Branca Freitas de Oliveira

Viscoelastic failure analysis of composite plates and shells

The present paper describes a numerical method for modeling the failure behavior of composite laminates in the presence of large displacements and creep. Geometrical nonlinear finite elements are used for the discretization. The modeling of material behavior includes thermal, hygroscopic and viscoelastic effects, using an efficient state variable representation. Incremental damage is determined and used to calculate a modified stiffness matrix. Thus, the procedure can be used to analyze, for example, buckling, creep buckling and creep buckling including damage. A detailed formulation, computational procedures and examples to check the accuracy of the code are presented.

Modeling of the Mechanical Behavior of Metallic Foams: Damage Effects at Finite Strains

This work applies Gurson model to the simulation of damage in metallic foams. In the first part, two approaches for Gurson model application, i.e., the hypoelastic and the hyperelastic formulations are compared. Then, a study on the minimum number of cells to be employed in an experimental test to adequately simulate the behavior of the cellular material in tension is presented. Finally, considering that compression is the dominant loading in impact situations, a simulation including damage effects of a compression test on a cellular metal sample is shown and compared with experimental results.

Nonlinear viscoelastic analysis of thin-walled beams in composite material

Abstract Laminated composites of polymeric matrix show anisotropic viscoelastic behaviour, enhanced by temperature and humidity effects. The consideration of anisotropy and viscoelasticity are important for the determination of deformations and, as a consequence, of deformation-related phenomena, as elastic and creep buckling. This paper studies the behaviour of thin-walled beams of composite material under flexure and buckling, taking account of creep effects. The analysis uses a nonlinear viscoelastic finite element code with shell elements, whose basic formulation is given. The use of shell elements allows a better representation of constitutive properties and boundary conditions. Comparison with available analytical results is made for several cases like flexure of an I beam, buckling of beam columns and lateral buckling of this beams. The results show good correlation.

Finite element simulation of a rollover protective structure

Purpose – The purpose of this paper is to present a finite element simulation to validate the strength and energy absorption capacity of a rollover protective structure (ROPS) of agricultural tractors. The test consists of four steps: rear loading, crushing of the rear columns, side loading and crushing of the front columns. In this study a new design of a cabin for narrow tractors was simulated and from it the computational test was run for validation of the cabin. The simulation was performed using ANSYS software, considering the nonlinear characteristics of the materials, since during the test the plastic limit is reached. With the computational results, it was possible to predict the behavior of the structure before the real test. These results were used to propose design and materials changes that significantly improved the energy absorption, making it more efficient. The proposed cabin design reaches the energies and forces required in each step of the computational simulation of the ROPS test and t...

Aging Damage Effects on Composite Structures

High performance composite materials have many well‐known structural advantages. On the other hand, questions about durability and aging are still a subject of discussion and much needed research. As a consequence, in the last few years there have been available a growing amount of information on composites aging considering effects as temperature, oxidation, UV radiation, permanent loading etc. Much of this research is performed at the materials level, by materials scientists and it is not directly applicable to the design stage. In this paper, we use an analytical‐numerical framework for the compilation, interpretation and application of experimental data to actual engineering analysis and design. The formulation includes elastic anisotropic relations; aging viscoelastic anisotropic constitutive equations in terms of state variables; age‐adjusted failure and degradation criteria; all in a setting of large displacements with small strains. These equations are set in a form adequate for numerical analysis...

An analytical and numerical framework for evaluating axial compression of finite dimensional metallic foams by generalization of buckling effects

ABSTRACT In this work, the overall behavior of finite dimensional metallic foam specimens under axial compression is derived from a mixed analytical and numerical framework based on the buckling pattern that the walls of the foam exhibit in this situation. Instead of a full simulation of the entire foam, the present approach requires the simulation of three compression cases of representative volume elements and works on a finite dimensional context instead of the infinite periodic boundary consideration. Results obtained with the present framework are compared with those obtained using full finite element simulation, exhibiting excellent agreement even at very large strains.

Metallic Foam Density Distribution Optimization Using Genetic Algorithms and Voronoi Tessellation

Metallic foams have a very particular structure due to their high specific stiffness. Density plays an important role on their structural response and is also determinant to the foam’s weight. The main goal of this paper is to find an ideal density distribution to open-cell metallic foams in order to achieve optimized structural performance. A density distribution optimization using an irregular description of the foam by a Voronoi tessellation and a genetic algorithm for the numerical optimization is presented in this work. The structural analysis is performed with linear elastic beam finite elements and the foam structure is modeled as a Voronoi tessellation. The density is related to the number of Voronoi seeds, which may configure lighter or denser foams and vary throughout the model. The minimization and maximization of stiffness were analyzed for different structural applications in order to demonstrate the capability of the developed methodology.

Compression Behaviour of Finite Dimensional Cellular Metals by Generalization of Cell Buckling Effects

Metal foams are materials of recent developments and applications that show interesting combinations of physical and mechanical properties. Foams are commonly used as of passive safety components due to their high capacity of energy absorption under impact conditions. In this work the foam is represented as a cellular material with a regular structure and specimens of a cellular metal are used to study the foam behaviour. Considering that compression is the dominant loading in impact situations, the deformation behaviour of finite dimensional cellular metal specimens under compression is investigated. The specimen deformation configuration is determined by means of fundamental buckling effects on cells walls evaluated from simple representative volume elements. Damage effects under a finite strain context are included together with self-contact considerations. The overall behaviour of a finite specimen is derived from an analytical and numerical framework based on the boundary conditions present on the foam. The main advantage of this method is the capability of determine the full behaviour of a complex foam configuration with only simple case analyses, with low computational cost.

Computational Analysis of Loading–Unloading and Non-homogeneity Effects in Metallic Hollow Sphere Structures

Cellular metals can be applied in crash absorbers, heat exchangers and heat isolators, lightweight structures, acoustic and vibration damping. Cellular materials with a fairly uniform microstructure may be obtained with metallic hollow spheres welded or bonded. Such materials are known as MHSS (Metallic Hollow Sphere Structures). Crash absorbers, that act in compression and can be severely deformed, suffer severe deformation and some level of ductile damage can be expected. To consider this damage effect, the Gurson model is employed in this paper. Numerical simulation via finite elements is employed to study the mechanical behaviour of MHSS, focusing on two major aspects: first, the effect of previous compression and load reversal on voids nucleation; second, the consideration of nonuniform material properties of the metallic spheres. Numerical results are compared with experimental data.