Browsing by Author "Valido, A. J."
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- Biomechanical and physicochemical characterization of the rat’s osteoporotic bonePublication . Santos, C.; Carmezim, M. J.; Valido, A. J.; Colaço, B.; Fernandes, M. H.; Gomes, P.Osteoporosis is a clinical condition characterized by an increased risk of bone fracture. Structural fragility seems to result from a reduction in bone mass due to a decreased mineral content and impaired/altered protein synthesis. Despite the established knowledge on osteoporosis, little is known on how these compositional changes may affect the bone microarchitecture throughout different bones, and how these correlate with the attained increase of the fracture risk. This work aims to examine the relation between bone material properties and skeletal fragility in the femur, tibia, maxilla, mandible and calvarial structure of an animal model representative of the human osteoporotic condition, i.e., the ovariectomized Wistar rat. Two groups of animals, with 3 and 6 months following the induction of the osteoporotic condition were established, and bone tissue properties were compared to age-matched healthy controls. The biomechanical performance of different bones was tested by three-point bending. Structural (load-bearing capacity and stiffness), geometric (cross-sectional area, cortical sectional area, and moment of inertia) and material (modulus of elasticity and maximum elastic stress) properties were also evaluated, while morphological characterization was performed by FEGSEM. The organic content was determined by ash measurements and confocal Raman microscopy was used to evaluate compositional differences. Reported analyses showed that osteoporotic bones presented significant mechanical and morphological differences when compared with those from age-matched controls. Moreover, osteoporotic bones displayed a high ultimate tensile strength and were found to be more brittle than control bones. Attained differences could be related to the reduction in the collagen content, verified in osteoporotic conditions. These results highlight the role of the collagenous component in the structural and material properties of trabecular bone, and may account for the attained reduced bone mass in osteoporotic conditions.
- Limiting performance analysis of a vehicle restraint systemPublication . Moita, Paulo; Cardoso, J. B.; Valido, A. J.
- Optimal design of thin-walled laminated beams with geometrically nonlinear behaviourPublication . Valido, A. J.; Cardoso, João B.; Moita, Paulo P.The purpose of this paper is to present a finite element model for optimal design of composite laminated thin-walled beam structures, with geometrically nonlinear behaviour, including post-critical behaviour and accounting warping deformation. A general continuum formulation is presented for the structural nonlinear analysis, based on the virtual work principle, and using the Updated Lagrangean procedure to describe the deformation of the structure. In order of defining the post-critical behaviour, a generalized displacement control method has been implemented. The thin-walled beam cross-section is considered as made from an assembly of flat layered laminated composite panels. The cross-section bending-torsion properties are integrals based on the cross-section geometry, on the warping function and on the individual stiffness of the laminates that constitute the cross-section. In order to determine its bending-torsion properties, the cross-section geometry is discretized by quadratic isoparametric finite elements. Along its axial direction, the beam is modelled throughout two-node Hermitean finite elements with seven degrees-of-freedom a node. Design sensitivities are imbedded into the finite element modelling and assembled in order to perform the design sensitivity analysis of various structural performance measures by using the adjoint method. As design variables one considers laminate thickness, lamina orientations and the global cross-section geometry. This geometry is defined by the position of master nodes related to the cross-section finite element mesh. Design optimization is performed throughout nonlinear programming techniques.