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Vólos, Greece

Bassi A.,University of Brescia | Aravas N.,University of Thessaly | Aravas N.,Mechatronics Institute | Genna F.,University of Brescia
European Journal of Mechanics, A/Solids | Year: 2012

A methodology for the numerical solution of discretized boundary value problems that involve rate-independent, elastic-plastic finite-strain models is developed. The formulation is given in terms of a structural Linear Complementarity Problem. A methodology for the determination of bifurcation and limit points along an equilibrium path is described. The proposed method is suited particularly for plasticity models that involve yield surfaces with singular points (corners, edges, apexes, etc.). © 2011 Elsevier Masson SAS. All rights reserved.


Bassi A.,University of Brescia | Aravas N.,University of Thessaly | Aravas N.,Mechatronics Institute | Genna F.,University of Brescia
European Journal of Mechanics, A/Solids | Year: 2012

A methodology for the numerical integration of rate-independent, elastic-plastic finite-strain models is developed. The methodology is based on the idea of local linearization of the yield surface that was proposed in Maier (1969), adopted as the basis for an integration scheme in Hodge (1977), and developed further in Franchi and Genna (1984, 1987), so far for small-strain problems only. The proposed algorithm is based on the solution of a local Linear Complementarity Problem and is suited particularly for plasticity models that involve yield surfaces with singular points (corners, edges, apexes, etc.). © 2011 Elsevier Masson SAS. All rights reserved.


Danas K.,Laboratoire Of Mecanique Des Solides | Aravas N.,University of Thessaly | Aravas N.,Mechatronics Institute
Composites Part B: Engineering | Year: 2012

A new constitutive model for elasto-plastic (rate-independent) porous materials subjected to general three-dimensional finite deformations is presented. The new model results from simple modifications of an earlier model of Kailasam and Ponte Castañeda (1997, 1998) [40,41] so that it reproduces the exact spherical and cylindrical shell solution (composite sphere and composite cylinder assemblage) under purely hydrostatic loadings, while predicting (by calibration) accurately the void shape evolution according to the recent "second-order" model of Danas and Ponte Castañeda [17]. Furthermore, the present model is based on a rigorous homogenization method which is capable of predicting both the constitutive behavior and the microstructure evolution of porous materials. The microstructure is described by voids of arbitrary ellipsoidal shapes and orientations and as a result the material exhibits deformation-induced (or morphological) anisotropy at finite deformations. This is in contrast with the well-known Gurson [32] model which assumes that the voids remain spherical during the deformation process and thus the material remains always isotropic. The present model is implemented numerically in a finite element program where a three-dimensional thin-sheet (butterfly) specimen is subjected to a combination of shear and traction loading conditions in order to examine the effect of stress triaxiality and shearing upon material failure. The ability of the present model to take into account the nontrivial evolution of the microstructure and especially void shape effects leads to the prediction of material failure even at low stress triaxialities and small porosities without the use of additional phenomenological damage criteria. At high stress triaxialities, the present model gives similar predictions as the Gurson model. © 2011 Elsevier Ltd. All rights reserved.


Toumanidou Th.,Institute for Bioengineering of Catalonia | Spyrou L.A.,Mechatronics Institute | Aravas N.,Mechatronics Institute | Aravas N.,University of Thessaly
10th International Workshop on Biomedical Engineering, BioEng 2011 | Year: 2011

Mechanical characteristics of orthopaedic fixators, such as stiffness and stability, directly influence the mechanobiological environment in which the bone is healed. A comprehensive understanding of the sensitivity of the Ilizarov Apparatus (IA) to configuration parameters is the key to its biomechanical analysis. In this study, detailed 3D Finite Element (FE) models of two-ring and four-ring configurations of the IA are developed and studied with respect to axial stiffness. A parametric analysis is carried out on the four-ring configuration and the effects of the ring diameter, the wire diameter, and pretension level on the frame stiffness are studied. Both configurations are found to exhibit a non-linear stiffness behavior under axial compression. It is found that the addition of levels of fixations, i.e., rings and wires, augments fixator's stability. The analysis shows that decreasing the rings size and increasing the wires diameter and pretension levels provides greater frame stiffness. The results are in good agreement with the findings reported in literature. © 2011 IEEE.


Spyrou L.A.,University of Thessaly | Spyrou L.A.,Mechatronics Institute | Aravas N.,University of Thessaly | Aravas N.,Mechatronics Institute
Journal of Applied Mechanics, Transactions ASME | Year: 2011

A three-dimensional constitutive model for muscle and tendon tissues is developed. Muscle and tendon are considered as composite materials that consist of fibers and the connective tissues and biofluids surrounding the fibers. The model is nonlinear, rate dependent, and anisotropic due to the presence of the fibers. Both the active and passive behaviors of the muscle are considered. The muscle fiber stress depends on the strain (length), strain-rate (velocity), and the activation level of the muscle, whereas the tendon fiber exhibits only passive behavior and the stress depends only on the strain. Multiple fiber directions are modeled via superposition. A methodology for the numerical implementation of the constitutive model in a general-purpose finite element program is developed. The current scheme is used for either static or dynamic analyses. The model is validated by studying the extension of a squid tentacle during a strike to catch prey. The behavior of parallel-fibered and pennate muscles, as well as the human semitendinosus muscle, is studied. © 2011 American Society of Mechanical Engineers.

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