2,879 research outputs found
Explicit finite element implementation of a shape memory alloy constitutive model and associated analyses
Shape memory alloys (SMA) represent an important class of smart metallic materials employed in various innovative applications thanks to their unique thermomechanical behavior. Since the 1980s, several SMA constitutive models have been proposed and implemented into both commercial and academic ïŹnite element analysis software tools. Such models have demonstrated their reliability and robustness in the design and optimization of a wide variety of SMA-based components. However, most models are implemented using implicit integration schemes, thus limiting their applicability in highly nonlinear analyses. The objective of this work is to present a novel explicit integration scheme for the numerical implementation of the three-dimensional Souza-Auricchio model, a phenomenological model able to reproduce the primary SMA responses (i.e., pseudoelasticity and shape memory eïŹect). The model constitutive equations are formulated by adopting the continuum thermodynamic theory with internal variables, following a plasticity-like approach. An elastic predictor-inelastic corrector scheme is here used to solve the time-discrete non-linear constitutive equations in the explicit framework. The proposed algorithm is investigated through several benchmark boundary-value problems of increasing complexity, considering both pseudoelastic and shape memory response in quasi-static conditions; a comparison with an implicit integration scheme is also performed. Such numerical tests demonstrate the ability of the algorithm to reproduce key material behaviors with eïŹectiveness and robustness. Particularly, the analysis of SMA cables demonstrates the eïŹectiveness of the explicit algorithm to solve complex problems involving widespread nonlinear contact, which prevent the convergence of the implicit scheme. Details such as mass-scaling options are also discussed
Multi-objective optimization of nitinol stent design
Nitinol stents continuously experience loadings due to pulsatile pressure, thus a given stent design should possess an adequate fatigue strength and, at the same time, it should guarantee a sufficient vessel scaffolding. The present study proposes an optimization framework aiming at increasing the fatigue life reducing the maximum strut strain along the structure through a local modification of the strut profile.The adopted computational framework relies on nonlinear structural finite element analysis combined with a Multi Objective Genetic Algorithm, based on Kriging response surfaces. In particular, such an approach is used to investigate the design optimization of planar stent cell.The results of the strut profile optimization confirm the key role of a tapered strut design to enhance the stent fatigue strength, suggesting that it is possible to achieve a marked improvement of both the fatigue safety factor and the scaffolding capability simultaneously. The present study underlines the value of advanced engineering tools to optimize the design of medical devices
Lamotrigine versus valproic acid as first-line monotherapy in newly diagnosed typical absence seizures: an open-label,randomized, parallel-group study
Purpose: To compare the efficacy of lamotrigine
(LTG) and valproic acid (VPA) in newly diagnosed children and
adolescents with typical absence seizures.
Methods: A randomized, open-label parallel-group design
was used. After undergoing an awake video-EEG recording,
which included one to two trials of 3 min of hyperventilation
and intermittent photic stimulation, eligible patients were randomized
to receive LTG or VPA. LTG was initiated at a daily
dose of 0.5 mg/kg for 2 weeks in two divided doses, followed by
1.0 mg/kg/day for an additional 2 weeks. Thereafter, doses were
increased in 1-mg/kg/day increments every 5 days until seizures
were controlled, intolerable adverse effects occurred, or a maximum
dose of 12 mg/kg/day had been reached. VPA was equally
uptitrated according to clinical response, starting at 10 mg/kg
and increasing by 5 mg/kg/24 h every 3 days, if required, to a
maximum of 30 mg/kg/day in three divided doses. Patients were
seen in the clinic every month for â€12 months.The primary efficacy
end point at each visit was seizure freedom, defined as lack
of clinically observed seizures since the previous visit and lack
of electroclinical seizures during ambulatory 24-h EEG testing
and a video-EEG session with hyperventilation.
Results: Thirty-eight children (17 boys, 21 girls), aged from 3
to 13 years (mean, 7.5 years), all newly diagnosed with childhood
or juvenile typical absence seizures, were enrolled. After 1 month
of treatment, 10 (52.6%) of 19 children taking VPA and one
(5.3%) of 19 taking LTG were seizure free (p = 0.004). By
the 3-month follow-up, 12 (63.1%) children taking VPA and
seven (36.8%) taking LTG were controlled (p = 0.19). After 12
months, 13 children taking VPA (dose range, 20â30 mg/kg/day;
mean serum level, 76.8 mg/L; range, 51.4â91 mg/L) and 10
taking LTG (dose range, 2â11 mg/kg/day; mean serum level,
8.1 mg/L; range, 1.1â18 mg/L) were seizure free (p=0.51). Side
effects were mostly mild and transient and were recorded in two
(10.6%) children treated with VPA and in six (31.8%) treated
with LTG.
Conclusions: Both VPA and LTG can be efficacious against
absence seizures, although VPA shows a much faster onset of
action, at least in part because of its shorter titration schedule.
KeyWords: LamotrigineâValproic acidâTypical absencesâ
Monotherapy.
Valproic acid (VPA) and ethosuximide (ESM) have
been shown to be equally effective as monotherapy for
typical absence seizures (1,2), and, at present, they are generally
considered first-choice drugs for this seizure type.
VPA controls absences inâŒ75% of patients, in addition to
being effective against generalized tonicâclonic seizures
(70%) and myoclonic seizures (75%). However, its use
may involve safety risks for postmenarchal women (3).
ESM produces complete control of absences in 70% of
treated patients (4,5), but it is unsuitable as monotherapy
Accepted Ma
A focal plane detector design for a wide-band Laue-lens telescope
The energy range above 60 keV is important for the study of many open
problems in high energy astrophysics such as the role of Inverse Compton with
respect to synchrotron or thermal processes in GRBs, non thermal mechanisms in
SNR, the study of the high energy cut-offs in AGN spectra, and the detection of
nuclear and annihilation lines. Recently the development of high energy Laue
lenses with broad energy bandpasses from 60 to 600 keV have been proposed for a
Hard X ray focusing Telescope (HAXTEL) in order to study the X-ray continuum of
celestial sources. The required focal plane detector should have high detection
efficiency over the entire operative range, a spatial resolution of about 1 mm,
an energy resolution of a few keV at 500 keV and a sensitivity to linear
polarization. We describe a possible configuration of the focal plane detector
based on several CdTe/CZT pixelated layers stacked together to achieve the
required detection efficiency at high energy. Each layer can operate both as a
separate position sensitive detector and polarimeter or work with other layers
to increase the overall photopeak efficiency. Each layer has a hexagonal shape
in order to minimize the detector surface required to cover the lens field of
view. The pixels would have the same geometry so as to provide the best
coupling with the lens point spread function and to increase the symmetry for
polarimetric studies.Comment: 10 pages, 9 figure
The gamma-ray burst monitor for Lobster-ISS
Lobster-ISS is an X-ray all-sky monitor experiment selected by ESA two years
ago for a Phase A study (now almost completed) for a future flight (2009)
aboard the Columbus Exposed Payload Facility of the International Space
Station. The main instrument, based on MCP optics with Lobster-eye geometry,
has an energy passband from 0.1 to 3.5 keV, an unprecedented daily sensitivity
of 2x10^{-12} erg cm^{-2}s$^{-1}, and it is capable to scan, during each orbit,
the entire sky with an angular resolution of 4--6 arcmin. This X-ray telescope
is flanked by a Gamma Ray Burst Monitor, with the minimum requirement of
recognizing true GRBs from other transient events. In this paper we describe
the GRBM. In addition to the minimum requirement, the instrument proposed is
capable to roughly localize GRBs which occur in the Lobster FOV (162x22.5
degrees) and to significantly extend the scientific capabilities of the main
instrument for the study of GRBs and X-ray transients. The combination of the
two instruments will allow an unprecedented spectral coverage (from 0.1 up to
300/700 keV) for a sensitive study of the GRB prompt emission in the passband
where GRBs and X-Ray Flashes emit most of their energy. The low-energy spectral
band (0.1-10 keV) is of key importance for the study of the GRB environment and
the search of transient absorption and emission features from GRBs, both goals
being crucial for unveiling the GRB phenomenon. The entire energy band of
Lobster-ISS is not covered by either the Swift satellite or other GRB missions
foreseen in the next decade.Comment: 6 pages, 4 figures. Paper presented at the COSPAR 2004 General
Assembly (Paris), accepted for publication in Advances in Space Research in
June 2005 and available on-line at the Journal site
(http://www.sciencedirect.com/science/journal/02731177), section "Articles in
press
Topiramate in children and adolescents with epilepsy and mental retardation: a prospective study on behavior and cognitive effects.
The aim of the present study was to assess the behavioral and cognitive effects following treatment with topiramate in children and
adolescents with epilepsy with mild to profound mental retardation. The study group comprised 29 children, 16 males and 13 females,
aged 3 to 19 years, affected by partial (4) and generalized (25) crypto/symptomatic epilepsy and mental retardation (7 mild, 5 moderate,
15 severe, 2 profound), who were administered topiramate (TPM) as add-on therapy to their baseline antiepileptic treatment. At baseline,
3 months, 6 months, and 12 months, parents or caregivers of each patient were administered a questionnaire based on the Holmfrid
Quality of Life Inventory. After a 3-month follow-up, the add-on topiramate caused overall mild to moderate cognitive/behavioral worsening
in about 70% of children and adolescents with mental retardation and epilepsy. After 6 and 12 months of follow-up, global worsening
persisted in 31 and 20.1% of cases, respectively. In conclusion, this trial confirms that TPM can have significant adverse cognitive
and behavioral side effects, even in mentally disabled children and adolescents.
2007 Elsevier Inc. All rights reserved
Reversible Stress-Driven and Stress-Free Two-Way Shape Memory Effect in a Sol-Gel Crosslinked Polycaprolactone
The two-way shape memory effect is the ability of a material to change its shape between two configurations upon application and removal of a stimulus, and, among shape memory polymers, it is featured only by few systems, such as semicrystalline networks. When studied under tensile conditions, it consists of elongation-contraction cycles along cooling and heating across the crystallization and melting region, typically under the application of a constant load. However, recent studies on crosslinked semicrystalline co-polymers demonstrate that also a completely stress-free, or self-sustained, two-way effect may be achieved through specific thermomechanical cycles. This effect is currently regarded with interest for the development of intrinsically reversible sensors and actuators, and it may also be displayed by simpler materials, as homopolymer-based semicrystalline networks. Only seldom articles investigate this possibility, therefore in this work the two-way shape memory behavior is studied on a poly(e-caprolactone) system, crosslinked by means of a sol-gel approach. The effect is studied both under stress-driven and stress-free condition, by applying properly set-up thermo-mechanical histories. The results allow to describe the effect as a function of temperature, to reveal the dependence on specific testing parameters and to compare the extent of the reversible strain variation under these two conditions
Stress-Free Two-Way Shape Memory Effect of Poly(ethylene glycol)/ Poly(epsilon-caprolactone) Semicrystalline Networks
In this work, poly(ethylene glycol) (PEG)/poly(epsilon- caprolactone) (PCL) semicrystalline networks were prepared by photo-cross-linking of methacrylated macromonomers with different molecular weights and in different proportions to obtain amphiphilic materials capable of displaying properly designed shape memory effects. Networks based on PCL 10 kDa and PEG 3 kDa showed suitable thermal and mechanical properties with well-separated crystallization and melting regions to achieve a self-standing two-way shape memory effect. Particularly, after the application of a specific thermomechanical history, these materials are capable of cyclically changing their shape between two configurations upon cooling-heating cycles in the absence of any external load applied. The effect of the composition of the networks and of the employed thermomechanical parameters, such as the applied strain and the actuation temperature, was investigated to shed light on the shape memory mechanism for this class of materials, which are considered promising for applications in the biomedical field and as reversible actuators for soft robotics
Development status of a Laue lens project for gamma-ray astronomy
We report the status of the HAXTEL project, devoted to perform a design study
and the development of a Laue lens prototype. After a summary of the major
results of the design study, the approach adopted to develop a Demonstration
Model of a Laue lens is discussed, the set up described, and some results
presented.Comment: 11 pages, 11 figures, 2007 SPIE Conference on Optics for EUV, X-Ray,
and Gamma-Ray Astronomy II
Multi-material 3D printed shape memory polymer with tunable melting and glass transition temperature activated by heat or light
Shape memory polymers are attractive smart materials that have many practical applications and academic interest. Three-dimensional (3D) printable shape memory polymers are of great importance for the fabrication of soft robotic devices due to their ability to build complex 3D structures with desired shapes. We present a 3D printable shape memory polymer, with controlled melting and transition temperature, composed of methacrylated polycaprolactone monomers and N-Vinylcaprolactam reactive diluent. Tuning the ratio between the monomers and the diluents resulted in changes in melting and transition temperatures by 20, and 6 °C, respectively. The effect of the diluent addition on the shape memory behavior and mechanical properties was studied, showing above 85% recovery ratio, and above 90% fixity, when the concentration of the diluent was up to 40 wt %. Finally, we demonstrated multi-material printing of a 3D structure that can be activated locally, at two different temperatures, by two different stimuli; direct heating and light irradiation. The remote light activation was enabled by utilizing a coating of Carbon Nano Tubes (CNTs) as an absorbing material, onto sections of the printed objects
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