Andrea Avanzini

Biomechanical Evaluation of Ascending Aortic Aneurysms

The biomechanical properties of ascending aortic aneurysms were investigated only in the last decade in a limited number of studies. Indeed, in recent years, there has been a growing interest in this field in order to identify new predictive parameters of risk of dissection, which may have clinical relevance. The researches performed so far have been conducted according to the methods used in the study of abdominal aortic aneurysms. In most cases, uniaxial or biaxial tensile tests were used, while in a smaller number of studies other methods, such as opening angle, bulge inflation, and inflation-extension tests, were used. However, parameters and protocols of these tests are at present very heterogeneous in the studies reported in the literature, and, therefore, the results are not comparable and are sometimes conflicting. The purpose of this review then thence to provide a comprehensive analysis of the experimental methodology for determination of biomechanical properties in the specific field of aneurysms of the ascending aorta to allow for better comparison and understanding of the results.

Levator ani deformation during the second stage of labour

A very important medical problem for females is urinary incontinence, sometimes associated with faecal incontinence and pelvic organ prolapse. One of the most common reasons these issues are increasing is clearly the muscle damage during childbirth. This article focusses on understanding the complex behaviour of the levator ani muscles involved in the second stage of labour. A geometrical model obtained from a 23-year-old nulliparous woman was used to simulate childbirth. Several assumptions were introduced in order to simplify the problem without significantly affecting the global response of the system. An anisotropic hyperelastic model was used to characterize the material behaviour; the muscle fibres were assumed to be mostly orientated circumferentially. In addition, particular attention was also put to the boundary conditions of the model. The introduction of the constraints imposed by the coccyx bone in the central area of the levator ani group represents one the most important improvement compared to previous computational models. The maximum deformation and stress were found in the pubococcygeus muscle of the levator ani group. A stretch value close to 2.2 was determined by considering different material parameters. The results seem convincing with respect to medical observation and previous analysis. However, there are still some limitations concerning the material definition and the geometry and trajectory of the head that can be further improved.

Integrated Experimental and Numerical Comparison of Different Approaches for Planar Biaxial Testing of a Hyperelastic Material

Planar biaxial testing has been applied to a variety of materials to obtain relevant information for mechanical characterization and constitutive modeling in presence of complex stress states. Despite its diffusion, there is currently no standardized testing procedure or a unique specimen design of common use. Consequently, comparison of results obtained with different configurations is not always straightforward and several types of optimized shapes have been proposed. The purpose of the present work is to develop a procedure for comprehensive comparison of results of biaxial tests carried out on the same soft hyperelastic material, using different types of gripping methods and specimen shapes (i.e., cruciform and square). Five configurations were investigated experimentally using a biaxial test rig designed and built by the authors, using digital imaging techniques to track the displacements of markers apposed in selected positions on the surfaces. Then, material parameters for a suitable hyperelastic law were determined for each configuration examined, employing an inverse method which combines numerical simulations with the finite element method (FEM) and optimization algorithms. Finally, efficiency of examined biaxial configurations was assessed comparing stress reductions factor, degree and uniformity of biaxial deformation, and operative strain ranges.

Structural analysis of a stented pericardial heart valve with leaflets mounted externally

Our aim was to understand the structural and functional behaviour of a pericardial heart valve with biological leaflets attached externally to a stent. To our knowledge, there is little if any literature concerning these kinds of bioprosthetic heart valves, while there is more concerning bioprosthetic heart valves with leaflets mounted internally. We studied the problem using a finite element approach considering leaflets and stent interaction, the influence of leaflet anisotropy and stent stiffness, by comparing quasi-static and dynamic loadings. Although we considered the problem to be symmetric and fluid–structure interaction was not implemented, we believe that our results could be a solid basis for valve optimization. We found regions of high stress concentration at the commissure near the stent tip and at the base of the leaflet cusp. The structural behaviour in the first region was complex, while the stress in the second region acted radially because of high bending. Although leaflet tissue anisotropy and stent stiffness exerted a significant influence on the structural and functional behaviours, they had a contrasting effect on leaflet stress state, coaptation and valve opening. Therefore, a good optimization should take into account both structural and functional requirements when tuning tissue properties and stent stiffness.

A computational procedure for life assessment of UHP reciprocating seals with reference to fatigue and leakage

A calculation procedure is shown for assessing the working life of reciprocating plastic seals for Ultra High Pressure (UHP) applications. Two main damage phenomena are considered, low cycle fatigue and wear, associated with structural failure or leakage respectively. An integrated approach, based on a finite element analysis of the seal, is proposed to evaluate both fatigue failure and leakage occurrence. The model takes into account non-linear effects due to seal contact interactions, material cyclic behaviour and its dependence on hydrostatic pressure. Fatigue life is predicted by combining equivalent strain range calculated in the critical points with life curve of the material. Leakage prediction is made by calculating a leakage index, based on instroke and outstroke pressure gradients and related to contact pressure profile change due to wear. Seal design can be optimised by choosing seal material and geometry so to delay fatigue failure and leakage, scheduling their occurrence at the same time.

Shape memory behavior of epoxy-based model materials: Tailoring approaches and thermo-mechanical modeling

A series of structurally related epoxy resins were prepared as model systems for the investigation of the shape memory response, with the aim to assess the possibility of tailoring their thermo-mechanical response and conveniently describing their strain evolution under triggering stimuli with a simple thermoviscoelastic model. The resins formulation was varied in order to obtain systems with controlled glass transition temperature and crosslink density. The shape memory response was investigated by means of properly designed thermo-mechanical cycles, which allowed to measure both the ability to fully recover the applied strain and to exert a stress on a confining medium. The results were also compared with the predictions obtained by finite element simulations of the thermo-mechanical cycle by the employ of a model whose parameters were implemented from classical DMA analysis.

Failure analysis and life prediction of polymeric rollers for industrial applications

The present study investigates the behaviour of polymeric rollers employed in a cam mechanism of an industrial machine, consisting of two concentrically fitted rings made of polyurethane and short fibre-reinforced PEEK. A cyclic mechanical characterisation of the materials was at first carried out by strain controlled fatigue tests. The safe range of working conditions of the rollers in terms of rotating speed and contact load was then investigated by rolling contact tests carried out on roller prototypes with a dedicated test rig. These tests allowed obtaining important information about the different damage mechanisms occurring in the component: above a critical working temperature, rollers failure resulted from rings unfitting due to excessive deformations, while fatigue crack nucleation in the outer ring at the interface with the inner one was observed below this threshold, at high contact load levels. No contact fatigue damage appeared. FEM models of the roller were also developed, based on cyclic material data, in order to analyse the cyclic stress and strain field and to predict the fatigue life, finding a satisfactory agreement with the experimental rolling contact tests results.