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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.

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Cismasiu, C., J. R. G. Ferreira, and H. B. Rebelo. "Modelação tridimensional de ondas de choque em LS-DYNA." Construção Magazine.86 (2018): 24-28.
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Santos, F. A., H. Rebelo, M. Coutinho, L. S. Sutherland, C. Cismasiu, I. Farina, and F. Fraternali. "Low velocity impact response of 3D printed structures formed by cellular metamaterials and stiffening plates: PLA vs. PETg." Composite Structures (2020): 113128. AbstractWebsite

This work studies the low-velocity impact response of 3D-printed layered structures made of thermoplastic materials (PLA and PETg), which form sacrificial claddings for impact protection. The analyzed structures are composed of crushable cellular cores placed in between terminal stiffening plates. The cores tessellate either honeycomb hexagonal unit cells, or hexagonal cells with re-entrant corners, with the latter exhibiting auxetic response. The given results highlight that the examined PETg protectors exhibit higher energy dissipation ratios and lower restitution coefficients, as compared to PLA structures that have the same geometry. It is concluded that PETg qualifies as an useful material for the fabrication of effective impact protection gear through ordinary, low-cost 3D printers.

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.

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Mihali, A., H. B. Rebelo, C. Cismaşiu, and N. H. Shaker. "Impact of building model complexity on predicting external explosion consequences." Engineering Structures. 339 (2025): 120534. AbstractWebsite

Accurate blast models are essential for disaster management and emergency preparedness. Semi-empirical methods, which rely on free-field assumptions, struggle to provide precise data for complex building shapes because they ignore shock wave reflections. This research numerically investigates the effects of an explosion on a large building of complex geometrical configuration using blastFoam, estimating the façade damage and assessing risks to occupants. To evaluate the importance of accurately modelling the building’s geometry, four levels of detail were considered. Additionally, simulations were also performed using the Load Blast Enhanced (LBE) method from LS-DYNA to compare the estimates of this faster semi-empirical approach with those obtained through CFD analyses. The findings reveal that simpler CFD models are adequate for façade analysis and injury assessment around the building but fall short for predicting injury distribution within interior spaces or between buildings. The LBE underestimates both structural damage and human injury levels, while detailed CFD highlights the importance of accounting for interior walls and windows to enhance blast pressure predictions.

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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.

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.

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.

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Gomes, G. J., Válter Lúcio, and Corneliu Cismasiu. "Full Scale Blast testing of a High-Performance Energy-Absorbing System." PROTEDES2022. Região Sul da Ordem dos Engenheiros, Lisboa 2022.
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Mihali, Alin, Hugo B. Rebelo, and Corneliu Cismasiu. "Explosion consequences assessment in buildings with complex geometries." 19th International Symposium on Interaction of the Effects of Munitions with Structures. Bonn, Germany 2024.
Rebelo, Hugo Bento, Filipe Amarante dos Santos, Corneliu Cismasiu, and Duarte Santos. "Exploratory study on geodesic domes under blast loads." International Journal of Protective Structures (2019).
de Gomes, Gabriel Jesus, Valter José Guia da Lúcio, Corneliu Cismasiu, and José Luis Mingote. "Experimental Validation and Numerical Analysis of a High-Performance Blast Energy-Absorbing System for Building Structures." Buildings. 13.601 (2023): 1-20.
Gomes, G., H. Rebelo, V. Lúcio, C. Cismasiu, and J. Mingote. "Experimental Research and Development on Blast Resistant Structures." Advances on Testing and Experimentation in Civil Engineering, Springer Tracts in Civil Engineering. Switzerland: Springer Nature, 2023. 1-20.
Cismaşiu, C., A. Narciso, and F. Amarante dos Santos. "Experimental Dynamic Characterization and Finite Element Updating of a Footbridge Structure." Journal of Performance of Constructed Facilities.10.1061/(ASCE)CF.1943-5509.0000615 (2014).Website
Rebelo, H. B., A. Gregório, P. A. R. Rosa, D. Lecompte, and C. Cismaşiu. "Experimental characterization of 3D-printed PLA under uniaxial high strain-rate loading." AuxDefense 2018 – 1st World Conference on Advanced Materials for Defense. Lisbon 2018. auxdefense_extabstract_v2.pdf
Rebelo, H. B., D. Lecompte, C. Cismasiu, A. Jonet, B. Belkassem, and A. Maazoun. "Experimental and numerical investigation on 3D printed PLA sacrificial honeycomb cladding." International Journal of Impact Engineering. 131 (2019): 162-173.Website
Cismasiu, Corneliu, Filipe Amarante P. dos Santos, and Ana I. M. Rodrigues. "Experimental and FE updating techniques for the unseating vulnerability assessment of a footbridge structure." The 4th International Conference on Dynamics, Vibration and Control. Shanghai, China: Shanghai Institute of Applied Mathematics and Mechanics, 2014. icdvc_2014.pdf
Joaquim, Ana, Corneliu Cismasiu, Filipe Santos, and Elsa Caetano. "Estimation of the tensile force in the stay-cables of Salgueiro Maia Bridge using ambient vibration tests." ISDAC2017 - International Symposium on the Dynamics and Aerodynamics of Cables. Porto: FEUP, 2017. artigo_isdac_v5_ec.pdf
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Santos, Filipe, Corneliu Cismasiu, Ildi Cismasiu, and Chiara Bedon. "Dynamic Characterisation and Finite Element Updating of a RC Stadium Grandstand." Buildings. 8.10 (2018): 141, 1-19.
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.

Gomes, G. J., V. J. G. Lúcio, and C. Cismasiu. "Development of a high-performance blast energy-absorbing system for building structures." AuxDefense2022 - 3rd World Conference on Advanced Materials for Defense. Guimarães, Portugal 2022.
de Gomes, Gabriel Jesus, Valter José Guia da Lúcio, and Corneliu Cismasiu. "Development of a high-performance blast energy-absorbing system for building structures." International Journal of Protective Structures. 15.3 (2024): 484-508. AbstractWebsite

Shock absorbers have been widely used in the automotive and aeronautical industries for many years. Inspired on these devices, the paper presents an analytical and numerical assessment of a high performance protective system for building structures against blast loads, which is composed of a shielding element connected to the main structure, at the floor levels, through ductile Energy Absorbing Connectors (EACs). The EACs exploit the external tube inversion mechanism to absorb a significant part of the imparted kinetic energy from the blast wave. While the system prototype has been developed in laboratory, it was characterized and tested in a full-scale blast testing campaign. A validated finite element model was used next to analyze its performance in a more demanding design scenario. The introduction of EACs notably reduces the peak horizontal loads and the kinetic energy transferred to the protected structure, being expected a significant reduction of the stresses in the supporting vertical elements, in addition to the protection of structural and non-structural members. These results encourage further studies of the presented protective system that can be potentially employed for a large variety of blast threat scenarios, especially when increasing the stand-off is not a possible/viable option and sensitive facilities have to be protected.

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Amarante dos Santos, Filipe, Corneliu Cismasiu, and Francisco Braz Fernandes. "Cyclic Instability of Shape-Memory Alloys in Seismic Isolation Systems." International Journal of Structural Glass and Advanced Materials Research (2018): 1-14.