Measurement Of Strain Fields Research Articles

3D strain heterogeneity and fracture studied by X-ray tomography and crystal plasticity in an aluminium alloy

Strong correlations between measured strain fields and crystal plasticity finite element (CP-FE) predictions based on the real microstructure are found for a plane strain tensile specimen made of 6016 T4 aluminium alloy. This is achieved using multimodal X-ray lab tomography giving access to both the initial grain structure and the strain evolution. The real microstructure of the central region of interest (ROI) of the undeformed specimen is obtained non destructively using lab-based diffraction contrast tomography (DCT) and meshing. An in situ tensile test, using absorption contrast tomography (ACT) is then performed for twelve loading increments up to fracture. Taking advantage of the plane strain condition, the evolution of the internal strain field is measured by two-dimensional digital image correlation (DIC) in the material bulk using the natural speckle provided by intermetallic particles. Early strain heterogeneities in the form of slanted bands, that are spatially stable over time, are revealed and the fracture path – determined from the post mortem scan – is found to coincide with the bands exhibiting maximum strain. CP-FE simulations are performed on the meshed microstructure of the specimen acquired by DCT and are compared with image correlation measurements. The measured strain fields are well described by 3D CP-FE predictions, whilst it is shown that neither a macroscopic anisotropic plasticity model nor a CP-FE simulation with random grain orientations could reproduce the measurements.

Influence of loading conditions on acquired AE signals in biocomposites

Composite materials are increasingly used in lightweight transportation systems and civil engineering due to increasing weight constraints and installation costs. In the case of transportation systems such as in the aerospace and automobile industries, the use of composite materials reduces the weight, which is reflected in increasing the transportable load and reducing fuel consumption, and therefore the mechanical performance of the material is a very important and desired attribute. Biocomposites reinforced with natural fibers, such as flax, are gaining attention in engineering applications due to their sustainable and mechanical properties. This paper's research was designed to investigate the AE signals at different loading conditions of V-notched biocomposite specimens. Applying different loading cases, ranging from pure tension to pure shear, provided an understanding of the material's response under diverse stress states. The multi-axial strength and failure characteristics of biocomposites are evaluated. Digital image correlation technique provided measurement of strain fields complemented with AE data. AE signals have proven effective in capturing the onset and progression of different failure sources in composite material.

Identification of Moving Train Axle Loads for Simply Supported Composite Beam Bridges in Urban Rail Transit

With the rapid urbanization and expansion of rail transit systems, the axle loads of trains, which are a critical aspect of their configuration, have significantly increased. This increase poses substantial potential threats to the safety and service life of existing bridges within urban rail transit networks. Therefore, it is imperative to develop methods for monitoring and identifying train axle loads. In this study, a strain field measurement scheme was devised and implemented for an operational simply supported composite beam bridge in urban rail transit. This involved numerical modeling and validation of the bridge’s structural response, followed by the calculation of strain influence lines at specific measurement points. Subsequently, a method for identifying train axle loads, considering the dynamic amplification effect, was established. This method integrates principles from strain influence lines with mathematical optimization techniques. Specifically, the axle loads of locomotives on the forward AC03 type trains of Shanghai Metro Line 3 were found to conform to a logarithmic normal distribution model, while those of middle carriages followed a normal distribution model. Their respective mean axle loads were determined as 9.64 t and 10.77 t, with a shared variance of 0.8. Similarly, the axle loads of locomotives and middle carriages on reverse AC03-type trains also followed normal distribution models, with identical mean values around 10.5 t. The variances for axle loads of locomotives and middle carriages of reverse trains were found to be 1.36 and 0.8, respectively. The developed method effectively enables the monitoring of train axle loads and assessment of their impact on bridge structures, therefore enhancing safety and operational reliability within urban rail transit systems.

In-situ detection of delamination reinitiation in carbon fiber reinforced polymers post barely visible impact damage

In this study, advancements are presented in the in-situ detection of delamination reinitiation from Barely Visible Impact Damage (BVID) in composite materials, utilizing enhancements in Digital Image Correlation (DIC) techniques during a Compression After Impact (CAI) test. The study measured strain fields in the longitudinal, transverse, and shear directions, focusing specifically on the point of highest out-of-plane displacement to identify the onset of delamination propagation from BVID sites generated at different impact energy levels. By correlating the measured strains with the peak out-of-plane displacement, a unique determination of onset damage reinitiation associated with BVID during CAI testing was achieved. This method introduces a refined in-situ assessment technique for structural integrity, allowing for the early detection of critical damage propagation in composite materials.

Application of Digital Image Correlation to Small Punch Test for Determination of Stress–Strain Properties

AbstractThe analysis of the small punch test is based on the force on the moving punch and the deflection data acquired at a single point of the specimen bottom. However, the spatial distribution of stress and strain at any given instant is non-uniform and its variations with increase in punch penetration are quite complex. In this work, the digital image correlation (DIC) technique is integrated with small punch test for in-situ full field strain measurement in the bottom surface of the specimen. The DIC results reveal that with the progression of deformation, the peak equivalent plastic strain at the bottom surface shifts from the center of the specimen to a characteristic radial location, where the strain rapidly builds up and concentrates leading to instability and cracking. Combining DIC-based strain results with finite element model-based stress estimation at the characteristic radial location, a methodology for determining the stress–strain curve from small punch test is formulated and the outcomes are compared with tensile test results for four different metallic alloys.

Full-field, frequency-domain comparison of simulated and measured human brain deformation.

We propose a robust framework for quantitatively comparing model-predicted and experimentally measured strain fields in the human brain during harmonic skull motion. Traumatic brain injuries (TBIs) are typically caused by skull impact or acceleration, but how skull motion leads to brain deformation and consequent neural injury remains unclear and comparison of model predictions to experimental data remains limited. Magnetic resonance elastography (MRE) provides high-resolution, full-field measurements of dynamic brain deformation induced by harmonic skull motion. In the proposed framework, full-field strain measurements from human brain MRE in vivo are compared to simulated strain fields from models with similar harmonic loading. To enable comparison, the model geometry and subject anatomy, and subsequently, the predicted and measured strain fields are nonlinearly registered to the same standard brain atlas. Strain field correlations , both global (over the brain volume) and local (over smaller sub-volumes), are then computed from the inner product of the complex-valued strain tensors from model and experiment at each voxel. To demonstrate our approach, we compare strain fields from MRE in six human subjects to predictions from two previously developed models. Notably, global values are higher when comparing strain fields from different subjects ( ) than when comparing strain fields from either of the two models to strain fields in any subject. The proposed framework provides a quantitative method to assess similarity (and to identify discrepancies) between model predictions and experimental measurements of brain deformation, and thus can aid in the development and evaluation of improved models of brain biomechanics.

Tiny Bubbles: Measuring Strain Fields and Missing Atoms in Nanoscale He Bubbles via High-Resolution STEM Techniques

Tiny Bubbles: Measuring Strain Fields and Missing Atoms in Nanoscale He Bubbles via High-Resolution STEM Techniques

Misconception of the Use of 2D Plane Stress Solutions for Free Surfaces of Cracked Bodies and Its Significance for Full Field Measurement Techniques

Although 2D fracture mechanics solutions have been successfully applied, many analyses are affected by the wrong presumption of the 2D plane stress state at the free surface intersected by a crack front. The significance of this presumption and the related confusion have grown rapidly with the progress of strain field measurement techniques, such as the digital image correlation. The article demonstrates this wrong presumption on two of the most commonly measured and studied entities, which are the plastic zone in front of the crack tip and the crack tip opening displacement. 3D stress and strain solution is necessary to be performed. A realistic crack front curvature is required to be considered. Only such solutions can be used for comparison with the experimentally obtained strain fields. Other solutions are wrong and can bring confusion, wrong conclusions about material behavior, and slow down the progress in the field of fracture mechanics due to the incompatibility of the presumed stress state and the stress state present in experiments.

Wireless strain-field monitoring system for motion recognition via direct-ink-writing sensor-array

Large-area strain-monitoring is highly desired in various engineering applications. Despite the fact that the commercial strain gauges have been successfully used for strain measurement, there is still big challenge for this manually-pasting based solution especially for large-area monitoring. First, the wires of multiple commercial strain gauges for large-area monitoring will exhibit remarkably abundant and complex, and eventually reduce the reliability and convenience of the measurement system. Second, the commercial terminal instrument is usually function-redundant with complicated electronic configuration to match the low-sensitive metal-foil strain gauge. Meanwhile, the wire-based signal acquisition and processing system substantially limit the compatibility for moving objects. Here we take the advantages of direct-ink-writing (DIW) 3D printed sensor array, thereby propose a wireless-based solution for large-area strain field measurement, data transmission and processing. The sensitivity of the printed sensing unit is one order of magnitude higher than that of traditional strain gauges, while the sensor array layout is optimized for compatibility to the strain orientation and detecting domain. A high-frequency signal acquisitor is designed and assembled with a wireless transmitter for rapid multi-point strain detection and real-time data transmission. A remote data receiver and processing terminal are developed and integrated for twin-visualization of the detected strain field. In particular, the detected strain data are processed to construct a cascaded support-vector-machine (SVM) classifier, that successfully identify motion states of moving vehicles in high precision. This study provides insight into exploring the advantages of DIW printed sensor arrays combined with specifically-designed wireless system and strain-based dynamic recognition.

Direct modeling of non-uniform strain field of heterogeneous materials

Engineering materials have a heterogeneous microstructure that induces non-uniform deformations (NUDs) even under uniform stress. Therefore, the development of NUDs in heterogeneous materials under a macroscopically non-uniform stress is characterized by the interactions between micro- and macroscopic NUDs. In this study, we attempt to directly describe microscopic NUDs by introducing a constitutive equation for the strain field. Using polycrystalline metal, which is a well-known heterogeneous material, we performed the uniaxial tensile tests on specimens with curved gauge sections. During modeling, the constitutive equation without nonlocal effect was first fitted based on the distribution of the local equivalent stress and strain. The strain calculated using the fitted equation was regarded as the strain for the homogeneous material, whereas the strain deviating from the fitted equation was assumed to be induced by the microscopic heterogeneity of the material. The respective strains were represented using different constitutive models, and the parameters employed in the models were fitted separately using the experimentally measured strain field. The plastic compliance gradient was employed as a material parameter quantifying the microscopic NUDs. Subsequently, a multiaxial elastoplastic nonlocal constitutive equation was formulated based on the nonlocally extended J2 flow theory. Finite element method simulations introducing the proposed nonlocal effect indicated that a scale-dependent NUD characterized by the interactions between micro- and macro-NUDs could be represented using the proposed model. Furthermore, the proposed model could predict the experimentally observed strain field characterized by a polycrystalline microstructure, such as the dispersion of the strain concentration at the curved gauge section and the appearance of NUDs even under uniform stress.

Biaxial Extension of Cruciform Specimens: Embedding Equilibrium Into Design and Constitutive Characterization

BackgroundMain desired features of biaxial tests are: uniformity of stresses and strains; high strain levels in gauge areas; reliable constitutive parameters identification. Despite cruciform specimen suitability to modern tensile devices, standard testing techniques are still debated because of difficulties in matching these demands.ObjectiveThis work aims at providing rational performance objectives and efficient cruciform specimens shapes in view of constitutive characterization.MethodsObjective performance is evaluated along particular lines lying on principal directions in equibiaxial tensile tests. A rich specimen profile geometry is purposely optimized via finite elements analysis by varying cost function and material compressibility. Experimental tests, monitored via digital image correlation, are carried out for validation.ResultsNew shapes are designed and tested in a biaxial tensile apparatus and show to perform better than existing ones. Elastic parameter identification is efficiently performed by only exploiting full field strain measurements along statically significant lines.ConclusionsSmall gauge areas and small fillet radii cruciform specimens approach the ideal deformation behaviour. For the constitutive parameters identification in planar tensile experiments, it suffices to monitor strains along the gauge lines.

On the high fracture toughness of wood and polymer-filled wood composites – Crack deflection analysis for materials design

Cracks oriented in the toughest direction across the grain of wood (0°) tend to deflect at 90° to the precrack rather than extending in 0° direction. Fracture toughness data across the grain are therefore difficult to interpret. Crack growth mechanisms and effects from replacing wood pore space with a polymer are investigated. Crack growth is analyzed in four-point bending fracture mechanics specimens of birch and two different polymer-filled birch composites using strain-field measurements and finite element analysis (FEA). Calibrated cohesive zone models in both precrack and 90°-directions describe fracture process zone properties in orthotropic FEA-models. Conditions for 0° crack penetration versus 90° crack deflection are analyzed based on cohesive zone properties. Stable, subcritical crack deflection takes place at low load, reduces crack tip stress concentration, and contributes to high structural toughness, provided the 90° toughness is not too low. Polymer-filled neat birch composites have the best structural toughness properties in the present investigation, since 90° toughness is not compromised by any chemical treatment.

Investigation of the large-signal electromechanical behavior of ferroelectric HfO2–CeO2 thin films prepared by chemical solution deposition

Investigation of the large-signal electromechanical behavior of ferroelectric HfO2–CeO2 thin films prepared by chemical solution deposition

Fracture behavior of double edge notch AlSi10Mg alloy fabricated by laser powder bed fusion

In the present study, the fracture behavior of additively manufactured (AM) metal parts has been investigated. To this aim, double edge notch tension (DENT) specimens have been printed using AlSi10Mg alloy based on the laser powder bed fusion (PBF) technique. Particularly, DENT specimens have been fabricated with three different ligament lengths and three different printing directions for each ligament length. In this study, we used the concept of essential work of fracture to describe the fracture behavior AMed parts. According to a series of tests under static loading conditions, influences of the ligament length and printing direction on the structural integrity of the DENT specimens have been determined. According to the experimental practices, levels of ductility and size of plastic zone in the ligament region have been documented. Here, we utilized digital image correlation technique to measure strain fields on the surface of the examined samples. Moreover, numerical simulations have been conducted to simulate the stress distribution from DENT specimens. In addition, visual inspection of the fracture surfaces has been conducted using a free-angle observation system. The outcome of this study can be used for future designs of PBF printed parts with a better structural performance.

High sensitivity strain measurement on a vibrating saxophone reed using stroboscopic digital image

In this paper, we present a study of different optical textures and techniques to apply very fine, non-specular reflecting random speckle texture, which makes it possible to measure surface strains with a sensitivity better than 0.01 % strain on a vibrating object. The method is based on triggered stroboscopic digital image correlation, which is able to obtain time-resolved full-field measurements of displacement and strain fields. The method is demonstrated on reeds of the saxophone to investigate their role in sound production and playability. Such high-sensitivity measurements allow the detection of subtle effects, such as artefacts on synthetic saxophone reeds caused by the production process.

In-situ measurements of temperature and strain fields of large-diameter concrete-filled steel tube columns and the refined finite element modeling

Concrete-filled steel tube (CFST) columns are prevalent in transmission towers and arch bridges due to their cost-effectiveness and superior mechanical performance. The growing megastructures lead to large-diameter CFST components, which are exposed to the risk of early-age thermal concrete cracking. In this study, temperature and strain fields of two 2.1 m-dia. CFST columns were monitored in-situ. Within 30 h, the peak temperature and tensile strain at the centroid reached 97.0°C and 400 με, respectively. Meanwhile, temperature fields of eight scaled CFST columns were also monitored and compared. In addition, ultrasonic pulse velocity test was performed to detect the early-age thermal cracking. To predict the temperature and strain evolutions accurately, a refined numerical model is proposed, considering coupled mechanical, hydration and thermal processes. It is revealed that the model could accurately predict the temperature and strain evolutions of CFST columns. Furthermore, experiments and numerical simulations illustrate that thermal cracking would likely be triggered in CFST components with diameter greater than 1.2 m.

Digital twin and cross-scale mechanical interaction for fabric rubber composites considering model uncertainties

Digital twin ushers in growth as one cutting-edge technology that enables high-precision, informative and real-time interaction of advanced composites, influencing the paradigm of composite analysis and design in aerospace, automotive, intelligent electronics and other fields. The online and fine evolution of digital twins at scale face challenges due to creating scalable predictions, updates, and controls with coupled information in workflows. In this work, an interactive digital twin methodology for complex composite structures (CCSDT) based on machine learning is introduced. To demonstrate the framework's feasibility and applicability, a hybrid architecture comprising hierarchical deep neural networks (H-DNNs), statistical inference and cross-scale physical constraints is proposed considering the fabric rubber composites (FRC) in aerospace field. The framework predicts 3D displacement and stress fields directly from sensing features and can incorporate dynamic updating and evaluation of computational models into data assimilation. Real-time prediction and uncertainty quantification are verified through synthetic and experimental data, and the capability of CCSDT in perception and decision is demonstrated by a case of cruise state monitoring. The comparison between the measured strain field and the post-processing predicted strain field shows the extensibility of the direct prediction results. These results provide guidance for the development of composite digital twins, stimulating the potential for cost-effective and efficient digital twin services.

Real-time Strain Field Measurement Based on Dense Fiber Bragg Gratings Array

Real-time Strain Field Measurement Based on Dense Fiber Bragg Gratings Array

Fibre optic sensing for strain-field measurement in geotechnical laboratory experiments

Strain measurement inside the soil body in three-dimensional (3D) experiments is a real challenge for physical modellers in geotechnics. The use of fibre optic sensing offers the possibility of continuous measurement of the strain at depth with high spatial and temporal resolution in small-scale laboratory experiments. Despite the technology not being fully ready yet for centrifuge experiments, this is an important development in geotechnics. This paper explores the capacities of distributed fibre optic sensing (DFOS) as a solution for direct soil strain profile measurement at depth. A series of small-scale plane-strain experiments is used to simulate the simple case of the formation of a downwards subsidence. DFOS cables are laid in the soil specimen, and strains are directly compared with the soil movement, recorded with cameras through particle image velocimetry. Results indicate the ability of DFOS in detecting soil movements and underline the typical signature strain profile expected during this type of experiments. Finite-element analyses are carried out to further underpin the consequences of performing these tests at 1g and extend the findings to potential applications in 3D and on the geotechnical centrifuge. This shows promising results for detection of soil strain profiles inside the soil body in 3D.

Importance of TEM sample thickness for measuring strain fields

Transmission electron microscopy (TEM) has emerged as a valuable tool for assessing and mapping strain fields within materials. By directly analyzing local atomic spacing variations, TEM enables the precise measurement of local strain with high spatial resolution. However, it is standard practice to use thin specimens in TEM analysis to ensure electron transparency and minimize issues such as projection artifacts and contributions from multiple scattering. This raises an important question regarding the extent of structural modification, such as strain relaxation, induced in thin samples due to the increased surface-to-volume ratio and the thinning process. In this study, we conducted a systematic investigation to quantify the influence of TEM sample thickness on the residual strain field using deformed Fe-based and Zr-based metallic glasses as model systems. The samples were gradually thinned from 300 nm to 70 nm, and the same area was examined using 4D-STEM with identical imaging settings. Our results demonstrate that thinning the sample affects the atomic configuration at both the short-range (SR) and medium-range (MR) scales. Consequently, when the sample is thinned too much, it no longer preserves the native deformation structure. These findings highlight the critical importance of maintaining sufficient TEM sample thickness for obtaining meaningful and accurate strain measurements.