Core Spreading Research Articles

Failure mechanism and plastic hinge of RC joints under impact load acting on column ends

Impact loads often act on columns end during accidents such as vehicle impacts and terrorist attacks. The resulting deformations lead to a redistribution of internal forces in the member as well as a reduction in the load carrying capacity. The integrity of the core area of the joint is threatened, which could trigger a progressive collapse. This study numerically investigated the damage and deformation of reinforced concrete (RC) beam-column joints when they are impacted on the column end. The resistance mechanism of joints and the influence of impact location as well as impact velocity was explored. Besides, the deformation capacity of RC beam-column joints under impact loading was quantified using equivalent plastic hinges. It was found that the damage and plastic deformation is concentrated at the impacted column end. The horizontal reaction force at the beam end provides resistance and plays the role as a bearing. The column bottom reaction force is in the same direction as the impact force. However, it is much smaller than the beam end reaction force. The resistance mechanisms within the joints can be categorized into strut-and-tie near the loading point and arches in the mid-span of the beam and column. The core area rotates and has a significant lateral displacement towards the impact location, whether it is loaded at the beam end or the column end. As the impact location moves away from the core aera, the impacted column end exhibit more bending failure. With increasing impact velocity, the impact force, reaction force and total energy absorption of the joint increase. The deformation increases, with more damage occurring in the core area and spreading to the beam. In addition, the plastic curvature was used to describe the actual plastic deformation of the joints, whose maximum occurring at the interface where the impacted column end meets the core area. A formula for the equivalent plastic hinge length of RC joints was proposed, taking into account the effects of impact location and impact velocity.

Interaction of C, N and O interstitial solute atoms with screw dislocations in HfNbTaTiZr high entropy alloy

Using density functional theory (DFT), we characterize the interaction of screw dislocations with interstitial impurities, in particular oxygen, nitrogen and carbon, in the HfNbTaTiZr alloy of body-centered cubic (BCC) structure. Considering different configurations representative of the disordered solid solution alloy, we evidence a noticeable core spreading of the screw dislocation whether with or without solute, associated with a wide range in dislocation-solute interaction energy, which is found to be mainly attractive for the three investigated impurities. This interaction energy is shown to be highly influenced by the type of the metal atoms in the neighbourhood of the solute. Notably the more BCC elements are present around the solute (either Nb or Ta), the more repulsive this interaction is.

The Full Non-linear Vortex Tube-Vorton Method: the pre-stall condition

The present hybrid vortex tube-vorton method is based entirely on the Full Multi-wake Vortex Lattice Method (FMVLM) concepts, which means detaching vorticity with precise vortex strength and orientation along all separation lines between each discretized element of a shell-body, including all external edges. Since the classic Vortex Particle Method (VPM) is unstable by itself because it does not conserve the total amount of circulation as time evolves (Kelvin’s circulation theorem), an isolated Vortex (regularized) Filament Method (VFM) approach is implemented to obtain advection of vorticity, while the induced velocity field is obtained through its corresponding full vorton cloud. Further, a novel vortex squeezing/stretching scheme for such a vortex cylinder-sphere approach is proposed based on variation in time for vortex volumes in order to precisely (zero residual) conserve both circulation and vorticity at each time step (for each detached vortex element), while the viscous effect can be accounted for via the Core Spreading Method (CSM).

Development of a formulation to predict molten core spreading in an LWR severe accident

Relocated corium into the Primary Containment Vessel (PCV) needs to be properly cooled to avoid or mitigate molten-core concrete interactions (MCCI) in the PCV in order to maintain its supporting capability for the reactor pressure vessel and to suppress combustible/non-condensable gas releases. To know how effective the cooling is, it became important to know the geometry of the relocated corium. The present study chooses to focus on the “Wet Cavity” strategy and to build a reliable tool to evaluate the corium coolability in such a case. To achieve this goal, a previously developed formulation built to predict the corium geometry under the “Dry Cavity” strategy was extended to the conditions used in the “Wet Cavity” strategy. This extension includes the effect of solidification and cooling from the water by using a newly developed expression for the dimensionless thickness s*, the water subcooling, and the melt’s super-heat. After the validation of the extended formulation was confirmed, potential restrictions and limitations were investigated.

A comparative study of circular and rectangular bended plunging jets

In the present work we study the topology, mixing properties, turbulence quantities, dependence on the outlet geometry of a sharp-edged orifice plunging jet which first issues horizontally in air and then plunges in a water pool. The investigated orifices shapes are circular and rectangular. Data are acquired at different Reynolds numbers in the range 11000–25000, based on the orifice diameter (equal to 2 cm) and on the average exit velocity, as derived from flow rate measurements. Velocity fields in vertical and horizontal planes are measured using planar time-resolved Particle Image Velocimetry. Results show a clear asymmetry of the cross-velocity profiles both in circular and rectangular cases, with the latter that revealed a shape which is Reynolds number dependent. Axial velocity decays, potential core lengths and spreading rates highlight an opposite trend between the two jet geometries, thus suggesting a higher mixing for the lowest Reynolds number circular jet and the highest rectangular one. Plunging angle shows a dependency on Reynolds number. Moreover, it seems to play a role in the evolution of the upper and lower side of the jet due to the onset of a co-flow. Ambient mass entrainment points out the different interactions of the two plunging jets with the ambient flow: in circular case, it entrains fluid from the surroundings, from horizontal to vertical planes in streamwise direction, while in rectangular one it ejects flow from vertical to horizontal planes. Finally, Strouhal numbers are derived for main vortices frequencies along jet centerline, other than upper and lower sides.

Study of alkaline carbonate cooling to mitigate Ex-Vessel molten corium accidents

To mitigate adverse effects from molten corium following a reactor pressure vessel failure (RPVF), some new reactor designs employ a core catcher and a sacrificial material (SM), such as ceramic or concrete, to stabilize the molten corium and avoid containment breach. Existing reactors cannot easily be modified to include these SMs but could be modified to allow injectable cooling materials. Current reactor designs are limited to using water to stabilize the corium, but this can create other issues such as reaction of water with the concrete forming hydrogen gas. The novel SM proposed here is a granular carbonate mineral that can be used in existing light water reactor plants. The granular carbonate will decompose when exposed to heat, inducing an endothermic reaction to quickly solidify the corium in place and producing a mineral oxide and carbon dioxide. Corium spreading is a complex process strongly influenced by coupled chemical reactions, including decay heat from the corium, phase change, and reactions between the concrete containment and available water. A recently completed Sandia National Laboratories laboratory directed research and development (LDRD) project focused on two research areas: experiments to demonstrate the feasibility of the novel SM concept, and modeling activities to determine the potential applications of the concept to actual nuclear plants. Small-scale experiments using lead oxide (PbO) as a surrogate for molten corium demonstrate that the reaction of the SM with molten PbO results in a fast solidification of the melt due to the endothermic carbonate decomposition reaction and the formation of open pore structures in the solidified PbO from CO2 released during the decomposition. A simplified carbonate decomposition model was developed to predict thermal decomposition of carbonate mineral in contact with corium. This model was incorporated into MELCOR, a severe accident nuclear reactor code. A full-plant MELCOR simulation suggests that by the introduction of SM to the reactor cavity prior to RPVF ex-vessel accident progression, e.g., core-concrete interaction and core spreading on the containment floor, could be delayed by at least 15 h; this may be enough for additional accident management to be implemented to alleviate the situation.

An atomistic study of the influence of carbon on the core structure of screw dislocation in BCC Fe and its consequences on non-Schmid behavior

In the present work, we report on the effect of introducing carbon atom in the core of the screw dislocations in body centered iron (Fe) using classical interatomic potentials available in open literature. Reconstruction of dislocation core from easy to hard core configuration is observed irrespective of the initial position of the carbon atom, which eventually settles in the dislocation core occupying the prismatic sites formed by the rows of Fe atoms in hard core. Loading is carried out parallel and perpendicular to the Burgers vector to understand the effect of carbon on the non-Schmid characteristics of the slip. Twinning–antitwinning (T/AT) asymmetry of CRSS is observed when the dislocation core symmetry of the easy core configuration is retained after the core reconstruction occurs. The CRSS of dislocation in both easy and hard core configuration is sensitive to the loading conditions. The asymmetry in CRSS and choice of glide planes are correlated with the dislocation core spreading onto the {1 1 0} planes in the 〈1 1 1〉 zone under the influence of the applied loads.

Thermodynamic model for polymorphic dislocation core spreading within hexagonal close packed metals

An exactly solvable thermodynamic model for the core structure of $\ensuremath{\langle}a\ensuremath{\rangle}$-type screw dislocations in $\ensuremath{\alpha}$-Ti is introduced and used to explore the role in which polymorphic core spreading contributes to the free energy of the dislocation. Each segment of the dislocation core is assumed to be in one of three possible configurations, and the free energy differences per segment between dislocation core spreadings are taken as parameters. It is shown that thermal fluctuations between the core spreadings should be common, even at room temperature, and that these fluctuations can contribute significantly to the free energy. It is also shown that under some circumstances, the core structures can display morphological switching wherein the stable core spreading changes morphology with a change in temperature. The implications for dislocation dynamics and for modeling finite temperature dislocation core properties using molecular dynamics simulations are considered.

Development of a Formulation to Predict Molten Core Spreading in an Lwr Severe Accident

Development of a Formulation to Predict Molten Core Spreading in an Lwr Severe Accident

Basal dislocations in MAX phases: Core structure and mobility

The MAX phases, layered ternary carbides and nitrides, plastically deform via exclusively basal slip systems. This constraint leads to their well-documented plastic anisotropy and deformation mechanisms such as kinking. Basal dislocations govern the incipient plastic deformation, and therefore understanding them is a key step to utilizing the unique properties of MAX phases. As such, we performed the first fully atomistic calculations of the core structures and mobilities of basal dislocations using a bond-order interatomic potential. Calculations show that the weakly bonded A-layer can have a large impact on core structure, with several configurations exhibiting non-planar cores and being an order of magnitude less mobile than their planar counterparts. Additionally, due to the unique stacking sequence and stacking faults in MAX phases, basal dislocations preferentially nucleate and glide as same-signed pairs on adjacent slip planes. These pairs dissociate as zonal dislocations over much larger distances, enabling greater core spreading and therefore increased mobility. Edge pairs have Peierls stresses more closely resembling those in metals than in ceramics, suggesting they control the incipient plasticity of MAX phases at lower temperatures. Overall, these results provide a foundation for better understanding MAX phase incipient plasticity through basal dislocations and thus enabling them to be better engineered for specific applications.

Hardening induced by dislocation core spreading at disordered interface in Cu/Nb multilayers

Atomistic scale interface structures could tailor the dislocation behaviors at interface, as well as interface strengthening. In this work, we design a disordered interface by introducing an ultrathin amorphous layer in the Cu/Nb multilayers. Compared to the sharp interface, the disordered interface can smear the impinging dislocation with relaxing the in-plane and out-of-plane Burgers vectors, thus inducing a remarkable hardening in the Cu/Nb multilayers. The findings not only prove the hardening effect induced by dislocation interface smearing, but also shed light on developing advanced materials with ultrahigh hardness via disordered interface.

Fluid–structure coupling in time domain for dynamic stall using purely Lagrangian vortex method

This paper presents the purely fast Lagrangian vortex method (FLVM) for the simulation of the external incompressible flows past heaving and pitching bodies with high-frequency oscillation. The Nascent vortex element is introduced to the flow field to retain the Lagrangian characteristics of the solver. The viscous effect is modeled using a core spreading method coupled with the splitting and merging spatial adaptation scheme. The particle’s velocity is calculated using Biot–Savart formulation. To accelerate computation, a fast multipole method (FMM) is employed. The validity of FLVM solver is verified by temporal and spatial convergence studies for the case of flows past an impulsively started cylinder at the Reynolds numbers ranging from 50 to 9500. The accuracy of FLVM is then confirmed for the simulation of flows around the pitching flat plate and oscillating airfoil. The time history of drag and lift coefficients and the vorticity contours show a good agreement with those reported in literature. Furthermore, the FLVM is employed to determine the flutter derivatives and flutter speed of an oscillating flat plate. Results are compared with theoretical solutions based on Theodorsen’s function. In general, the results agree well with those obtained by the inviscid theory.

On the glide of [100] dislocation and the origin of ‘pencil glide’ in Mg2SiO4 olivine

ABSTRACTOlivine with chemical composition (Mg,Fe)2SiO4 is a silicate which is supposed to strongly constrain the flow of the Earth’s upper mantle under thermal convection. Its mechanical properties are thus of primary importance. Slip systems in olivine involve two types of dislocations with [100] and [001] Burgers vectors. In this study, we report atomistic modelling of screw dislocations with [100] Burgers vector and their intrinsic properties. We show that the [100] screw dislocation core exhibits several configurations corresponding to spreading in different planes and different relative stabilities. At low pressure, we identify a clear tendency for core spreading in (010). At higher pressure, relevant for the Earth’s mantle, we show that pressure promotes a change in core configuration with spreading into equivalent {021} planes. Based on the systematic investigation of the minimum energy path between the different configurations, we show that the variability of core structures allows complex glide paths which has been described at the macroscopic level as ‘pencil glide’. Our results suggest that the pencil glide is more efficient at high pressure.

A numerical study on jet characteristics under different supercritical conditions for engine applications

Fuel injection is one of the key factors that influence the energy conversion efficiency and emission level of engines. In this study, supercritical injection and mixing under different conditions were investigated using large eddy simulation method. Real-fluid thermodynamic models based on the Peng-Robinson equation of state, including the volume-translation method, were established, and the adjusted pressure implicit split operator algorithm was developed to handle solution instability due to thermodynamic nonidealities. Density variations, potential core lengths, spreading rate, and turbulent velocity fluctuations were analyzed. The effects of injection temperature and ambient pressure on supercritical jet disintegration were studied. The results show that both high injection temperature and elevated ambient pressure can facilitate jet mixing, and the potential core length and spreading rate are sensitive to the injection temperature near pseudo-boiling point. The pseudo-boiling process, which results in a large density gradient and isobaric heat capacity in the jet surface, delays the shrinkage of the radial boundary of the potential core and leads to small spreading angles of jets. The turbulent velocity fluctuations reveal significant anisotropy in the mixing layer of supercritical jets, and a large density gradient resulting from pseudo-boiling behavior damps radial turbulent velocity fluctuations at the surface of jets.

Hydrogen distribution induced screw dislocation core spreading in tungsten

The underlying physical mechanisms of the retention and self-clustering of hydrogen(H) in tungsten(W) are important in understanding the radiation damage and plasma trapping of W-based components in divertors, in which the influence of dislocations has not been sufficiently addressed. For this purpose, quantum mechanics/molecular dynamics(QM/MD) simulations are performed to investigate the distribution of hydrogen(H) clusters along screw dislocation lines in W, where the spreading of the screw dislocation core is first observed introduced by the H clusters. This core spreading constitutes an essential factor that could account for the hydrogen embrittlement phenomenon.

Size effect of layer thickness on stress fields due to interface core-spreading dislocation arrays in multilayers

The elastic fields due to interface dislocation arrays with spreading core in multilayers are derived analytically by means of the superposition of Green function of individual compact dislocations. The numerical results for Cu-Nb multilayers with dislocation arrays demonstrate that: (1) There exists a critical layer thickness (CLT) in describing interface shear stress. The maximum interface shear stress (MISS) increases with the layer thickness as it exceeds CLT; while the MISS decreases rapidly with the layer thickness as it is thinner than CLT. (2) Both the density and core width of interface dislocations have salient effect on the stress fields. The overall stress field is confined by high density of dislocations, and the stress singularity is released by core spreading. (3) A new dislocation prefers to glide in the middle region between the two adjacent interface dislocations by means of Peach-Kohler (P-K) force acting on the dislocation and elastic energy of dislocations. These findings would provide foundations to deepen the understanding of microscopic plastic deformation mechanisms and their related macroscopic mechanical properties of metallic anisotropic multilayers.

Unraveling the dislocation core structure at a van der Waals gap in bismuth telluride

Tetradymite-structured chalcogenides such as bismuth telluride (Bi2Te3) are of significant interest for thermoelectric energy conversion and as topological insulators. Dislocations play a critical role during synthesis and processing of such materials and can strongly affect their functional properties. The dislocations between quintuple layers present special interest since their core structure is controlled by the van der Waals interactions between the layers. In this work, using atomic-resolution electron microscopy, we resolve the basal dislocation core structure in Bi2Te3, quantifying the disregistry of the atomic planes across the core. We show that, despite the existence of a stable stacking fault in the basal plane gamma surface, the dislocation core spreading is mainly due to the weak bonding between the layers, which leads to a small energy penalty for layer sliding parallel to the van der Waals gap. Calculations within a semidiscrete variational Peierls-Nabarro model informed by first-principles calculations support our experimental findings.

Numerical Study on the Flow Characteristics of 2-D Free Jet at a Low Reynolds Number

Two-dimensional free jets have been numerically studied in the laminar flow regime. The numerical solution was obtained by solving the incompressible Navier-Stokes equations with SMAC finite-difference method. The variation of the axial velocity, potential core and jet spreading width were investigated to make clear the characteristics of free jet at a low Reynolds number. It was found that the fully developed region where the self-similarity is satisfied exists in the downstream of the collapsing point of the potential core. The variation of the core is depended on the Reynolds number. It showed that the half-width of geometry is useful in identifying the similarity. The predicted velocity distributions were in good agreement with those of experimental and theoretical results.

Dislocation Interaction and V-Shaped Growth of the Distorted Structure During Nanoindentation of Cu20Ni20Al20Co20Fe20 (high-entropy alloy)-Coated Copper: A Molecular Dynamics Simulation-Based Study

In this paper, the deformation behavior of Cu20Ni20Al20Co20Fe20 high-entropy alloy-coated single-crystal Cu substrate which undergoes nanoindentation has been investigated under molecular dynamic simulation with embedded-atom method potential. The dynamic structural evolutions under nanoindentation are presented using centrosymmetry parameter analysis, common neighbor analysis and radial distribution function plots. In the initial level of nanoindentation, the interface deformation is greatly confronted by the confined V-shaped growth of the distorted structure. But the sudden discrete dislocation burst account for avalanche break-down in the interface layer, which further get influenced by the evolution of multiple dislocation nodes that is significantly governed by core spreading, extended misfit dislocation generation and relative rotation. In the meanwhile, the subsequent generation of dislocation locks, complicated multiple dislocation loops, dislocation junctions and limited cross-slip in wide stacking faults (SFs) hasten the work hardening and in turn slows down the deformation progress. On the other hand, the intermediate appearance of narrow SFs and slip bands significantly reduces the work hardening rate that increases the optimum fracture strain value of the specimen. Moreover, the overall increase in dislocation density and dislocation length leads to a significant growth in dislocation sources which leads to forest hardening in the later stage.

Plastic flow resistance of NiTiCu shape memory alloy-theory and experiments

The NiTiCu alloys belong to a class of materials with excellent shape memory properties. The limitations in shape memory properties arise due to onset of slip at interfaces and also in austenite domains. As slip mediated plasticity is a source of irreversibility, it is imperative to understand the glide resistance of austenite which can be rather complex. In this paper, we develop a model to precisely derive the CRSS for slip substantiated with the uniaxial loading experiments on single crystals in a wide range of orientations employing Digital Image Correlation. We illustrate the core spreading of {011}<001> dislocations and evaluate the role of non-Schmid stress components which introduces profound anisotropy in CRSS levels. The model matches the experimental findings in single crystals with excellent agreement. The theory and experiments show significant crystal orientation dependence of plasticity which must be taken into account when designing with these alloys.