Regions Of High Strain Research Articles

Elastic Localization With Particular Reference to Tape-Springs

Abstract Many elastic systems localize under applied displacement, precipitating into regions of lower and higher strain; further displacement is accommodated by growth of the high strain region at a constant load. Such systems can be studied as propagating instabilities, focusing on the work required to propagate the high strain region, or as two-phase energy minimization problems. It is shown that the Maxwell “equal-areas” construction, and the related common tangent construction, provide the solution to either approach. A new, graphical, proof of the Maxwell equal-areas construction using total strain energy diagrams is presented. Tape-springs are investigated as a case study, with localization presenting as the formation of elastic folds—developable regions with high curvature. One notable property of tape-spring folds is that the fold radius is approximately equal to the initial transverse radius. This result was first proven by Rimrott, and later improved by Calladine and Seffen. A further improvement is obtained here by application of the common tangent construction, and all solutions are shown to be approximations to the Maxwell equal-areas construction in the limit of zero thickness.

Preparing gradient microstructure to achieve enhanced performance in GH4586 superalloy: Experiments and models

In this present study, a combined method of controlling strain deformation (CSD) and gradient-temperature heat treatment (GTHT) was adopted to prepare gradient microstructure in GH4586 superalloy. Consequently, it brought enhanced tensile strength from 1313.1±18.7 MPa at intermediate transition (IT) region to 1506.3±15.7 MPa at high-strain and low-temperature (HS-LT) region, presenting an improvement of 11.6%. And it also enhanced fracture toughness from 77.7±0.3 MPa⋅m1/2 at IT region to 86.7±3.7 MPa⋅m1/2 at low-strain and high-temperature (LS-HT) region, presenting an improvement of 14.7%. The formation of gradient microstructure along the longitudinal direction of CSD+GTHT-treated sample was thoroughly investigated from perspectives of precipitates distribution, activated slip systems and dislocations evolution. During CSD, the size and volume fraction of γ′ precipitates decreased under the effect of strain-induced dissolution behavior. The activation of (1‾ 11)[101] and (11‾ 1)[011] slip systems dominated dislocation movement at each region and there retained increasing dislocation density due to more extensively activated slip systems at HS region. During GTHT, the high-density dislocations promoted recrystallization while the concentratedly distributed γ′ precipitates obstructed movement of recrystallized front, leading to formation of heterogeneous structure with high-density grain boundaries and dislocations at HS-LT region. Finally, a model reflecting the effect of grain boundary strengthening, precipitation strengthening and dislocation strengthening on mechanical property of gradient microstructure was established and successfully applied to property prediction of GH4586 superalloy.

Full-field stress-strain analysis of octagonal shape PMMA using reflection photoelasticity

This research involves stress-strain analysis in poly(methyl methacrylate) (PMMA) octagonal shape compressed by hydraulic system. The observation of uniaxially stressed samples was carried out using reflection photoelasticity technique. The results of fringe order number and principal stress angle were used to determine stress and strain distribution in the samples by shear difference method. It was found that high stress-strain region was close to the contact point where compressive force was applied but it was zero at the edge of the samples. The area of high strain region increased as the magnitude of force was increased but it decreased as the dimension of octagonal shape was increased.

Multiscale homogenization of thermo-mechanical viscoelastic response of 3D orthogonal composites with time-dependent CTEs

Viscoelastic behavior of the matrix affects the thermomechanical response of polymer composites. This study investigated the structural effects of the time-dependent thermal expansion response of 3D orthogonal woven composite (3DOWC) by combining experiments and finite element modeling (FEM), incorporating the viscoelasticity of its constituents. The deformation characteristics at three orthogonal cross-sections, in the presence of structure-related strain fields at various temperatures, were investigated using digital image correlation method. Findings reveal heterogeneity and localization of the thermal strain, characterized by periodic stripes with alternating high strain regions on the resin and low strain regions on the yarn. This localization intensifies at higher temperatures, but diminishes with increased distance from the interface. Furthermore, the composites exhibit time-dependent thermal expansion behavior derived from the time-independent thermal expansion of its constituents, which cannot be captured in classic elastic material model. FEM reveals that thermal stresses are concentrated at the interfaces between constituents with significant differences in their coefficients of thermal expansion (CTE), particularly at the interface between the resins and axial yarns. The maximum thermal stress occurs in the binder yarns over the considered temperature range due to their low volume content, which should be considered in practical applications.

Rapid screening of creep resistance in additive manufactured Ti-6Al-4V alloy

In this study, rapid screening of creep resistance for additively manufactured (AM) materials is enabled using an accelerated creep test (ACT) called the dynamic negative stepped test (DNST). Additively manufactured components are on the rise in aerospace systems, particularly turbine engine parts. It can be challenging to identify the AM process parameters that correspond to high-temperature strength and creep resistance. Tests are needed that rapidly screen and qualify creep resistance. In DNST, the temperature is held constant, and the specimen is taken through a series of load holds from high to low. The DNST contains two distinct characteristics: (1) the hold duration for each step is “dynamically” controlled to ensure the minimum-creep-strain-rate is achieved at every step, and (2) the “descending” profile maximizes damage accumulation while remaining on the edge of instability. The DNST assesses a material's ability to resist creep damage accumulated across multiple deformation mechanisms. In this study, electron beam melted Ti–6Al–4V, built at six (6) directions: 0°, 30°, 45°, 60°, 90°, and V, is tested using a four-step DNST at 650 °C. Analysis of the minimum-creep-strain-rate (at each step), creep ductility, and rupture were performed. A strain energy density (SED) benchmark metric was established to rapidly screen and classify creep resistance using an “Ashby-like” plot that balanced energy dissipated and energy dissipation rate. The creep resistance was found to be ranked from V, 60, 45, 30, 90, and 0 in descending order. Microstructural and crystallographic characterization revealed grain refinement progressing from the strain-free to the high-strain regions, indicative of dynamic recrystallization (DRX). Further observations revealed that the material's energy capacity increased with the alignment of the grains' long axis and tensile direction.

Experimental study on dynamic deformation and breaking mechanism of high-temperature hard rock cutting by abrasive water jet

The development of deep geothermal resources encounters challenges related to high temperatures and hard reservoir rocks. Abrasive water jets (AWJ) offer a potential solution to enhance the efficiency of breaking high-temperature hard rocks in deep ground. This mainly originated from their combined characteristics of jet cooling impact and high-speed abrasive grinding. In order to investigate the dynamic deformation process and rock-breaking mechanism of high-temperature hard rocks cut by AWJ, laboratory tests were conducted on high-pressure abrasive water jets cutting granite, sandstone, and marble. Rock strains were monitored using dynamic strain gauges and the digital image correlation (DIC) technique. The results indicate that in the process of abrasive jet cutting high-temperature hard rock, the rock strain is divided into three stages: compression deformation, deformation release, and stable deformation. The strain response zone can be divided into strain concentration zone, strain transition zone, and strain weak response zone. The high strain region and heat exchange region of high-temperature hard rock during AWJ cutting process are almost consistent. Jet vaporization weakens the water cushion effect during the water hammer stage, leading to high strain concentration at the stagnation point of the jet. During the stagnation stage, the fluid weakens the binding of particles, the abrasive becomes more divergent, and the range of high strain response region expands. Unlike the dynamic strain generated by jet erosion of hard rock, when jet cutting rocks, the strain in the vertical jet direction is generally greater than that along the jet direction. The cutting effect increases with the increase of rock temperature. When the rock temperature is 300 °C, the cutting depth of sandstone, granite, and marble increases by 50 %, 120 %, and 180 %, respectively. Under the combined effects of high-frequency jet impact, high-speed abrasive grinding, and thermal stress, hard rock undergoes varying degrees of damage and failure.

Formation of plate-like high-strain region in polycrystalline α-titanium and its relation to high-strain bands on surface

The investigation of heterogeneous deformation mechanisms in materials is important for understanding fracture mechanisms. In this study, uniaxial tensile deformation of polycrystalline α-titanium was numerically reproduced by three-dimensional crystal plasticity finite element analysis, and the following were revealed for the first time: (i) plate-like high-strain regions form inside the specimen, which are typically observed as band-like high-strain regions (high-strain bands: HSBs) on the surface of and cross-sections in α-titanium specimen, and (ii) the formation of HSBs also affects the work-hardening rate near the yielding point. The reasons for the difference in the formation mechanism of HSBs between the surface and inside were also discussed. The results obtained showed that active slip systems that contribute to the formation of HSBs may differ between the surface and inside of specimen, and strain in HSBs on the specimen surface is higher than that inside. The distributions of HSBs at the surface of and inside the specimen are determined by distributions of resolved shear stress. These results are important for understanding the differences in the fracture mechanisms between the surface and interior of materials.

Morphological disparity and structural performance of the dromaeosaurid skull informs ecology and evolutionary history

Non-avialan theropod dinosaurs had diverse ecologies and varied skull morphologies. Previous studies of theropod cranial morphology mostly focused on higher-level taxa or characteristics associated with herbivory. To better understand morphological disparity and function within carnivorous theropod families, here we focus on the Dromaeosauridae, ‘raptors’ traditionally seen as agile carnivorous hunters.We applied 2D geometric morphometrics to quantify skull shape, performed mechanical advantage analysis to assess the efficiency of bite force transfer, and performed finite element analysis to examine strain distribution in the skull during biting. We find that dromaeosaurid skull morphology was less disparate than most non-avialan theropod groups. Their skulls show a continuum of form between those that are tall and short and those that are flat and long. We hypothesise that this narrower morphological disparity indicates developmental constraint on skull shape, as observed in some mammalian families. Mechanical advantage indicates that Dromaeosaurus albertensis and Deinonychus antirrhopus were adapted for relatively high bite forces, while Halszkaraptor escuilliei was adapted for high bite speed, and other dromaeosaurids for intermediate bite forces and speeds. Finite element analysis indicates regions of high strain are consistent within dromaeosaurid families but differ between them. Average strain levels do not follow any phylogenetic pattern, possibly due to ecological convergence between distantly-related taxa.Combining our new morphofunctional data with a re-evaluation of previous evidence, we find piscivorous reconstructions of Halszkaraptor escuilliei to be unlikely, and instead suggest an invertivorous diet and possible adaptations for feeding in murky water or other low-visibility conditions. We support Deinonychus antirrhopus as being adapted for taking large vertebrate prey, but we find that its skull is relatively less resistant to bite forces than other dromaeosaurids. Given the recovery of high bite force resistance for Velociraptor mongoliensis, which is believed to have regularly engaged in scavenging behaviour, we suggest that higher bite force resistance in a dromaeosaurid taxon may reflect a greater reliance on scavenging rather than fresh kills.Comparisons to the troodontid Gobivenator mongoliensis suggest that a gracile rostrum like that of Velociraptor mongoliensis is ancestral to their closest common ancestor (Deinonychosauria) and the robust rostra of Dromaeosaurus albertensis and Deinonychus antirrhopus are a derived condition. Gobivenator mongoliensis also displays a higher jaw mechanical advantage and lower resistance to bite force than the examined dromaeosaurids, but given the hypothesised ecological divergence of troodontids from dromaeosaurids it is unclear which group, if either, represents the ancestral condition. Future work extending sampling to troodontids would therefore be invaluable and provide much needed context to the origin of skull form and function in early birds. This study illustrates how skull shape and functional metrics can discern non-avialan theropod ecology at lower taxonomic levels and identify variants of carnivorous feeding.

Investigation of Viscoelastic-Plastic Properties of Fresh Cemented Gangue Fly Ash Backfill Slurries

In underground filling mining, freshly prepared cemented gangue-fly ash backfill (CGFB) slurries are typically piped into the gobs. The rheological properties of backfill slurry during pipeline transportation have a direct impact on the transportation characteristics, which in turn affect pipeline blockage and wear. In this paper, the rheological behavior and viscoelastic-plastic properties of CGFB during pipeline transportation are investigated. The effects of different solid content and cement content on resistivity were tested experimentally, and their viscoelasticity and plasticity were analyzed. The results show that with the increase in solid phase content and cement content, the viscosity, yield stress, and energy storage modulus of the materials showed an increasing trend. The viscosity and yield stress of the material both increased, reaching 32.77% and 51.22%, respectively. It was found by the dynamic shear test that in the low-strain region, the material showed a more significant elastic nature of the solid, while in the high-strain region, the viscosity of the material gradually increased. Cement has a substantially lower resistivity than fly ash and gangue, and with the increase in solid concentration, the resistivity of the material shows an increasing trend. With the increase in cement content, the resistivity generally shows a decreasing trend, but it should be noted that the resistivity change trend may tend to stabilize after the cement content exceeds 12%. The study’s findings can aid in understanding the rheological properties of CGFB and its viscoelastic-plastic behavior during the underground filling and conveying process, which can provide a reference basis for research and application in related fields.

Strain partitioning dominated growth of strike-slip fault: Insight from the Tan-Lu fault in Eastern China

The dynamic variations of the strain field and associated tectonic deformation during fault evolution can provide useful insights into the fault growth mechanisms. As one of the longest strike-slip fault system in the world, the ∼3000-km length Tan–Lu fault zone (TLFZ) in eastern China, is ideal for studying the mechanisms of fault propagation and linkage. This study investigated the Cenozoic Liaodong Bay subbasin, which hosts main structures of four faults in the central TLFZ, using structural analysis and strain calculations based on abundant seismic data. Our new results reveal three stages of fault growth (D1, D2, and D3) during the Cenozoic. Strain partitioning occurred during D1, with non-coaxial deformation within the high-strain regions leading to the growth of individual fault segments. D2 involved the redistribution of strain partitioning, with localized strain overlapping along fault tips to produce both left- and right-stepping fault sets. The locally increased simple-shear component favored the development of right-stepping fault arrays. Fault linkage appears to be achieved in two ways: 1) left-stepping, dextral faults were linked by folds dominated by pure shear along overlapping fault tips, and 2) fault segments were linked via fractures developed in simple-shear-dominated right-stepping overlap zones. D3 was characterized by the continued accumulation of simple shear, which resulted in fault linkage through fractures and formed continuous faults with high strain. Therefore, we propose a tectonic model in which strain partitioning results in linkage of segmented faults along fault tips. This model takes into account the spatial and temporal variations in fault geometry and strain type during the growth of the strike-slip fault, provides theoretical support for the structural patterns observed in natural faults worldwide.

Influence of power dissipation value and deformation activation energy on recrystallization in compression deformation behavior of Mg-Li-Zn-Y alloy

The thermal compression deformation behavior of Mg-6Li-11Zn-2.5Y alloy was systematically investigated by using the isothermal hot compression tests using the Gleeble-3500 thermal mechanical physical simulation system with the conditions of the temperature of 513–593 K and the strain rate of 0.01–10 s−1. The constitutive model and thermal processing map were established. The variation pattern of the unstable region under different strains was analyzed and the safe processing region was determined. The influence of different power dissipation values and deformation activation energies on the dynamic recrystallization mechanism was clarified. The results demonstrate that the deformation activation energy is 156.08 kJ/mol. The unstable region gradually expands from the low temperature and low strain region to the high temperature and high strain region with the strain increasing. In the unstable region, adiabatic shear band pear in α-Mg phase, of which the main reason is the poor processing formability in the unstable region. The dominant nucleation mechanism is discontinuous dynamic recrystallization under the conditions of high power dissipation and low deformation activation energy. While, under the conditions of low power dissipation and high deformation activation energy, the nucleation mechanism is mainly driven by the particle stimulated nucleation.

Experimental study of the effect of crack distribution on the failure mechanism of sandstone specimens based on inclination angles and number of parallel flaws

Discontinuous joints are prevalent in engineered rock masses and play a significant role in the stability of the rock mass. This study aims to analyze the impact of the inclination angle and number of prefabricated flaws on the crack evolution and failure pattern of sandstone specimens. Uniaxial compression tests, along with acoustic emission technology and digital image technology, were employed to monitor and analyze the effects. The findings indicate that: (1) With the increase in the flaw inclination angle, the damage mode of the specimen transitions from tensile to compressive-shear failure. The localized high-strain region on the surface of the specimen predicts the propagation path for the formation of macroscopic cracks. (2) When the number of prefabricated flaws is small, the flaws mainly expand through tensile wing cracks. As the number of flaws increases, the inner flaw tip does not produce cracks. Instead, the failure of the entire specimen occurs along the direction of the outer flaw's tensile wing crack, with the inner flaw running through it. (3) The winged tensile crack is the first crack to appear in all rock samples, regardless of the flaw initiation angles. Finally, the stress intensity factor at the crack tip under uniaxial compression conditions, without considering the closure effect, was expressed based on fracture mechanics theory. The crack initiation angle was then calculated. The results of the theoretical calculation of the initiation angle were found to be consistent with the test results. These research findings can serve as theoretical references and provide insights into the failure mechanisms of cracked rocks and the development of disaster control methods in rock engineering.

(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.

Strain-Induced Reversible Motion of Skyrmions at Room Temperature.

Magnetic skyrmions are topologically protected swirling spin textures with great potential for future spintronic applications. The ability to induce skyrmion motion using mechanical strain not only stimulates the exploration of exotic physics but also affords the opportunity to develop energy-efficient spintronic devices. However, the experimental realization of strain-driven skyrmion motion remains a formidable challenge. Herein, we demonstrate that the inhomogeneous uniaxial compressive strain can induce the movement of isolated skyrmions from regions of high strain to regions of low strain at room temperature, which was directly observed using an in situ Lorentz transmission electron microscope with a specially designed nanoindentation holder. We discover that the uniaxial compressive strain can transform skyrmions into a single domain with in-plane magnetization, resulting in the coexistence of skyrmions with a single domain along the direction of the strain gradient. Through comprehensive micromagnetic simulations, we reveal that the repulsive interactions between skyrmions and the single domain serve as the driving force behind the skyrmion motion. The precise control of skyrmion motion through strain provides exciting opportunities for designing advanced spintronic devices that leverage the intricate interplay between strain and magnetism.

Effect of freezing and thawing process on fracturing mechanism and acoustic emission characteristics of notched semi-circular bend specimens of red sandstone

Understanding the effect of freeze-thaw (F-T) cycles on the fracture mechanism of rocks under mode-I loading is exceptionally prominent in addressing the underlying factors of rock engineering under a frosty environment. A three-point bending test was carried out on sandstone specimens exposed to 0, 10, 20,40, 60 and 80F-T cycles to achieve the fracture parameters. Moreover, the digital image correlation (DIC) and acoustic emission (AE) methods were employed to reveal the progressive failure in different loading stages. The results divulged that the F-T cycles significantly influenced the fracture parameters, including fracture toughness (KIC) and fracture energy (Gf). The average KIC values have given a negative linear relation with increasing F-T cycles. The percentage of KIC drop with F-T cycles is 9.78 %, 12.20%, 18.56%, 27.2%, and 31.60% for samples treated with 10, 20, 40, 60, and 80 cycles, respectively, while the fracture energy (Gf) gives a decline-rise trend from 0 to 10 and 20 to 80 cycles. The AE signals have demonstrated an apparent stage characteristics, and a sudden rise has occurred in the AE count rate at peak loading, revealing the fracture zone of rock under loading. The DIC effectively revealed the progressive failure pattern, especially at the later loading stages. When the load progressed to around 80–90% of maximum loading, a diverse high-strain region appeared at the tipping point, and the high-strain area continued to develop. The results of this study can effectively address the complex fracturing phenomenon of rocks and provide valuable insights into construction and engineering operations in cold climates.

Radial plastic flow machining: A new process for fabricating gradient-structured terminals in one step

Radial Plastic Flow Machining (RPFM) is a novel severe plastic deformation process. By designing a specific plastic flow channel, the material is allowed to flow radially into the predetermined channel under the combined action of extrusion and cutting. A restricting ring is placed in the channel to form a copper terminal structure in one step. This study investigates in detail the formation mechanism, mechanical properties, and thermal stability of gradient structured copper terminals generated under different extrusion angles. It was found that as the extrusion angle increased, the high strain region slightly decreased, while the low strain region increased, allowing more material to be radially extruded. This indicates that tensile strength and toughness can be balanced by controlling different extrusion angle parameters. In performance testing, the study showed that the hardness of the copper terminals prepared by RPFM was more than double that of the original copper material, and the maximum tensile strength also doubled. Moreover, the performance of the copper terminals annealed at 150∘C was similar to that of untreated copper terminals, demonstrating good thermal stability at this temperature. The study suggests that RPFM is not only an effective process for manufacturing gradient structured copper terminals in one step, but it also provides a new method for manufacturing copper terminals with excellent comprehensive performance.

Net section tension capacity of high strength steel single shear bolted connections

Single shear bolted connections under tension usually exhibit out-of-plane deformation due to the inherent load eccentricity, resulting in significant stress concentration around the bolt holes and a stress gradient through the plate thickness. This effect is particularly pronounced in high-strength steel (HSS) connections due to the relatively lower ductility and tensile-to-yield strength ratio of HSS. However, it remains unclear whether this issue can lead to premature fracture in high strain regions before effective stress redistribution develops across the net section, subsequently reducing the ultimate load capacity of the HSS connections. To examine the above design issue, an experimental and numerical study on single shear bolted connections was conducted and reported in this paper. A total of 25 connections were tested, among which 17 were made of HSS (Q690 and Q960), and 8 were made of Q345 mild steel (MS). Based on a validated finite element (FE) model, a comprehensive numerical analysis was carried out to interpret the structural behaviour of the specimens and explore the influence of various design parameters on the ultimate tension capacity of single shear bolted connections. The findings revealed that, despite the existence of out-of-plane deformation in the steel plate, all connections (either MS or HSS) investigated in this study reached the theoretical net section resistance (the product of the net section area and the ultimate tensile strength). This is because (1) the relatively low ductility and tensile-to-yield strengths ratio of HSS materials are still sufficient to allow an effective stress redistribution across both the transverse net section and the plate thickness, and (2) the biaxial stress state around the bolt holes enhances the tension capacity of the net section. Finally, a statistical evaluation of the combined test and numerical data showed that the existing design equations for predicting the net section tension capacity of HSS (specified in EN 1993-1-12:2007 and prEN 1993-1-1:2020) can be safely applied to the design of HSS single shear bolted connections, even in scenarios where the tensile-to-yield strengths ratio of HSS is as low as 1.06.

Twin domain and antiphase boundaries in microcrystals of Κ-phase Ga2O3

The structural properties of twin domain boundaries (TDB) and antiphase boundaries (APB) in individual thin, hexagonal prismatic microcrystals of κ-Ga2O3 grown on GaN/sapphire template with HVPE were investigated with electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The microcrystals were composed from the domains with three in-plane 120° rotational orientations. It was found that every individual twin domain contains a parallel array of APBs of a high density stretched in the [010] direction. APBs possess steps or interruption and can form double oppositely shifted spatially separated layers (APB dipoles). TDBs on majority of their length are incoherent being not strictly flat and serve as the border for the APBs interruptions. Panchromatic cathodoluminescence mapping of the microcrystals revealed that not all TDBs and APBs reduced its intensity. The interruptions and steps of APBs were proposed to be the main origin of the excess charged carrier recombination. A model of the atomic structure in the vicinity of the defects is proposed and the assumption about the formation of local high strain regions and dangling bonds was made.

In situ characterization of thermomechanically loaded solution strengthened, nanocrystalline nickel alloys

Nanocrystalline (NC) Ni-40Cu and Ni-40Cu-0.6P (at.%) alloys were mechanically loaded in tension at ambient (23 °C) and elevated (150 °C) temperatures with the deformed nanostructure captured by in situ transmission electron microscopy coupled with digital image correlation for data mining high strain regions prior to catastrophic failure. The addition of the P provided grain boundary partitioning that stabilized the NC structure under both loading cases, which was not found to be the case for the binary alloy. While mechanical strength softening was observed in each of the alloys upon thermomechanical loading, the retention of strength was substantially higher in the ternary alloy than its binary counterpart. Digital image correlation was found to be a useful means for image mining to identify different regions where failure mechanisms were initiated. Intergranular failure was observed as the dominant failure mechanism in all specimens with the binary alloy revealing a coarser fracture profile as compared to a finer fracture profile in the ternary alloy. Atomistic simulations are used to understand P solute strengthening of the grain boundaries against this fracture failure mode.

Numerical analysis of the mechanical and electrical properties of (RE)BCO tapes with multiple edge cracks

One of the leading causes of critical current degradation in rare-earth barium–copper-oxide tapes is the micro-cracks produced by mechanical slitting. These cracks are scattered near the edge of the tape and vary in length and angle. In this work, a tape model with multiple edge cracks is established. Under tensile loading, the effects of the Poisson ratio, crack length, crack angle, crack spacing, and geometric mutation between cracks on the stress intensity factor are investigated using the extended finite element method (XFEM). Tensile experiments were conducted at room temperature to investigate the crack propagation behavior of tapes with multiple edge cracks. The results show that the stress intensity factor obtained using XFEM is more informative than the analytical solution, which ignores the Poisson effect. The stress intensity factor is sensitive to crack length and angle variations and exhibits an evident jump characteristic when a geometric mutation occurs. The jump level strongly depends on the geometric difference. The jump location is the initiation site for crack propagation, which is consistent with the experiment results. The strain analysis of the tape implies that high-strain regions exist at the crack tip before the tensile strain reaches the irreversible strain limit. The critical strain of crack propagation is closely related to the form of crack distribution. It dominates the irreversible strain limit of critical current degradation, which facilitates understanding the early degradation of critical current. Finally, some engineering suggestions are given.