Felipe Gabaldón

Finite element models for mechanical simulation of coronary arteries

Mechanical simulation of tissue in the walls of coronary arteries may provide valuable quantitative information for medical practice, such as understanding the evolution of stenosis, angioplasty processes, and placement of stents and possible restenosis. The material constitutive models which represent the mechanical response to strain are highly nonlinear as well as anisotropic, thus precluding standard finite element formulations with isotropic or linear elastic materials. Further, in order to study phenomena such as stenosis or restenosis, they must include essential phenomena in biological soft tissues such as growth and remodeling, as well as the consideration of initial stresses and strains. This paper discusses these issues and proposes some mathematical models for their mechanical simulation within a finite element framework. Some preliminary applications to the study of clinical cases are shown.

A computational procedure for prediction of ballasted track profile degradation under railway traffic loading

A computational procedure is developed in the present paper, allowing the prediction of the ballasted track profile degradation under railway traffic loading. In this procedure, an integration of the short-term and long-term mechanical processes of track deterioration is taken into account, using a track degradation model. This degradation model is incorporated into a finite element code where two modes of calculation are implemented: the “implicit mode” concerns the short-term track deterioration, in which the hypoplastic model is used for the ballast layer and the dynamic response to an instantaneous train axle passage is obtained to serve as input data for the “explicit mode”, which concerns the simulation of long-term track deterioration, using the accumulation model for ballast layer. The whole procedure is illustrated on the prediction of the ballasted track profile degradation of a track section of 100 m. The results show a significant influence of the type of track geometry defects and the vehicle velocity on the evolution of track deterioration and the capability of the proposed procedure in reproducing the track profile degradation.

Fluid-solid interaction in arteries incorporating the autoregulation concept in boundary conditions.

In pre-surgery decisions in hospital emergency cases, fast and reliable results of the solid and fluid mechanics problems are of great interest to clinicians. In the current investigation, an iterative process based on a pressure-type boundary condition is proposed in order to reduce the computational costs of blood flow simulations in arteries, without losing control of the important clinical parameters. The incorporation of cardiovascular autoregulation, together with the well-known impedance boundary condition, forms the basis of the proposed methodology. With autoregulation, the instabilities associated with conventional pressure-type or impedance boundary conditions are avoided without an excessive increase in computational costs. The general behaviour of pulsatile blood flow in arteries, which is important from the clinical point of view, is well reproduced through this new methodology. In addition, the interaction between the blood and the arterial walls occurs via a modified weak coupling, which makes the simulation more stable and computationally efficient. Based on in vitro experiments, the hyperelastic behaviour of the wall is characterised and modelled. The applications and benefits of the proposed pressure-type boundary condition are shown in a model of an idealised aortic arch with and without an ascending aorta dissection, which is a common cardiovascular disorder.