An efficient numerical integration algorithm for the single mode compressible Leonov model – Complas XI

In this contribution, an algorithm for numerical integration of the Leonov elastoviscoplastic model is proposed. The operator split methodology and the Newton-Raphson method are used to derive the state update algorithm and obtain the numerical solution of the discretized evolution equations. Particular effort is devoted to the reduction of the number of required residual equations in order to have a more efficient numerical implementation. The consistent tangent module is expressed in a closed form as a result of the exact linearization of the discretized evolution equations. The performance of the algorithm is validated through comparison with existing experimental data

NURBS distance fields for extremely curved cracks

This paper presents the first methodology that combines a meshless method and the exact representation of cracks using Non-Uniform Rational B-Splines (NURBS). The methodology consists on developing an enrichment function based on distance functions to NURBS curves.The examples show the potential of the proposed approach and demonstrate the applicability to problems involving complex cracks that appear in sol-gel films

Oxygen reduction reaction features in neutral media on glassy carbon electrode functionalized by chemically prepared gold nanoparticles

Gold nanoparticles (AuNPs) were prepared by chemical route using 4 different protocols by varying reducer, stabilizing agent and solvent mixture. The obtained AuNPs were characterized by transmission electronic microscopy (TEM), UV-Visible and zeta potential measurements. From these latter surface charge densities were calculated to evidence the effect of the solvent mixture on AuNPs stability. The AuNPs were then deposited onto glassy carbon (GC) electrodes by drop-casting and the resulting deposits were characterized by cyclic voltammetry (CV) in H2SO4 and field emission gun scanning electron microscopy (FEG-SEM). The electrochemical kinetic parameters of the 4 different modified electrodes towards oxygen reduction reaction (ORR) in neutral NaCl-NaHCO3 media (0.15 M / 0.028 M, pH 7.4) were evaluated by rotating disk electrode voltammetry and subsequent Koutecky-Levich treatment. Contrary to what we previously obtained with electrodeposited AuNPs [Gotti et al., Electrochim. Acta 2014], the highest cathodic transfer coefficients were not obtained on the smallest particles, highlighting the influence of the stabilizing ligand together with the deposits morphology on the ORR kinetics

The quest for stiff, strong and tough hybrid materials: An exhaustive exploration

Howto arrange soft materials with strong but brittle reinforcements to achieve attractive combinations of stiffness, strength and toughness is an ongoing and fascinating question in engineering and biological materials science. Recent advances in topology optimization and bioinspiration have brought interesting answers to this question, but they provide only small windows into the vast design space associated with this problem. Here, we take a more global approach in which we assess the mechanical performance of thousands of possible microstructures. This exhaustive exploration gives a global picture of structure-property relationships and guarantees that global optima can be found. Landscapes of optimum solutions for different combinations of desired properties can also be created, revealing the robustness of each of the solutions. Interestingly, while some of the major hybrid designs used in engineering are absent from the set of solutions, the microstructures emerging from this process are reminiscent of materials, such as bone, nacre or spider silk.</p

Bio-inspired nacre-like composites via simple, fast, and versatile techniques such as doctor-blading

Theoretical and experimental studies show that the high performance of biological composites such as nacre and bone originates from a sophisticated microstructure, where hard and stiff inclusions form a staggered, brick wall-like structure within a softer and more deformable matrix. This morphology results in outstanding combinations of stiffness, strength and toughness, and therefore it is very attractive to duplicate it in engineering composites. Here, we demonstrate how simple, fast and versatile techniques such as doctor-blading can be used to make such bio-inspired composites. We fabricated and characterized composites made of micron-sized alumina tablets embedded in epoxy matrices. Scanning Electron Microscopy (SEM) images show that the tablets are well dispersed, aligned, and staggered through the polymer matrix resulting in a nacre-like material. The tensile behavior of these composites shows a good combination of stiffness, strength and energy dissipation. We also developed finite element models of the staggered microstructure, which properly capture the interactions between inclusions and the effects of mineral concentration. These models can be used to optimize the microstructure and fully harness the nacre-like structure and mechanisms, in new materials with applications in aerospace, defense or biomedical engineering.</p

Nacre-like materials using a simple doctor blading technique: Fabrication, testing and modeling

The remarkable mechanical performance of biological materials such as bone, nacre, and spider silk stems from their staggered microstructure in which stiff and strong reinforcements are elongated in the direction of loading, separated by softer interfaces, and shifted relative to each other. This structure results in useful combinations of modulus, strength and toughness and therefore is increasingly mimicked in bio-inspired engineering composites. Here, we report the use of a simple and versatile technique based on doctor-blading to fabricate staggered composites of microscopic alumina tablets with high alignment in a chitosan matrix. Tensile tests on these nacre-like materials show that the modulus and strength of the composite films are enhanced by the incorporation of ceramic tablets, but only up to 15 vol% after which all properties degrade. This phenomenon, also reported in the past for most of nacre-like materials, composed of micro/nano tablets, obtained from different techniques, has been limiting our ability to produce large volumes of high-performance nacre-like materials. Examination of the structure of the films revealed that at lower tablet concentrations the tablets are well-aligned and well dispersed thorough the volume of the film. At 15 vol% and beyond, we observed tablet misalignment and clustering. In order to investigate the impact of these imperfections on material performance we developed large scale finite element models representative of the structure of the composite films. These models show that the mechanical performance significantly degrades with tablet misalignment, and especially at high tablet concentrations. The simulations along with the SEM images therefore quantitatively explain the experimental trends, e.g. the degradation of mechanical properties at high tablet contents.</p

Carving 3D architectures within glass: Exploring new strategies to transform the mechanics and performance of materials

Combining high strength, hardness and high toughness remains a tremendous challenge in materials engineering. Interestingly nature overcomes this limitation, with materials such as bone which display unusual combinations of these properties in spite of their weak constituents. In these materials, highly mineralized “building-blocks” provide stiffness and strength, while weak interfaces between the blocks channel non-linear deformation and trigger powerful toughening mechanisms. This strategy is also exploited in multilayered ceramics, fiber-reinforced composites, and more recently in topologically-interlocked materials. In this work we apply these concepts to the toughening of glass panels by incorporating internal architectures carved within the material using three-dimensional laser engraving. Glass is relatively stiff and hard but it has no microstructure, no inelastic deformation mechanism, low toughness and poor resistance to impacts. We demonstrate how introducing controlled architectures in glass completely changes how this material deforms and fails. In particular, our new architectured glass panels can resist about two to four times more impact energy than plain glass. Our architectured glass also displays non-linear deformation, progressive damage and failure contained within a few building blocks. This work demonstrates how micro-architecture, bio-inspiration and top-down fabrication strategies provide new pathways to transform the mechanics and performance of materials and structures.</p

Discrete element models for the deformation and fracture of biological composites

We use the discrete element method to capture the deformation and fracture behavior of staggered composites with up to 10,000 inclusions. We find that in tension strain localization is accelerated by statistical variations in the microstructure, but that this effect can be offset with strain hardening at the interfaces. We also measure the fracture toughness of this microstructure using a notched compact tension geometry and J-integrals. The fracture model captures the effects of bridging, inelastic process zone, statistics and hardening on the crack resistance curve. These findings provide a better understanding of the mechanics of natural composites, and can serve as guidelines for the design of bio-inspired materials</p

Overcoming the brittleness of glass through bio-inspiration and micro-architecture

Highly mineralized natural materials such as teeth or mollusk shells boast unusual combinations of stiffness, strength and toughness currently unmatched by engineering materials. While high mineral contents provide stiffness and hardness, these materials also contain weaker interfaces with intricate architectures, which can channel propagating cracks into toughening configurations. Here we report the implementation of these features into glass, using a laser engraving technique. Three-dimensional arrays of laser-generated microcracks can deflect and guide larger incoming cracks, following the concept of 'stamp holes'. Jigsaw-like interfaces, infiltrated with polyurethane, furthermore channel cracks into interlocking configurations and pullout mechanisms, significantly enhancing energy dissipation and toughness. Compared with standard glass, which has no microstructure and is brittle, our bio-inspired glass displays built-in mechanisms that make it more deformable and 200 times tougher. This bio-inspired approach, based on carefully architectured interfaces, provides a new pathway to toughening glasses, ceramics or other hard and brittle materials.</p