The concept of Isogeometric Analysis recently introduced by Hughes et al. 2005 is herein applied to the study of structural vibrations. In this framework, the favourable behaviour of the method is ...verified and compared with some classical finite element results. Numerical experiments are shown for structural one-, two- and three-dimensional problems in order to test the performance of this promising technique in the field of the analysis of natural frequencies and modes.
•We apply isogeometric collocation techniques to spatial Timoshenko rods.•We solve the strong form equations of the problem in both displacement-based and mixed formulations.•We prove that mixed ...collocation schemes are locking-free independently of the polynomial degrees for the unknown fields.•Numerical experiments confirm the accuracy and efficiency of the considered methods.
In this work we present the application of isogeometric collocation techniques to the solution of spatial Timoshenko rods. The strong form equations of the problem are presented in both displacement-based and mixed formulations and are discretized via NURBS-based isogeometric collocation. Several numerical experiments are reported to test the accuracy and efficiency of the considered methods, as well as their applicability to problems of practical interest. In particular, it is shown that mixed collocation schemes are locking-free independently of the choice of the polynomial degrees for the unknown fields. Such an important property is also analytically proven.
The employment of shape memory alloys (SMA) in a large number of applications in the fields of aeronautical, biomedical, and structural engineering has been the motivation for an increasing interest ...in the direction of a correct and exhaustive modeling of their macroscopic behaviour.
Many models for SMA available in the literature consider fully reversible phase transformations (i.e. no permanent inelastic strains), which are proved by experiments to be sometimes a not fully realistic approximation. In this paper we propose a new three-dimensional model which is capable of including permanent inelastic effects combined with a good description of pseudo-elastic and shape-memory behaviours. Moreover, we report the numerical results from a number of both uniaxial and non-proportional biaxial tests, which aim at assessing model features and performance.
Abstract Prosthesis positioning in transcatheter aortic valve implantation procedures represents a crucial aspect for procedure success as demonstrated by many recent studies on this topic. Possible ...complications, device performance, and, consequently, also long-term durability are highly affected by the adopted prosthesis placement strategy. In the present work, we develop a computational finite element model able to predict device-specific and patient-specific replacement procedure outcomes, which may help medical operators to plan and choose the optimal implantation strategy. We focus in particular on the effects of prosthesis implantation depth and release angle. We start from a real clinical case undergoing Corevalve self-expanding device implantation. Our study confirms the crucial role of positioning in determining valve anchoring, replacement failure due to intra or para-valvular regurgitation, and post-operative device deformation.
Until recently, heart valve failure has been treated adopting open-heart surgical techniques and cardiopulmonary bypass. However, over the last decade, minimally invasive procedures have been ...developed to avoid high risks associated with conventional open-chest valve replacement techniques. Such a recent and innovative procedure represents an optimal field for conducting investigations through virtual computer-based simulations: in fact, nowadays, computational engineering is widely used to unravel many problems in the biomedical field of cardiovascular mechanics and specifically, minimally invasive procedures. In this study, we investigate a balloon-expandable valve and we propose a novel simulation strategy to reproduce its implantation using computational tools. Focusing on the Edwards SAPIEN valve in particular, we simulate both stent crimping and deployment through balloon inflation. The developed procedure enabled us to obtain the entire prosthetic device virtually implanted in a patient-specific aortic root created by processing medical images; hence, it allows evaluation of postoperative prosthesis performance depending on different factors (e.g. device size and prosthesis placement site). Notably, prosthesis positioning in two different cases (distal and proximal) has been examined in terms of coaptation area, average stress on valve leaflets as well as impact on the aortic root wall. The coaptation area is significantly affected by the positioning strategy ( − 24%, moving from the proximal to distal) as well as the stress distribution on both the leaflets (+13.5%, from proximal to distal) and the aortic wall ( − 22%, from proximal to distal). No remarkable variations of the stress state on the stent struts have been obtained in the two investigated cases.
We investigate the use of Isogeometric Analysis for the model construction and simulation of aortic valve closure. We obtain converged results and compare with benchmark finite element analysis. We ...find that Isogeometric Analysis is capable of attaining the same accuracy with models consisting of two orders of magnitude fewer nodes than finite element models; analogous savings are observed also in terms of analysis time. Model construction and mesh refinement are likewise performed more efficiently with Isogeometric Analysis.
5α-reductase inhibitors are regarded as a promising chemoprevention strategy to reduce the incidence and delay the progression of prostate cancer. Landmark clinical trials have shown the ...chemopreventive potential of these drugs, but they appear to be mostly effective in mild tumors and have also been correlated with a higher prevalence of advanced prostate cancer. Hence, the use of 5α-reductase inhibitors for prostate cancer chemoprevention has become a controversial issue. The effects of these drugs on prostate cancer growth remain incompletely understood, but they are thought to promote apoptosis in the tumor. Additionally, 5α-reductase inhibitors induce global prostate shrinkage, which decreases the tumor-inhibiting effect of the mechanical stress accumulated in prostatic tissue due to common prostate enlargement with age. Thus, the competition between this mechanical effect and apoptotic upregulation may explain the controversial outcomes of 5α-reductase inhibitors on prostate cancer. Here, we extend our mechanically-coupled model of prostate cancer growth by including the mechanical and apoptotic action of 5α-reductase inhibitors and explore their combined effect on an aggressive tumor in silico. Our simulations show that the apoptotic boost dominates in the first months of therapy but the long-term outcome of 5α-reductase inhibitors depends on its competition with a decrease in hydrostatic stress caused by prostate shrinkage, which favors tumor growth. By combining moderate or strong prostate shrinkage with mild or intense apoptotic upregulation, our simulations show different tumor growth dynamics ranging from long-term inhibition of prostate cancer growth to rapidly growing large tumors, which may evolve towards advanced disease. Thus, our proposed mechanism for the action of 5α-reductase inhibitors may contribute to resolve the controversy around the use of these drugs for chemoprevention and to gain insight on prostate cancer dynamics during its use. The computational technology used herein could also assist physicians to monitor prostatic tumors during 5α-reductase inhibitor therapy and enable the early identification of responders from non-responders in a patient-specific manner.
•We study mechanic and apoptotic effects of 5α-reductase inhibitors on prostate cancer.•We derive an organ-scale, patient-specific computational model for this treatment.•Mechanical drug effects are parameterized with data from large clinical studies.•Our simulations show that apoptotic effects dominate in the short-term.•Drug-induced prostate shrinkage may overcome the apoptotic effect long-term.
In the present paper we investigate the capability of finite element methods to correctly reproduce the stability range of finite strain problems in the incompressible regime. To this end, we develop ...a numerical scheme, obtained combining a stream function formulation with an isogeometric NURBS approach, which is able to sharply estimate the stability limits of the continuous problem. Using such a method, we show a pair of benchmark problems on which various well-known finite element methods largely fail in approximating the correct stability range.
We present a reformulation of the classical Timoshenko beam problem, resulting in a single differential equation with the rotation as the only primal variable. We show that this formulation is ...equivalent to the standard formulation and the same types of boundary conditions apply. Moreover, we develop an isogeometric collocation scheme to solve the problem numerically. The formulation is completely locking-free and involves only half the degrees of freedom compared to a standard formulation. Numerical tests are presented to confirm the performance of the proposed approach.
PDF
