Search

Use of resorbable external reinforcement of the pulmonary autograft during the Ross operation has been suggested, but the differential regional potential for dilation of the aorta, mainly regarding the neo-root and the neo-Valsalva sinuses, represents an unresolved issue. Auxetic materials could be useful in preventing dilation given their favorable mechanical properties. We designed a composite semiresorbable armoured bioprosthesis constituted by polydioxanone and expanded polytetrafluoroethylene and evaluated its effectiveness as a pulmonary autograft reinforcement device in an animal model of the Ross procedure.An experimental model of the Ross procedure was performed in 20 three-month-old growing lambs. The pulmonary autograft was alternatively nonreinforced (control group n = 10) or reinforced with composite bioprosthesis (reinforced group n = 10). Animals were followed up during growth for 6 months by angiography and echocardiography. Specific stainings for extracellular matrix and immunohistochemistry for metalloproteinase-9 were performed.Reference aortic diameter increased from 14 ± 1 mm to 19 ± 2 mm over 6 months of growth. In the control group, pulmonary autograft distension (28 ± 2 mm) was immediately noted, followed by aneurysm development at 6 months (40 ± 2 mm, P.001 vs reference). In the reinforced group, an initial dilation to 18 ± 1 mm was detected and the final diameter was 27 ± 2 mm (42% increase). Two deaths due to pulmonary autograft rupture occurred in the control group. On histology, the control group showed medial disruption with connective fibrous replacement, whereas in the reinforced group compensatory intimal hyperplasia was present in the absence of intimal tears. The bioprosthesis promoted a positive matrix rearrangement process favoring neoarterialization and elastic remodeling as demonstrated on specific staining for elastin collagen and metalloproteinase-9.The device adapted and functionally compensated for the characteristics of autograft growth, guaranteeing a reasonable size of the autograft at 6 months, but more important, because the device is biocompatible, it did not disrupt the biological process of growth or cause inflammatory damage to the wall.

Inverse problems form an essential mathematical apparatus for dealing composite materials. They allow to derive the macroscopic behavior of complex heterogeneous media from the knowledge of their physical properties and geometrical features exhibited at the microscale level, thus being at the basis of design strategies for conceiving new materials in which selected electromagnetic, thermo mechanical, or chemical performances can be drastically and helpfully enhanced. Self-healing materials, bio-inspired composites with encapsulated healing agents able to repair the damage, as well as nanocomposites, such as colloids, gels, copolymers and carbon nanotubes, are all main actors on the scene of materials science and engineering. All these issues are common to modeling and inverse problems and constitute three distinct mathematical problems: homogenization, localization, and optimization. Hence, these models and the related inverse problems form the topic of the present article, where after giving a concise introduction to the complementary theories of homogenization and localization, two relevant heuristic micromechanical models based on the engaging of microstructural fabric tensor measures are illustrated in detail, to analyze anisotropic nonhomogeneous porous media and hyperelastic fiber-reinforced materials. In addition, remarkable analytical methods for solving elastic problems for composite materials and notes on promising issues on topology optimization problems have been finally discussed. Keywords: composites materials; homogenization; inverse problems; topology optimization; analytical elastic solutions; fabric tensor

In the present work buckling stresses of prismatic flat and stiffened shell structures are derived within the framework of the Kantorovich approach, making reference to both Von Karman and Koiter–Sanders theories, the latter exploiting the Green–Lagrange strain tensor which is needed if the expected buckling modes involve comparable in-plane and out-of-plane displacements. Additionally, in order to highlight the contribution of each nonlinear term of the Koiter–Sanders model, two further intermediate choices are also considered, namely an enhanced Von Karman model and a spurious model which collects selected terms from different theoretical approaches, generally adopted in literature with the aim of simplifying the numerical analyses. The obtained results show how in buckling problems where the weight of in-plane displacements cannot be neglected the Von Karman model tends to overestimate the critical load, while the three considered alternative models result substantially equivalent, at least from the practical standpoint.

An analytical approach for the buckling analysis of rectangular plates under mixed boundary conditions is presented. In order to solve the partial differential equation governing the problem at hand a method of separation of variables is here adopted, by introducing the displacement field as a result of the scalar product of two vectors which combine prescribed and unknown scalar functions. By following this strategy, exact buckling solutions for a wide class of problems, in which mixed boundary conditions can be assigned relaxing some usual constraints, are determined, and buckling load of plates, where biaxial tensile and compressive loads are applied in presence of piecewise clamped and partially supported sides, obtained analytically. Several cases of engineering interest are finally analyzed, and comparisons of the theoretical outcomes with literature data and Finite Element-based numerical results are also shown, in order to highlight the effectiveness of the proposed strategy.

Abstract The demand for pipe materials that can withstand increasing hydrostatic pressures continues to grow as oil and gas exploration takes us into deeper and deeper waters. As water depth increases, failure by hydrostatic collapse becomes more likelyas the limiting failure mode for pipeline design. In general, pipes for deepwater applications possess a diameter to thickness ratio in a region where failure is dominated by both instability and plastic collapse. This implies that, prior to failure, the compressive yield strength of the material must be exceeded, followed by ovalisation and further local yielding. To enable effective design work to be performed, and proven during service, it is necessary to ensure that the mechanical and dimensional properties of the pipe material are controlled within very strict boundaries and reliable and accurate formulae are required to take into account these factors. However, mechanical equations for this simple failure mode have not been satisfactory established. Current existing formulations use basic mechanical expressions that are calibrated to some limited test data. However none of these expressions examine the actual mechanics of the failure mechanism. This paper presents an investigation into the mechanics of this specific problem and permits a simple assessment of the effects of geometrical and material data on the collapse pressure of moderately thick tubes. An analytical expression has been generated and correlated to extensive test data and finite element models, proving the accuracy of the expression. The resulting formula also allows a deeper understanding of the effect pipe shape has on collapse and opens to the door for manufacturers to further enhance pipe material performance. Despite the complexity of the collapse mechanisms this work presents a concise yet rigorous and effective analytical approach. The work presented in this paper will form the basis of future investigations that will enhance the understanding of this failure mode in pipelines. This will lead to increased pipeline safety while allowing the boundaries of pipeline application to be challenged, enabling hydrocarbon recovery in deeper and deeper waters. Introduction Since the early 1990's the export of hydrocarbons through deepwater trunklines has been seriously considered. In deep and ultra-deeep waters the diameter of trunklines coupled with the hydrostatic pressure tends to lead to failure of the pipeline by external collapse. This failure mode is an instability phenomenon that is governed by the geometry of the pipeline and its material properties. Failure of a pipeline section is through a subtle combination of its properties and as such the exact limiting collapse pressure is very difficult to determine through simple mathematics. Finite element techniques can allow the estimation of collapse where the actual geometry and material properties are known but in reality shape and properties vary along the pipe length, around the circumference as well as through the wall thickness. Despite these complications relationships do exist that provide adequate prediction of collapse pressure and these are discussed in the next section. However, each of these relationships has its route in the same fundamental, simple mathematics with expressions generated to blend transitions between elastic collapse for thin walled structures and plastic collapse for thicker sections. None of these relationships actually seeks to understand the actual mechanics of this failure mode however, their accuracy and applicability is generally improved through calibration of the output with actual test data.

On the basis of a straightforward analytical approach which has been recently proposed by the present authors, a comparison with numerical procedures to predict plastic collapse in circular rock tunnels is reported. In fact, numerical modeling of the evolution of progressive failure leading to collapse in tunnels is a quite complicated matter and requires great care in modeling the problem and interpretating the results. In order to provide a guide to engineers facing rock limit state problems, a few examples by means of three commercial packages are presented, discussed in details and confronted with the analytical results.

A mechanical analysis of the conductive keratoplasty on hyperopic eyes has been carried out, and the attention has been focused on incorporating the actual viscoelastic properties of the human corneal tissue and on the stress gradients induced by the intervention. By avoiding unnecessary complications which may obscure the essential behaviour of the model, the results are in very good agreement with the clinical and experimental findings and suggest that the major role in the commonly observed decrease of the initial degree of the refractive correction might be played by the stress gradients at the intervention spots, which are likely to influence the wound-healing. The study aims to contribute some firm mechanical roots to the predictability of the outcome of an increasingly popular technique that, notwithstanding several advantages with respect to ablative interventions, at present cannot be considered completely satisfactory.

With particular interest on total hip arthroplasty (THA), optimization of orthopedic prostheses is employed in this work to minimize the probability of implant failure or maximize prosthesis reliability. This goal is often identified with the reduction of stress concentrations at the interface between bone and these devices. However, aseptic loosening of the implant is mainly influenced by bone resorption phenomena revealed in some regions of the femur when a prosthesis is introduced. As a consequence, bone resorption appears due to stress shielding, that is to say the decrease of the stress level in the implanted femur caused by the significant load carrying of the prosthesis due to its higher stiffness. A maximum stiffness topological optimization-based (TO) strategy is utilized for non-linear static finite element (FE) analyses of the femur-implant assembly, with the goal of reducing stress shielding in the femur and to furnish guidelines for re-designing hip prostheses. This is accomplished by employing an extreme accuracy for both the three- dimensional reconstruction of the femur geometry and the material properties maps assigned as explicit functions of the local densities.

Bone is a dynamic material, able to destroy and create new tissue by responding to changes of physiological and mechanical stimuli. When loads overcome the elastic threshold, the bone tissue exhibits the formation of microcracks, whose size can increase as function of the load history, so activating a new bone remodeling process. In the present study the elastoplastic Gurson’s model is used to describe the evolution of microcracks in the bone, in terms of strain and volume fraction evolutions with the time. Applying a monoaxial, triaxial and hydrostatic time-periodical load histories to an osteon unit assumed to be obeying to the Gurson’s model, it is finally shown that the non-linear stress-strain behaviour leads to a some interesting results about the corresponding non-linear evolution of void growth in the bone unit.

The present work is aimed at deriving assessment and design formulae for determining the elastic–plastic response and the ultimate compressive strength of circular concrete columns confined by Fiber Reinforced Polymers (FRP). To achieve this, a constructive method for obtaining closed-form elastic and post-elastic solutions for Functionally Graded Material Cylinders (FGMCs), constituted by an isotropic central core and n arbitrary cylindrically orthotropic hollow phases, is proposed. In the first part of the paper, under the hypotheses of axis-symmetrical boundary conditions, elasticity and perfect bond between the phases, new analytical solutions for self-equilibrated axial forces applied at the extremities of the object are derived. In particular, the rather general mathematical approach has been based on a strategy already proposed by some of the authors in a previous work, and here extended to anisotropic hollow phases. The key of the involved method is to reduce the differential Boundary Value Problem (BVP) to the equivalent linear algebraic one, by means of a special matrix-like arrangement of the governing equations and invoking the Complex Potential Theory for anisotropic materials. The obtained general solution has been then easily particularized to the two phase FGMC representing the circular concrete column confined by FRP sheets. In the second part of the paper, the above mentioned solutions for the concrete column are “moved” within the post-elastic range and we investigate the evolution of the stress field in the solid components when the concrete core is characterized by an Intrinsic Curve or Schleicher-like elastic–plastic behavior endowed with associate flow rule, and the FRP cylindrically orthotropic hollow phases obey to an elastic–brittle Tsai–Hill anisotropic yield criterion. At the end, the elastic and post-elastic response of the overall solid and predictive formulae for estimating the failure mechanism, in terms of concrete ultimate compressive strength, confining pressure and strain at failure, are derived. The obtained results are finally compared with several experimental literature data, highlighting the very good agreement between the analytical predictions and experimental tests.

It is well known that the design of submarine pipelines relies on accurate test results for the local buckling collapse of pipes subjected to bending loading. The present paper analyses apparently anomalous values of axial tensile and compressive strains from recent test results in comparison to the values that would be expected on the basis of simple bending theory. This could have important consequences for the efficacy of the design factors derived using these results. The cause of the differences between the strain values obtained in the tests and those expected on the basis of simple bending theory are explained using finite element modelling. The differences result from the type of collars and supports commonly used in bending tests, the effects of which persist for a greater length along the test pipe than has hitherto been assumed. In general, it is pointed out that the application of the simplified engineering theory of bending can be erroneous when ovalisation is imposed or, on the contrary, the boundary conditions of the section are restrained from ovalising deformations. The influence of the D / t ratio is also analysed. The results contribute to the understanding of a crucial limit state for the design of onshore and offshore pipelines.

In the framework of heterogeneous materials, the purpose of the present work is to furnish a general approach to the construction of exact elastic solutions for FGM cylinders made of a central core and n arbitrary cylindrical hollow homogeneous and isotropic phases. The hypothesis of axi‐symmetric boundary conditions is here assumed in order to analyze a class of elastic problems characterized by non‐decaying selected mechanical quantities, and in particular the axial strains e33 in the radial direction, with x3 being the axis of the laminated cylinder. Example applications are finally illustrated, with reference to the problem of interpreting strain measurements obtained by using optical fibre sensors.

The proposal of the present work is to furnish a general approach to construct exact elastic solutions for FGM cylinders, made of a central core and n arbitrary cylindrical hollow homogeneous and isotropic phases. The hypothesis of axis-symmetrical boundary conditions is here assumed in order to analyze a class of elastic problems which present no-decaying of selected mechanical quantities and in particular of the axial strains ϵ 33 in the radial direction, being x 3 the axis of the laminated cylinder. To construct a robust mathematical procedure for obtaining exact elastic solutions for axis-symmetrical n-plies-Functionally Graded Material Cylinders (n-FGMCs), a theorem is first given for qualifying the space of the solutions and then their mathematical form is identified, when the object exhibits no-decaying of the axial strain. By starting from the classical Boussinesq-Somigliana-Galerkin vector and specializing it to torsionless composite cylinders characterized by no-decaying of the axial strain, a s...

In recent years the use of distributed optical fiber sensors for measurements of strain in beams, by means of the Brillouin scattering effect, has been proposed. Several works pointed out the practical difficulty of this kind of measurement, connected both to theoretical and to experimental problems, e.g. mechanical characterization of optical fibers, decaying of strains in the protective coatings, spatial resolution of the Brillouin scattering, brittleness of the glass core, elastic–plastic response of the polymeric jackets, end effects and the different responses of the fiber for dilatation and contraction. Dealing with each of the above problems still requires a great research effort. However, recent literature shows that distributed optical fiber measurement techniques are extremely useful for finding qualitative responses in terms of strains. Indeed, in spite of the above-mentioned uncertainties, the great advantage of the proposed distributed measurement of strains remains evident for the safety assessment of large structures, such as bridges, tunnels, dams and pipelines, over their whole lifetimes. In view of this, in the present paper the detection of defects or damage in bending beams—by using distributed optical fiber sensors in a method based on time domain stimulated Brillouin scattering—is proposed. In particular, laboratory tests were carried out to measure the strain profile along a steel beam. Two tests were performed: the first one involves an integral steel beam, while the second experiment is performed on a damaged beam. Comparison between these two tests allows the detection of the position and the establishing of bounds on the size of the defect. At the end, the quality and accuracy of the measurements are discussed and a sensitivity analysis of the strain readings taking into account the bonding conditions at the interface between the structure and the fiber is also carried out by means a parametric numerical simulation.

The use of distributed optical fibre sensors for strain measurements in beams, by means of Brillouin scattering effect, has been proposed recently. Several researchers have stressed the theoretical and practical difficulties related to this kind of measurement. These include the mechanical characterization of optical fibres, the decay of strains in the protective coatings, the spatial resolution of the Brillouin scattering, the brittleness of the glass core, the elastic – plastic response of the coatings, the end effects, and the different effects of strain readings in dilatation or in contraction. A solution to each of the above-cited problems would entail further research effort. However, all the works pointed out that the qualitative strain response of bending beams is clearly accounted for by distributed optical fibre sensors. In spite of the above-mentioned uncertainties, the distributed nature of the sensor makes it very attractive when safety assessment of large structures, such as bridges, tunnels, dams or pipes, is involved. In the present paper, the detection of defects or damage in bending beams by means of distributed optic-fibre sensors is proposed. In particular, the fibre-optics distributed sensor has been used for measurement of the deformation of a steel beam in experimental laboratory tests. Comparison of the experimentally measured strains, carried out on both damaged and undamaged beams, revealed the presence and position of defects in the beam. Quality and accuracy of the measurements carried out with distributed optical-fibre sensors are discussed, focusing on the applicability of the identification method.