1,721,370 research outputs found
Residual Stresses Induced by Cold Expansion of Adjacent and Cut-Out Holes
No full textFatigue life of fastener holes can be enhanced via a cold-expansion process to introduce a compressive residual stress field around the hole edge and to reduce crack growth propagation. Considering that aerospace components contain multiple rows of holes, the present investigation focuses on the evaluation of the three-dimensional residual stress distribution in adjacent cold-expanded (CE) holes. The redistribution of residual stresses caused by a cut introduced between two adjacent holes was also investigated. Finite element (FE) analysis and experimental technique were used to assess the residual stress distribution in a 6082-T6 aluminum plate with two adjacent holes expanded sequentially at 4 % nominal interference. The influence of center-to-center distance between holes was explored to assess the optimal level of separation between adjacent holes. Results suggested that residual stresses near second CE hole are markedly lower than those of first CE hole and that a cutting process does not affect the beneficial compressive residual stress around CE holes. These effects may delay fatigue crack propagation from CE holes or cut-out holes
Fatigue crack growth through particulate clusters in polycarbonate material
No full textThe interaction of a crack with a perfectly bonded inclusion or a cluster of inclusions in polycarbonate matrix was investigated through both numerical simulations and fatigue tests. Stress intensity factors (K(I)) were evaluated by boundary element method for several particle sizes, position and finally for inclusion cluster as a precursor study for the experiments. The numerical simulation has shown the crack tendency to circumvent the inclusions with consequential reduction of the growth rate. Fatigue crack growth tests were carried out on several particle-filled specimens at constant value of the applied stress intensity factor range (Delta K(Iapp)) highlighting the crack delay due to the presence of the stiff second phase. The experiments demonstrated that the inclusion effect on the crack growth rate can be explained with a model based on the crack shielding effect in which the particle would act to reduce the effective stress intensity factor at crack tip (K(Ieff)). Finally, the crack growth rate was predicted with an analytical model, and then compared to that obtained by the fatigue testing. Possible explanations for differences are discussed. (C) 2010 Elsevier Ltd. All rights reserved
In Silico Analysis of the MitraClip in a Realistic Human Left Heart Model
Full text linkMitral valve regurgitation is a common heart valve disorder associated with significant morbidity and mortality. Transcatheter mitral valve repair using the MitraClip device has emerged as a safe and effective alternative for patients unsuitable for conventional surgery. However, the structural and hemodynamic implications of MitraClip implantation in the left ventricle have not been extensively explored. This study aimed to assess the structural and hemodynamic performance of the MitraClip device using a high-fidelity model of the human heart, specifically focusing on a healthy mitral valve geometry. The implantation of the MitraClip device was simulated using the finite element method for structural analysis and the lattice Boltzmann method for computational flow analysis. MitraClip implantation induced geometrical changes in the mitral valve, resulting in local maxima of principal stress in the valve leaflet regions constrained by the device. Hemodynamic assessment revealed slow-moving nested helical flow near the left ventricular wall and high flow velocities in the apex regions. Vorticity analysis indicated abnormal hemodynamic conditions induced by the double-orifice area configuration of the mitral valve after MitraClip implantation. By predicting possible adverse events and complications in a patient-specific manner, computational modeling supports evidence-based decision making and enhances the overall effectiveness and safety of transcatheter mitral valve repairs
Viola ucriana. – In PASTA S., PEREZ-GRABER A., FAZAN L. & MONTMOLLIN B. DE (Eds), The Top 50 Mediterranean Island Plants, UPDATE 2017: http://top50.iucn-mpsg.org/species/15. IUCN/SSC/ Mediterranean Plant Specialist Group. Neuchâtel (Switzerland). E-book and on line. 141 pp. top50.iucn-mpsg.org.
No full textRegional variation of wall shear stress in ascending thoracic aortic aneurysms
No full textThe development of an ascending thoracic aortic aneurysm is likely caused by excessive hemodynamic loads exerted on the aneurysmal wall. Computational fluid-dynamic analyses were performed on patient-specific ascending thoracic aortic aneurysms obtained from patients with either bicuspid aortic valve or tricuspid aortic valve to evaluate hemodynamic and wall shear parameters, imparting aneurysm enlargement. Results showed an accelerated flow along the outer aortic wall with helical flow in the aneurysm center for bicuspid aortic valve ascending thoracic aortic aneurysms. In a different way, tricuspid aortic valve ascending thoracic aortic aneurysms exhibited normal systolic flow without substantial secondary pattern. Analysis of wall shear parameters evinced a high and locally varying wall shear stress on the outer aortic wall and high temporal oscillations in wall shear stress (oscillatory shear index) on either left or right side of aneurysmal aorta. These findings may explain the asymmetric dilatation typically observed in ascending thoracic aortic aneurysms. Simulations of a hypertensive scenario revealed an increase in wall shear stress upon 44% compared to normal systemic pressure models. Computational fluid-dynamics-based analysis may allow identification of wall shear parameters portending aneurysm dilatation and hence guide preventative intervention
Sicily: the island that didn't know to be an archipelago
Full text linkRecent geological studies demonstrated that most of Sicily was still under water during lower Pliocene, with the exception of the NE and the SE corners of the island (Peloritani Mts and Hyblaean Plateau, respectively). This geological evidence, so far not considered sufficiently by the scholars of the Sicilian flora, poses many still open questions on how and where many ancient lineages and Palaeogene relicts, currently found on the island, managed to survive. Purpose of this paper is to review the potential significance of isolation and ecological differentiation for the local floristic diversity and the evolution of narrow endemism in the Sicilian flora. In particular, the following drivers of Sicilian floristic patterns are considered: geographical segregation and age of the Sicilian terrains; climate variability and heterogeneity; geological patchiness; human influence on habitat fragmentation
Multiscale Modelling of Platelet Activation in Membrane Oxygenation Systems
No full textOne of the most common cause of failure in oxygenator systems consist of the thrombogenic risk associated with the interaction between the blood and the engineering material. For this reason, the use of membrane oxygenators is restricted to only 2-4 weeks in the extracorporeal membrane oxygenation system (ECMO). Using computational fluid dynamics, this study aims to investigate the mechanisms underlying the shear-mediated platelet activation in an oxygenation system. This was accomplished by first developing a multiscale model for the oxygenator system, followed by the estimation of the thrombogenic risk by mathematical modelling. High values of particle stress were found at inlet and outlet of blood flow. Computational flow analysis has revealed important insights into the thrombogenic mechanisms, suggesting that numerical analysis can be an effective tool to improve the design of oxygenator systems and reduce the time-to-market and animal trials
On the Modeling of Left Atrial Appendage Inversion: a Reverse Growth Analysis
Full text linkAtrial fibrillation (AF) can lead to thromboembolic events due to an increased
propensity for clot formation in the left atrial appendage (LAA). Current methods for reducing
the risk of these events, such as surgical exclusion and percutaneous occlusion, have limitations
that restrict their applicability and efficacy. Sulkin et al. [1] previously demonstrated the clinical
feasibility of a novel procedure involving partial inversion of the LAA to occlude the atrium
appendage.
This study aimed to explore left atrial appendage inversion (LAAI) on four patient-specific
LAA morphologies, each representing a distinct morphological variant: chicken wing, cactus,
windsock, and cauliflower. Left atrial geometries were extracted from CT images and then used
as input for patient-specific finite element analysis simulations of LAAI. The latter was
simulated by pulling the elements at the LAA tip along a predefined path to mimic the inversion.
The deformed configuration was then analyzed to map the stress field and establish a stressresorption relationship.
Folded LAA wall results in a transition from tensile to compressive stress distribution, which
can induce tissue resorption in the inverted appendage. This compressive stress distribution is
linked to the stretch distribution (λ) generated in the folded LAA. To define a stress-resorption
relationship, the growth and reverse growth kinematics proposed by Lee et al. [2] was adopted.
This approach involves a reversible growth multiplier (θ) that represents the combined effects
of elastic deformation and tissue growth/reverse growth., The trend of θ based on the straindriven growth model was derived using the Ogden constitutive model. The value of θ depended
to λ, and tissue resorption was triggered beyond a stretch threshold. Thus, we concluded that λ
generated in the LAAI region acts as a remodeling stimulator. This occurred until λ decrease
below a minimum value and θ converged to a final value following an exponential decay trend.
Our findings resulted in a stress-growth model, applied in four LAA morphologies, that can
model tissue resorption over time as compressive tensile gradually relax in the inverted LAA
Botanical excursions in central and Western Sicily - Field Guide For the 60th IAVS Symposium
No full textPlant succession on Sicilian terraces
Full text link189 phytosociological relevés have been made in five areas of Sicily, three on volcanic substrates and two on limestones, to study plant community succession trends within abandoned terraced vineyards and cereal fields. Disturbance status and abandonment age was recorded for each sample plot. DCA of these relevés suggests that four are the most important factors driving succession, i.e. geological substrate, bioclimate, age of abandonment and disturbance. In most cases - if there are some dispersal centres near the old fields - undisturbed and less disturbed terraces evolve quite rapidly towards the local potential “climax” community. Different trends have been recorded on the areas which underwent and still undergo frequent and severe disturbance: succession is biased or even blocked in a steady state, so that grassland, garrigue, mantle or herb-dominated communities prevail. The species composition of the plant communities subject to disturbance seems to be strongly connected with the time gap between abandonment and first disturbance occurrence. The “mixed” ecological-structural-phytosociological approach here adopted to describe plant communities was quite effective to interpret abandoned terraces communities and their dynamism
- …
