth and propagation of crack damage in asphalt concrete. The study results demonstrate that the visual mesoscopic FEMs used can be an effective means for further research on the strength mechanisms of asphalt concrete at the mesoscale.

An investigation of interface transferring mechanism of surface-bonded fiber Bragg grating sensors.

Surface-bonded fiber Bragg grating sensor has been widely used in measuring strain in materials. The existence of fiber Bragg grating sensor affects strain distribution of the host material, which may result in a decrease in strain measurement accuracy. To improve the measurement accuracy, a theoretical model of strain transfer from the host material to optical fiber was developed, incorporating the influence of the fiber Bragg grating sensor. Subsequently, theoretical predictions were validated by comparing with data from finite element analysis and the existing experiment [F. Ansari and Y. Libo, J. Eng. Mech. 124(4), 385-394 (1998)]. Finally, the effect of parameters of fiber Bragg grating sensors on the average strain transfer rate was discussed.

High-accuracy biaxial optical extensometer based on 2D digital image correlation

In two-dimensional digital image correlation (2D-DIC), the optical axis of the camera must be exactly perpendicular to the object surface to avoid the generation of out-of-plane displacement, which leads to low strain accuracy. In this work, we first studied the strain accuracy of a common optical extensometer when the optical axis of the camera is not perpendicular to the specimen surface. Derivation reveals that even a very small oblique angle θ will introduce a considerable strain error and that an opposite oblique angle θ generates an opposite strain error of the same magnitude. Therefore, taking the strain average of these two optical extensometers can almost entirely eliminate such effect of non-perpendicularity, which has the same form as the optical extensometer realized by dual-reflector imaging. Because the transverse strain of a specimen is less than its axial strain, the measurement of transverse strain requires higher strain-measurement accuracy. With the aid of a self-designed right-angled apparatus, we conveniently extend this 1D extensometer to a 2D situation, which is called the biaxial optical extensometer. Uniaxial tensile tests of a stainless-steel specimen are conducted to evaluate the strain accuracy of the biaxial optical extensometer quantitatively. Experimental results show that the transverse and axial strains obtained using the proposed biaxial extensometer are in good agreement with those obtained using strain gauges and that the proposed extensometer achieves higher strain accuracy compared to the 3D-DIC extensometer.

Distortion Correction Protocol for Digital Image Correlation after Scanning Electron Microscopy: Emphasis on Long Duration and Ex-Situ Experiments

Recently, scanning electron microscopy (SEM) has been used for digital image correlation (DIC), as micrographs can be acquired with high magnification, providing improved resolution to quantify strain heterogeneities. However, it has been shown by researchers that SEM images can be problematic due to inherent electromagnetic distortions that are not present in optical images. Drift, spatial distortions, and magnification uncertainties are the main issues that can seriously affect the accuracy of localized strain measurements. The present work focuses on long duration experiments, for which images are taken days or weeks apart. We have proposed a systematic procedure to reduce drift, correct spatial distortion, and account for magnification variations between pairs of acquired images. Additionally, SEM parameters are discussed and chosen to increase the signal-to-noise ratio and improve the accuracy of the DIC measurements. The spatial distortion correction increases the repeatability of the correlated values and the precision of the measurements. The implementation for this type of correction is done by applying the measured distortion gradient of a certified grid onto the DIC strain field. The magnification adjustment increases the reliability of the strain maps, ensuring the measurements are in agreement with the actual strain induced during the experiment. We have presented a systematic protocol for ex-situ DIC experiments within the SEM and some basic cross-check procedures that can be performed to evaluate the reliability of the reference grid and the precision of the final strain map.

Two-dimensional Moiré phase analysis for accurate strain distribution measurement and application in crack prediction.

Aimed at the low accuracy problem of shear strain measurement in Moiré methods, a two-dimensional (2D) Moiré phase analysis method is proposed for full-field deformation measurement with high accuracy. A grid image is first processed by the spatial phase-shifting sampling Moiré technique to get the Moiré phases in two directions, which are then conjointly analyzed for measuring 2D displacement and strain distributions. The strain especially the shear strain measurement accuracy is remarkably improved, and dynamic deformation is measurable from automatic batch processing of single-shot grid images. As an application, the 2D microscale strain distributions of a titanium alloy were measured, and the crack occurrence location was successfully predicted from strain concentration.

Comparison between point and long‐gage FBG‐based strain sensors during a railway bridge load test

AbstractStrain is a key parameter in laboratory and bridge load testing. The selection of a strain sensor depends on several factors, including the aim of the test and the specimen material. The application of the right sensor is vital to obtain accurate readings, especially in the case of heterogeneous materials such as concrete. This paper focuses on long‐gage and point fiber Bragg grating‐based strain sensors and their possible applications on concrete elements. First, strain sensors are described, after which long‐gage and point fiber Bragg grating strain sensors are compared in a concrete specimen test, a concrete column test and static and dynamic load tests on a concrete railway bridge. Results show that although it is advisable to use long‐gage sensors when monitoring heterogeneous materials, there are some particular cases were both sensors type can provide accurate strain measurements.

Micro/Nano-scale Strain Distribution Measurement from Sampling Moiré Fringes

This work describes the measurement procedure and principles of a sampling moiré technique for full-field micro/nano-scale deformation measurements. The developed technique can be performed in two ways: using the reconstructed multiplication moiré method or the spatial phase-shifting sampling moiré method. When the specimen grid pitch is around 2 pixels, 2-pixel sampling moiré fringes are generated to reconstruct a multiplication moiré pattern for a deformation measurement. Both the displacement and strain sensitivities are twice as high as in the traditional scanning moiré method in the same wide field of view. When the specimen grid pitch is around or greater than 3 pixels, multi-pixel sampling moiré fringes are generated, and a spatial phase-shifting technique is combined for a full-field deformation measurement. The strain measurement accuracy is significantly improved, and automatic batch measurement is easily achievable. Both methods can measure the two-dimensional (2D) strain distributions from a single-shot grid image without rotating the specimen or scanning lines, as in traditional moiré techniques. As examples, the 2D displacement and strain distributions, including the shear strains of two carbon fiber-reinforced plastic specimens, were measured in three-point bending tests. The proposed technique is expected to play an important role in the non-destructive quantitative evaluations of mechanical properties, crack occurrences, and residual stresses of a variety of materials.

Micro/Nano-scale Strain Distribution Measurement from Sampling Moiré Fringes.

This work describes the measurement procedure and principles of a sampling moiré technique for full-field micro/nano-scale deformation measurements. The developed technique can be performed in two ways: using the reconstructed multiplication moiré method or the spatial phase-shifting sampling moiré method. When the specimen grid pitch is around 2 pixels, 2-pixel sampling moiré fringes are generated to reconstruct a multiplication moiré pattern for a deformation measurement. Both the displacement and strain sensitivities are twice as high as in the traditional scanning moiré method in the same wide field of view. When the specimen grid pitch is around or greater than 3 pixels, multi-pixel sampling moiré fringes are generated, and a spatial phase-shifting technique is combined for a full-field deformation measurement. The strain measurement accuracy is significantly improved, and automatic batch measurement is easily achievable. Both methods can measure the two-dimensional (2D) strain distributions from a single-shot grid image without rotating the specimen or scanning lines, as in traditional moiré techniques. As examples, the 2D displacement and strain distributions, including the shear strains of two carbon fiber-reinforced plastic specimens, were measured in three-point bending tests. The proposed technique is expected to play an important role in the non-destructive quantitative evaluations of mechanical properties, crack occurrences, and residual stresses of a variety of materials.

Proof of concept of wireless TERS monitoring

Temporary earth retaining structures (TERS) help prevent collapse during construction excavation. To ensure that these structures are operating within design specifications, load forces on supports must be monitored. Current monitoring approaches are expensive, sparse, off-line, and thus difficult to integrate into predictive models. This work aims to show that wirelessly connected battery powered sensors are feasible, practical, and have similar accuracy to existing sensor systems. We present the design and validation of ReStructure, an end-to-end prototype wireless sensor network for collection, communication, and aggregation of strain data. ReStructure was validated through a six months deployment on a real-life excavation site with all but one node producing valid and accurate strain measurements at higher frequency than existing ones. These results and the lessons learnt provide the basis for future widespread wireless TERS monitoring that increase measurement density and integrate closely with predictive models to provide timely alerts of damage or potential failure.

Strain Transfer Analysis of a Clamped Fiber Bragg Grating Sensor

Clamped fiber Bragg grating (FBG) sensors have been widely applied in engineering strain measurements due to their advantages of high flexibility and efficiency. However, due to the existence of the interlayer, the strain measured by the encapsulated FBG sensor is not equal to the strain of the host material, which causes strain measurement errors. In this paper, the strain transfer analysis of a clamped FBG sensor based on the shear-lag theory is conducted to improve the accuracy of strain measurements. A novel theoretical model for the axial strain distribution of a clamped FBG sensor is proposed. It is also discussed how the gauge ratio and interlayer thickness affect the strain transfer rate. The accuracy of the proposed theoretical model is verified by experimental tensile tests. The theoretical value of the strain transfer rate matches well with the tested value.

Mini-tensile specimen application for sheets characterization

There are many cases when there is a shortage of the experimental material for detailed analysis and then small size specimens techniques becomes essential. The current paper deals with investigations of mini-tensile tests (MTT) application to metal sheets characterization. In the case of metal sheets assessment the most common are tensile tests for Lankford parameters and strain hardening determination. As most of the processes are not quasi-static and constant strain rate processes, thus assessment of strain rate hardening is also crucial part of the characterization. Previously developed and verified testing procedure of M-TTs for bulk materials is applied here for steel sheet made of DC01 characterization. Tests under quasi-static and dynamic loading conditions are carried out in order to describe above mentioned properties at room temperature. Accurate strain measurement is carried out with digital image correlation systems and results obtained with M-TTs are going to be confronted with standard size specimens’ results.

Measurement of high temperature full-field strain up to 2000 °C using digital image correlation

Understanding the deformation and strain at elevated temperature is a critical factor for the stability of aerodynamic shape, and an important consideration for the thermal protection system design. However, accurate measurement of deformation and strain at high temperatures is a challenge. Here, we present a measurement study for full-field strain mapping up to 2000 °C using digital image correlation (DIC) method, which mainly depends on the quality of speckle patterns on the specimen surface. In our study, the strain values are analyzed by DIC method while specimens are heated using a large electric current. Improvements in filtering and speckling allow the measured temperatures using this method to reach 2000 °C. We confirmed the validity of this method by comparison of measured Young’s modulus values with reference data for Inconel 718 Ni-based superalloy and graphite at different temperatures. Additionally, the full-field strain and Young’s modulus were demonstrated for a carbon fiber-reinforced carbon (C/C) composite uniaxial tensile specimen at 2000 °C.

Metallographic Analysis of Nakajima Tests for the Evaluation of the Failure Developments

The material characterisation in sheet metal forming is significantly improved with the introduction of optical measurement systems. The optical measurement is based on the digital image correlation (DIC) technique, which provides accurate strain measurement of the local strain distribution on specimens. The changes of the stochastic distribution due to the forming are therefore used in order to measure displacements and deformations. An acknowledged application of the DIC technique is the evaluation of the forming limit curve (FLC) for sheet metals. Typically, the FLC is evaluated by conducting Nakajima tests. The Nakajima test is a stretch forming-based test and allows the analysis of stretching at different stress conditions. A previous investigation based on pattern recognition on Nakajima tests has shown that for a conventional deep drawing steel DC04 the pattern evolution on the strain distribution during the test can be examined in order to predict the material failure. However, even if the investigation shows promising results, the correspondence between pattern on the surface and changes in the material structure is missing. The aim of the present work is an analysis of the surface structure of Nakajima tests at different stress states and drawing depths. The correlation between surface modifications and failure behaviour is conducted on the deep drawing steel DC04. The scanning electron microscope (SEM) (Merlin Zeiss GmbH) is employed in order to investigate the evolution of the surface at different strain conditions. The results of the surface and the metallographic investigation are finally compared with the forming limit prediction of the FLC and the failure behaviour at different stress conditions is discussed.

Rate effects on transformation kinetics in a metastable austenitic stainless steel

In this study, the effects of strain rate on the mechanical properties and the strain-induced austenite-to-martensite transformation in type 201 austenitic stainless steel (SS201) were investigated. This grade was selected as a low-cost stainless steel with good lightweighting potential for automotive applications. The material was tested in tension at a quasi-static rate (5×10-2 s-1), two low-intermediate rates (100 s-1 and 101 s-1), and a high rate (5×102 s-1). 3D digital image correlation was used to enable accurate strain measurements during mechanical testing. Magnetic induction and X-ray diffraction were used ex-situ of deformation to measure the volume fraction of martensite formed at each strain rate, for different plastic strain levels. The effects of strain rate on deformation-induced martensite formation and on the stress/strain behavior was determined in this study, and was compared to results reported in the literature for 300 series austenitic stainless steels. The results show a favourable response for the SS201, which exhibits a substantial increase in strength and energy absorption at high rates without compromising tensile ductility.

Self-calibration single-lens 3D video extensometer for high-accuracy and real-time strain measurement.

The accuracy of strain measurement using a common optical extensometer with two-dimensional (2D) digital image correlation (DIC) is not sufficient for experimental applications due to the effect of out-of-plane motion. Although three-dimensional (3D) DIC can measure all three components of displacement without introducing in-plane displacement errors, 3D-DIC requires the stringent synchronization between two digital cameras and requires complicated system calibration of binocular stereovision, which makes the measurement rather inconvenient. To solve the problems described above, this paper proposes a self-calibration single-lens 3D video extensometer for non-contact, non-destructive and high-accuracy strain measurement. In the established video extensometer, a single-lens 3D imaging system with a prism and two mirrors is constructed to acquire stereo images of the test sample surface, so the problems of synchronization and out-of-plane displacement can be solved easily. Moreover, a speckle-based self-calibration method which calibrates the single-lens stereo system using the reference speckle image of the specimen instead of the calibration targets is proposed, which will make the system more convenient to be used without complicated calibration. Furthermore, an efficient and robust inverse compositional Gauss-Newton algorithm combined with a robust stereo matching stage is employed to achieve high-accuracy and real-time subset-based stereo matching. Tensile tests of an Al-alloy specimen were performed to demonstrate the feasibility and effectiveness of the proposed self-calibration single-lens 3D video extensometer.

Investigation on Micro-Tensile Test System Platform Used for Test of the Weld

In this paper, the mechanical properties of electron beam welding TC4 titanium alloy in different regions of the welded joint were studied by using the Micro-tensile test device and digital image correlation method. In the research, the problem of sample's gripping, strain measurement accuracy, load measurement and lens distortion verification are solved. The main work is as follows: The digital image correlation method, which is non-contact, full field measurement, simple, easy operation and high accuracy, is used to measure the strain of the micro tensile specimen, the fine speckle technique for micro mechanical property testing is developed, furthermore, the measurement accuracy of the digital image correlation method for the micro sample is discussed. Finally, the Micro-tensile test system platform is built which can be used to test the micro mechanical properties of the weld.

Increasing the Measurement Dynamic Range of Rayleigh-Based OFDR Interrogator Using an Amplifying Add-On Module

An add-on amplifying module is presented, capable of significantly extending the measurement dynamic range of fiber-optic distributed strain/temperature interrogators, which are based on Rayleigh backscattering and optical frequency-domain reflectometry. Since Rayleigh backscattering in single-mode optical fibers is very weak, any optical loss along the sensing fiber, originating from bad connectors and/or micro-and macro-bending, especially under embedding, may jeopardize accurate strain measurements due to a deteriorated signal-to-noise ratio. It is shown that adding an optical amplifier between the interrogator output and the sensing fiber, together with an appropriate return path, has improved the one-way tolerable loss by 5 dB. In principle, better results can be obtained with higher optical gains until either optical reflections and backscattering into the interrogator increase its noise floor or other noise sources saturate the available signal-to-noise ratio.

A benchmark facility for guided elastic wave actuation by piezoelectric discs

Key aspects of piezoelectric actuation of circular-crested guided elastic waves (ccGEW) in homogeneous plates are investigated experimentally and studied with finite element analysis (FEA). The specific implementation of fiber-optic strain transduction presented offers virtually distortion-free conversion of radial GEW displacements with a flat transfer characteristic and negligible interference with the wave to be studied. Due to a concentric arrangement of actuator disc, fiber-optic transducer and circular plate, a shuttling GEW pulse packet can be retained over an exceptionally long period of time. Hence accurate long term measurements of the excited surface strain in response to piezoelectrical actuation becomes feasible for specific GEW modes. Presumably, traveling-wave-like behavior of compressional ccGEW pulses including the transition of converging into expanding wave packets around the symmetry axis is observed for the first time. Minor deviations between FEA predictions and measurement results can be traced back to imperfections of the experimental setup or inaccurate material data. The presented approach may be used for thorough characterization of piezoelectric disc actuators and studies of ccGEW propagation in various homogeneous materials.