Publications

Export 56 results:
Sort by: Author Title Type [ Year  (Asc)]
1999
Freitas, J. A. T., C. Cismasiu, and Z. M. Wang. "Comparative analysis of hybrid-Trefftz stress and displacement elements." Archives of Computational Methods in Engineering. State of the art reviews. 6.1 (1999): 35-39.
Freitas, J. A. T., and C. Cismaşiu. "Numerical implementation of hybrid-{T}refftz displacement elements." Computers & Structures. 73 (1999): 207-225. Abstract

The numerical implementation of the displacement model of the hybrid-{T}refftz finite element formulation is presented. The geometry of the supporting element is not constrained a priori. Unbounded, non-convex and multiply connected elements can be used. The approximation basis is naturally hierarchical and very rich. It is constructed on polynomial solutions of the governing differential equation, and extended to include the particular terms known to model accurately important local effects, namely the singular stress patterns due to cracks or point loads. Numerical and semi-analytical methods are used to compute the finite element matrices and vectors, all of which present boundary integral expressions. Appropriate procedures to store, manipulate and solve symmetric highly sparse systems are used. The characteristics of the finite element solving system in terms of sparsity and conditioning are analysed, as well as its sensitivity to the effects of mesh distortion, incompressibility and rotation of the local reference systems. Benchmark tests are used also to illustrate the performance of the element in the estimation of displacements, stresses and stress intensity factors.

2000
Cismaşiu, C. The hybrid-{T}refftz Displacement Element for Static and Dynamic Structural Analysis Problems. Lisboa, Portugal: Instituto Superior Técnico, 2000. Abstract

The displacement model of the hybrid-{T}refftz finite element formulation is applied to the solution of geometrically and physically linear static and dynamic problems. As the approximation bases solve locally the governing system of differential equations, the errors in the approximation affect only the implementation of the boundary conditions. Potential and elastostatic problems are used to illustrate the enforcement of the boundary conditions and the convergence of the solutions in energy, stresses and displacements, under both p- and h-refinement sequences and their insensitivity to mesh distortion, incompressibility and positioning of the coordinate system of the approximation basis. Also illustrated is the use of elements with arbitrary geometry and the efficiency that can be reached by including in the bases the solutions associated with dominant local effects, in particular those associated with singular stress fields. An adaptive p-refinement algorithm that exploits the naturally hierarchical nature of the approximation bases is presented and assessed. The formulation is generalised for elastodynamic analysis in the frequency domain of both bounded and unbounded domains, which are modelled either with absorbing boundary conditions or with semi-infinite elements that satisfy the Sommerfeld condition. The performance of the formulation is illustrated with tests on the convergence of the solutions in energy, stresses and displacements and on their insensitivity to mesh distortion, wave length and position of the absorbing boundary, for a wide spectrum of forcing frequencies and under both p- and h-refinement sequences.

2001
Freitas, J. A. T., and C. Cismaşiu. "Developments with hybrid-{T}refftz stress and displacement elements." Computer Assisted Mechanics and Engineering Sciences. 8 (2001): 289-311. Abstract

The paper reports on the work on hybrid-{T}refftz finite elements developed by the Structural Analysis Research Group, ICIST, Technical University of Lisbon. A dynamic elastoplastic problem is used to describe the technique used to establish the alternative stress and displacement models of the hybrid-{T}refftz finite element formulations. They are derived using independent time, space and finite element bases, so that the resulting solving systems are symmetric, sparse, naturally $p$-adaptive and particularly well suited to parallel processing. The performance of the hybrid-{T}refftz stress and displacement models is illustrated with a number of representative static and dynamic applications of elastic and elastoplastic structural problems.

2002
2003
Freitas, J. A. T., and C. Cismaşiu. "Adaptive p-refinement of hybrid-{T}refftz finite element solutions." Finite Elements in Analysis and Design. 39 (2003): 1095-1121. Abstract

An adaptive p-refinement procedure for the implementation of the displacement model of the hybrid-{T}refftz finite element formulation is presented. The procedure is designed to select and implement automatically the degrees of freedom in the domain (displacements) and on the boundary (surface forces) of the element to attain a prescribed level of accuracy. This accuracy is measured on the strain energy of the system for a prescribed finite element mesh. Local measures of error can be easily accounted for. The performance of the adaptive procedure suggested is illustrated using two-dimensional potential problems.

Freitas, J. A. T., and C. Cismaşiu. "Hybrid-{T}refftz displacement element for spectral analysis of bounded and unbounded media." International Journal of Solids and Structures. 40 (2003): 671-699. Abstract

The hybrid-{T}refftz displacement element is applied to the elastodynamic analysis of bounded and unbounded media in the frequency domain. The displacements are approximated in the domain of the element using local solutions of the wave equation, the Neumann conditions are enforced directly and the surface forces are approximated on the Dirichlet and inter-element boundaries of the finite element mesh. Two alternative elements are developed to model unbounded media, namely a finite element with absorbing boundaries and an unbounded element that satisfies explicitly the Sommerfeld condition. The finite element equations are derived from the fundamental relations of elastodynamics written in the frequency domain. The numerical implementation of these equations is discussed and numerical tests are presented to assess the performance of the formulation.

Silva, M. A. G., C. Cismaşiu, and C. G. Chiorean. "Ballistic Simulation of Impact on Composite Laminates." Proceedings of the International Conference Constructions 2003. Vol. 2. Cluj-Napoca, Romania: The Technical University of Cluj-Napoca, 2003. 139-146. Abstract

The paper reports on numerical simulation of impact problems on fiber reinforced plastic composite laminated plates reinforced with Kevlar 29. The ballistic impact caused by STANAG-2920 projectile is analyzed to obtain an estimate for the V50 and the global damage. All estimate have been carried out using the finite difference numerical code AUTODYN-3D, are compared with the experimental data to illustrate the performance of the simulation. Good correlation between resulting simulations and experimental results is demonstrated both in terms of deformation and damage of the laminates and ballistic performance.

Silva, M. A. G., C. Cismaşiu, and C. G. Chiorean. "Low velocity impact on laminates reinforced with {P}olyethylene and {A}ramidic fibres." Computational Methods in Engineering and Science. Proceedings of the 9th International Conference EPMESC IX. Eds. V. P. Iu, L. N. Lamas, Y. - P. Li, and K. M. Mok. Macao, China: A.A.Balkema Publishers, 2003. 843-849. Abstract

The present study reports low velocity impact tests on composite laminate plates reinforced either with Kevlar 29 or Dyneema. The tests are produced using a Rosand Precision Impact tester. The experimental results obtained for Kevlar 29 are simulated numerically. The deflection history and the peak of the impact force are compared with experimental data and used to calibrate the numerical model.

2004
2005
Silva, M. A. G., C. Cismaşiu, and C. G. Chiorean. "Numerical simulation of ballistic impact on composite laminates." International Journal of Impact Engineering. 31 (2005): 289-306. Abstract

The paper reports experimental and numerical simulation of ballistic impact problems on thin composite laminated plates reinforced with Kevlar 29. Ballistic impact was imparted with simulated fragments designed in accordance with STANAG-2920 on plates of different thickness. Numerical modelling was developed and used to obtain an estimate for the limit perforation velocity V50 and simulate failure modes and damage. Computations were carried out using a commercial code based on nite differences and values obtained are compared with the experimental data to evaluate the performance of the simulation. Good correlation between computational simulation and experimental results was achieved, both in terms of deformation and damage of the laminates. Future work is advanced to include the interposition of an outer ceramic layer as well as examining the influence of dry-wet and temperature cycles on the mechanical strength of the plates and their temporal evolution under accelerated ageing.

2007
Santos, F. P., and C. Cismasiu. "Shape memory alloys in structural vibration control." EVACES'07 - Experimental Vibration Analysis for Civil Engineering Structures. FEUP, Porto, Portugal 2007. Abstract

The unique superelastic behaviour exhibited by shape memory alloys (SMAs) allows the material to recover after withstanding large deformations. This recovery takes place without any residual strains, while dissipating a considerable amount of energy. This property makes the SMAs particularly suitable for applications in vibration control devices. Numerical models, calibrated with experimental laboratory tests, are used to investigate the dynamic response of vibration control devices. These devices are built up of austenitic superelastic wires. The energy dissipation and re-centring capabilities, important features of these devices, are clearly illustrated by the numerical tests. One of these devices is tested as a seismic passive vibration control system in a simplified numerical model of a railway viaduct.

2008
Cismasiu, Corneliu, and Filipe Amarante P. dos Santos. "Numerical simulation of superelastic shape memory alloys subjected to dynamic loads." Smart Materials and Structures. 17 (2008): 025036 (12pp). AbstractWebsite

Superelasticity, a unique property of shape memory alloys (SMAs), allows the material to recover after withstanding large deformations. This recovery takes place without any residual strains, while dissipating a considerable amount of energy. This property makes SMAs particularly suitable for applications in vibration control devices. Numerical models, calibrated with experimental laboratory tests from the literature, are used to investigate the dynamic response of three vibration control devices, built up of austenitic superelastic wires. The energy dissipation and re-centering capabilities, important features of these devices, are clearly illustrated by the numerical tests. Their sensitivity to ambient temperature and strain rate is also addressed. Finally, one of these devices is tested as a seismic passive vibration control system in a simplified numerical model of a railway viaduct, subjected to different ground accelerations.

dos Santos, Amarante F. P., and C. Cismasiu. "Comparison Between Two {SMA} Constitutive Models for Seismic Applications." Twelfth Conference on Nonlinear Vibrations, Dynamics, and Multibody Systems. Blacksburg, VA 24061 2008. Abstract
n/a
2010
dos Santos, Amarante F. P., and C. Cismasiu. "Comparison Between Two SMA Constitutive Models for Seismic Applications." Journal of Vibration and Control. 16 (2010): 897-914. AbstractWebsite

This paper analyses and compares the dynamic behavior of superelastic shape memory alloy (SMA) systems based on two different constitutive models. The first model, although being able to describe the response of the material to complex uniaxial loading histories, is temperature and rate independent. Thesecond model couples the mechanical and kinetic laws of the material with a balance equation considering the thermal effects. After numerical validation and calibration, the behavior of these two models is tested in single degree of freedom dynamic systems, with SMAs acting as restoring elements. Different dynamic loads are considered, including artificially generated seismic actions, in a numerical model of a railway viaduct. Finally, it is shown that, in spite of its simplicity, the temperature- and rate-independent modelproduces a set of very satisfying results. This, together with its robustness and straightforward computational implementation, yields a very appealing numerical tool to simulate superelastic passive control applications.

Cismasiu, Corneliu, and Filipe Amarante Dos P. Santos. "Shape Memory Alloys." Ed. Book Corneliu edited by: Cismasiu. ISBN: 978-953-307-106-0. Croatia: Scyio, Publishing, 2010. 127-154. Abstract
n/a
by Cismasiu, Edited Corneliu Shape Memory Alloys. Scyio, 2010.Website
2011
de Freitas, J., I. Moldovan, and C. Cismaşiu. "Hybrid-Trefftz displacement element for poroelastic media." Computational Mechanics (2011): 1-15. AbstractWebsite

The elastodynamic response of saturated poroelastic media is modelled approximating independently the solid and seepage displacements in the domain and the force and pressure components on the boundary of the element. The domain and boundary approximation bases are used to enforce on average the dynamic equilibrium and the displacement continuity conditions, respectively. The resulting solving system is Hermitian, except for the damping term, and its coefficients are defined by boundary integral expressions as a Trefftz basis is used to set up the domain approximation. This basis is taken from the solution set of the governing differential equation and models the free-field elastodynamic response of the medium. This option justifies the relatively high levels of performance that are illustrated with the time domain analysis of unbounded domains.

dos Santos, Amarante F. P., and C. Cismasiu. "Bridge hinge-restrainers built up of NiTi superelastic shape-memory alloys." Smart Structures and Materials (SMART'11). 5th ECCOMAS Thematic Conference on Smart Structures and Materials SMART'11. Saarbrücken, Germany 2011. Abstractsantos_2011.pdf

n/a

2012
Cismasiu, C., and Amarante F. P. dos Santos. "Towards a semi-active vibration control solution based on superelastic shape memory alloys." 15th WCEE. Lisbon, Portugal 2012. 2012_wcee_0379.pdf
2013
dos Santos, Amarante F. P., and C. Cismasiu. "Bridge Hinge-Restrainers Built up of NITI Superelastic Shape-Memory Alloys." New Trends in Smart Technologies . Eds. Christian Boller, and Hartmut Janocha. Saarbrücken: Fraunhofer Verlag, 2013. 195-203.
and Amarante dos Santos, F. P. and Cismaşiu, Pamies Teixeira C. J. "Semi-active vibration control device based on superelastic NiTi wires." Structural Control and Health Monitoring. 20 (2013): 890-902.Website
Cismasiu, Corneliu, and Filipe Pimentel Amarante dos Santos. "Shape Memory Alloys in Structural Vibration Control. Research at UNIC/DEC/FCT/UNL." International Conference "Tradition and Innovation". 60 Years of Civil Engineering Higher Education in Transilvania. Cluj-Napoca, Romania: UTCN, 2013. c60.pdf