In-plane Strain Field Research Articles

A possible explanation for double-peak structure in strain dependence of critical current density in REBa2Cu3O coated conductors

REBa2Cu3O coated conductors (REBCO-CCs; RE: rare-earth element or Y) have garnered considerable attention in high-field magnet applications. However, the twin structure existing in the ab plane of REBCO complicates the dependence of critical current density on temperature, magnetic field, and uniaxial strain: . In particular, the mechanism for the appearance of anomalous double-peak structure in is unclear.In this study, to understand the complicated strain dependence of critical current density, we measured of REBCO-CCs with c-axis-correlated pins or with random pins as a function of in-plane uniaxial strain and magnetic field applied parallel to the c axis. In the case of (Y,Gd)BCO-CC with BaZrO3 nanorods, we only observed a mountain-shaped response of under the entire magnetic field. However, GdBCO-CC without artificial pinning centers, where random pinning is dominant, exhibited a valley-shaped response at low fields and then a mountain-shaped response at high fields. These results suggest that the variation in the pinning mechanism (correlated- or random pins) plays a crucial role in the absence/presence of double-peak structure in REBCO-CCs.We developed a phenomenological model for in which (i) strain dependence of superconducting properties in two domains of REBCO and (ii) ideal behavior of correlated- or random pinning are considered. Consequently, we observed that the valley-shaped response of can be realized in the case of ideal random pinning with limited parameters, while ideal correlated pinning always exhibits a mountain-shaped response of . Furthermore, a crossover from a valley-shaped response under low fields to a mountain-shaped response under high fields in the strain dependence of is qualitatively consistent with our observations. This qualitative agreement between our model and experimental data validates the efficacy of our model to provide a clear understanding of the strain dependence of in REBCO-CCs, including the anomalous double-peak structure.

Mechanical Respond and Failure Mode of Large Size Honeycomb Sandwiched Composites under In-Plane Shear Load.

The present work focuses on the in-plane shear respond and failure mode of large size honeycomb sandwich composites which consist of plain weave carbon fabric laminate skins and aramid paper core. A special size specimen based on a typical element of aircraft fuselage was designed and manufactured. A modified in-plane shear test method and the corresponding fixture was developed. Three large size specimens were tested. The distributed strain gauges were used to monitor the mechanical response and ultimate bearing capacity. The results show that a linear respond of displacement and strain appears with the increase of the load. The average shear failure load reaches 205.68 kN with the shear failure occurring on the face sheet, and the maximum shear strain monitored on the composite plate is up to 16,115 με. A combination of theoretical analysis and finite element method (FEM) was conducted to predict the shear field distribution and the overall buckling load. The out-of-plane displacement field distribution and in-plane shear strain field distribution under the pure shear loading were revealed. The theoretical analysis method was deduced to obtain the variation rule of the shear buckling load. A good agreement was achieved among the experiment, theoretical analysis, and FEM results. It can be concluded that the theoretical analysis method is relatively conservative, and the FEM is more accurate in case of deformation and strain. The results predicted by h element and p element methods are very close. The results of the study could provide data support for the comprehensive promotion of the design and application of honeycomb sandwich composites.

Modeling of atomistic scale shear failure of Ag/MgO interface with misfit dislocation network

Metal/ceramic interfaces have broad applications and misfit dislocation network (MDN) is a prominent feature of the equilibrium metal/ceramic interfaces. As one main failure mode, interface shear failure is strongly affected by the motion of MDN. In this work, we investigate the equilibrium interface structure and shear failure of Ag/MgO interface via atomistic simulation method. Periodically distributed in-plane strain field caused by MDN and severe strain concentration at dislocation node regions are found by strain analysis. During interface shearing, these dislocation nodes act as strong pinning points to the gliding motion of MDN, which leads to bending of dislocation lines. Besides, energy analysis shows the interface shear stress is largely dependent on the variation of misfit dislocation energy. To understand interface shear failure under more complex conditions, we study the effect of model thickness and shear direction further. Due to transformation of nodal structure, the shear strength of thick model eventually decreases by almost a quarter; shear failure along the direction of Burgers vector is found to be energetically favored, with the lowest interface shear strength. This work reveals the crucial role of MDN in interface shear process, and the theoretical understanding gives some hints to metal/ceramic interface design.

Room temperature creep mechanism of a Pb-Sn-Sb lead alloy

Lead alloys are the most common materials adopted for the production of subsea power cable sheathing. The sheathing is a layer of stable and watertight metal, which serves to prevent the electrical failure of the cable. During the predicted operational life of the cables of several decades, these experience strains due to the installation process, the oceanic currents and the thermal expansion of the cable. The low melting temperature of such alloys, around 600 K, imply that creep deformation will occur when subjected to loading even at room temperature. The goal of the present study is to investigate the tensile behavior of the Pb-Sb-Sn alloy of interest in order to predict the correlation between strain rate and stress level. A mechanical characterization was performed through tensile testing at different strain rates of specimens cut from power cable sheathing. Due to the extreme ductility of the material, the use of digital image correlation was necessary to compute an acceptable approximation of the in-plane strain field on the surface of the specimens. The results were implemented in finite element method environment using Abaqus and Isight to calibrate a creep model able to reproduce at best the behavior of the material. Such model was also positively tested in the case of a relaxation test. In addition, a tensile test of several steps at different loads was executed with the aim of extrapolating and interpreting the steady state creep exponents at different creep regimes and the indications that these can provide on the deformation mechanisms of the alloy.

A novel approach for the investigation of low energy ice impacts

Soft body impacts are one of the main sources of damage for aircraft during flight due to the high relative velocities. However, a hailstone can also hit a structure at low velocity (e.g. during parking, taxi and terminal flight phases) causing barely visible damages. In this research work, a new approach is proposed for the investigation of low energy ice impacts. A drop tower apparatus was modified to accommodate specifically developed ice impactors. High-speed Digital Image Correlation was employed to measure the targets out-of-plane deformations and in-plane strain fields. Different ice impactors geometries, target materials and energy levels were considered. From results, it has been demonstrated that this methodology is able to capture the effect of ice impacting at low speed on targets.

Combined simulation and optical measurement technique for investigation of system effects on components solder fatigue

Among others, physics of failure related concepts are being developed to address the thermo-mechanical reliability challenges in automotive electronics. Limitations in particular applied for finite element (FE-) analyses are models of limited size, which rarely address the system character of failure. Also in testing a system view on fully mounted electronic control units (ECU) and loaded by environmental and active loading cannot be taken performing end-of-life tests for time limitations. Accelerated testing is done instead, however, mostly on board level.To overcome some of these limitations, a combined measuring-simulation technique is being developed, which is described in the paper. System level view on boards mounted in automotive ECUs is taken by a newly developed high-precision optical deformation measuring system. The multi-sensor measuring method combines a chromatic sensor for topography and warping analysis with an optical sensor for in-plane deformation and strain field analysis. By this combination, a high resolution can be reached for all three components of displacement vectors. Additionally, software tools allow the determination of derived quantities like strains, local curvatures and local warpage radius. The latter can be taken as input for FE-simulations. It is shown that some components, in particular QFNs, are sensitive to thermally induced cyclic warpage even if the corresponding bending deflections are in the micrometers range. Worst case in-plane stretching and cyclic warpage of a board mounted in an ECU have been measured. By corresponding simulations on QFN solder fatigue, mounted on a special test board, the critical fatigue life can be determined dependent on the interaction to the case, which differs by several hundred percent from a free-standing assembly.

Structural and electronic transformation in low-angle twisted bilayer graphene

Experiments on bilayer graphene unveiled a fascinating realization of stacking disorder where triangular domains with well-defined Bernal stacking are delimited by a hexagonal network of strain solitons. Here we show by means of numerical simulations that this is a consequence of a structural transformation of the moiré pattern inherent to twisted bilayer graphene taking place at twist angles θ below a crossover angle . The transformation is governed by the interplay between the interlayer van der Waals interaction and the in-plane strain field, and is revealed by a change in the functional form of the twist energy density. This transformation unveils an electronic regime characteristic of vanishing twist angles in which the charge density converges, though not uniformly, to that of ideal bilayer graphene with Bernal stacking. On the other hand, the stacking domain boundaries form a distinct charge density pattern that provides the STM signature of the hexagonal solitonic network.

Mechanical characterization of tetraxial textiles

In this paper, the mechanical properties of different tetraxial fabrics are investigated. Fabrics were produced using an innovative loom capable of weaving four threads at the same time with complete discretion of yarn type and count. The experimental investigation deals with in-plane and out-of-plane mechanical testing of tetraxial fabrics, as well as yarns made of four different materials (polyethylene terephthalate, glass, aramid, and basalt). The digital image correlation technique was used to measure the in-plane strain field for both uniaxial and biaxial tensile tests. The extensive experimental campaign allowed for a complete mechanical characterization of this novel fabric architecture including interlacement of different yarns.

Damage zone development in heterogeneous asphalt concrete

Measurements of the in-plane strain field around a notch tip were performed using digital image correlation (DIC) to provide an estimate of the Fracture Process Zone (FPZ) developing in a pre-notched asphalt concrete semi-circular bend sample under three-point bending. A magnification of 8µm/pixel was used in the DIC optical set up in order to be able to resolve strains separately in the aggregate particles and in the asphalt matrix regions between particles. A highly inhomogeneous strain field resulting from the composite nature of the asphalt concrete was seen. Aggregate particles were observed to have almost no strain at all in any of the cases investigated herein (temperatures of −12°C and 25°C and loading rates between 0.07mm/min and 50mm/min), while significant strains developed in the viscous asphalt binder in the vicinity of the notch tip. Estimation of the FPZ size was done using two possible proposed methods: one involving a threshold strain value and one involving the change of correlation coefficient used in the DIC minimization process. Results from both approaches were used to rank the size of the estimated FPZ as influenced by a number of variables, namely, asphalt recycled content, temperature, and applied displacement rate. The results indicate that the damage zones develop primarily between the aggregates and an increase in loading rate or recycled content has a similar effect as a reduction in temperature: more localized strains/damage and a reduction in the apparent FPZ size. Finally, it was observed that the smaller the extent of the FPZ the less tortuous the crack path becomes.

Analysis of dynamic bending test using ultra high speed DIC and the virtual fields method

Full-field deformation response of a copper bar impacted in a three point bending setup is studied. Ultra-high speed photography (frame rate: 5×106 fps) in conjunction with digital image correlation is utilized to study the in-plane deformation kinematics of the impacted specimen over 25.4μs duration after the impact. Distributions of temporal derivatives, i.e. strain rate, velocity and acceleration, over the area of interest are computed using the in-plane displacement and strain fields measured from image correlation. Complex deformation patterns resulting from wave propagation and inertial loading are studied and discussed. Having deformed the specimen well into the plastic regime, the idea initially proposed by Pierron et al. (2011) is extended to identify visco-plastic constitutive response of the examined material by incorporating an analysis based on the method of virtual fields. Accordingly, a strain rate sensitive constitutive model for the copper specimen is identified by reconstructing the inertia and the impact forces using full-field acceleration and reaction force measurements, respectively.

Integrated modelling of a 6061-T6 weld joint: From microstructure to mechanical properties

Welding can highly modify the mechanical properties of materials due to the extreme thermal solicitations applied. For precipitation hardened materials, such as aluminium alloy 6xxx, a welding operation implies a modification of the microstructural state and, consequently, of the mechanical properties, both phenomena being highly nonlinear. The purpose of this paper is to propose a methodology to predict the post-welding mechanical properties of a welded joint. For this, three models are coupled: (i) a thermal finite element model of the welded structure that allows the prediction of the material’s thermal history at every point; (ii) a precipitation model to predict the microstructural state in the joint using the thermal history; and (iii) a mechanical model to link the microstructural state to the mechanical properties, i.e. hardness, yield limit and hardening. A coupling between these models and a finite element commercial code is then performed to predict the precipitation state and mechanical properties of a 6xxx-T6 aluminium alloy after welding. To validate this methodology a tensile test is performed on a specimen extracted from a 6061-T6 welded plate. Using Digital Image Correlation, the in-plane strain fields across the weld are measured and compared with the finite element simulation of the tensile test, thereby providing good prediction.

Near-surface mechanical heterogeneities in a dissimilar aluminum alloys friction stir welded joint

The local mechanical properties of a dissimilar friction stir welded AA-2024-T3/AA-2198-T3 joint were documented during a uniaxial tensile test. High-resolution digital image correlation was performed during monotonic tensile tests to capture the local in-plane strain fields of the heterogeneous macrostructure of the weld. In the shoulder-affected region, banded macrostructures with heterogeneous mechanical properties were found. They were related to pronounced textures regions, which can be associated to strain-rate gradient during one rotation of the tool. The banded macrostructures in the nugget region were observed to be responsible for early plasticity in the joint and ultimately to be the fracture location of the weld. The heterogeneous mechanical response of the joint was also investigated by microhardness measurements. Differences were found between hardness and local tensile properties, demonstrating microhardness measurements can be misleading and only direct high-resolution digital image correlation techniques can document the mechanical behavior of materials having complex and heterogeneous micro-/macrostructures.

Strain field of the monovacancy in silicene: First-principles study

The in-plane strain fields of single-vacancy silicene with different monovacancy (MV) concentrations, as well as the corresponding electronic band structures, are investigated by using the first-principle calculations. Firstly the self-healing MV is found to be the most stable ground structure in silicene, which is different from the other 2D hexagonal honeycomb materials, e.g. graphene, h-BN. In the isolated MV center, the bonds along the pentagons are compressed, creating a compress field, and those close to the distorted hexagons are stretched, creating a stretch field. As the MV concentration increasing, the interacted compress field tends to corrugate the defected silicene, while the interacted stretch field impacts little on the low-buckled structure. Especially, the corrugation presents in those supercells with small MV concentration, just as the (4, 5), (4, 6), (4, 7), (4, 8) supercells. The corrugations approach zero at both low and high MV concentrations, and the (4, 6) supercell with a MV concentration of about 0.021, has a peak value of 3.23Å. The electronic calculations show that the linear dispersion at Γ point in pristine silicene is broken by the lower lattice symmetry of the self-healing MV reconstruction, which translates it into metal as well.

Polarization rotation associated critical phenomena in epitaxial PbTiO3 thin films near room temperature

Strain-driven and temperature-driven monoclinic-orthorhombic phase transition in epitaxial PbTiO3 exhibit similar behavior under electric field, i.e., polarization discontinuity is reduced at the first-order ferroelectric-ferroelectric transition whose latent heat vanishes at a critical point. Due to critical phenomena the energy barrier for polarization rotation significantly diminishes, and hence thermodynamic response functions tend to diverge in the induced monoclinic states. Phenomenological calculations show that dielectric and piezoelectric properties are highly tunable by in-plane strain and electric field, and large electromechanical response may occur in epitaxial PbTiO3 thin films at room temperature. Phenomenological calculations show that large electrocaloric responsivity can also be expected at room temperature by manipulating the phase transition.

In-Plane Strain Field Sensor Based on the Semiconductor Film

The ZnO semiconductor multicrystalline film is utilized as the sensing material, and a sensors array is demonstrated in this paper. Based on the coupling effect of piezoelectric and semiconducting, an ultra-high sensitivity to the deformation is obtained that the gauge factor of the single units is derived up to 199, which is 100 times of that of the commercial foil gage (gauge factor = 2). After calibration on every sensing unit, the distribution of the uniform and non-uniform strain applied on the device is measured and mapped by the sensors array successfully. The results show a good application of the device on the deformation field sensing by contact test method.

Sub-micron strain analysis of local stick-slip motion of individual shear bands in a bulk metallic glass

Nanodot deposition on a side surface of a rectangular sample and digital image correlation are used to quantify the in-plane strain fields associated with the propagation of a shear band in a PdNiP bulk metallic glass, induced by rolling. Within the resolution of the method related to an average inter-dot distance of 100 nm, deformation is found to be highly localized at the shear bands, while alternating areas with a size of 100–400 nm with opposite local shear strains are found. This phenomenon substantiates a local stick-slip nature of shear band propagation during the metallic glass deformation, even during rolling.

Full-Field Measurement at the Weave Scale in Hemp/Epoxy Composite Using Digital Image Correlation

This paper presents the strain field analysis of a woven hemp composite. Two different stacking sequences were tested: [0/90]7 and [±45]7. Tensile tests were coupled with a digital image correlation method in order to measure in-plane longitudinal, transverse and shear strain fields. Strong heterogeneities were observed in both stacking sequences. The obtained spatial resolution was fine enough to allow the comparison of maximum and minimum deformation areas in strain fields with the corresponding weave architecture. It was observed that strain levels were directly linked with strand positions. Results gave, for the studied woven hemp composite, a complete analysis of the local deformation mechanisms.

Interior strain analysis of a woven composite beam using X-ray computed tomography and digital volumetric speckle photography

Materials such as fiber reinforced composite are inherently 3D in nature. Its internal stress/strain fields are very complex even under the simplest loading condition. There is no effective means of measuring interior strain other than placing isolated sensors. In this paper we introduce a new strain measuring technique called digital volumetric speckle photography whereby we can probe the interior strain field in a whole field manner. We apply it to the determination in a woven fiber composite beam under 3-point bend. The in-plane strain field of three transverse sections and two longitudinal sections are determined using the technique. In addition we show that one of the principal strain fields of the mid-longitudinal section tends to predict the impending failure of the beam.

Uneven intensity change correction of speckle images using morphological Top-Hat transform in digital image correlation

By comparing two digital speckle images recorded before and after deformation, two-dimensional digital image correlation (DIC) method can accurately determine the in-plane displacement fields and strain fields. In a practical measurement, however, the variance of light source intensity, location and direction will cause the random uneven intensity change of the random speckle images and will lead to the obvious measurement error. Numerical simulation experiment is first carried out to analyse the influence of the recorded speckle images undergoing uneven light variation on DIC measurement accuracy. Then, a correction method for speckle images with uneven intensity change is proposed based on morphological Top-Hat transform. In addition, quantitative measurements of both in-plane rotation of a rigid body and three-point bending beam are investigated experimentally by DIC to verify the feasibility of the correction method. Experimental results show that the measurement accuracy of DIC is improved dramatically after the procedure of uneven light variation correction.

Monitoring archaeological wooden structures: Non-contact measurement systems and interpretation as average strain fields

Abstract Large wooden structures of cultural value, such as the Vasa ship, suffer from increasing deformation over time, caused by creep in the wood members, time-dependent deformation of joints and damage accumulation. In this paper, we discuss the suitability of the different available measurement techniques to monitor the increasing deformation and the structural health of wooden structures. In particular, a monitoring system needs to be able to measure the exact same point at different times to measure displacements. For large wooden objects with limited surface texture, the best accuracy for displacement measurements is currently achieved by monitoring well-defined targets with laser-assisted total stations. Furthermore, we emphasize the relation between displacements and deformation and advocate translating the raw displacement data into more meaningful average strain fields. A straightforward method to compute the in-plane average strain field is presented, and illustrated for the Vasa ship. The strain fields can indicate areas with localized strain, caused e.g. by weak zones in the structure with increased creep, damage or cracking. Such zones can then be more closely inspected and considered for improved support.