Full-field Methods Research Articles

Identification of Apple Fruit-Skin Constitutive Laws by Full-Field Methods Using Uniaxial Tensile Loading.

The protective and preservative role of apple skin in maintaining the integrity of the fruit is well-known, with its mechanical behaviour playing a pivotal role in determining fruit storage capacity. This study employs a combination of experimental and numerical methodologies, specifically utilising the digital image correlation (DIC) technique. A specially devised inverse strategy is applied to evaluate the mechanical behaviour of apple skin under uniaxial tensile loading. Three apple cultivars were tested in this work: Malus domestica Starking Delicious, Malus pumila Rennet, and Malus domestica Golden Delicious. Stress-strain curves were reconstructed, revealing distinct variations in the mechanical responses among these cultivars. Yeoh's hyperelastic model was fitted to the experimental data to identify the coefficients capable of reproducing the non-linear deformation. The results suggest that apple skin varies significantly in composition and structure among the tested cultivars, as evidenced by differences in elastic properties and non-linear behaviour. These differences can significantly affect how fruit is handled, stored, and transported. Thus, the insights resulting from this research enable the development of mathematical models based on the mechanical behaviour of apple tissue, constituting important data for improvements in the economics of the agri-food industry.

In situ biaxial loading and multi-scale deformation measurements of nanostructured materials at the CoSAXS beamline at MAX IV Laboratory.

Characterization of the mechanical response of polymers and composite materials relies heavily on the macroscopic stress-strain response in uniaxial tensile configurations. To provide representative information, the deformation process must be homogeneous within the gauge length, which is a condition that is rarely achieved due to stress concentration or inhomogeneities within the specimen. In this work, the development of a biaxial mechanical testing device at the CoSAXS beamline at MAX IV Laboratory is presented. The design facilitates simultaneous measurement of small- and wide-angle X-ray scattering (SAXS/WAXS), allowing assessment of the microstructural configuration before, after and during the continuous deformation process at multiple length scales. The construction also supports multiple deformation conditions, while guaranteeing stability even at high loads. Furthermore, the mechanical experiments can be complemented with spatially resolved mesoscopic surface deformation measurements using 3D-surface digital image correlation (DIC). Polycarbonate (PC) was used to demonstrate the varied material response to multi-axial deformation, as PC is isotropic with a high glass transition temperature (∼150°) and high strength. As a result, a clear correlation between full-field methods and the microstructural information determined from WAXS measurements is demonstrated. When a uniaxial load is applied, homogeneous strain regions could be observed extending perpendicular to the applied load. When a secondary axial load was added (biaxial mode), it was observed that high strain domains were created near the centre of the sample and at the boundaries after yield. With increased strain, the deformation in the main deformation direction also increases. Mechanical reliability was demonstrated by carrying out static loading of polyacrylonitrile-based carbon fibre (CF) bundles. As a result, the nonlinear stiffening behaviour typically observed in CFs was seen, while no evidence of the creation of new voids during loading was observed. The results support the reliability and broad applicability of the developed technique.

Assessment of the equivalent inclusion method for the numerical homogenization of fibrous composites

For the homogenization of fiber composites, the equivalent inclusion method is proposed as an alternative to costly full-field methods and less accurate mean-field or effective-field approaches. We take advantage of the slenderness of the inhomogeneities to overcome the “curse of dimensionality” that was evidenced previously for this method. The resulting method is shown on a number of examples to be both accurate and computationally efficient. It is also extremely versatile, as it applies to a large variety of physical problems (beyond electric conductivity considered here).

Noninvasive quantification of cerebrovascular pressure changes using 4D Flow MRI.

Hemodynamic alterations are indicative of cerebrovascular disease. However, the narrow and tortuous cerebrovasculature complicates image-based assessment, especially when quantifying relative pressure. Here, we present a systematic evaluation of image-based cerebrovascular relative pressure mapping, investigating the accuracy of the routinely used reduced Bernoulli (RB), the extended unsteady Bernoulli (UB), and the full-field virtual work-energy relative pressure ( WERP) method. Patient-specific in silico models were used to generate synthetic cerebrovascular 4D Flow MRI, with RB, UB, and WERP performance quantified as a function of spatiotemporal sampling and image noise. Cerebrovascular relative pressures were also derived in 4D Flow MRI from healthy volunteers ( ), acquired at two spatial resolutions (dx = 1.1 and 0.8 mm). The in silico analysis indicate that accurate relative pressure estimations are inherently coupled to spatial sampling: at dx = 1.0 mm high errors are reported for all methods; at dx = 0.5 mm WERP recovers relative pressures at a mean error of 0.02 ± 0.25 mm Hg, while errors remain higher for RB and UB (mean error of -2.18 ± 1.91 and -2.18 ± 1.87 mm Hg, respectively). The dependence on spatial sampling is also indicated in vivo, albeit with higher correlative dependence between resolutions using WERP (k = 0.64, R2 = 0.81 for dx = 1.1 vs. 0.8 mm) than with RB or UB (k = 0.04, R2 = 0.03, and k = 0.07, R2 = 0.07, respectively). Image-based full-field methods such as WERP enable cerebrovascular relative pressure mapping; however, accuracy is directly dependent on utilized spatial resolution.

Constitutive modeling of the anterior cruciate ligament bundles and patellar tendon with full-field methods

Anterior cruciate ligament (ACL) injury rates are rising, and there is little consensus about what puts someone at risk for an ACL injury. Finite element models provide an effective platform for systemically determining the effect of possible injury risk factors on ACL strain concentrations. However, the accuracy of a finite element model relies on the accuracy of the material models used in its construction. Standard material characterization methods require an assumption of both deformation and material homogeneity, but our recent work has demonstrated that structural features like the shape of the ligament–bone attachment (enthesis) and collagen fiber splay (material direction heterogeneity) create unavoidable deformation heterogeneity. Hence, in this study, full-volume, full-field, finite deformation methods were used to build material models for the ovine ACL bundles and patellar tendon. Specifically, displacement-encoded magnetic resonance imaging (MRI) was used to measure five full-volume deformation fields of each ligament (n=5 specimens of each) under tension, and the virtual fields method (VFM) was used to determine material model parameters with these data. This method accounts for strain heterogeneity, principal material direction heterogeneity (fiber splay), and enthesis shape. While most constitutive parameters were consistent among all specimen groups, parameters describing the degree of anisotropy, or collagen fiber alignment, showed statistically significant differences between groups. This work demonstrates that (when strain heterogeneity and structural properties are accounted for) ligament material properties are deterministic and ligament material microstructure is detectable with mesoscale measures of mechanical function.

Spatial light interference microscopy: principle and applications to biomedicine.

In this paper, we review spatial light interference microscopy (SLIM), a common-path, phase-shifting interferometer, built onto a phase-contrast microscope, with white-light illumination. As one of the most sensitive quantitative phase imaging (QPI) methods, SLIM allows for speckle-free phase reconstruction with sub-nanometer path-length stability. We first review image formation in QPI, scattering, and full-field methods. Then, we outline SLIM imaging from theory and instrumentation to diffraction tomography. Zernike's phase-contrast microscopy, phase retrieval in SLIM, and halo removal algorithms are discussed. Next, we discuss the requirements for operation, with a focus on software developed in-house for SLIM that enables high-throughput acquisition, whole slide scanning, mosaic tile registration, and imaging with a color camera. We introduce two methods for solving the inverse problem using SLIM, white-light tomography, and Wolf phase tomography. Lastly, we review the applications of SLIM in basic science and clinical studies. SLIM can study cell dynamics, cell growth and proliferation, cell migration, mass transport, etc. In clinical settings, SLIM can assist with cancer studies, reproductive technology, blood testing, etc. Finally, we review an emerging trend, where SLIM imaging in conjunction with artificial intelligence brings computational specificity and, in turn, offers new solutions to outstanding challenges in cell biology and pathology.

Stochastic evaluation of stress and strain distributions in duplex steel

Austenite–ferrite duplex steels generally consist of two differently textured polycrystalline phases with different glide mechanisms. For estimating the effective mechanical behavior of heterogeneous materials, there exist well established approaches, two of which are the classes of mean-field and full-field methods. In this work, the local fields resulting from these different approaches are compared using analytical calculations and full-field simulations. Duplex steels of various textures measured using X-ray diffraction are considered. Special emphasis is given to the influence of the crystallographic texture on the stress and strain distributions.

A Review: Anomaly-Based versus Full-Field-Based Weather Analysis and Forecasting

AbstractComparisons between anomaly and full-field methods have been carried out in weather analysis and forecasting over the last decade. Evidence from these studies has demonstrated the superiority of anomaly to full field in the following four aspects: depiction of weather systems, anomaly forecasts, diagnostic parameters, and model prediction. To promote the use and further discussion of the anomaly approach, this article summarizes those findings. After examining many types of weather events, anomaly weather maps show at least five advantages in weather system depiction: 1) less vagueness in visually connecting the location of an event with its associated meteorological conditions, 2) clearer and more complete depictions of vertical structures of a disturbance, 3) easier observation of time and spatial evolution of an event and its interaction or connection with other weather systems, 4) simplification of conceptual models by unifying different weather systems into one pattern, and 5) extension of model forecast length due to earlier detection of predictors. Anomaly verification is also mentioned. The anomaly forecast is useful for raising one’s awareness of potential societal impact. Combining the anomaly forecast with an ensemble is emphasized, where a societal impact index is discussed. For diagnostic parameters, two examples are given: an anomalous convective instability index for convection, and seven vorticity and divergence related parameters for heavy rain. Both showed positive contributions from the anomalous fields. For model prediction, the anomaly version of the beta-advection model consistently outperformed its full-field version in predicting typhoon tracks with clearer physical explanation. Application of anomaly global models to seasonal forecasts is also reviewed.

Digital image processing methods for the evaluation of optical anisotropy effects in tempered architectural glass using photoelastic measurements

In the present paper, optical anisotropy effects in architectural glass are evaluated using digital image processing. Hereby, thermally toughened glass panes were analyzed quantitatively using a circular polariscope. Glass subjected to externally applied stresses or residual stresses becomes birefringent. Polarized light on birefringent materials causes interference colors (iridescence), referred to as anisotropies, which affect the optical appearance of glass panes in building envelopes. Thermally toughened glass, such as toughened safety glass or heat strengthened glass, show these iridescences due to thermally induced residual stress differences. RGB-photoelastic full-field methods allow the quantitative measurement of anisotropies, since the occurring interference colors are related to the measured retardation values. By calibrating the circular polariscope, retardation images of thermally toughened glass panes are generated from non-directional isochromatic images using computer algorithms. The analysis of the retardation images and the evaluation of the anisotropy quality of the glass is of great interest in order to detect and sort out very low quality glass panes directly in the production process. Therefore, in this paper retardation images are acquired from different thermally toughened glass panes then different image processing methods are presented and applied. It is shown that a general definition of exclusion zones, e.g. near edges is required prior to the evaluation. In parallel, the limitations in the application of first-order statistical and threshold methods are presented. The intend of the investigation is the extension of the texture analysis based on the generation of Grey Level Co-occurrence Matrices, where the spatial arrangement of the retardation values is considered in the evaluation. For the first time, the results of textural features of different glass pane formats could be compared using reference areas and geometry factors. By reduction of the original image size, the computation time of textural analysis algorithms could be remarkably speeded up, while the textural features remained the same. Finally, the knowledge gained from these investigations is used to determine uniform texture features, which also includes the pattern of anisotropy effects in the evaluation of thermally toughened glass. Together with a global evaluation criterion this can now be implemented in commercial anisotropy measurement systems for quality control of tempered architectural glass.

MTEX2Gmsh: a tool for generating 2D meshes from EBSD data

In material sciences applied to crystalline materials, such as metals or ceramics, the grain morphology (size and shape) and the crystallographic texture are of great importance for understanding the macroscopic behaviour of the materials. Micromechanics of polycrystalline aggregates consists in evaluating the thermo-mechanical behaviour of the aggregates at their grain scale. If the investigated material is subjected to macroscopic deformation, the local strain can be obtained either experimentally, thanks to full-field measurement methods such as microgrid technique or Digital Image Correlation (DIC), or thanks to numerical simulation of the microstructure. The latter needs to take into account the mechanical heterogeneities (due to the different constituents) and the anisotropy of each phase, depending on its crystalline orientation. Orientation Imaging Microscopy (OIM), usually made from Electron Backscatter Diffraction (EBSD), is now widely used as a characterization technique. Indeed, it is in great interest for investigating the grain morphology and local crystal orientations in crystalline materials. Raw EBSD data can be considered as matrices of measurements of crystallographic data: each dot contains information about the phase and its orientation at the corresponding position. In order to perform Finite Element Analysis (FEA) on a polycrystal, one needs to first generate a mesh based on either EBSD or reconstructed grains. In this mesh, the Grain boundaries (GBs) must be accurately described since they play an important role in the overall behaviour of aggregates. To the best authors' knowledge, it appears that no existing tool for generating meshes from EBSD data is able to provide a robust grain description (e.g. suitable for any kind of phase and geometry) together with customizable features (e.g. variable element sizes). The proposed software, named MTEX2Gmsh works regardless the number of phases and the symmetries of those phases. In addition, it provides a smooth and accurate definition of the GBs. It is based on the MTEX toolbox for Matlab and the Gmsh software. MTEX2Gmsh allows to mesh the volume with a couple of options, such as: - increasing element size with increasing distance from the grains boundaries; - element type (tetrahedron, wedge or brick elements); - nesting the Region of Interest (ROI) into a larger medium.

Full-Field Measurement of Residual Stresses in Composite Materials Using the Incremental Slitting and Digital Image Correlation Techniques

The slitting method is a widely used destructive technique for the determination of residual stresses. Because of the rich data content of the full-field methods, optical techniques such as digital image correlation (DIC) are replacing strain gages for surface measurements. The objective of the current paper is to overcome the difficulties that arise in using the DIC technique combined with the slitting method. The present noise, low signal-to-noise ratio, and systematic errors are the main impediments to the use of DIC in the slitting method. An approach based on the eigenstrain concept was exploited to ascertain the optimum region of interest (ROI) for the analysis. After that, a robust procedure was implemented to utilize the DIC method while excluding the rigid body motion and rotation artifacts from the obtained displacements. Different slitting steps may cause dissimilar rigid body motions and rotations of the specimen. The proposed method was able to eliminate all of these different shears and stretches in the images simultaneously. The slitting experiment was conducted on a symmetric cross-ply composite specimen, and the slit progressed down to half the thickness. Although some rigid body motions were large, the method managed to exclude all of them for eight slitting steps. A comparison made between the results of the current method and those of the strain gage technique shows that they are in acceptable agreement with each other, and this full-field method can be extended to smaller scales or other destructive techniques.

2D Kinematic Quantification of Soil Particles around Growing Plant Root based on Optical Mechanics

The quantitative kinematic description of the surrounding soil particles during root growth is a technical challenge and biologically important. In this study, a two-dimensional camera-based imaging system was used to observe micro scale interactions between plant roots and soil particles. Maize root tip was imaged during ingress into the soils. This produced a series of two-dimensional images that represent temporal resolution of the geometric soil and root configurations at the micrometer scale. These images were used as inputs for full-field kinematic quantification methods, which enabled the analysis of two-dimensional deformation of the soils around elongating root. Correlation-based discrete object tracking and contour updating were used to track the shapes and the locations of soil particles and soil aggregates, while incremental digital image correlation was proposed to extract deformation and strain field within soil particle and soil aggregate. These techniques allowed the full-field displacements and strains of the soil to be quantified and the changes in shapes of soil particles to be visualized. Experimental results show that the presented shape tracking scheme, incremental digital image correlation and the research findings will be useful for the measurement and quantification of soil particle kinematics of soil-root physical interactions.

Towards Criteria Characterizing the Metrological Performance of Full-field Measurement Techniques

Users of full-field measurement methods like Digital Image Correlation (DIC) often aim to perform measurements with the best trade-off between spatial resolution, bias and measurement resolution. Whenever two full-field methods are compared, it is essential that these criteria are taken into consideration. Recently a metrological efficiency indicator for full-field measurements has been proposed and discussed. This indicator combines measurement resolution and spatial resolution. It has been shown to be invariant to the subset size in the case of Local DIC. The goal of this article is to discuss a method, which determines both the spatial and the measurement resolutions for a given bias for two different DIC methods, in order to obtain the metrological efficiency indicator for each of these methods. The benefit of this indicator is that it does not depend on setting parameters such as the subset size, which are chosen by the user. As such, it can be considered as intrinsic to each technique, thus enabling fair comparison. Local DIC and triangular finite element based Global DIC will be the subject of this investigation. With this setting, their respective subset and triangular element sizes will be related to the spatial resolution of both methods for a given acceptable bias. By using the metrological efficiency indicator, the performance of the two methods will be compared and discussed to a new level of detail. Generally speaking, the indicator shows that the metrological performance of both methods is similar, confirming their popularity. However, it will be shown that, depending on the choice of what an acceptable bias is, one of the method may be preferred to another. The results show that for the specific DIC versions used in the study, for cases for which a significant bias is acceptable, Local DIC outperforms Global DIC, while the opposite is true in the case for which the bias requirements are more stringent. Finally, the quadratic versions of both DIC versions are shown to significantly outperform their respective linear versions.

Investigation of the Tensile Behavior of Recycled Asphalts in the Small-Strain Domain with the Grid Method

Under mechanical loading, asphalt mixtures exhibit in their bulk heterogeneous strain fields characterized by localized gradients concentrated within the binder. Measuring such fields constitutes a challenge for the full-field measurement methods currently used in the experimental mechanics community. This is particularly true when the objective is to measure strains in the linear viscoelastic domain, which is characterized by low strain levels (about some 200 μe). In the present study, the strain distribution is measured on the surface of several recycled asphalt pavements (RAP) subjected to direct tensile tests. Four asphalt mixtures incorporating respectively 0%, 20%, 40% and 100% of RAP are studied. The tests are performed using a servo-controlled machine for rheological tests (MAER) and the local strain fields are measured using the Grid Method (GM). Out-of-Plane Motions (OPM) and camera noise are the main causes of disturbance that significantly affect the strain measurements. A method is proposed here to compensate the OPM. It is specially dedicated to bituminous mixtures. Sensor noise is filtered over time to improve the measurement resolution. Obtained results indicate that such compensations allow the use of GM to obtain quantitative measurements of the asphalt deformation in the small-strain domain. Finally, the behavior of the different samples are compared and the effect of RAP inclusion on the local strain distribution is observed and characterized.

Double wavelength combined electronic speckle pattern interferometry–moiré system for full-field strain analysis on electronic components

Determination of thermal dissipation capability and thermomechanical behavior of electronic components is becoming a relevant topic in view of the fact that thermal management can strongly affect reliability and lifetime of the component. The level of miniaturization, which is achieved in current electronics, requires that no-contact analysis must be privileged in view of the fact that any contact with the chip can significantly alter the heat capacity of the component and to introduce a systematic bias in the measurements. Moreover, the high complexity of electronics components suggests that full-field methods should be adopted in order to take into account about differences in terms of thermomechanical response in different areas of the same sample. In this article, an original optical set-up including a speckle interferometer and a projection moiré branch was developed; this system comprises two different laser sources emitting at two different wavelengths. Recording by a color camera allows detecting simultaneously the speckle pattern and the projected fringes. The whole information is successively separated in the post-processing stage and this allows obtaining in-plane and out-of-plane displacement fields. The system demonstrated its capability to determine dynamic response of the LM1084 analyzed component and, moreover, to detect the presence of a functional damage.

Bonding in B2 and B2+: Insights from full configuration interaction and valence bond studies

Full Configuration Interaction (Full-CI) and Valence Bond Self-Consistent Field (VBSCF) methods have been used to study the electronic structure and bonding in B2 and B2+ molecules. The bonding analysis based on these calculations shows that the B2 molecule is stabilised due to the formation of a double σ bond, one strong σ-bond together with one second weaker σ-bond, and two weak π bonds. Upon ionization one π electron is removed from the system and B2+ is formed, which has a one electron σ bond, instead of a π bond. It has been shown that a few carefully chosen VB configurations are enough to describe the bonding; with these structures, geometrical parameters as well as dissociation energies of these unusual molecular species are in agreement with full-CI results.

Application toward Confocal Full-Field Microscopic X-ray Absorption Near Edge Structure Spectroscopy.

Using X-ray absorption near edge structure (XANES) spectroscopy, information on the local chemical structure and oxidation state of an element of interest can be acquired. Conventionally, this information can be obtained in a spatially resolved manner by scanning a sample through a focused X-ray beam. Recently, full-field methods have been developed to obtain direct 2D chemical state information by imaging a large sample area. These methods are usually in transmission mode, thus restricting the use to thin and transmitting samples. Here, a fluorescence method is displayed using an energy-dispersive pnCCD detector, the SLcam, characterized by measurement times far superior to what is generally applicable. Additionally, this method operates in confocal mode, thus providing direct 3D spatially resolved chemical state information from a selected subvolume of a sample, without the need of rotating a sample. The method is applied to two samples: a gold-supported magnesia catalyst (Au/MgO) and a natural diamond containing Fe-rich inclusions. Both samples provide XANES spectra that can be overlapped with reference XANES spectra, allowing this method to be used for fingerprinting and linear combination analysis of known XANES reference compounds.

Output-only full-field modal testing

Operational modal analysis has become the focus of much research attention in the last two decades. Instead of an artificial force, the ambient excitation is considered as white-noise input to the structure and modal properties are calculated only from measured responses. In terms of the measurement technique, full-field optical methods, for example: electronic speckle pattern interferometry and digital image correlation have become popular and there is now much interest in applying these methods in structural dynamics. In this case the generated data is a full displacement map of the object, therefore there is no necessity to select specific measurement locations in order to visualise the deformation. However, there are generally large volumes of data to be processed, which makes the computation expensive and time-consuming, especially for engineering structures with large surface areas. Thanks to image decomposition techniques, huge amounts of data can be compressed into tens of shape descriptors with acceptably small distortion. In this paper, operational modal analysis and full-field methods are combined together, and the analysis is done in the shape descriptor domain to reduce the required computation time. Simulated responses from a finite element model of a clamped plate (under random excitation) serve to illustrate the methodology. Several different operational modal analysis methods are applied to analyse the data, and results are provided for purposes of comparison.

Simultaneous measurement of contour and micro-deformation using full-field optical methods

Simultaneous measurement of contour and micro-deformation using full-field optical methods

Influence of the Analysis Window on the Metrological Performance of the Grid Method

This paper deals with the grid method in experimental mechanics. It is one of the full-field methods available for estimating in-plane displacement and strain components of a specimen submitted to a load producing slight local deformation. This method consists in, first, depositing a regular grid on the surface of a specimen, and, second, comparing images of the grid before and after deformation. A possibility is to perform windowed Fourier analysis to measure these deformations as changes of the local grid aspect. The aim of the present study is to investigate the choice of the analysis window and its influence on the metrological performances of the grid method. Two aspects are taken into account, namely the reduction of the harmonics of the grid line profile, which are not pure sine because of manufacturing constraints, and the transfer of the digital noise from the imaged grid to the mechanical measurements. A theoretical study and a numerical assessment are presented. In addition, the interested reader can find in this paper a calculation of the Wigner---Ville transform of a triangular function which, to the best of the present authors' knowledge, is not available in the existing literature.