Determination Of Strain Fields Research Articles

Assessment of Digital Image Correlation Effectiveness and Quality in Determination of Surface Strains of Hybrid Steel/Composite Structures.

The application of the digital image correlation (DIC) contactless method has extended the possibilities of reliable assessment of structure strain fields and deformations throughout the last years. However, certain weak points in the analyses using the DIC method still exist. The fluctuations of the results caused by different factors as well as certain deficiencies in the evaluation of DIC accuracy in applications for hybrid steel/composite structures with adhesive joints are one of them. In the proposed paper, the assessment of DIC accuracy based on the range of strain fluctuation is proposed. This relies on the use of a polynomial approximation imposed on the results obtained from the DIC method. Such a proposal has been used for a certain correction of the DIC solution and has been verified by the introduction of different error measures. The evaluation of DIC possibilities and accuracy are presented on the examples of the static tensile tests of adhesively bonded steel/composite joints with three different adhesives applied. The obtained results clearly show that in a non-disturbed area, very good agreement between approximated DIC and FEM results is achieved. The relative average errors in an area, determined by comparison of DIC and FEM strains, are below 15%. It is also observed that the use of approximated strains by polynomial function leads to a more accurate solution with respect to FEM results. It is concluded that DIC can be successfully applied for the analyses of hybrid steel/adhesive/composite samples, such as determination of strain fields, non-contact visual detection of faults of manufacturing and their development and influence on the whole structure behavior during the strength tests, including the elastic response of materials.

Addressing the hydrogen entrapment in Al2024 alloys using scanning transmission electron microscopy and density functional theory

The effect of precipitate coherency in an aged aluminum (Al) 2024 alloy is investigated using scanning transmission electron microscopy (STEM) and density functional theory (DFT) methods. The focus of the study is on understanding the effects of hydrogen (H2) entrapment in the alloy, which leads to the degradation of its corrosion resistance. We employed STEM to analyze the structure and strain properties of the alloy at high spatial resolution. This has been achieved by applying the geometrical phase analysis (GPA) to the acquired HRSTEM images. The determination of strain field around the precipitates was utilized to gauge the degree of coherency at the interface of the precipitates with Al matrix. We also applied density functional theory (DFT) simulations to investigate the interfacial energy at the interfaces of the precipitates with the Al matrix. The obtained results of the DFT simulations showed that the interfacial energy at the semi-coherent precipitates was determined to be 0.06 eV per atom, which was higher compared to the coherent precipitates. Our findings suggest that the semi-coherent precipitates have a less stable interface with the Al matrix. Furthermore, the study revealed that the H2 entrapped at the semi-coherent precipitate regions exhibits a binding energy which is 3.2 eV lower than that of the coherent precipitates. This implies that hydrogen binds more strongly at the semi-coherent precipitate cites, which enhances the process of the alloy embrittlement.

Characterization of fracture process in polyolefin fibre-reinforced concrete using ultrasonic waves and digital image correlation

This study explores the monitoring of the fracture process in concrete beams and aims to characterize the evolution of damage in polyolefin fibre-reinforced concrete beams by utilizing the integrated application of two measurement techniques, digital image correlation and ultrasonic testing. The interpretation of registered wave time histories data was provided by the calculation of the magnitude-phase-composite metrics. An efficient procedure for the determination of strain field and the identification of crack height was subsequently developed. The results of the study show that the energy and phase shifts of ultrasonic waves can be used as autonomous damage indicators in a system for the real-time monitoring of a fracture process.

Determination of strain fields on two-dimensional images using the STC method

In this paper, a novel method called the Strain Tensor using Computational Fourier transform moiré (STC) method is proposed to obtain strain maps in a 2D lattice image such as an HRTEM or HAADF-STEM image. The method is a modification of the CFTM method, but it has two essential merits. Firstly, the method provides a 2 × 2 strain tensor using two independent Bragg peaks, and can hence be used to calculate both the axial and shear components, whereas the CFTM method can be used to calculate only the axial strain. Secondly, the STC method can provide more reliable results than the CFTM method by excluding the weak Bragg peaks of low signal/noise ratio on a digital diffractogram. In addition, the STC method is better than the GPA method because it does not require an unwrapping process. In this work, the STC method is demonstrated using a computer-generated image, showing that the method matches exactly with the CFTM method. Furthermore, the results for an experimental image reveal that the STC method can be an alternative method for strain analysis when the CFTM method cannot provide reliable results owing to weak Bragg peaks of low signal/noise ratio.

Strain field determination in III–V heteroepitaxy coupling finite elements with experimental and theoretical techniques at the nanoscale

AbstractWe are briefly reviewing the current status of elastic strain field determination in III–V heteroepitaxial nanostructures, linking finite elements (FE) calculations with quantitative nanoscale imaging and atomistic calculation techniques. III–V semiconductor nanostructure systems of various dimensions are evaluated in terms of their importance in photonic and microelectronic devices. As elastic strain distribution inside nano-heterostructures has a significant impact on the alloy composition, and thus their electronic properties, it is important to accurately map its components both at the interface plane and along the growth direction. Therefore, we focus on the determination of the stress-strain fields in III–V heteroepitaxial nanostructures by experimental and theoretical methods with emphasis on the numerical FE method by means of anisotropic continuum elasticity (CE) approximation. Subsequently, we present our contribution to the field by coupling FE simulations on InAs quantum dots (QDs) grown on (211)B GaAs substrate, either uncapped or buried, and GaAs/AlGaAs core-shell nanowires (NWs) grown on (111) Si, with quantitative high-resolution transmission electron microscopy (HRTEM) methods and atomistic molecular dynamics (MD) calculations. Full determination of the elastic strain distribution can be exploited for band gap tailoring of the heterostructures by controlling the content of the active elements, and thus influence the emitted radiation.

Optical-Digital Method for the Determination of Strain Fields in Local Areas of Reinforced-Concrete Bridges

By using the procedure of digital correlation image processing, we develop a portable optical-digital correlator for the determination of the distribution of strains in reinforced-concrete beams under static loading. We present the distributions of strains for a longitudinal beam in the region of the joint obtained in the course of experimental investigations of a model of the bridge.

Quality assessment for strain field determination based on the NISLT approach

By determining the state of strain, the status of a structure can be effectively evaluated. Strain field determination based on the non-iterative solution for linear transformations (NISLT) approach has been recently proposed to successfully identify rigid-body motion and natural deformation, which are coupled in the displacement fields in strain calculation. However, the NISLT approach cannot provide a direct measure of the quality of estimated parameters. This study has rigorously derived an analytical expression of the covariance matrix of linear transformation parameters using the NISLT approach. The derivation was used to determine the quality of the displacement field and can be used to estimate the quality of the determined parameters. The derivation was verified in numerical and laboratory experiments, which demonstrated that the estimated maximum principal strains differed from the originally assumed principal strain (real strain) within 1.96 times of the standard deviation (1.96σ). Therefore, the derived quality assessment of the NISLT approach provided a fair estimation of how accurate the strain would be under a certain displacement accuracy. The quality of the structure’s integrity monitoring system could thus be regulated, which is essential in the planning of structural monitoring tasks.

Non-destructive strain determination based on phase measurement and radial basis function

A method of non-destructive strain field determination based on the combination of the temporal phase measurement interferometry (TPMI) with radial basis function (RBF) was proposed in this paper. The quality of unwrapped phase obtained by TPMI was improved after RBF interpolation. Then accurate displacements can be obtained, followed by the strain components calculated by employing point-wise local least-squares fitting technique. Optimal sampling intervals of images with different sizes were given based on numerical experiments. For validation, the normal strain distributions in y direction of an aluminum plate subjected to uniform tensile loading were determined by the proposed technique and compared with values measured by strain gauge. The results are in good agreement with each other. Besides, strain distribution of a three-point bending beam with pre-notch was evaluated by the proposed method and the strain gauge technique. Comparison of the results reveals the effectiveness and feasibility of the proposed method.

Determination of Strain Field on the Superior Surface of Excised Larynx Vocal Folds Using DIC

The objective of the present study was to quantify the mechanical strain and stress in excised porcine larynges during self-oscillation using digital image correlation (DIC) method. The use of DIC in the excised larynx setup may yield accurate measurements of the vocal fold displacement field. Exvivo animal larynx. Measurements were performed using excised porcine larynges on a humidified flow bench, equipped with two high-speed cameras and a commercially available DIC software. Surface deformations were calculated from digital images recorded at 3000 frames per second during continuous self-oscillation for four excised porcine larynges. Larynx preparation consisted of removing the supraglottal wall and the false folds. DIC yielded the deformation field on the superior visible surface of the vocal folds. Measurement data for adducted and freely suspended vocal folds were also used to estimate the distribution of the initial prephonatory strain field. An isotropic constitutive law, the polymer eight-chain model, was used to estimate the surface distributions of planar stresses from the strain data. The Lagrangian normal strain values were between ∼16% and ∼29% along the anterior-posterior direction. The motion of material points on the vocal fold surface described an elliptical trajectory during oscillation. A phase difference was observed between the anterior-posterior and the medial-lateral component of the displacement. The strain data and eight-chain model yielded a maximum stress of ∼4kPa along the medial-lateral direction on the superior surface. DIC allowed the strain field over the superior surface of an excised porcine larynx to be quantified during self-oscillation. The approach allowed the determination of the trajectory of specific points on the vocal fold surface. The results for the excised larynx were found to be significantly different than previous results obtained using synthetic replicas. The present study provides suggestions for future studies in human subjects.

Impact of lens distortions on strain measurements obtained with 2D digital image correlation

The determination of strain fields based on displacements obtained via digital image correlation (DIC) at the micro-strain level (≤1000μm/m) is still a cumbersome task. In particular when high-strain gradients are involved, e.g. in composite materials with multidirectional fibre reinforcement, uncertainties in the experimental setup and errors in the derivation of the displacement fields can substantially hamper the strain identification process. In this contribution, the aim is to investigate the impact of lens distortions on strain measurements. To this purpose, we first perform pure rigid body motion experiments, revealing the importance of precise correction of lens distortions. Next, a uni-axial tensile test on a textile composite with spatially varying high strain gradients is performed, resulting in very accurately determined strains along the fibers of the material.

Effects of Joint Thickness, Adhesion and Web Shells to the Face Shell Bedded Concrete Masonry Loaded in Compression

The Australian masonry standard allows either prism tests or correction factors based on the block height and mortar thickness to evaluate masonry compressive strength. The correction factor helps the taller units with conventional 10 mm mortar being not disadvantaged due to size effect. In recent times, 2–4 mm thick, high-adhesive mortars and H blocks with only the mid-web shell are used in masonry construction. H blocks and thinner and higher adhesive mortars have renewed interest of the compression behaviour of hollow concrete masonry and hence is revisited in this paper. This paper presents an experimental study carried out to examine the effects of the thickness of mortar joints, the type of mortar adhesives and the presence of web shells in the hollow concrete masonry prisms under axial compression. A non-contact digital image correlation technique was used to measure the deformation of the prisms and was found adequate for the determination of strain field of the loaded face shells subjected to axial compression. It is found that the absence of end web shells lowers the compressive strength and stiffness of the prisms and the thinner and higher adhesive mortars increase the compressive strength and stiffness, while lowering the Poissons ratio

Effects of joint thickness, adhesion and web shells to the face shell bedded concrete masonry loaded in compression

The Australian masonry standard allows either prism tests or correction factors based on the block height and mortar thickness to evaluate masonry compressive strength. The correction factor helps the taller units with conventional 10 mm mortar being not disadvantaged due to size effect. In recent times, 2-4 mm thick, high-adhesive mortars and H blocks with only the mid-web shell are used in masonry construction. H blocks and thinner and higher adhesive mortars have renewed interest of the compression behaviour of hollow concrete masonry and hence is revisited in this paper. This paper presents an experimental study carried out to examine the effects of the thickness of mortar joints, the type of mortar adhesives and the presence of web shells in the hollow concrete masonry prisms under axial compression. A non-contact digital image correlation technique was used to measure the deformation of the prisms and was found adequate for the determination of strain field of the loaded face shells subjected to axial compression. It is found that the absence of end web shells lowers the compressive strength and stiffness of the prisms and the thinner and higher adhesive mortars increase the compressive strength and stiffness, while lowering the Poisson's ratio.

Determination of strain field and heterogeneity in radial forging of tube using finite element method and microhardness test

Utilizing the Finite Element Method (FEM), the strain field in the radial forging process of tube is calculated at different process conditions and compared with the experimental results achieved using the microhardness test. The effect of various process parameters such as friction, axial feed, back push and front pull forces and die angles on the strain field are investigated. Using the results of the analysis, it is shown that the deformation inhomogeneity, introduced by an Inhomogeneity Factor (IF), is maximum in the inner zone of tube, while the minimum and the maximum effective strains are appeared at the inner zone of tube and about the core of the tube thickness, respectively. Also, it was found that inhomogeneity decreases by increasing the die angle, decreasing the back push force and decreasing the friction factor.

Using Digital Image Correlation Techniques and Finite Element Models for Strain-Field Analysis of a Welded Aluminium Structure

The use of aluminium in lightweight structures and corrosive environments is continuously increasing. Welding is often used to assemble different aluminium components and it is of great importance to take into account the deteriorating effect the welding operation on the aluminium’s strength when designing these structures. In the present study, the strain-fields of welded aluminium tensile specimens have been analysed by digital image correlation (DIC) techniques, micro hardness measurements and finite element (FE) modelling. The stress-strain curve of the weld material, the heat affected zone (HAZ) and the base material have been determined by tensile tests. The material properties and the extension of the HAZ were also correlated to the micro hardness. The experimental data has been used as an input to the material models of the FE analysis. The study shows that using FE-analysis in combination with strain-field determination by DIC is very powerful. The resulting strain-fields are easily compared and the FE-analysis is verified in a straightforward way. Furthermore, the results of the study suggest that micro hardness measurements could be used to derive the strength of the material affected by the welding operation.

Assessment of Measuring Errors in Strain Fields Obtained via DIC on Planar Sheet Metal Specimens with a Non-Perpendicular Camera Alignment

The determination of strain fields based on displacement components obtained via 2D-DIC is subject to several errors that originate from various sources. In this contribution, we study the impact of a non-perpendicular camera alignment to a planar sheet metal specimen’s surface subject to biaxial loading conditions. The errors are estimated in a numerical experiment. To this purpose, deformed images - that were obtained by imposing finite element (FE) displacement fields on an undeformed image - are numerically rotated for various Euler angles. It is shown that a 3D-DIC stereo configuration induces a substantial compensation for the introduced image-plane displacement gradients. However, higher strain accuracy and precision are obtained - up to the level of a perfect perpendicular alignment - in a proposed ”rectified” 2D-DIC setup. This compensating technique gains benefit from both 2D-DIC (single camera view, basic amount of correlation runs, no cross-camera matching nor triangulation) and 3D-DIC (oblique angle compensation).

Error estimation in measuring strain fields with DIC on planar sheet metal specimens with a non-perpendicular camera alignment

The determination of strain fields based on displacement components obtained via 2D-DIC is subject to several errors that originate from various sources. In this contribution, we study the impact of a non-perpendicular camera alignment to a planar sheet metal specimen's surface. The errors are estimated in a numerical experiment. To this purpose, deformed images – that were obtained by imposing finite element (FE) displacement fields on an undeformed image – are numerically rotated for various Euler angles. It is shown that a 3D-DIC stereo configuration induces a substantial compensation for the introduced image-plane displacement gradients. However, higher strain accuracy and precision are obtained – up to the level of a perfect perpendicular alignment – in a proposed “rectified” 2D-DIC setup. This compensating technique gains benefit from both 2D-DIC (single camera view, basic amount of correlation runs, no cross-camera matching nor triangulation) and 3D-DIC (oblique angle compensation). Our conclusions are validated in a real experiment on SS304.

Experimental and numerical analysis of the forming process of a CP titanium scoliotic instrumentation

Abstract This paper presents a finite element model to simulate and optimize the bending forming process of a Commercially Pure (CP) titanium part for instrumentation intended to correct scoliotic deformations. The numerical tool is used as a technological indicator, in particular to anticipate tool dimensions compensating for the springback after bending. The mechanical behaviour of material is studied from uniaxial tensile tests with an image analysis measurement method for the determination of strain fields. The experimental results are then used to identify the intrinsic behaviour of the CP titanium and the parameters of an anisotropic elastic–plastic model. The numerical simulation is carried out on a reference part manufactured by the industrial partner. The study reveals that the numerical outline is in agreement with the measurements carried out on the part if the springback after bending is taken into account. The influence of the opening angle of the punch and die on this phenomenon is finally discussed through a parametric study.

EDITORIAL

EDITORIAL

Mechanisms of scoliosis surgical correction related to Cotrel-Dubousset and in situ contouring techniques

This paper presents a finite element model to simulate and optimize the bending forming process of a Commercially Pure (CP) titanium part for instrumentation intended to correct scoliotic deformations. The numerical tool is used as a technological indicator, in particular to anticipate tool dimensions compensating for the springback after bending. The mechanical behaviour of material is studied from uniaxial tensile tests with an image analysis measurement method for the determination of strain fields. The experimental results are then used to identify the intrinsic behaviour of the CP titanium and the parameters of an anisotropic elastic–plastic model. The numerical simulation is carried out on a reference part manufactured by the industrial partner. The study reveals that the numerical outline is in agreement with the measurements carried out on the part if the springback after bending is taken into account. The influence of the opening angle of the punch and die on this phenomenon is finally discussed through a parametric study.

Quantitative determination of strain fields around Ni 4Ti 3 precipitates in NiTi

High-resolution transmission electron microscopy and image-processing techniques are used to measure the strain fields surrounding coherent Ni 4Ti 3 precipitates in an austenitic Ni 51Ti 49 matrix. Images are recorded in the [1 1 1 ¯ ] B2 and the [1 0 1 ¯ ] B2 zones, and the {1 1 0} B2 interplanar spacings are used to determine the strain induced by both small (50 nm diameter) and large (300 nm diameter) precipitates. From these observations, the maximum strain in the surrounding matrix is mapped and identified as compressive or tensile. Interactions between strain fields of different precipitates are also investigated. A simple model for the observed strain is proposed and compared to the classical Eshelby solution for an ellipsoidal inclusion.