Explicit Integration Research Articles

Investigation of high strain rate effects on strain-hardening cementitious composites using Voronoi-cell lattice models

Strain-hardening cementitious composites (SHCC) have been reported in several studies to exhibit in several studies to exhibit high toughness even under extreme dynamic loads. To understand such dynamic behavior of SHCC, this study extends a previously developed rate-dependent lattice model by introducing a non-local formulation of the fiber-matrix interface behavior. The Voronoi-cell lattice model, representing the cementitious matrix, employs an explicit time integration scheme and a visco-plastic unit to account for the inertia effect and Stefan effect. Additionally, randomly-distributed short fibers were modeled using an analytical model of shear stress transfer at the fiber-matrix interface. The proposed numerical model successfully simulated the direct tensile behavior and multiple cracking patterns of SHCC, revealing distinct rate-dependent failure characteristics. Through a simple parameter fitting process, the model accurately predicted the dynamic increase factor (DIF) of strength and strain capacity of high-speed tensile experiments. Furthermore, the numerically evaluated rate dependency of the fiber-matrix interface exhibited identical trends to previous experimental studies. Finally, the influences of fluctuations in the applied strain rate during actual high-speed loading tests and the random dispersion of fibers were assessed.

Explicit and Implicit Integration of Constitutive Equations of Chaboche Isotropic-Kinematic Hardening Material Model

The proper selection of the material model and its parameters in numerical simulations of the material response under loading is a very important task. In the cyclic load one should assume different phenomena, e.g. ratchetting effect, the mean stress relaxation, creep, etc. The Chaboche kinematic hardening model is mainly applied in order to capture the Bauschinger effect due to its good description of the material behaviour under the cyclic loading. The modified Chaboche isotropic-kinematic hardening model is considered in this paper in order to simulate the strain-controlled and stress-controlled uniaxial cyclic tension-compression test. The implicit and explicit integration procedures are tested here, showing advantages and disadvantages of both methods. It is shown that both approaches provide similar results. The implicit integration method of constitutive equations is unconditionally stable and thus less calculation steps are required in comparison to the explicit procedure. The implicit and explicit integration of the Chaboche model including isotropic and kinematic hardening are then implemented for the 3D case in commercial ABAQUS program in the form of the user material procedure. The correctness of results obtained for the user material model is verified by comparison to results for commercially implemented Chaboche model.

A highly efficient computational approach for fast scan-resolved simulations of metal additive manufacturing processes on the scale of real parts

This article proposes a novel high-performance computing approach for the prediction of the temperature field in powder bed fusion (PBF) additive manufacturing (AM) processes. In contrast to many existing approaches to part-scale simulations, the underlying computational model consistently resolves physical scan tracks without additional heat source scaling, agglomeration strategies or any other heuristic modeling assumptions. A growing, adaptively refined mesh accurately captures all details of the laser beam motion. Critically, the fine spatial resolution required for resolved scan tracks in combination with the high scan velocities underlying these processes mandates the use of comparatively small time steps to resolve the underlying physics. Explicit time integration schemes are well-suited for this setting, while unconditionally stable implicit time integration schemes are employed for the interlayer cool down phase governed by significantly larger time scales. These two schemes are combined and implemented in an efficient fast operator evaluation framework providing significant performance gains and optimization opportunities. The capabilities of the novel framework are demonstrated through realistic AM examples on the centimeter scale including the first scan-resolved simulation of the entire NIST AM Benchmark cantilever specimen, with a computation time of less than one day. Apart from physical insights gained through these simulation examples, also numerical aspects are thoroughly studied on basis of weak and strong parallel scaling tests. As potential applications, the proposed thermal PBF simulation approach can serve as a basis for microstructure and thermo-mechanical predictions on the part-scale, but also to assess the influence of scan pattern and part geometry on melt pool shape and temperature, which are important indicators for well-known process instabilities.

Explicit and structure-preserving exponential wave integrator Fourier pseudo-spectral methods for the Dirac equation in the simultaneously massless and nonrelativistic regime

Explicit and structure-preserving exponential wave integrator Fourier pseudo-spectral methods for the Dirac equation in the simultaneously massless and nonrelativistic regime

Phase-field description of fracture in NiTi single crystals

A phase-field model for thermomechanically-induced fracture in NiTi at the single crystal level, i.e., fracture under loading paths that may take advantage of either of the functional properties of NiTi–superelasticity or shape memory effect–, is presented, formulated within the kinematically linear regime. The model accounts for reversible phase transformation from austenite to martensite habit plane variants and plastic deformation in the austenite phase. Transformation-induced plastic deformation is viewed as a mechanism for accommodation of the local deformation incompatibility at the austenite–martensite interfaces and is accounted for by introducing an interaction term in the free energy derived based on the Mori–Tanaka and Kröner micromechanical assumptions and the hypothesis of martensite instantaneous growth within austenite. Based on experimental observations suggesting that NiTi fractures in a stress-controlled manner, damage is assumed to be driven by the elastic energy, i.e., phase transformation and plastic deformation are assumed to contribute in crack formation and growth indirectly through stress redistribution. The model is restricted to quasistatic mechanical loading (no latent heat effects), thermal loading sufficiently slow with respect to the time rate of heat transfer by conduction (no thermal gradients), and a temperature range below Md, which is the temperature above which the austenite phase is stable, i.e., stress-induced martensitic transformation is suppressed. The numerical implementation of the model is based on an efficient scheme of viscous regularization in both phase transformation and plastic deformation, an explicit numerical integration via a tangent modulus method, and a staggered scheme for the coupling of the unknown fields. The model is shown able to capture transformation-induced toughening, i.e., stable crack advance attributed to the shielding effect of inelastic deformation left in the wake of the growing crack under nominal isothermal loading, actuation-induced fracture under a constant bias load, and crystallographic dependence on crack pattern.

Peridynamic modelling of dynamic damage and fragmentation of cracked solids during impact contact

Dynamics damage of solids exhibit unique characteristics that are absent in static analysis. The presence of pre-existing cracks further increases the complications. To address this issue, the present study focuses on modelling dynamic damage of cracked solids during impact contact. The model is formulated based on the state-based peridynamics. It incorporates a modified damage model and employs the short-range force approach to capture the impact damage. The damping force is introduced to eliminate numerical oscillation around cracks. Then, the adaptive explicit time integration is introduced to efficiently solve the peridynamic equations. Model performance is evaluated through benchmark tests. Next, we simulate impact between two spheres with pre-existing cracks. Our focus lies in comprehending the mechanisms by which material properties and crack patterns affect damage. Further sensitivity analysis reveals that the kinetic energy of the impact system is significantly affected by the pre-existing cracks and geometric parameters of the cracks, including their number, length, angle, and distribution. The interplay of crack interaction and the gradual connection of the damaged region plays a vital role in the phenomena. The presented study offers insights into damage evolution during impact contact and provides guidelines for assessing impact damage in structures with pre-existing defects.

Improved explicit quartic B-spline time integration scheme for dynamic response analysis of viscoelastic systems

A quartic B-spline based explicit scheme is extended for dynamic analysis of viscoelastic systems. In the calculation formulation, different integration approximation schemes for exponential damping models and other damping models are presented. The mathematical derivation of the convolution solving strategy is formulated for viscoelastic systems. For the damping models composed of exponential function, this scheme only requires once convolution calculation in each time step, even though the dynamic equations are used twice. This significantly reduces the calculation time for the damping models composed of exponential function. Numerical examples show that the proposed quartic B-spline based explicit scheme achieves higher accuracy and efficiency compared to other competitive explicit schemes, not only for damping models composed of exponential functions, but also for other viscoelastic damping models.

Improved modal analyses using the novel quadrilateral overlapping elements

In this work, the novel quadrilateral overlapping element incorporated with the harmonic trigonometric functions is presented to reduce the efforts in mesh division for modal analysis. The new partition of unity functions in the overlapping element are cubic polynomials. Therefore, low-order polynomials can be utilized to augment local approximation space without the linear dependence problem. Moreover, the original polynomial approximation spaces are further enriched by the harmonic trigonometric functions without introducing additional rank deficiencies in the system matrices. Thus, the presented enriched overlapping finite element method (EOFEM) provides very precise eigenfrequencies and eigenmodes by using a very coarse mesh. However, the shape functions in the presented EOFEM do not possess the Kronecker delta function property. Hence, a conforming method that modifies the shape functions rather than the variational principle is proposed to impose the essential boundary conditions. Several numerical examples in two dimensions are then investigated to elaborate on the performance of the presented EOFEM in modal analyses. Besides, a block-diagonal lumped mass matrix is presented and further extended to the structure vibration analysis by incorporating the explicit time integration method. It is proven that the enriched overlapping element is computationally more efficient than the quadratic finite element and the original overlapping element.

Implicit Surface Tension for SPH Fluid Simulation

The numerical simulation of surface tension is an active area of research in many different fields of application and has been attempted using a wide range of methods. Our contribution is the derivation and implementation of an implicit cohesion force based approach for the simulation of surface tension effects using the Smoothed Particle Hydrodynamics (SPH) method. We define a continuous formulation inspired by the properties of surface tension at the molecular scale which is spatially discretized using SPH. An adapted variant of the linearized backward Euler method is used for time discretization, which we also strongly couple with an implicit viscosity model. Finally, we extend our formulation with adhesion forces for interfaces with rigid objects. Existing SPH approaches for surface tension in computer graphics are mostly based on explicit time integration, thereby lacking in stability for challenging settings. We compare our implicit surface tension method to these approaches and further evaluate our model on a wider variety of complex scenarios, showcasing its efficacy and versatility. Among others, these include but are not limited to simulations of a water crown, a dripping faucet, and a droplet toy.

Rate-dependent lattice modeling of textile reinforced cementitious composites under tensile loading

This paper presents a lattice-based modeling technique to simulate the direct tensile behavior of textile reinforced cementitious composites (TRCC) under quasi-static and dynamic loadings. To analyze the reinforcing effect of textiles in TRCC, the fiber bridging force and the shear stress transmission at the fiber-matrix interface are non-locally modeled. Along with an appropriate scheme for explicit time integration, visco-plastic rheological units are introduced to depict the dynamic behavior of the cementitious matrix. The model results are compared with those of both static and dynamic experimental studies. It is demonstrated that the model well describes the strain hardening and multiple cracking behavior of TRCC and effectively captures the dynamic behavior. Proper stress transfer formulation of the fiber-matrix interface is essential for simulating the crack patterns, including crack distributions and openings, observed in the experiments. Furthermore, the effect of the strain rate on these interfacial properties of TRCC is investigated in the dynamic analysis.

Angular Momentum Conserving Limiters for the Euler Equations

Limiters that are used to stabilise the discontinuous Galerkin method fail to preserve its angular momentum conservation property. We propose a simple technique that restores the conservation of angular momentum while preserving the second-order accuracy of the scheme when paired with an explicit time integrator. Numerical experiments confirm the conservation property of the technique paired with two limiters [Giuliani and Krivodonova. 2018. “Analysis of Slope Limiters on Unstructured Triangular Meshes.” Journal of Computational Physics 374: 1–26; Giuliani and Krivodonova. 2019. “A Moment Limiter for the Discontinuous Galerkin Method on Unstructured Triangular Meshes.” SIAM Journal on Scientific Computing 41 (1): A508–A537]. We show that the technique eliminates numerical artifacts that arise from limiting solutions to the Euler equations.

One for all and all for one: Increasing the adaptive capacity of households and communities through a public work programme

This article uncovers the mechanisms shaping the impact of the public work component of Ethiopia’s Productive Safety Net Programme (PSNP) on beneficiaries and communities’ food security and vulnerability to various shocks. Using three waves of a national representative household survey, this study provides quantitative evidence on the pathways through which the social protection intervention affects direct beneficiaries and their community peers. The empirical findings show that the PSNP beneficiaries are less likely to be food insecure and to experience harvest losses in the aftermath of droughts. Notably, the beneficial effects of the programme partially spill over to the direct participants’ community peers. This is likely to be due the nature of the public works implemented through the programme, such as the integrated community-based watershed development, including soil and water conservation measures and rangeland management in pastoral areas. Additionally, no significant impacts have been found when households self-report stresses unrelated to droughts. Our findings are expected to inform the debate on the effectiveness of the PSNP and other adaptive social protection programmes. From a policy perspective, they suggest the explicit integration of environmental and climate considerations into the design of social protection interventions targeting poor agricultural households highly vulnerable and exposed to weather shocks.

Appearing shock waves in Arruda – Boyce incompressible rod

The Arruda – Boyce incompressible hyperelastic potentials are used for analyzing one-dimensional acoustic wave propagation in a semi-infinite nonlinearly elastic rod. It has been found that during the propagation of the delta-like pulses, the following phenomena are observed, (i) the shock wave fronts arise; (ii) the pulses spread out due to physical dispersion; (iii) the mechanical energy of the considered system decreases with time, resulting in a decrease of the pulse magnitudes; and, at the same time (iv) thermal energy arises and increases with time, that is caused by formation and propagation of shock wave fronts. The analysis utilizes a combined method consisting of the explicit time integration technique coupled with the finite element method for spatial discretization.

Numerical analysis of non-proportional biaxial reverse experiments with a two-surface anisotropic cyclic plasticity-damage approach

This paper deals with the numerical analysis of ductile damage and fracture behavior under non-proportional biaxial reverse loading conditions. A two-surface anisotropic cyclic elastic–plastic-damage continuum model is adequately presented, which takes into account the Bauschinger effect, the stress-differential effect, and the change of hardening rate after reverse loading. An efficient Euler explicit numerical integration algorithm, based on the inelastic (plastic or plastic-damage) predictor-elastic corrector approach, is utilized to analyze the stress and finite strain loading histories. Detailed discussions are provided on different numerical integration-related consistent tangent operators that achieve convergence within the global Newton–Raphson scheme. The proposed continuum model is implemented into the commercial software Ansys as a user-defined subroutine (UMAT). Furthermore, the novel non-proportional biaxial tensile reverse experiments are performed to validate the proposed continuum model. The associated numerical simulations investigate the stability and accuracy of the proposed algorithm and material model.

Modelling of shakedown, ratcheting and liquefaction of saturated granular soils using kinematic hardening with memory surface

The present paper proposes a kinematic hardening model with a memory surface to more accurately reproduce cyclic hardening and the behaviour of saturated granular soils subjected to cyclic loads. The new cyclic hardening law offers improved accuracy in predicting soil ratcheting in drained conditions and failures due to liquefaction in undrained conditions. Simulating these two aspects has been challenging due to issues such as significant loss of accuracy and numerical instability, particularly under extended (or a large number of) cyclic loads. To deal with the challenge, in addition to the novel theoretical framework, we also developed a robust sub-stepping algorithm based on an explicit stress integration scheme with automatic error control. The proposed model is compared with several existing models and its performance is evaluated by conducting sensitivity analyses and performing validations against a wide range of laboratory test results.

Three-Dimensional Variational Multi-Doppler Wind Retrieval over Complex Terrain

Abstract The interaction of airflow with complex terrain has the potential to significantly amplify extreme precipitation events and modify the structure and intensity of precipitating cloud systems. However, understanding and forecasting such events is challenging, in part due to the scarcity of direct in situ measurements. Doppler radar can provide the capability to monitor extreme rainfall events over land, but our understanding of airflow modulated by orographic interactions remains limited. The SAMURAI software is a three-dimensional variational data assimilation (3DVAR) technique that uses the finite element approach to retrieve kinematic and thermodynamic fields. The analysis has high fidelity to observations when retrieving flows over a flat surface, but the capability of imposing topography as a boundary constraint is not previously implemented. Here, we implement the immersed boundary method (IBM) as pseudo-observations at their native coordinates in SAMURAI to represent the topographic forcing and surface impermeability. In this technique, neither data interpolation onto a Cartesian grid nor explicit physical constraint integration during the cost function minimization is needed. Furthermore, the physical constraints are treated as pseudo-observations, offering the flexibility to adjust the strength of the boundary condition. A series of observing simulation sensitivity experiments (OSSEs) using a full-physics model and radar emulator simulating rainfall from Typhoon Chanthu (2021) over Taiwan are conducted to evaluate the retrieval accuracy and parameter settings. The OSSE results show that the strength of the IBM constraints can impact the overall wind retrievals. Analysis from real radar observations further demonstrates that the improved retrieval technique can advance scientific analyses for the underlying dynamics of orographic precipitation using radar observations.

Stereo-EEG-guided network modulation for psychiatric disorders: Interactive holographic planning

BackgroundConnectomic modeling studies are expanding understanding of the brain networks that are modulated by deep brain stimulation (DBS) therapies. However, explicit integration of these modeling results into prospective neurosurgical planning is only beginning to evolve. One challenge of employing connectomic models in patient-specific surgical planning is the inherent 3D nature of the results, which can make clinically useful data integration and visualization difficult. MethodsWe developed a holographic stereotactic neurosurgery research tool (HoloSNS) that integrates patient-specific brain models into a group-based visualization environment for interactive surgical planning using connectomic hypotheses. HoloSNS currently runs on the HoloLens 2 platform and it enables remote networking between headsets. This allowed us to perform surgical planning group meetings with study co-investigators distributed across the country. ResultsWe used HoloSNS to plan stereo-EEG and DBS electrode placements for each patient participating in a clinical trial (NCT03437928) that is targeting both the subcallosal cingulate and ventral capsule for the treatment of depression. Each patient model consisted of multiple components of scientific data and anatomical reconstructions of the head and brain (both patient-specific and atlas-based), which far exceed the data integration capabilities of traditional neurosurgical planning workstations. This allowed us to prospectively discuss and evaluate the positioning of the electrodes based on novel connectomic hypotheses. ConclusionsThe 3D nature of the surgical procedure, brain imaging data, and connectomic modeling results all highlighted the utility of employing holographic visualization to support the design of unique clinical experiments to explore brain network modulation with DBS.

Teledentistry Acceptability Among Latina/o/x Adults in Rural Southwest Virginia

Abstract: Inequitable access to dental care remains a barrier to optimal health for rural populations. With the COVID-19 pandemic, teledentistry to improve the delivery of oral health care has been emphasized. Few previous investigations of teledentistry acceptability have included Latina/o/x populations in rural areas of the United States. We recruited Latina/o/x adults (N=91) in rural southwest Virginia to participate in a survey to assess correlates related to teledentistry acceptability. More than half of participants (57%) reported no interest in using teledentistry if it were available. Having a yearly household income above $24,000 and not having dental insurance were positively associated with telehealth acceptability (p=.04 and p=.01, respectively). Inequitable access to dental care may persist even with broad availability of teledentistry services unless there is explicit integration of health equity.

A Fully Explicit Lattice Spring Elastoplastic Solid Model for Fluid Structure Interaction Applications

The Fluid Structure Interaction (FSI) solver based on mesh-less methods developed at University of Nottingham Malaysia has been proven to be successful in solving hydroelastic problems involving elastic body. In this paper, the possibility of modelling elastocplastic material behaviour using mesh-less method is further explored. Elastoplastic material bahaviour is essential for modelling ductile fracture in a wide range of engineering problems. The use of a type of nonlocal lattice spring model (LSM) based on fully explicit integration scheme in modelling elastoplastic material behaviour is presented in this work. Specifically, the return mapping algorithm is incorporated in the solid solver for modelling J2 plasticity. The effect of time step size and particle spacing on the accuracy of the solver are investigated by checking the predicted strain against theoretical solution. It is found out that the strain results obtained by varying the time step size are somewhat consistent with the theoretical solution. The predicted strain also converges to the theoretical result as the particle spacing is refined.

The characteristics of audiovisual temporal integration in streaming-bouncing bistable motion perception: considering both implicit and explicit processing perspectives.

This study explored the behavioral and neural activity characteristics of audiovisual temporal integration in motion perception from both implicit and explicit perspectives. The streaming-bouncing bistable paradigm (SB task) was employed to investigate implicit temporal integration, while the corresponding simultaneity judgment task (SJ task) was used to examine explicit temporal integration. The behavioral results revealed a negative correlation between implicit and explicit temporal processing. In the ERP results of both tasks, three neural phases (PD100, ND180, and PD290) in the fronto-central region were identified as reflecting integration effects and the auditory-evoked multisensory N1 component may serve as a primary component responsible for cross-modal temporal processing. However, there were significant differences between the VA ERPs in the SB and SJ tasks and the influence of speed on implicit and explicit integration effects also varied. The aforementioned results, building upon the validation of previous temporal renormalization theory, suggest that implicit and explicit temporal integration operate under distinct processing modes within a shared neural network. This underscores the brain's flexibility and adaptability in cross-modal temporal processing.