Design of thermoelastic multi-material structures with graded interfaces using topology optimization

A level set based shape and topology optimization framework is used to study the effect of graded interfaces in the optimization process of micro-architectured multi-materials. In contrast to previous studies interfaces are considered as smooth transition between phases instead of a sharp delimitation between two phases. A study on extreme thermoelastic properties of 2D isotropic composites is achieved and optimal design are presented. The study shows how taking into account smooth interfaces can influence the optimal design of these materials

Environmental protection of titanium alloys in centrifugal compressors at 500°C in saline atmosphere

The use of the titanium alloy Ti-6246 (Ti–6Al–2Sn–4Zr–6Mo, wt-%) for gas turbine compressors allows an increase in working temperature and stress level. Under severe service conditions, the material experiences combined high temperature and high mechanical stress and, in saline atmospheres, stress corrosion cracking (SCC) can occur, leading to catastrophic mechanical failure. The present study was performed to evaluate the potential of several surface treatments to protect Ti-6246 alloy, after salt deposit, from hot salt SCC at temperatures ?500°C and 500 MPa static mechanical stress conditions. Shot peening, thermal oxidation and metal–ceramic coatings were investigated. Experimental results confirm the existence of brittle stress corrosion phenomena marked by a low residual elongation of test samples and the presence of oxides on the fracture surfaces. Both shot peening and metal–ceramic coatings increase the hot salt SCC resistance of the alloy. Times to rupture were improved by a factor of 3 for shot peening and by a factor of 10 for metal–ceramic coatings. Inversely, the time to rupture of preoxidised alloys has been halved compared with uncoated alloys. As well as these interesting quantitative results, structural studies of metal–ceramic coatings showed that they are mechanically and chemically compatible with the titanium alloy substructure and should work under severe thermomechanical stresses and aggressive atmospheres

Mechanical adhesion of SIO2 thin film on a polymeric substrate under compressive stress

International audienceTo ensure good adhesion between a 200 nm silicon dioxide layer and a 4.5 μm thick hardcoat polymeric coating, a better understanding of mechanisms of adhesion at this interface is needed. To reach this purpose, quantification of adhesion is performed by analyzing SiO2 buckle morphologies generated under compressive stress. This adhesion test was chosen for its representativeness of defects observed in real life. Interfacial toughness can be determined by applying Hutchinson & Suo model. This analytical model involves accurate value of elastic modulus Ef of SiO2 thin film. Small dimensions at stake make characterization of elastic modulus challenging. First part of the study focuses on using both nano-indentation and AFM to attempt assessment of SiO2 thin film elastic modulus. Results showed significant influence of substrate for both techniques. Impact on mechanical properties between SiO2 thin films with different intrinsic stresses was also investigated and suggests that higher density of SiO2 thin film leads to higher elastic modulus. Compression tests resulted in formation of straight-sided buckles that evolve into telephone cords upon unloading. Numerical simulation and Digital Image Correlation were implemented to ensure homogeneous strain of substrate and favor regular distribution of buckles. Values of energy release rates of SiO2 / Hardcoat range from 2.7 J/m² to 8.9 J/m², depending on moduli values found on wafer or lens substrate

Implicit Representations of the Human Intestines for Surgery Simulation

International audienceIn this paper, we propose a modeling of the intestines by implicit surfaces for abdominal surgery simulation. The difficulty of such a simulation comes from the animation of the intestines. As a matter of fact, the intestines are a very long tube that is not isotropically elastic, and that bends over itself at various spots, creating multiple self-contacts. We use a multiple component model for the intestines: The first component is a mechanical model of their axis; the second component is a specific sphere-based model to manage collisions and self-collisions; and the third component is a skinning model to define their volume. This paper focuses on the better representation for skinning the intestines. We compare two implicit models: Surfaces defined by point-skeletons and convolution surfaces. A direct application of this simulation is the training of a typical surgical gesture to move apart the intestines in order to reach certain areas of the abdomen

Shape and topology optimization considering anisotropic features induced by additive manufacturing processes

International audienceThis article considers the modelling of the effective properties of the constituent material of structures fabricated by additive manufacturing technologies; the influence of these properties on the design optimization process is analyzed, and the opportunities that they offer in this context are investigated. On the one hand, emphasizing on the case where the particular material extrusion techniques are used for the construction, we propose a model for the anisotropic material properties of shapes depending on the (user-defined) trajectory followed by the machine tool during the assembly of their $2d$ layers. On the other hand, we take advantage of the potential of additive manufacturing technologies for constructing very small features: we consider the optimization of the infill region of a shape with given external contour with the goal to improve at the same time its lightness and its robustness. The optimized and constraint functionals of the domain involved in the shape optimization problems in both contexts are rigorously analyzed, notably by relying on the notion of signed distance function. Eventually, several numerical experiments are conducted in two dimensions to illustrate the main points of the study

FE simulation of interfacial delamination between SiO2 thin film and polymeric substrate

International audienceOphthalmic lenses are made of plastic polymeric substrates usually coated with functional treatments composed of 5 to 15 layers, ranging from micrometers to nanometers. Each of these interfaces may lead to delamination due to poor adhesion, and therefore affect the vision and comfort of wearers. The interface between the anti-reflective stack and the hardcoat is particularly sensitive because of chemical and mechanical contrast of its materials. To better understand mechanisms that lead to loss of adhesion between the SiO2 anti-reflective layer deposited on the anti-scratch hardcoat, compression experiments are performed to induce buckling of SiO2 layer. A Finite Element Model is developed to refine characterization of interfacial properties. Simulation of buckling phenomenon and delamination of SiO2 layer under compressive stress is proposed. Interfacial properties between SiO2 layer and hardcoat are modeled using cohesive elements following a traction separation law. Geometry of buckle yielded by the model is compared against experimental results to adjust the numerical model and access interfacial properties

Robust And Scalable Learning Of Complex Dataset Topologies Via Elpigraph

Large datasets represented by multidimensional data point clouds often possess non-trivial distributions with branching trajectories and excluded regions, with the recent single-cell transcriptomic studies of developing embryo being notable examples. Reducing the complexity and producing compact and interpretable representations of such data remains a challenging task. Most of the existing computational methods are based on exploring the local data point neighbourhood relations, a step that can perform poorly in the case of multidimensional and noisy data. Here we present ElPiGraph, a scalable and robust method for approximation of datasets with complex structures which does not require computing the complete data distance matrix or the data point neighbourhood graph. This method is able to withstand high levels of noise and is capable of approximating complex topologies via principal graph ensembles that can be combined into a consensus principal graph. ElPiGraph deals efficiently with large and complex datasets in various fields from biology, where it can be used to infer gene dynamics from single-cell RNA-Seq, to astronomy, where it can be used to explore complex structures in the distribution of galaxies.Comment: 32 pages, 14 figure

Understanding mechanisms of adhesion of SiO2 thin films evaporated on a polymeric substrate

A better understanding of mechanisms of adhesion between a 200 nm thick silicon dioxide layer and a 4.5 µm thick polymeric hardcoat is indispensable for an efficient adhesion at the interface. To reach this purpose, focus is placed on two axes: finding an applicable and effective method to quantify adhesion and in parallel, characterizing mechanical properties of materials composing the system. The second axis is needed to obtain data to feed modeling codes, enabling a better analysis of the adhesion experiment. Modulus of modified Si02 was found to be roughly 20% higher than reference Si02, by nanoindentation. AFM experiments showed no difference between modified and reference Si02• Currently, an investigation to detect cracks at the interface of interest for micro-tensile test is ongoing. Adhesion tests, such as micro-compression will be performed as well

Mechanical adhesion of SiO2 thin films onto polymeric substrates

Quantification of adhesion between a 200 nm silicon dioxide layer and a 4.5 μm thick polymeric coating was performed by analysing the SiO2 buckle morphologies generated under compressive stress. Impacts of mechanical properties of SiO2 layers, as well as a surface pretreatment on adhesion, are shown. Interfacial toughness of both configurations are assessed using the Hutchinson and Suo model, which involves buckle dimensions determined in situ by an optical profilometer, and elastic modulus Ef, of the SiO2 thin films, characterised by nanoindentation. The surface pretreatment led to initiation of buckling at a higher strain. The same trend is observed for a layer with a lower stiffness and residual stress