Abstract

Roughly speaking, granular media exhibit three basic scales: the specimen scale, the contact scale, and an intermediate scale made up of a set of adjoining particles. In this lecture, we will discuss this latter scale, in a two dimensional context. More specifically, the granular assembly can be regarded as a two phases medium. Grain column like patterns (force chains) develop within the medium, participating actively to its mechanical strength. These columns are surrounded by grain loops, made up of 3, 4, 5, or 6 grains (larger grain loops are much less frequent). According to the number of constituting grains, the mechanical properties of grain loops are very different. In particular, 6 grains loops are prone to deform, contributing locally to a change in the void ratio. On the contrary, 3 grains loops deform just a little, but resist quite well to a deviatoric stress. According to the initial porosity of the assembly, and depending upon the loading path considered, the nature of grain loops surrounding force chains is versatile, with continuous transition mainly from 3 grains loops to 6 grains loops (or vice versa). This is a new route to investigate from a microstructural point of view why a granular assembly may be destabilized, leading to a localized or diffuse failure pattern. In addition, these ingredients are shown to give rise to a microstructural interpretation of the socalled critical state, according to the failure mode taking place.

Abstract

Abstract

Abstract

Among the numerous strategies available for suppressing/decreasing delamination of laminated composite materials, the current work is focused on the tufting process. A literature study is carried out to underscore the advantages of this technology over alternative manufacturing techniques.  The current paper presents with a mesomechanical composite modeling approach, based on a MMF (MicroMechanics of Failure) micromechanics scheme.  The type of composite material analyzed here is a conventional ply laminate tufted along its thickness.  In terms of mesomechanics, a new unit cell is described and formulated. In order to assess the accuracy of the current approach, three theory-experiment correlations studies have been carried out, the results obtained being quite accurate.

Abstract

The non-linear behaviour ocurring in symmetrical angle-ply laminates formed by unidirectional layers of carbon fiber reinforced polymeric matrix (CFRP), known as pseudo-ductility, has been previously studied numerically and experimentally under different uniaxial, biaxial and bending loads. This phenomenon is associated with different mechanisms which affect the mechanical behaviour of the material: large permanent strains, fiber reorientation and matrix cracking. The present study is based on a microscale approach through simulations based on the Finite Element Method (FEM), with the aim of predicting the homogenized response of these laminates subjected to multiaxial in-plane. The micromechanical model is based on a Representative Volume Element (RVE) where Periodic Boundary Conditions (PBC) are applied, together with a non-linear material model of the matrix and a cohesive interface between fibre and matrix. In addition, the implementation of the model is updated to be able to reproduce any plane stress state in a [±45]_S laminate, without the need to add other meso- or macroscale models.

Abstract

The prediction of local field statistics from effective properties is an open problem in the field of micromechanics. Partial information on the local field statistics is accessible from homogenization assumptions. In particular, exact phase-wise second moments of stresses can be calculated analytically from the effective strain energy density. In recent years, full-field calculations have become efficient enough to sample large ensembles of microstructures in the plastic regime (e.g. Gehrig et. al [4]). In the present work, the maximum entropy method known from statistical thermodynamics is used to estimate first and second moments of local stresses from known eigenstrain distributions. The simple and refined formulations of the maximum entropy method proposed by Kreher and Pompe [9] are considered. While the simple method yields satisfactory results for a large amount of material classes (cf. Krause and Böhlke [7]), we prove that it does not respect the linearity of the eigenstrain problem. We further show that neither method corresponds to the exact second moments of stresses known from the effective strain energy density. By incorporating additional information, we find an improved maximum entropy method. As an example, we analyze stress fluctuations in polycristalline titanium.For the exact analytical solution and the maximum entropy methods, we use the singular approximation and the Hashin-Shtrikman bounds. For comparison, we numerically approximate full-field statistics using an FFT approach. In all methods, the stress fluctuations caused by the anisotropy of the single crystal strongly influence the elastic-plastic transition.

Abstract

This research shows a novel in-situ experimental approach to characterize a carbon-epoxy unidirectional composite system interface. Computational models were implemented to support the experimental campaign and identify the interface properties. After validating the computational model with experimental results, it was employed to study the behaviour of interface debonding under both mode I and mode II damage. By utilizing the validated model, it becomes possible to assess the consistency of the proposed experimental approach in characterizing the interface, with the aim of generating in future an analytical model able to predict interface characteristics. 

Abstract

Abstract

This work presents the design and manufacture of the structural tail (subchassis+seat) of a racing motorbike which has participated in the MotoStudent 2016 edition, held in October 2016 on the Alcañiz circuit, representing the University of Malaga. Two motorcycles were presented, one inside the Petrol category (4 stroke engine and 250 cc) and another one in the Electric category. It is a self-supporting structure that replaces the use of the traditional subchassis and seat, in order to save weight. As a material we selected a 2x2 twill carbon fabric 3K, 200 g/m2 and Resoltech 1070/1074 epoxy resin. The selected manufacturing process has been manual molding and vacuum bag. Taking into account the estimated fiber percentage, the properties of the elemental sheet of composite material have been estimated using micromechanical models. These properties have been implemented in the finite element program Patran-Nastran for the optimization of the part. Once it has been designed, a model of the part has been fabricated on MDF wood which has been used to manufacture the mold of the piece in fiberglass. The designed part was homologated by the organization of the competition. The developed part is 50% lighter that previous solution. The manufactured mold allows to produce more pieces with the consequent cost reduction.