Microcracking in piezoelectric materials by the Boundary Element Method

A 3D boundary element model for piezoelectric polycrystalline micro-cracking is discussed in this contribution. The model is based on the boundary integral representation of the electro-mechanical behavior of individual grains and on the use of a generalized cohesive formulation for inter-granular micro-cracking. The boundary integral formulation allows to address the electro-mechanical boundary value problem in terms of generalized grain boundary and inter-granular displacements and tractions only, which implies the natural inclusion of the cohesive laws in the formulation, the simplification of the analysis pre-processing stage, and the reduction of the number of degrees of freedom of the overall analysis with respect to other popular numerical methods

Use of natural resins in repairing damaged timber beams – An experimental investigation

Different techniques including the application of steel elements, composite materials and polymeric resins have been used in the past to repair damaged timber beams. However, there is a growing need to replace these materials with those with minimal environmental impact. In addition, stringent requirements of conservation authorities on the compatibility between repair and parent materials have also necessitated search for innovative repair materials for timber beams. Therefore, an increasing shift of focus towards the use of materials derived from natural sources in repairing and reinforcing timber structures is currently experienced. This paper presents the results of an exploratory study on the use of natural resins (rosin and bone glue) in repairing oak timber beams. 15 oak timber beams with cross section dimensions of 67 x 67 mm and 1100 mm in length were tested in four-point bending to failure. Undamaged, damaged (unrepaired) and damaged but repaired timber beams (with rosin and bone glue) were tested. The effectiveness of the repair material and technique was analysed based on the bending capacity and mid span deflection at failure. The initial results show negligible effectiveness of rosin in repairing timber beams. In fact, about 16% reduction (average) in load carrying capacity with a corresponding 5% decrease (average) in maximum displacement was recorded. Relatively higher level of effectiveness was recorded with the use of bone glue (about 10 % average increase in load carrying capacity). However, over 30% corresponding average increase in the maximum displacement was also recorded. Further work investigating different repair techniques and other natural resins is presently underway

A three-dimensional boundary element model for the analysis of polycrystalline materials at the microscale

A three-dimensional multi-domain anisotropic boundary element formulation is presented for the analysis of polycrystalline microstructures. The formulation is naturally expressed in terms of intergranular displacements and tractions that play an important role in polycrystalline micromechanics, micro-damage and micro-cracking. The artificial morphology is generated by Hardcore Voronoi tessellation, which embodies the main statistical features of polycrystalline microstructures. Each crystal is modeled as an anisotropic elastic region and the integrity of the aggregate is restored by enforcing interface continuity and equilibrium between contiguous grains. The developed technique has been applied to the numerical homogenization of SiC and the obtained results agree very well with available data

Intergranular damage and fracture in polycrystalline materials. A novel 3D microstructural grain-boundary formulation

The design of advanced materials requires a deep understanding of degradation and failure pro- cesses. It is widely recognized that the macroscopic material properties depend on the features of the microstructure. The knowledge of this link, which is the main subject of Micromechanics [1], is of relevant technological interest, as it may enable the design of materials with specific requirements by means of suitable manipulations of the microstructure. Polycrystalline materials are used in many technological applications. Their microstructure is characterized by the grains morphology, size distribution, anisotropy, crystallographic orientation, stiffness and toughness mismatch and by the physical-chemical properties of the intergranular interfaces. These aspects have a direct influence on the initiation and evolution of micro-damage, which is also sensitive to the presence of micro-imperfections. Any theory trying to explain the failure mechanisms in these materials must then accommodate a relevant number of parameters. In this study, a novel 3D grain-boundary micro-mechanical model for the analysis of intergranular degradation and failure in polycrystalline materials is presented. The microstructure is generated by means of Voronoi tessellations, able to retain the main statistical features of polycrystals. The formulation is built on a boundary integral representation of the elastic problem for the crystals, that are modeled as 3D anisotropic elastic domains with arbitrary orientation [2]. This representa- tion involves only mechanical variables at the grains interfaces, i.e. displacement jumps and trac- tions, that play an important role in the micromechanics of polycrystals. The aggregate integrity is restored by enforcing suitable intergranular conditions. The onset and evolution of intergranular damage is modeled using an extrinsic irreversible cohesive law, able to address mixed-mode fail- ure conditions. Upon interface failure, a non-linear frictional contact analysis is used, to address separation, sliding or sticking between the formed micro-crack surfaces. The incremental-iterative algorithm for tracking the micro-evolution is presented. Several numerical tests on pseudo and fully three-dimensional microstructures are discussed. The present formulation is a promising tool in the framework of multiscale analysis of degradation and failure in polycrystalline materials

Computational aeroelastic analysis of wings based on the structural discontinuous Galerkin and aerodynamic vortex lattice methods

An original computational framework for the aeroelastic analysis of wings featuring general transverse section is developed. The framework is based on the coupling between a novel discontinuous Galerkin structural model and an aerodynamic vortex lattice method, which is implemented in both the planar and non-planar version. The structural model, which constitutes the novelty of the present work, allows generalized kinematics and is thus able to capture higher-order structural deformation modes. With respect to other more used structural representations, the discontinuous Galerkin approach is based on the use of discontinuous basis functions and suitably-defined boundary terms to enforce the inter-element continuity and boundary conditions. Such features naturally enable high-order accuracy, ease of parallelization and, specifically for this work, straightforward coupling with the vortex lattice method. The framework is validated through benchmark tests, providing favourable matching with reference literature data

Effects of voids and flaws on the mechanical properties and on intergranular damage and fracture for polycrystalline materials

It is widely recognized that the macroscopic material properties depend on the features of the microstructure. The understanding of the links between microscopic and macroscopic material properties, main topic of Micromechanics, is of relevant technological interest, as it may enable the deep understanding of the mechanisms governing materials degradation and failure. Polycrystalline materials are used in many engineering applications. Their microstructure is determined by distribution, size, morphology, anisotropy and orientation of the crystals. It worth noting that also the physical-chemical properties of the intergranular interfaces, as well as the presence of micro-imperfections within the microstructure, have to be taken into account, as they may have to a strong influence on onset and evolution of damage

Porosity effects on elastic properties of polycrystalline materials: a three-dimensional grain boundary formulation

Polycrystalline materials are widely used in many technological applications of engineering interest. They constitute an important class of heterogeneous materials, and the investigation of the link between their macro and micro properties, main task of the micromechanics [1], is of relevant technological concern. The internal structure of a polycrystalline material is determined by the size and the shape of the grains, by their crystallographic orientation and by different type of defects within them. In this sense, the presence of internal voids, pores, is important to take into account in the determination of the polycrystalline aggregate properties. Porosity exists in almost all materials to some extent and in particular in the polycrystalline ones; it is strictly depending by the conditions in which their construction techniques are set. However, sometimes it is desired for other than structural reasons such us, for example, heat transfer properties, radar reflection etc. For this reason the effects of porosity should be of concern to any polycrystalline material developed for a design. In particular, the macroscopic effects of the pores on polycrystalline materials elastic properties is of high interest and the Young and shear modulus are the major parameter to analyze in this case. In this study the influence of porosity presence on the elastic proprieties of polycrystalline materials is investigated and a 3D grain boundary micro mechanical model for the analysis of porosity in polycrystalline materials is used [2]. Therefore, the volume fraction of porosity, pore size and their distribution can be varied to better simulate the response of a real porous materials to a given load. The formulation is built on a boundary integral representation of the elastic problem for the single grain, that is modelled as 3D linearly elastic orthotropic domain with arbitrary spatial orientation. The artificial polycrystalline morphology is represented using the Voronoi Tessellation. This algorithms, in fact, is widely recognised and used for the generation of microstructural model and it is simple to generate the statistical features of polycrystalline microstructures. The formulation is expressed in terms of intergranular fields, namely displacement and traction that play an important role in polycrystalline micromechanics

Core-Shell Charge Transfer in Plasmonic Fe@Ag Nanoparticles on MgO Film

In this work we report the interfacial charge transfer between the Fe core and Ag shell in self-organized nanoparticles on MgO films on Mo(001). Predeposited Fe nanoparticles organize in a square network with long-range order on the oxide surface guided by the MgO coincidence lattice. When Ag is added, it covers the Fe nanoparticles, forming a shell. By means of XPS and UPS we show that a charge transfer occurs between the Fe core and the Ag shell, determining the oxidation of part of the Fe atoms and a negative charging of the Ag shell. This is confirmed by band bending and core level shifts. As a consequence of the Fe@Ag morphology and composition the plasmonic response of the nanoparticles is modified with respect to pure Ag nanoparticles