Accurate Strain Measurements Research Articles

Tunable lasing in cholesteric liquid crystal elastomers with accurate measurements of strain.

We report wide range and reversible tuning of the selective reflection band of a single crystal cholesteric liquid crystal elastomer (CLCE). The tuning is the result of mechanical shortening of the helical pitch achieved by imposing a uniform uniaxial strain along the helical axis. On doping the CLCE sample with a laser dye, we observe lasing from the CLCE in both glassy and rubbery states. By changing the cholesteric pitch, mechanical compression provides tuning of the laser emission from the dye doped CLCE over a significant part of the fluorescence band of the laser dye. In this work we demonstrate for the first time that both the CLCE pitch and the lasing wavelength are linearly dependent on the strain imposed on the CLCE film.

Qualification of a distributed optical fiber sensor bonded to the surface of a concrete structure: a methodology to obtain quantitative strain measurements

Distributed optical fiber systems (DOFSs) are an emerging and innovative technology that allows long-range and continuous strain/temperature monitoring with a high resolution. Sensing cables are either surface-mounted or embedded into civil engineering structures to ensure long-term structural monitoring and early crack detection. However, strain profiles measured in the optical fiber (OF) may differ from the actual strain in the structure due to the shear transfer through the intermediate material layers between the OF and the host material (i.e., in the protective coating of the sensing cable and in the adhesive). Therefore, OF sensors need to be qualified to provide accurate quantitative strain measurements. This study presents a methodology for the qualification of a DOFS. This qualification is achieved through the calculation of the so-called mechanical transfer function (MTF), which relates the strain profile in the OF to the actual strain profile in the structure. It is proposed to establish a numerical modeling of the system, in which the mechanical parameters are calibrated from experiments. A specific surface-mounted sensing cable connected to an optical frequency domain reflectometry interrogator is considered as a case study. It was found that (i) tensile and pull-out tests can provide detailed information about materials and interfaces of the numerical model; (ii) the calibrated model made it possible to compute strain profiles along the OF and therefore to calculate the MTF of the system; (iii) the results proved to be consistent with experimental data collected on a cracked concrete beam during a four-point bending test. This paper is organized as follows: first, the technical background related to DOFSs and interrogators is briefly recalled, the MTF is defined and the above-mentioned methodology is presented. In the second part, the methodology is applied to a specific cable. Finally, a comparison with experimental evidence validates the proposed approach.

An educational intervention to help medical students achieve accurate and consistent measurement of longitudinal myocardial strain on transthoracic echocardiogram

An educational intervention to help medical students achieve accurate and consistent measurement of longitudinal myocardial strain on transthoracic echocardiogram

PP.08.21

Objective: The new cardiovascular magnetic resonance (CMR) T1 mapping sequences can be applied to quantify diffuse myocardial fibrosis. Also, new CMR analysis softwares allow for the accurate and reproducible measurement of myocardial strain. We aimed to use these two new developments in a pilot study in order to assess the association between presence of diffuse fibrosis and changes in myocardial strain. Design and method: 12 hypertensive patients (HT, 52 ± 7yrs, 10 males) with preserved ejection fraction and 12 controls (NT, 44 ± 10yrs, 8 males) underwent a CMR protocol at 3T that included black blood morphological sequences, cine sequences in the 2,3 and 4-chamber views as well as short axis series with typically 40 phases for each image acquisition, T1-mapping with two MOLLI acquisitions (3–3–5), before and 15 min after administration of a bolus of gadolinium-DTPA (0.1 mM/kg), and late gadolinium sequences in the same views as the cine sequences. All the scans were eventually analysed by two experienced observers with a dedicated software to obtain left ventricular volumes, ejection fraction and mass, precontrast myocardial T1 values, gadolinium partition coefficient (GPC) and extracellular volume fraction (ECV), a measure of diffuse myocardial fibrosis. Also, global longitudinal and radial myocardial strain and strain rate, as well as epicardium-endocardium strain gradient were measured. Results: All subjects had preserved systolic function and results are shown on the table. There was a trend towards increased ECV in HT though it did not reach statistical significance.Global londitudinal and radial strain (GLS, GRS), longitudinal strain rate (GLSR) and precontrast T1 were all lower in HT, while radial strain gradient between subendocardium and subepicardium (GRS endo-epi) was increased in HT. There was a significant inverse correlation of GLS and GRS with ECV, and of radial strain rate with ECV (all p < 0.05).Conclusions: In this pilot study we have found that there is an inverse correlation between global strain, a sensitive indicator of regional contractility, and myocardial extracellular volume fraction, a marker of diffuse fibrosis. This finding may have implications for early diagnosis of target organ damage in hypertensive patients.

High-accuracy two-dimensional full field strain measurement method using camera array

In recent years, the digital image correlation technique, due to its full-field, non-contact and large measurement, play an important role in non-contact large deformation measurement in many fields. Because of the strain resolution is limited by the camera resolution, compared with the strain gauges, the strain measurement accuracy of digital image correlation is not enough. To make the digital image correlation measurement technology in the strain resolution has big breakthrough, rely on hardware technology, will be a long waiting. In this paper, a camera array, combined with image stitching technology, was used to realize the high-resolution image acquisition and improve the strain measuring resolution of digital image correlation, so as to provide a high-accuracy strain measurement. Moreover, a novel calibration based image stitching method was proposed. The proposed method can maximize the use of spatial resolution of digital camera, and three-dimensional reconstruction of the overlapped areas can be achieved using the calibration results, this can be used to reduce the impact of out-of-plane motion on strain measurement.

The effect of pattern overlap on the accuracy of high resolution electron backscatter diffraction measurements

High resolution, cross-correlation-based, electron backscatter diffraction (EBSD) measures the variation of elastic strains and lattice rotations from a reference state. Regions near grain boundaries are often of interest but overlap of patterns from the two grains could reduce accuracy of the cross-correlation analysis. To explore this concern, patterns from the interior of two grains have been mixed to simulate the interaction volume crossing a grain boundary so that the effect on the accuracy of the cross correlation results can be tested. It was found that the accuracy of HR-EBSD strain measurements performed in a FEG-SEM on zirconium remains good until the incident beam is less than 18nm from a grain boundary. A simulated microstructure was used to measure how often pattern overlap occurs at any given EBSD step size, and a simple relation was found linking the probability of overlap with step size.

The influence of frame rate on two-dimensional speckle-tracking strain measurements: a study on silico-simulated models and images recorded in patients.

Ultrasound-derived myocardial strain can render valuable diagnostic and prognostic information. However, acquisition settings can have an important impact on the measurements. Frame rate (i.e. temporal resolution) seems to be of particular importance. The aim of this study was to find the optimal range of frame rates needed for most accurate and reproducible 2D strain measurements using a 2D speckle-tracking software package. Synthetic two dimensional (2D) ultrasound grey-scale images of the left ventricle (LV) were generated in which the strain in longitudinal, circumferential, and radial direction were precisely known from the underlying kinematic LV model. Four different models were generated at frame rates between 20 and 110 Hz. The resulting images were repeatedly analysed. Results of the synthetic data were validated in 66 patients, where long- and short-axis recordings at different frame rates were analysed. In simulated data, accurate strain estimates could be achieved at >30 frames per cycle (FpC) for longitudinal and circumferential strains. Lower FpC underestimated strain systematically. Radial strain estimates were less accurate and less reproducible. Patient strain displayed the same plateaus as in the synthetic models. Higher noise and the presence of artefacts in patient data were followed by higher measurement variability. Standard machine settings with a FR of 50-60 Hz allow correct assessment of peak global longitudinal and circumferential strain. Correct definition of the region of interest within the myocardium as well as the reduction of noise and artefacts seem to be of highest importance for accurate 2D strain estimation.

Capacitive soft strain sensors via multicore-shell fiber printing.

A new method for fabricating textile integrable capacitive soft strain sensors is reported, based on multicore-shell fiber printing. The fiber sensors consist of four concentric, alternating layers of conductor and dielectric, respectively. These wearable sensors provide accurate and hysteresis-free strain measurements under both static and dynamic conditions.

How to Define End-Diastole and End-Systole?: Impact of Timing on Strain Measurements

ObjectivesThis study aimed to investigate to what extent timing definitions influence strain measurements and which surrogates are reliable and feasible to define end-diastole (ED) and end-systole (ES) during speckle-tracking (STI) analysis. BackgroundCurrent STI-based strain measurements are highly automated. It remains unclear when a particular analysis software defines the zero baseline and the systolic strain measurement position. MethodsA total of 60 subjects (20 healthy volunteers, 20 patients with coronary artery disease, and 20 patients with typical left bundle-branch block) underwent a complete echocardiographic examination. In one-half of them, a real M-mode through the mitral valve was acquired for each electrocardiographic (ECG) lead of the echo machine. Timing of peak R and automatic ECG trigger were compared with mitral valve closure for every electrode. Mitral and aortic valve closure, as observed in the apical 3-chamber view, served as reference for ED and ES. With the use of these references, end-systolic global longitudinal strain (ES-GLS) and end-systolic segmental longitudinal strain (ES-SLS) longitudinal end-systolic strain were measured at baseline and after changing the definition of either ED or ES by ±4 frames. Furthermore, strain and volume curves derived from the same tracking, as well as the Doppler interrogation of the valves, were compared with the references. ResultsDepending on the selected lead, timing of the ECG-derived time markers changed considerably compared with mitral valve closure. Changing the definition of ED and ES resulted in significantly different ES-GLS and ES-SLS values in all subjects. ES-SLS in dyssynchronous hearts showed the highest sensitivity to timing definition. From all methods, spectral Doppler was the most reliable time marker in all subjects (p > 0.05). ConclusionsExact temporal definition of ED and ES has a major impact on the accuracy of strain measurements. After direct observation of the valves, Doppler evaluation is the best means for characterizing ED and ES for STI analysis.

On the strain measurement and stiffness calculation of carbon fibre reinforced composites under quasi-static tensile and tension-tension fatigue loads

The applicability of different strain measurement techniques for carbon/epoxy laminates under quasi-static tensile and tension-tension fatigue loads was studied. Strain gauges, mechanical extensometers, digital image correlation and 2 D camera systems were applied on laminates tested at angles of 0°, 45°, 60°, 90° and ±45°. In addition, displacements recorded by the servo-hydraulic piston were monitored and compared to local strain measurement techniques. Representative examples that illustrate characteristics and limits of each technique in quasi-static and fatigue tests are discussed. Influences of the respective method of strain measurement, the specimen surface, fibre direction and processes in the specimens during tests on the recorded stress-strain behaviour and on the calculated stiffness are presented. Recommendations for accurate strain measurement of anisotropic laminates based on the results are made.

An Improvement of the Time Dependent Method Based on the Coefficient of Correlation for the Determination of the Forming Limit Curve

Due to the global energy policy according to the aims of economic efficiency and environmental compatibility the production of lightweight constructions is nowadays a need in many fields of mechanical engineering, especially in automotive and aerospace sectors. The substitution of conventional deep drawing steels with modern sheet metals, i.e. aluminium or high strength steel, provides a positive contribution towards this trend. Prerequisite for an efficient application of these resources is a comprehensive material characterization including the analysis of the formability as well as the failure behaviour. Regarding the accurate prediction of forming behaviour, a robust failure criterion is necessary in order to perform the numerical simulation designs by using finite element method (FEM). The leading criterion used for sheet metal forming is the forming limit diagram (FLD). The FLD gives a graphical description of material formability by plotting the major strain values related to the minor strain values to failure in a curve called forming limit curve (FLC). With the improvement of the digital image correlation (DIC) techniques, which provide very accurate strain measurement, new methods relied on the strain history of the specimen during the test are proposed to determine the FLC. Within this paper, the time dependent method based on the coefficient of correlation is investigated. In particular an improvement dependent on experimental parameter is presented in order to increase its sensibility and reliability for different materials.

About the inevitable compromise between spatial resolution and accuracy of strain measurement for bone tissue: A 3D zero-strain study

The accurate measurement of local strain is necessary to study bone mechanics and to validate micro computed tomography (µCT) based finite element (FE) models at the tissue scale. Digital volume correlation (DVC) has been used to provide a volumetric estimation of local strain in trabecular bone sample with a reasonable accuracy. However, nothing has been reported so far for µCT based analysis of cortical bone. The goal of this study was to evaluate accuracy and precision of a deformable registration method for prediction of local zero-strains in bovine cortical and trabecular bone samples. The accuracy and precision were analyzed by comparing scans virtually displaced, repeated scans without any repositioning of the sample in the scanner and repeated scans with repositioning of the samples.The analysis showed that both precision and accuracy errors decrease with increasing the size of the region analyzed, by following power laws. The main source of error was found to be the intrinsic noise of the images compared to the others investigated. The results, once extrapolated for larger regions of interest that are typically used in the literature, were in most cases better than the ones previously reported. For a nodal spacing equal to 50 voxels (498µm), the accuracy and precision ranges were 425–692µε and 202–394µε, respectively. In conclusion, it was shown that the proposed method can be used to study the local deformation of cortical and trabecular bone loaded beyond yield, if a sufficiently high nodal spacing is used.

Stereo Image Based Motion Measurements in Fluids: Experimental Validation and Application in Friction Extrusion

Optical refraction at interfaces is a common issue when viewing a submerged specimen through a transparent window. The distortions that are introduced during such imaging must be minimized when employing stereo-vision systems to make accurate quantitative (a) position, displacement, velocity and strain measurements on structures immersed in fluids via marker tracking and (b) velocity measurements along particle paths in a fluid via particle tracking. In this study, for the first time an optical model with refraction at multiple media interfaces that was developed previously for stereo-vision based measurements in fluids is experimentally validated via (a) motion and strain measurements on an immersed, rotating surface and (b) particle tracking in a fluid. Consistent with the model, the calibration and reconstruction processes are developed and demonstrated experimentally to be effective in removing distortions. To demonstrate the utility of the method for accurate measurements on submerged objects, the authors measured positions and strains on a rotating, rigid surface through stereo-imaging of specific features. Results confirmed that the vision-based method is accurate and repeatable for measuring 3D positions and strains on submerged bodies. The methodology is then applied to the study of extrusion processes. Using a transparent small scale lab model extruder with a highly viscous fluid designed for use with stereo vision measurement systems, the 3D motions of neutrally buoyant spherical particles are measured during the extrusion process and compared to CFD simulation predictions. Results confirm that the dual calibration-stereo imaging approach is more accurate and effective for particle tracking than typical approaches used in such studies.

Experimental and modeling study on increasing accuracy of strain measurement by ion beam analysis in SiGe strained layer thin films

We have used Rutherford backscattering spectrometry to characterize Si/ε-Si0.8Ge0.2/Si strained-layer heterogeneous epitaxial structures. Two-dimensional yield mapping of backscattered 2MeV He atoms as a function of tilting angles around off-normal 〈110〉 axis are obtained. For Si/Si0.8Ge0.2/Si with buried strained layer, we observe distortion of (110) planar minimum in yield mapping from Ge. The distortion features slightly decreasing angular offsets with decreasing tilting away from 〈110〉 axis. Our finding suggests that strain measurements using one dimensional angular scan within or close to (100) plane will lead to strain underestimation. A two dimensional yield mapping is preferred over one dimensional angular scan for high accuracy in strain measurements, so the angular off-set can be measured in a region free of distortion. The experimental observations are in good agreement with our modeling prediction.

Effects of bonding layer on the available strain measuring range of fiber Bragg gratings

Stress-induced birefringence can lead to distortion in the reflection spectra of fiber Bragg grating (FBG) sensors, thereby resulting in the loss of accuracy and stability of strain measurements. The bonding layer is a direct factor in producing stress birefringence within FBGs. To assess the impacts quantitatively, a theoretical model that links the bonding layer and the reflection spectrum was established. At the same time, the finite element method, based on the theoretical model, was used to study the relationships between characteristics of the bonding layer and reflection spectrum in detail. The analytical results indicate that high elastic modulus and mismatched Poisson's ratio of bonding layer decrease the available strain measuring range of FBGs remarkably, and that unreasonable geometric parameters of the bonding layer should be avoided. In addition, a validation experiment was conducted and experimental results proved the prediction of the theoretical analysis. It can be concluded from the results that the bonding layer is the major limiting factor for the application of surface-bonded FBG sensors in large strain measurements. The bonding materials and bonding processes used in producing FBG sensors deserve serious consideration.

Mechanical Behavior and Anisotropy of Spin-Coated SU-8 Thin Films for MEMS

Spin-coated SU-8 thin films used in Microelectromechanical Systems (MEMS) devices are investigated using tensile experiments on freestanding microscale specimens to understand their mechanical response. Modified in situ optical microscope experiments are performed on 2 μm thick transparent SU-8 films in the strain rate range of 2×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> /s to 2×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> /s and accurate strain measurements are extracted using Digital Image Correlation. The effects of strain rate and anisotropy, on mechanical properties, in hard baked and non-hard baked films were studied in conjunction with Fourier Transform Infrared Spectroscopy. The Young's modulus and tensile strength were found to increase with strain rate significantly, with the stress vs. strain behavior changing from viscoelastic to linear elastic. Hard baking of the films resulted in better mechanical properties, viz., elastic modulus and tensile strength, due to increased cross-linking density. The average moduli for hard baked and non-hard baked films were 3.48±0.57 GPa and 2.92± 0.43 GPa, respectively, obtained from 30 experiments each, over the entire strain rate range. Furthermore, the spin-coat process used to make the films developed anisotropy in the plane of the films due to molecular orientation, which was independent of hard baking temperature and duration.

Measuring forming limit strains with digital image correlation analysis

Digital image correlation (DIC) analysis methods can provide very accurate strain measurement that is useful for identifying the onset of localized necking and thus determining the forming limit strains of sheet metals. Several different DIC procedures to establish a forming limit diagram (FLD) are compared including both position dependent and time dependent measurement methods where the latter is found to be more accurate for an Al–Mg–Si alloy. Since FLD standards are still being developed to include DIC methods, a new DIC-based time dependent measurement method for the forming limit strains is proposed in this research: (1) Surface topography is extracted from the DIC data and changes in surface height difference are obtained, of the test specimen; (2) The onset of localized necking is identified by the limit of stable flow of the surface height difference or the first derivative of the surface height difference near the development of a local neck when using the Marciniak test procedure. Compared to existing methods, this new procedure more accurately defines the onset of necking and the boundary of safe deformation of an FLD.

Full‐Field Measurements on Low‐Strained Geomaterials Using Environmental Scanning Electron Microscopy and Digital Image Correlation: Improved Imaging Conditions

ABSTRACTFull‐field strain measurement at microscale on geomaterials by means of hydromechanical in situ testing and imaging in an environmental scanning electron microscope and digital image correlation techniques is a challenging task because of both low magnitude of investigated strains (typically of the order of 10−3) and unfavourable imaging conditions. In view of improving strain measurement accuracy, three major sources of measurement errors are evaluated, and methods to minimise their effects are proposed. First, a fast and simple procedure to accurately quantify image noise is proposed and subsequently used to optimise various environmental scanning electron microscope setting parameters, such as dwell time, spot size, working distance and chamber pressure. Second, a specific procedure to limit magnification fluctuations to a sufficiently low level is described. Finally, digital image correlation systematic errors are quantified on real images, and several ways to reduce their amplitude are compared. The combination of these improvements finally allows us to reach an appropriate accuracy for overall strain measurements and characterisation of microscale heterogeneities.

Tensile Anisotropy of Cu/Al Clad Metals After Prestrain Using Digital Image Correlation

A dimensional deformation measurement system (XJTUDIC/VS) based on digital image correlation was employed to measure the deformation and strain field of Cu/Al clad metals made using the explosive welding technique. An extensometer with a gage length of 50 mm was used during the Q235 tensile test to verify the strain measurement accuracy of the system. The results demonstrate that the accuracy of the XJTUDIC/VS system is ≥0.5%, which is very close to that of the extensometer. The anisotropy of Cu/Al clad metals under uniaxial tension after 0, 1.5, 2.5, and 4.5% prestrain was investigated. With an increase in strain, the true stress-strain curve varied at different directions in the same prestrains. The strain hardening rate, r value (changes from 0.1 to 0.9), and n value (changes from 0.25 to 0.35) all strongly depended on loading directions and prestrains. The linear relations between longitudinal strain and transverse strain were also influenced by tensile directions and prestrains. Cu/Al clad metals generally presented strong anisotropy.

Digital Image Correlation Based Method of Crack Growth Rate and Fracture Toughness Measurements on Mini-Samples

Modern materials fabrication methods which utilize severe plastic deformation (SPD) do not often allow to obtain enough volume of material to prepare standardized samples for mechanical tests. Therefore, there is a need for mini-samples testing. Mini-samples tests require special approach in terms of precise strain measurements. Accurate strain measurements may be achieved by means of non-contact optical method, namely Digital Image Correlation (DIC). The aim of this work is to present the methodology and results of mini-samples tests in which displacement fields measurements performed by means of DIC and inverse method were utilized for calculation of stress intensity factors and crack tip position tracking. The influence of the subarea of optical measurements for which calculation were performed on the calculations results has been investigated during tests in SPD processed Al alloys and brittle WC-Co sinters produced by means of impact sintering method.