378 research outputs found
SOLID-SHELL FINITE ELEMENT MODELS FOR EXPLICIT SIMULATIONS OF CRACK PROPAGATION IN THIN STRUCTURES
Crack propagation in thin shell structures due to cutting is conveniently simulated
using explicit finite element approaches, in view of the high nonlinearity of the problem. Solidshell
elements are usually preferred for the discretization in the presence of complex material
behavior and degradation phenomena such as delamination, since they allow for a correct
representation of the thickness geometry. However, in solid-shell elements the small thickness
leads to a very high maximum eigenfrequency, which imply very small stable time-steps. A new
selective mass scaling technique is proposed to increase the time-step size without affecting
accuracy. New ”directional” cohesive interface elements are used in conjunction with selective
mass scaling to account for the interaction with a sharp blade in cutting processes of thin ductile
shells
Statistical extraction of process zones and representative subspaces in fracture of random composite
We propose to identify process zones in heterogeneous materials by tailored
statistical tools. The process zone is redefined as the part of the structure
where the random process cannot be correctly approximated in a low-dimensional
deterministic space. Such a low-dimensional space is obtained by a spectral
analysis performed on pre-computed solution samples. A greedy algorithm is
proposed to identify both process zone and low-dimensional representative
subspace for the solution in the complementary region. In addition to the
novelty of the tools proposed in this paper for the analysis of localised
phenomena, we show that the reduced space generated by the method is a valid
basis for the construction of a reduced order model.Comment: Submitted for publication in International Journal for Multiscale
Computational Engineerin
Aging concrete structures: a review of mechanics and concepts
The safe and cost-efficient management of our built infrastructure is a challenging task considering the expected service life of at least 50 years. In spite of time-dependent changes in material properties, deterioration processes and changing demand by society, the structures need to satisfy many technical requirements related to serviceability, durability, sustainability and bearing capacity. This review paper summarizes the challenges associated with the safe design and maintenance of aging concrete structures and gives an overview of some concepts and approaches that are being developed to address these challenges
Homogenization of discrete mesoscale model of concrete for coupled mass transport and mechanics by asymptotic expansion
Mass transport phenomenon in concrete structures is strongly coupled with
their mechanical behavior. The first coupling fabric is the Biot's theory
according to which fluid pressure interacts with solid stress state and
volumetric deformation rate of the solid induces changes in fluid pressure.
Another coupling mechanism emerges with cracks which serve as channels for the
fluid to flow through them and provide volume for fluid storage. Especially the
second coupling mechanism presents a challenge for numerical modeling as it
requires detailed knowledge about cracking process. Discrete mesoscale
mechanical models coupled with mass transport offer simple and robust way to
solve the problem. On the other hand, however, they are computationally
demanding. In order to reduce this computational burden, the present paper
applies the asymptotic expansion homogenization technique to the coupled
problem to deliver (i) continuous and homogeneous description of the
macroscopic problem which can be easily solved by the finite element method,
(ii) discrete and heterogeneous mesoscale problem in the periodic setup
attached to each integration point of the macroscale along with (iii) equations
providing communication between these two scales. The transient terms appear at
the macroscale only, as well as the Biot's coupling terms. The coupling through
cracking is treated at the mesoscale by changing conductivity of the conduit
elements according to the mechanical solution, otherwise the two mesoscale
steady state problems are decoupled and can be therefore solved in a sequence.
This paper presents verification studies showing performance of the homogenized
solution.Comment: 29 pages, 9 figure
Adaptive discretization refinement for discrete models of coupled mechanics and mass transport in concrete
An adaptive discretization refinement strategy for steady state discrete
mesoscale models of coupled mechanics and mass transport in concrete is
presented. Coupling is provided by two phenomena: the Biot's theory of
poromechanics and an effect of cracks on material permeability coefficient. The
model kinematics is derived from rigid body motion of Voronoi cells obtained by
tessellation of the domain. Starting with a coarse discretization, the density
of Voronoi generator points is adaptively increased on the fly in regions where
the maximum principal stress exceeds a chosen threshold. Purely elastic
behavior is assumed in the coarse discretization, therefore no transfer of
history/state variables is needed. Examples showing (i) computational time
savings achieved via the adaptive technique and (ii) an agreement of the
outputs from the fine and adaptive models during simulations of hydraulic
fracturing and three-point bending combined with a fluid pressure loading are
presented.Comment: 25 pages, 13 figure
Concrete microstructure homogenization technique with application to model concrete serviceability
Conventionally, mechanical properties of concrete are attained through experiment by leaving microstructural phases interaction in a black box. To fully understand concrete, it is necessary to bridge the gap between microstructure and macro properties. In this dissertation, with several models being given progressively, an innovative homogenization model of concrete is proposed in which concrete is regarded as cement and aggregate particles connected by interfacial transition zone (ITZ). Defined on a representative volume element (RVE), the relationship between microstructure and macro properties is established. The proposed model is validated by experimental results and then applied in the study of concrete serviceability. The concrete homogenization model includes RVE in two scale levels: cement paste RVE in microscale and concrete RVE in mesoscale. Cement paste RVE is composed by microstructural phases (water, unhydrated cement, calcium hydroxide, calcium silicate hydrate, etc.), which are determined by the validated three-dimensional (3D) cement hydration and microstructural development model HYMOSTRUC® or CEMHYD3D. The developed cement paste RVE at different hydration ages is transferred to a finite element method (FEM) model and upscaled by homogenization as inputs for concrete RVE in the mesoscale. Cement paste homogenization model is validated by the experimental study of nanosilica effects on the mechanical properties of cement paste. Concrete RVE can be generated by converting realistic or (re)constructed concrete material image into finite element environment. In this dissertation, cell operation method is presented to (re)construct concrete. The similarity between (re)constructed image and target image is verified by low-order correlation functions. In the discrete model of concrete, each cement paste element or each aggregate is treated as a discrete particle; and these particles are bonded together by equivalent ITZ. To simulate cracking and particle interaction, ITZ is represented by cohesive zone model (CZM) and contact mechanism. This dissertation will demonstrate that the concrete homogenization technique can capture the relationship between structure and material, and enable us to study concrete serviceability in view of microstructure evolution. As the applications of the proposed homogenization model of concrete, the following studies on concrete serviceability are carried out: deflection variation in reinforced concrete (RC) beams propagated from concrete microstructural variability, and mechanical consequences of concrete subjected to alkali-silica reaction (ASR). Due to the inherent uncertainty in concrete microstructure, variation of RC beam deflection is inevitable. For satisfactory use of RC members, it is necessary to incorporate uncertainty of concrete properties in deflection prediction. With the help of homogenization modeling, microstructural variability in concrete is projected to the deflection variation in RC beams. Alkali-silica reaction (ASR) is a kind of chemical reaction in concrete. Alkali in cement meets with reactive silica in aggregate and expanding gels are produced if there is enough water. The swelling of gels will induce stress and alter concrete microstructure. In some cases, this alteration includes cracking and expansion in concrete member. The condition becomes more complicated when the expansion caused by swelling gels is confined by reinforcement and prestress in concrete. Using the proposed homogenization technique, the mechanical consequence of ASR on concrete is simulated. ASR gels expansion is achieved by aggregate volume increase, which causes internal stress and deteriorates ITZ in concrete RVE model. The proposed mechanical model of concrete subjected to ASR is demonstrated on plain concrete specimens (prism and cylinder). The simulated cases are validated by experimental work by others. It is proved that the proposed ASR model by using concrete microstructure homogenization can stand with ASR chemical and diffusional models given by other researchers to predict the serviceability of concrete structure subjected to ASR
Homogenization of discrete diffusion models by asymptotic expansion
Diffusion behaviors of heterogeneous materials are of paramount importance in
many engineering problems. Numerical models that take into account the internal
structure of such materials are robust but computationally very expensive. This
burden can be partially decreased by using discrete models, however even then
the practical application is limited to relatively small material volumes.
This paper formulates a homogenization scheme for discrete diffusion models.
Asymptotic expansion homogenization is applied to distinguish between (i) the
continuous macroscale description approximated by the standard finite element
method and (ii) the fully resolved discrete mesoscale description in a local
representative volume element (RVE) of material. Both transient and
steady-state variants with nonlinear constitutive relations are discussed. In
all the cases, the resulting discrete RVE problem becomes a simple linear
steady-state problem that can be easily pre-computed. The scale separation
provides a significant reduction of computational time allowing the solution of
practical problems with a negligible error introduced mainly by the finite
element discretization at the macroscale.Comment: 33 pages, 12 figure
Računalna mehanika u znanosti i inženjerstvu – Quo vadis
Computational Mechanics has many applications in science and engineering. Its range of application has been enlarged widely in the recent decades. Hence, nowadays areas such as biomechanics and additive manufacturing are among the new research topics, in which computational mechanics helps solve complex problems and processes. In this contribution, these emerging areas will be discussed together with new discretization schemes, e. g. virtual element method and particle methods, whereby the latter need high performance computing facilities in order to solve problems such as mixing in an accurate way. Failure analysis of structures and components is another topic that is developing fast. Here, modern computational approaches rely on the phase field method that simplifies discretizations schemes. All these approaches and methods are discussed and evaluated by means of examples.Računalna mehanika ima široku primjenu u znanosti i inženjerstvu. Njeno područje primjene se znatno povećalo u zadnjim desetljećima. Danas polja kao biomehanika i aditivna proizvodnja nova su područja istraživanja u kojima računalna mehanika pomaže rješavati složene probleme i procese. U radu se razmatraju ova granična područja zajedno s novim diskretizacijskim postupcima kao što su metoda virtualnih elemenata i metoda čestica, gdje potonja zahtijeva moćnu računalnu opremu da bi se mogli točno riješiti problemi kao što je miješanje. Analiza oštećenja konstrukcija i njenih komponenata je drugo područje koje se brzo razvija, pa se ovdje moderni računalni postupci odnose na metodu faznih polja koja pojednostavljuje diskretizacijske sheme. Svi navedeni postupci i metode su razmatrani i vrednovani u numeričkim primjerima
Efficient computational mesoscale modeling of concrete under cyclic loading
Tesi amb diferents seccions retallades per drets de l'editor.Concrete is a complex material and can be modeled on various spatial and temporal scales. While simulations on coarse scales are practical for engineering applications, a deeper understanding of the material is gained on finer scales. This is at the cost of an increased numerical effort that can be reduced by the three methods developed and used in this work, each corresponding to one publication.
The coarse spatial scale is related to fully homogenized models. The material is described in a phenomenological approach and the numerous parameters sometimes lack a physical meaning. Resolving the three-phase mesoscopic structure consisting of aggregates, the mortar matrix and the interfaces between them allow to describe similar effects with simpler models. This work addresses two computational challenges related to mesoscale modeling.
First, aggregate particles take up a high volume fraction and an efficient particle-packing algorithm is required to generate non-overlapping, random esostructures. Enforcing an additional distance between the aggregates is essential to obtain undistorted meshes for finite element simulations, but further complicates the packing problem. An event-driven molecular-dynamics algorithm is applied to this problem that, in contrast to traditional methods, allows movement and a dense arrangement of the aggregates. This allows creating concrete mesostructures with realistic aggregate volume fractions.
The second challenge concerns stability problems in mesoscale simulations of concrete fracture. The geometric complexity and the combination of three material laws for each of the phases leads to numerical instabilities, even for regularized material models. This requires tiny time steps and numerous iterations per time step when integrated with a classic backward Euler scheme. The implicit–explicit (IMPL-EX) integration extrapolates internal variables that account for the nonlinear behavior. This linearizes the equations, provides additional robustness and a computational speedup. In combination with a novel time step control method, a three-dimensional mesoscale compression test is accelerated by a factor of 40, compared to an adaptive backward Euler algorithm.
The life time of concrete under cyclic loads is commonly predicted with empirical Wöhler lines. They relate the number of endured cycles with the applied load amplitude and can be included in constitutive formulations. They can, however, hardly be generalized to geometries and load configurations other than the ones tested.
On a finer temporal scale, fatigue failure is modeled by the accumulation of damage within each loading cycle. This resolves the whole process of failure, includes stress redistributions and size effects and can easily be extended to multiphysics phenomena.
The third computational challenge solved here is the efficient temporal integration that would not be feasible in a naive cycle-by-cycle integration of thousands or millions of cycles. The cost of evaluating a single cycle is reduced by reformulating the problem in the frequency space. It is sufficient to equilibrate the structure once for each Fourier coefficient which significantly speeds up this evaluation. The accumulated damage of one cycle is integrated in time using an adaptive cycle jump concept. For a two dimensional void test structure, the combination of both techniques leads to a 25 times faster simulation compared to the full integration.
These three main contributions decrease the numerical cost of mesoscale simulations, allow larger and more detailed models and are a basis to deepen the understanding of the complex failure patterns in concrete.El hormigón es un material complejo y puede ser modelado en varias escalas espaciales y temporales. Mientras que las simulaciones en escalas gruesas son prácticas para aplicaciones de ingeniería, se obtiene una comprensión más profunda del material en escalas más finas. Esto es a costa de un mayor esfuerzo numérico que puede ser reducido por los tres métodos desarrollados y utilizados en este trabajo, cada uno de los cuales corresponde a una publicación. La escala espacial gruesa está relacionada con modelos totalmente homogeneizados. El material se describe con un enfoque fenomenológico y los numerosos parámetros a veces carecen de significado físico. La resolución de la estructura mesoscópica trifásica formada por los áridos, la matriz de mortero y las interfaces entre ellos permite describir efectos similares con modelos más sencillos. Este trabajo aborda dos retos computacionales relacionados con el modelado a mesoescala. En primer lugar, las partículas agregadas absorben una fracción de gran volumen y se requiere un algoritmo eficiente de empaquetamiento de partículas para generar mesoestructuras aleatorias que no se solapen. Hacer cumplir una distancia adicional entre los agregados es esencial para obtener mallas no distorsionadas para simulaciones de elementos finitos, pero complica aún más el problema de empaquetado. A este problema se le aplica un algoritmo de dinámica molecular impulsado por eventos que, a diferencia de los métodos tradicionales, permite el movimiento y una disposición densa de los agregados. Esto permite crear mesoestructuras de hormigón con fracciones de volumen de agregado realistas. El segundo reto se refiere a los problemas de estabilidad en las simulaciones mesoescalares de fracturas de hormigón. La complejidad geométrica y la combinación de tres leyes materiales para cada una de las fases conduce a inestabilidades numéricas, incluso para modelos materiales regularizados. Esto requiere pequeños pasos de tiempo y numerosas iteraciones por paso de tiempo cuando se integra con un esquema clásico de Euler hacia atrás. La integración implícita- explícita (IMPL-EX) extrapola variables internas que dan cuenta del comportamiento no lineal. Esto linealiza las ecuaciones, proporciona robustez adicional y una aceleración computacional. En combinación con un nuevo método de control de paso en el tiempo, una prueba de compresión tridimensional de mesoescala es acelerada por un factor de 40, en comparación con un algoritmo adaptativo de Euler hacia atrás. La vida útil del hormigón bajo cargas cíclicas se predice comúnmente con las líneas empíricas de Wöhler. Relacionan el número de ciclos soportados con la amplitud de carga aplicada y pueden ser incluidos en formulaciones constitutivas. Sin embargo, difícilmente pueden generalizarse a geometrías y configuraciones de carga distintas a las probadas. En una escala temporal más fina, la falla por fatiga es modelada por la acumulación de daño dentro de cada ciclo de carga. Esto resuelve todo el proceso de fracaso, incluye redistribuciones de estrés y efectos de tamaño, y puede extenderse fácilmente a fenómenos multifísicos. El tercer reto computacional resuelto aquí es la integración temporal eficiente que no sería factible en una integración costosa de miles o millones de ciclos ciclo a ciclo. El costo de evaluar un solo ciclo se reduce reformulando el problema en el espacio de frecuencias. Es suficiente equilibrar la estructura una vez para cada coeficiente de Fourier, lo que acelera significativamente esta evaluación. El daño acumulado de un ciclo se integra en el tiempo utilizando un concepto de salto de ciclo adaptativo. Para una estructura de prueba de vacío bidimensional, la combinación de ambas técnicas conduce a una simulación 25 veces más rápida en comparación con la integración completa.Postprint (published version
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