Inhomogeneous Strain Field Research Articles

Deformation processes of polymer composites with stress concentrators under different reinforcement schemes

This paper is dedicated to an experimental investigation of an effect of a stress concentrator orientation on a mechanical behavior of polymer composites with various reinforcement schemes. Composite specimens have been made of layered carbon fiber reinforced polymer (CFRP) with lay-up patterns of [±45]16 and [0/90]16. The stress concentrators have been cut in a form of a rectangle (with rounded corners) orientated at angles of 0°, 45°, and 90° to a loading axis. The novel data about the composite’s mechanical behavior has been obtained by the digital image correlation (DIC) method, the acoustic emission (AE) method and the optical microscopy. Quasistatic tests have been carried out taking into account recommendations of the ASTM D5766, ASTM D3039, ASTM D3518. The effect of the stress concentrator orientation on the composite’s strength has been evaluated. It has been found that the stress concentrator does not affect the strength of the composite with the [±45]16 lay-up pattern. This peculiarity has been explained by the absence of the layers preventing the fibers system turning in the direction of the load application. The DIC method has allowed to study the evolution of inhomogeneous strain fields on the specimens’ surfaces. Computational simulations have been carried out within the elasticity stage. The results have shown that the strain fields obtained by both the DIC method and the numerical simulation were similar for the composites with the [0/90]16 reinforcement pattern. However, significant differences have been found for the [±45]16 reinforcement pattern, which have been explained by the development of inelastic zones near the stress concentrator even if the elastic stage is realized at the macrolevel. The using of the AE method and the optical microscopy has allowed to reveal typical mechanisms of the structural damage and to study their occurrence during the tests. It has been concluded that the orientation of the stress concentrator significantly influences the deformation processes of the CFRP with various reinforcement patterns.

Fatigue behavior of pultruded fiberglass tubes under tension, compression and torsion

This work is devoted to an experimental investigation of fatigue behavior of pultruded fiberglass tubes under uniaxial tension, compression and torsion. Static tests were carried out; a presence of postcritical deformation stage during torsion is noted. Regularities of inhomogeneous strain fields evolution are analyzed using digital image correlation method. Fatigue curves are built for four cyclic loading modes: tension-tension, compression-compression, tension-compression and torsion. An analysis of specimens' fractures is carried out, typical damaging mechanisms are revealed. Residual dynamic stiffness data is obtained and studied using a previously proposed fitting model. Results demonstrate model's high descriptive capability and its flexibility to describe two-staged and three-staged stiffness degradation curves. An influence of loading mode on a shape of these curves is found out. Model parameters' dependence on maximum stress value during the loading cycle is studied using the Pearson's correlation coefficient. The necessity of multiaxial fatigue behavior investigation of pultruded fiberglass tubes is concluded.

Uniaxial and biaxial experimental investigation of glassy polymers

The alignment of polymer chains within polycarbonate (PC) significantly influences the mechanical properties of the deformed specimen. This influence can be affected by both the global stretch rate and the type of loading. The objectives of the current work are to investigate uniaxial film, bar specimens, and biaxial thick-walled specimens using 3D-optical measurements.Our approach involves developing different methods to determine the local inhomogeneous strain fields for both thin and thick-walled uniaxial specimens. In the pure tension case and under the assumption of uniaxial plane stress, we utilize the strains to calculate the local volume ratio and, consequently, approximate the local true stress at different axial positions of the specimen. Additionally, we investigate the effect of variations in global stretch rates on local stresses. Furthermore, we explore induced anisotropy for different global stretching ratios.Another objective is to investigate biaxial specimens under sequential and simultaneous biaxial stretching conditions, examining the impact of both stretching types on the mechanical and structural properties of the material.

Site-specific reactivity of stepped Pt surfaces driven by stress release.

Heterogeneous catalysts are widely used to promote chemical reactions. Although it is known that chemical reactions usually happen on catalyst surfaces, only specific surface sites have high catalytic activity. Thus, identifying active sites and maximizing their presence lies at the heart of catalysis research1-4, in which the classic model is to categorize active sites in terms of distinct surface motifs, such as terraces and steps1,5-10. However, such a simple categorization often leads to orders of magnitude errors in catalyst activity predictions and qualitative uncertainties of active sites7,8,11,12, thus limiting opportunities for catalyst design. Here, using stepped Pt(111) surfaces and the electrochemical oxygen reduction reaction (ORR) as examples, we demonstrate that the root cause of larger errors and uncertainties is a simplified categorization thatoverlooks atomic site-specific reactivity driven by surface stress release. Specifically, surface stress release at steps introduces inhomogeneous strain fields, with up to 5.5% compression, leading to distinct electronic structures and reactivity for terrace atoms with identical local coordination, and resulting in atomic site-specific enhancement of ORR activity. For the terrace atoms flanking both sides of the step edge, the enhancement is up to 50 times higher than that of the atoms in the middle of the terrace, which permits control of ORR reactivity by either varying terrace widths or controlling external stress. Thus, the discovery of the above synergy provides a new perspective for both fundamental understanding of catalytically active atomic sites and design principles of heterogeneous catalysts.

Understanding the asymmetric orientations and stress states in polycrystalline NiTi SMA by in-situ synchrotron-based high-energy X-ray diffraction

The stress-induced martensite transformations (SIMTs) dramatically affect the recoverable strain and mechanical response of polycrystalline NiTi. An in-depth understanding of the propagation manner and orientation preference of SIMTs is therefore crucial. In this work, we present a unique asymmetric anisotropy of SIMTs and lattice strains induced by Lüders-type deformation in polycrystalline NiTi, achieved through a combination of in-situ synchrotron X-ray diffraction and uniaxial tensile loading and unloading experiments. Our experimental findings reveal that in polycrystalline NiTi under uniaxial deformation, the asymmetry of SIMTs is attributed to the inhomogeneous strain field caused by the Lüders-type mechanism. The asymmetrical SIMT starts with the forward Lüder band and disappears along with the backward Lüder band. The austenite with the favored orientation of ⟨110⟩A//loading direction (LD) transformed and recovered back at a higher rate compared to other orientations during both loading and unloading.

Dissipative Particle Dynamics Simulation for Reaction-Induced Phase Separation of Thermoset/Thermoplastic Blends.

Reaction-induced phase separation occurs during the curing reaction when a thermoplastic resin is dissolved in a thermoset resin, which enables toughening of the thermoset resin. As resin properties vary significantly depending on the morphology of the phase-separated structure, controlling the morphology formation is of critical importance. Reaction-induced phase separation is a phenomenon that ranges from the chemical reaction scale to the mesoscale dynamics of polymer molecules. In this study, we performed curing simulations using dissipative particle dynamics (DPD) coupled with a reaction model to reproduce reaction-induced phase separation. The curing reaction properties of the thermoset resin were determined by ab initio quantum chemical calculations, and the DPD parameters were determined by all-atom molecular dynamics simulations. This enabled mesoscopic simulations, including reactions that reflect the intrinsic material properties. The effects of the thermoplastic resin concentration, molecular weight, and curing conditions on the phase-separation morphology were evaluated, and the cure shrinkage and stiffness of each cured resin were confirmed to be consistent with the experimental trends. Furthermore, the local strain field under tensile deformation was visualized, and the inhomogeneous strain field caused by the phase-separated structures of two resins with different stiffnesses was revealed. These results can aid in understanding the toughening properties of thermoplastic additives at the molecular level.

Design of a Frequency Multiplier Based on Laterally Coupled Quantum Dots for Optoelectronic Device Applications in the Terahertz Domain: Impact of Inhomogeneous Indium Distribution, Strain, Pressure, Temperature, and Electric Field

Design of a Frequency Multiplier Based on Laterally Coupled Quantum Dots for Optoelectronic Device Applications in the Terahertz Domain: Impact of Inhomogeneous Indium Distribution, Strain, Pressure, Temperature, and Electric Field

Unique Identification of Stiffness Parameters in Hyperelastic Models for Anisotropic, Deformable, Thin Materials Based on a Single Experiment - A Feasibility Study Based on Virtual Full-Field Data

BackgroundCharacterizing material properties of thin sheets for design or manufacturing purposes is an essential concern in many engineering applications. This task is particularly challenging for materials with a pronounced anisotropic and nonlinear mechanical behavior.ObjectiveA hybrid, experimental-numerical approach for the characterization of the mechanical, nonlinear response of thin, anisotropic, deformable materials is proposed. In contrast to classical approaches where various biaxial tension tests are analyzed, the main goal here is the complete characterization based on one single experiment.MethodsThe proposed approach is based on a novel non-standard experimental setup which is on the one hand easy to install and use, and which on the other hand intentionally induces a strongly inhomogeneous strain field in the specimen capturing as many deformation modes and intensities as possible. The resulting displacement field can be measured using e.g., digital image correlation, and is then accessible to the parameter identification as full-field data. To allow for an efficient identification, an extended equilibrium gap method is presented, where unknown boundary force distributions applied in the experiment are computed iteratively. The approach’s feasibility is assessed through virtual full-field data obtained by numerical simulation of the proposed experimental setup using predefined parameter values and applying realistic noise. That way, a quantitative assessment of the method’s performance regarding two specifically chosen material models is enabled.ResultsProvided that the stiffness-related material parameters are indeed linear in the stress equations, a quadratic optimization problem can be constructed to allow for a unique identification of the parameter values. Analysis show that reference parameter values for calendered rubber as well as coated textile fabric can be identified, even when realistic noise is applied to the virtual test data.ConclusionBased on the presented investigations, the proposed method has been found to be feasible for the accurate identification of stiffness-related parameters of anisotropic, nonlinear thin sheets using a single experiment.

MULTIFRACTAL CHARACTERIZATION OF THE INHOMOGENEOUS STRAIN EVOLUTION OF THE DEHYDRATED COAL: INSIGHT FROM COAL MICROSTRUCTURE

Underground coal mining in China has gradually moved into deeper seams in recent years, which results in a higher ambient temperature in the mining space and significantly affects the mechanical behavior of coal. In this study, dehydrated coal samples were obtained at different temperatures ranging from 30[Formula: see text] to 70[Formula: see text], and the mechanical behavior of the dehydrated coal was investigated through compressive loading tests. The digital image correlation (DIC) method was used to acquire the strain field of coal, and a multifractal analysis was conducted to characterize the strain evolution of coal. The findings suggest that the increasing temperatures result in higher moisture desorption rates and greater volumetric contraction strain in coal. Furthermore, coal with higher moisture desorption exhibits higher peak stress and peak strains when subjected to compressive loading. The multifractal analysis of the inhomogeneous strain evolution indicates a gradual decrease in the parameter [Formula: see text] under compressive loading, followed by a sudden increase before reaching the failure point due to strain localization. The multifractal mechanism was further investigated, revealing that the inhomogeneous strain field of coal is inherently affected by the microstructure of coal. In addition, a mathematical model was proposed to elucidate the relationship between the inhomogeneous coal strain and the microstructure of coal. The result indicates that the inhomogeneity of the coal strain is directly associated with the multifractal singularity of the coal microstructure. Finally, the feasibility of using the multifractal parameter [Formula: see text] to identify coal strain localization has been demonstrated, indicating its potential value in aiding engineers to determine the SLZ in deep coal mines.

(Invited) Exciton Transport in Strained 2D Semiconductors

Being atomically thin, flexible, and exhibiting considerable light emission and ultrafast non-equilibrium dynamics, semiconducting transition metal dichalcogenides (TMDs) have been considered as promising candidates for next-generation optoelectronic devices. The optical and electronic properties of TMDs are governed by a rich landscape of tightly bound excitons, including regular bright excitons, as well as optically inaccessible dark exciton states. Recently, strain engineering of monolayer TMDs has been introduced to tune their optical properties, such as the exciton transition energy, exciton-phonon coupling, or the Stokes shift [1-3].Transport of charge carriers is crucial for nanoelectronics. In conventional materials, electronic transport can be conveniently controlled by external electric fields. However, the tightly bound excitons, being neutral particles, are only weakly affected by electrical fields. We demonstrate that mechanical strain can also be used to manipulate the transport of excitons in TMDs. To this end, we apply homogeneous tensile strain to a WS2 monolayer by bending the substrate [1], which causes a redshift of the X0 exciton photoluminescence. By measuring the spatiotemporal photoluminescence after near-resonant excitation with femtosecond laser pulses, we map the spread of excitons and extract the strain-dependent diffusion coefficient [4].Furthermore, we demonstrate the propagation of excitons in an inhomogeneous strain landscape. We create inhomogeneous tensile strain in TMD monolayers by transferring them onto patterned substrates with nanopillars or by a nanoimprint technique [5]. Due to the redshift of the exciton resonances with applied strain, excitons are expected to move towards high-strain regions in an inhomogeneous strain field - the so-called "funneling" effect. We verify this behavior for the "bright" TMD material monolayer MoSe2. In the case of the "dark" monolayer WS2, we observe exactly the opposite effect. Here, the excitons are expelled from the high-strain regions ("anti-funneling") [6]. By comparing our experimental results with a microscopic theory, we explain this observation by the drift of momentum-dark KΛ excitons, which, in contrast to bright excitons, shift to higher energies with strain.Our joint experiment-theory study highlights the dominant role of momentum-dark excitons for the dynamics in monolayer TMDs and provides crucial design guidelines for TMD devices based on exciton transport.

Inverse identification of plastic anisotropy through multiple non-conventional mechanical experiments

In theory, a single non-conventional mechanical experiment generating inhomogeneous strain fields enables to inversely identify an anisotropic yield function. However, several studies have shown that it is challenging to design such a sufficiently data-rich experiment, particularly when it must be conducted on a standard uniaxial tensile machine. Instead of relying on a single non-conventional uniaxial tensile experiment, combining multiple non-conventional experiments is proposed to inversely identify an advanced anisotropic yield function. The feasibility of the proposed method is verified through the inverse identification of the Yld2000-2d yield function using synthetically generated Digital Image Correlation (DIC) data. This approach accounts for the metrological aspects of DIC while avoiding potential experimental errors and uncertainties related to the selected material model. It has been demonstrated that when a single non-conventional tensile test is combined with a non-conventional biaxial tensile test, the inversely identified anisotropy parameters are in good agreement with those found through the conventional method. This finding is contingent on maintaining an equal contribution of the strain states from each experiment to the cost function. Excluding overlapping data points and assigning proper weights to the trustworthy data based on the strain state and the level of plastic deformation is found to be key. The numerical results are experimentally validated, thereby revealing the crucial role of the adopted anisotropic yield function. Furthermore, the uncertainty associated with the inversely identified parameters based on three repetitions of the same experiment is discussed.

Hole-Spin Driving by Strain-Induced Spin-Orbit Interactions.

Hole spins in semiconductor quantum dots can be efficiently manipulated with radio-frequency electric fields owing to the strong spin-orbit interactions in the valence bands. Here we show that the motion of the dot in inhomogeneous strain fields gives rise to linear Rashba spin-orbit interactions (with spatially dependent spin-orbit lengths) and g-factor modulations that allow for fast Rabi oscillations. Such inhomogeneous strains build up spontaneously in the devices due to process and cool down stress. We discuss spin qubits in Ge/GeSi heterostructures as an illustration. We highlight that Rabi frequencies can be enhanced by 1 order of magnitude by shear strain gradients as small as 3×10^{-6} nm^{-1} within the dots. This underlines that spins in solids can be very sensitive to strains and opens the way for strain engineering in hole spin devices for quantum information and spintronics.

Unveiling the dynamic softening mechanism via micromechanical behavior for a near-β titanium alloy deformed at a high strain rate

For near-β high-strength titanium alloys, rheological softening is the key step during thermal deformation to determine the microstructure and mechanical properties. Nevertheless, the intrinsic softening mechanism during the rheological stage is still an open question due to the special complexity of the process. In this study, the deformation process of near-β titanium alloy TC18 (Ti–5Al–5Mo–5V–1Cr–1Fe, wt.%) in a dual-phase region at a high strain rate was investigated. The micromechanical behavior of TC18 alloy during thermal deformation at 785 °C and 0.1/s was revealed by using in-situ high-energy X-ray diffraction (HEXRD). The stress partitioning and microstructure evolution mechanism for this alloy were discussed in detail. It has been confirmed that the main softening mechanisms of TC18 alloy during thermal deformation are dynamic recovery (DR) and continuous dynamic recrystallization (cDRX). The stress partitioning between constituent phases featured by higher stress in the α phase than in the β phase induces an inhomogeneous strain field near α phase lamellae to promote the dislocation pile-up in subgrains. The cDRX process of β grains is composed of dislocation slipping, DR and subgrains formation, and transformation of recrystallized grains with high-angle grain boundaries (HAGB). Schmid factor and micro stress have a remarkable effect on dynamic recrystallization (DRX) behavior. The soft <200>β//LD oriented grains are more likely to undergo cDRX than the hard <111>β//LD oriented grains. This work may have implications for understanding the deformation mechanism and provide a fundamental basis for selecting appropriate processing technology for near-β titanium alloys.

Engineering Dynamic Structural Color Pixels at Microscales by Inhomogeneous Strain-Induced Localized Topographic Change.

Structural colors in homogeneous elastomeric materials predominantly exhibit uniform color changes under applied strains. However, juxtaposing mechanochromic pixels that exhibit distinct responses to applied strain remains challenging, especially on the microscale where the demand for miscellaneous spectral information increases. Here, we present a method to engineer microscale switchable color pixels by creating localized inhomogeneous strain fields at the level of individual microlines. Trenches produced by transfer casting from 2.5D structures into elastomers exhibit a uniform structural color in the unstretched state due to interference and scattering effects, while they show different colors under an applied uniaxial strain. This programmable topographic change resulting in color variation arises from strain mismatch between layers and trench width. We utilized this effect to achieve the encryption of text strings with Morse code. The effective and facile design principle is promising for diverse optical devices based on dynamic structures and topographic changes.

Mechanical Properties Degradation of Fiberglass Tubes during Biaxial Proportional Cyclic Loading.

Composite structures during an operation are subjected to various types of external loading (impact, vibration, cyclic, etc.), which may lead to a decrease in mechanical properties. Previously, many experimental investigations of the mechanical behavior of composites under uniaxial cyclic loading were carried out. Acquisition of new data on the reduction of composite materials' mechanical characteristics under conditions of multiaxial cyclic loading, as well as verification of existing models for calculation of the residual properties, are relevant. Therefore, this work is devoted to the experimental investigation of the mechanical behavior of fiberglass tubes under proportional cyclic loading. Static and fatigue tests were carried out under tension with torsion conditions. Inhomogeneous strain fields were obtained using a non-contact optical video system VIC-3D. The structural damage accumulation processes were analyzed by an AMSY-6 acoustic emission signals recording system. Surface defects were determined using a DinoLite microscope. Residual dynamic elastic modules were calculated during fatigue tests, and fatigue sensitivity curves were built. Data was approximated using various models, and their high descriptive capability was revealed. Damage accumulation stages were determined. The dependence of the models' parameters on a stress state were observed. It was concluded that multiaxial cyclic loading leads to a significant decrease in mechanical properties, which should be taken into account in composite structure design.

Influence of plane-strain compression on the microstructure and tribological behavior of GUR 1050 UHMWPE

Ultra-high molecular weight polyethylene (UHMWPE) has been used as a bearing surface in orthopedic implants due to its outstanding physical and mechanical properties. Modifications in the structure of the polymer have a direct effect on its wear. In this work, plane-strain compression in a channel die was applied to induce microstructural changes in specimens of UHMWPE GUR 1050. These structural changes were characterized using a combined approach involving Raman spectroscopy and atomic force microscopy. These qualitative and quantitative characterization resulted in a valuable understanding of the changes in the material microstructure when subjected to plastic deformation. A molecular non-uniform alignment of the UHMWPE molecules, with fragmentation and kinking of polymer lamellae, was observed in the direction of material flow, perpendicular to the compressive load direction, following an inhomogeneous strain field generated by the mechanical compression. The microstructural analyses revealed an increased crystalline content and decreased intermediate phase while amorphous phase content remained unchanged, in all the regions of the deformed specimen. The tribological performance, evaluated by the scratch resistance force, decreased along the material flow direction and increased along the load direction in the deformed polymer compared to that of the uncompressed polymer. Plane-strain compression was able to modify the polymer microstructure, introducing directional anisotropy in its tribological behavior that can impact the wear performance of the material.

Strain registration in the gradient zone by two types of fiber-optic sensors

Reliable measurement of strain in various sections of the monitored structure is an important problem for creating Structural Health Monitoring systems. Fiber-optic sensors have great potential to solve this problem. This paper presents the results of experiments on strain measurements in zones of homogeneous and gradient strain distribution using point fiber-optic sensors based on Bragg gratings and distributed fiber-optic sensors based on Rayleigh backscattering. Two types of fiber-optic sensors have demonstrated the ability to measure an inhomogeneous strain field and a satisfactory correspondence of the measurement results.

Hierarchical multiscale crystal plasticity framework for plasticity and strain hardening of multi-principal element alloys

The multi-principal element alloys (MPEAs) exhibit the unprecedented combinations of the excellent mechanical properties, especially high strength and good ductility. However, the accurate and reasonable models for describing the mechanical behavior of MPEAs are still scarce due to their distinctive serious lattice distortion effects, which limit the performance prediction. Here, we develop a new general framework by combining the atomic simulation, discrete dislocation dynamics, and crystal plasticity finite element method, to study the strain-hardening behavior for MPEAs, which achieves the influence of the complex cross-scale factors, including the lattice distortion at the nanoscale and the dislocation hardening at the microscale, on the plastic deformation. Compared with the classic crystal plasticity finite element, the bottom-up hierarchical multiscale model could couple the underlying physical mechanisms from the nano-micron-meso scales and captures the inhomogeneous strain field induced by the serious lattice distortion, thus showing the high accuracy and ubiquitous availability for MPEAs. The result shows that the prediction of the strain-stress curve in the polycrystal MPEAs agrees well with the experimental result at the quasi-static tension, which verifies the accuracy of the proposed method. In addition to the dislocation evolution, the heterogeneous strain distribution combined with the significant change from the orientation of some grains could be an important reason for the enhanced strength at the micron scale. The present work not only gives an insight into the relationship between the multiscale microstructure and strain hardening considering the mechanistic linkages of the lattice distortion, dislocation behavior, and grain structure, but also provides a general approach to physically predict the mesoscopic mechanical response in MPEAs.

Mechanical behavior of woven CMCs under non-uniform stress and strain fields

Woven ceramic matrix composites (CMCs) hot-section components will sustain spatially non-uniform stress and temperature fields in service, and the mechanical response in CMCs under inhomogeneous stress and strain fields remains indistinct. This study focuses on one noteworthy aspect of this issue: the effects of weave architecture on the mechanical response of CMCs under off-axis and thermomechanical loading. To this end, the stress and strain evolution of wavy tows in CMCs are investigated by establishing a meso-scale finite element model, which reflects the real weave architecture. Besides, the model is verified by comparing the predicted results with the experimental results of a representative woven CMC. The strain and stress concentrations appear at the location of tow cross-overs because of the bending and straightening of tows, resulting in the woven composites strength may be inferior to that of laminate composites. Although the scope of this work is limited to one type of weaving, it is expected that the modeling method can be useful in examining other types of weaving.

Combining Unique Planar Biaxial Testing with Full-Field Thickness and Displacement Measurement for Spatial Characterization of Soft Tissues.

Soft tissues rely on the incredible complexity of their microstructure for proper function. Local variations in material properties arise as tissues develop and adapt, often in response to changes in loading. A barrier to investigating the heterogeneous nature of soft tissues is the difficulty of developing experimental protocols and analysis tools that can accurately capture spatial variations in mechanical behavior. In this article, we detail protocols enabling mechanical characterizations of anisotropic, heterogeneous soft tissues or tissue analogs. We present a series of mechanical tests designed to maximize inhomogeneous strain fields and in-plane shear forces. A customized, 3D-printable gripping system reduces tissue handling and enhances shear. High-resolution imaging and laser micrometry capture full-field displacement and thickness, respectively. As the equipment necessary to conduct these protocols is commercially available, the experimental methods presented offer an accessible route toward addressing heterogeneity. © 2022 The Authors. Current Protocols published by Wiley Periodicals LLC. Basic Protocol 1: Unique biaxial testing of soft tissues and tissue analogs Basic Protocol 2: Full-field thickness measurement of soft tissues and tissue analogs Support Protocol 1: Creating and speckling cruciform-shaped samples for mechanical testing Support Protocol 2: Creating custom gripping system to minimize sample handling.