Bibiana Luccioni

Coupled plastic-damaged model

Abstract A constitutive model that couples plasticity and damage is presented. The model is thermodynamically consistent and comes from a generalization of classical plasticity theory and isotropic damage theory of Kachanov. Coupling between plasticity and damage is achieved through a simultaneous solution of the plastic and the damage problem. After a description of the model, a numerical algorithm for the integration of the resulting constitutive equations is presented. It is an Euler Backward type of algorithm that is particularly suitable to solve plain stress non-linear problems with a 2D finite element program. The consistent stiffness matrix is also derived. The paper is completed with some application examples that show that the model presented accurately reproduces the behaviour of elastic-plastic-damaged materials.

Thermo-mechanic model for concrete exposed to elevated temperatures

Abstract A thermo-mechanical model for concrete subjected to high temperatures is presented in this paper. The model is based on a coupled plastic-damage model that has been extended to consider damage induced by high temperatures. The model is calibrated with experimental results of residual strength tests in concrete specimens. The paper is completed with application examples and comparisons with experimental results that validate the model presented. The model is also used to the damage assessment of a concrete structure that has been subjected to fire.

A directional damage model

A directional damage model for initially isotropic metals and geomaterials is presented in this paper. The model is based on a space transformation using an analogy with the theory of finite strains. As a result, damage is defined by a second order damage tensor. The basic equations of the model are derived from the thermodynamics of irreversible processes. A return-mapping algorithm is developed for the numerical integration of the model. Applications examples presented show that the model is able to reproduce damage-induced anisotropy, with directional stiffness degradation and hardening, under general loading conditions.

Defining Erosion Limit for Concrete

Numerical simulation is usually used for predicting the response of concrete and fiber reinforced concrete structures to blast or impact loads. Depending on both charge weight and standoff distance, blast loads can cause fracture and spalling of concrete. In order to numerically reproduce these effects, an erosion model can be used to remove from the calculation the elements that have reached certain criteria. This erosion model represents a numerical tool to avoid great distortion of Lagrange meshes. For this reason, its application to the simulation of a physical phenomenon requires the calibration with experimental results. A review of different erosion criteria and limits used by different authors to simulate concrete under blast loads is presented in this paper. Some application examples and comparisons with experimental results are developed to show the effect of erosion limit on damage results and the dependence on the materials properties, mesh size and scaled distance.

FE modeling of a closed box beam with piezoelectric fiber composite patches

A thin walled single-cell box beam with piezoelectric fiber composite patches bonded to its skin is analyzed. Patch fibers are oriented at ± 45° with respect to the beams longitudinal axis. Two types of connections are used to supply control signals to the patches. In the first case, upper and lower patches are connected in pairs. The second case involves independent control of each patch. A FE approach is used to model the host structure and the actuators. The analysis is made in the state-space using a state feedback control concept. A LQR control strategy is used. A uniform field material model is used to obtain the effective electromechanical properties of the patches, while the passive host structure material is modeled as isotropic linear elastic. Closed-loop root locus plots show a notable increment in damping ratios introduced by independent actuation with respect to coupled actuation. Frequency responses show the attenuation in tip twist and tip displacement while resonant conditions are investigated. Numerical results reveal that, in contrast to coupled actuation, independent actuation is able to control not only torsional modes but also flexional modes. However, as expected, due to the orientation of the patches, torsional vibration control exhibits a better performance. Finally, a transient analysis is performed and results for open and closed-loop systems are compared. A substantial attenuation of the closed-loop system with admissible voltages is attained.

Meso-scale fracture simulation using an augmented Lagrangian approach

A cohesive zone model implemented in an augmented Lagrangian functional is used for simulation of meso-scale fracture problems in this paper. The method originally developed by Lorentz is first presented in a rigorous variational framework. The equivalence between the stationary point of the one-field problem and the saddle point of the mixed formulation is proved by solving the double inequality of the mixed functional. An adaptation to simulate fracture phenomena in the meso-scale via mesh modification is also presented as an algorithm to insert zero-thickness interface elements based on Lagrange multipliers, boarding the non-trivial task of the field interpolation for different crack paths (plain and tortuous). A suitable tool to study the matrix fracture and debonding phenomena in composites with strongly different component stiffnesses that avoids ill-conditioning matrices associated with intrinsic cohesive zone models is obtained. The method stability is discussed using a simple patch test. Some numerical applications to fracture problems taking into account the mesostructure and, particularly, the study of transverse failure of longitudinal fiber reinforced epoxy and the fracture in concrete specimens are included in the paper. Comparing the numerical results with the experimental results obtained by other researchers, the paper introduces a discussion about the influence of coarse aggregate volume in meso-scale fracture mechanisms in concrete L-shaped specimens.

Effects of Large and Spread Explosives Loads

This paper presents experimental data and numerical analysis of the effects of detonating explosive charges ranging from 1000 to 26288 kg laid out on the ground. The charges consist of different ordnances widespread in a carpet-like form. Numerical analysis using ANSYS/Autodyn are carried out with a view to gain a better understanding of the blast loading resulting from the detonation of large masses of ordnance. Good correlation between the numerical results and experiments in terms of crater dimensions and blast wave parameters is obtained. The influence of the charge configurations on the blast wave parameters and crater shape are also investigated. While the cube root scaled distance works well for the evaluation of pressure and impulse values produced by a relatively compact charge layout, the scaled distance parameter has to be modified for cases where charges are spread in a carpet-like fashion.

Determinación de cargas generadas por explosiones en ambientes urbanos

El objetivo principal de este trabajo es la determinacion de la masa de explosivo y la ubicacion del foco de la explosion generada por un ataque terrorista en un ambiente urbano congestionado. La metodologia presentada es especialmente util cuando las dimensiones del crater generado por la explosion son inciertas o desconocidas. Se realizo un analisis computacional para la determinacion de presiones e impulsos en las fachadas de los edificios de una cuadra en un ambiente urbano real. Se realizaron simulaciones correspondientes a diferentes combinaciones de masa de explosivo - ubicacion del foco. Por otra parte, se realizo la evaluacion del dano producido en los edificios a traves de la utilizacion de las curvas de isodano que relacionan, en forma aproximada, las presiones e impulsos con los danos producidos. Posteriormente se definieron contornos de igual dano y se trazaron los mapas de isodano en toda la cuadra, para las diferentes alternativas analizadas. Por ultimo, a traves de la comparacion de los danos reales y los simulados computacionalmente, pudieron descartarse las alternativas no coincidentes.