Equivalence Theory Applied to Anisotropic Thin Plates

We extend the Equivalence Theory (ET) formulated by Absi [1] for the statics of isotropic materials to the statics and dynamics of orthotropic materials. That theory relies on the assumption that any real body modeling may be substituted by another one that, even though it may possibly have material constitutive laws and geomet- ric properties with no physical sense (like negative cross sections or Young modulus), is intended to be more advantageous for calculus. In our approach, the equivalence is expressed by equating both the effective strain energies of the two models and the material structural weights in dynamics [2]. We provide a numerical analysis of the convergence properties of ET approach while comparing its numerical results with those predicted by the analytical theory and the Finite Elements Method for thin plates. [1] E. ABSI, “La Theorie des Equivalences et Son Application a l’Etude des Ouvrages d’Art,” Série: Théories et Méthodes de Calcul, Annales de l’Institut Technique du Bâtiment et des Travaux Publics, Supplément No. 298, Octobre 1972. [2] M. Haddad, “Application de la Methode des Equivalences en Dynamique,” Rapport de Stage Master2 Recherche, l’Institut Supérieur de l’Aéronautique et de l’Espace (ISAE), Toulouse, 7 February 2010

War and deforestation in Sierra Leone

The impact of armed conflict on the environment is of major public policy importance. We use a geographically disaggregated dataset of civil war violence together with satellite imagery of land cover to test whether war facilitated or prevented forest loss in Sierra Leone. The conflict data set allows us to establish where rebel groups were stationed and where battles and attacks occurred. The satellite data enables to us to monitor the change in forest cover (total, primary, and secondary) in all of Sierra Leone's 151 chiefdoms, between 1990 (prior to the war) and 2000 (just prior to its end). The results suggest that conflict in Sierra Leone acted as a brake on local deforestation: conflict-ridden areas experienced significantly less forest loss relative to their more conflict-free counterparts

Approche fiabiliste pour l’optimisation locale d’un problème couplé fluide-structure

Le projet OSYCAF (Optimisation d’un Système Couplé fluide/structure représentant une Aile Flexible) a pour objectif de proposer une méthodologie d’optimisation multidisciplinaire dans un contexte aéronautique. Plus précisément, il s’agit d’optimiser une aile d’avion en tenant compte des interactions fluide-structure. Les modèles de mécanique des fluides et des structures sont des disciplines devant communiquer entre elles et avec l’optimiseur global. L’optimisation est réalisée sur deux niveaux : par rapport aux paramètres globaux, communs aux deux disciplines, et par rapport aux paramètres locaux, propres à chacune. Le travail présente l’optimisation de la structure par rapport aux paramètres locaux. Dans ce cadre, il est proposé d’introduire des incertitudes probabilistes permettant de tenir compte de contraintes de fiabilité

Sur la modélisation quasi-continue d'un modèle discret unidimensionnel de Cosserat

Certains problèmes liés à la discrétisation des milieux continus micropolaires (ou de Cosserat) linéairement élastiques ou, inversement, à la continualisation des milieux granulaires analogues, sont illustrés via l'analyse du comportement statique d'une simple chaîne monoatomique et la dérivation de son (ses) modèle(s) de poutre(s) équivalente(s) qui peut être soit locale (Timoshenko,Euler-Bernoulli) ou non-locale (Eringen,Kunin), en tenant compte de ses propriétés spectrales et analytiques ambigües

An atomistically-meaningful pseudocontinuum representation for the finite monatomic chain with harmonic nearest-neighbor interactions

An atomistically-meaningful pseudocontinuum representation for the nontrivial lattice dynamics of a fi- nite monatomic chain with linear elastic interactions between nearest neighbor atoms is analytically de- duced by mean of a dynamic mechanical analysis extending the memory-dependent pseudocontinuum viewpoint suggested in [M. Charlotte and L. Truskinovsky, Lattice dynamics from a continuum viewpoint , J. Mech. Phys. Solids, 60, pages 1508–1544 (2012)]. For a correct description of the lattice dynamics at its interstice length scale, the pseudocontinuum model integrates both the bulk and boundary inertial (heat- vibration) effects of the atomistic medium through specific modifications of the classical elastodynamic Newton’s law model: these modifications involve a generalization of the D’Alembert’s principle of inertial forces and Neumann-Robin’s boundary conditions, without increasing the number of initial and boundary conditions of the generic mechanical evolution problem, unlike all other generalized continuum models proposed in the literature up to this date. Owing to the spatially local and one-dimensional nature of the discrete and pseudocontinuum models, relationships are thus more clearly pinpointed between the elastodynamic normal stress field of that exact generalized continuum representation and the cohesive (or internal) and inertial forces operating at the lattice sites within the bulk of a finite-size monatomic chain and at its boundary

Stress-based topology optimization of compliant mechanisms using nonlinear mechanics

The present work demonstrates how a light structure can be easily designed through Topology Optimization even including complex analysis and sizing criteria such as hyperelastic Neo-Hookean materials for nonlinear analysis and aggregated stress constraints. The SIMP approach was adopted and two different strategies were analysed using an in house versatile MATLAB code. MMA was used as reference optimizer (in structural optimization) whereas a unified aggregation and relaxation method was adopted to deal with stress constraints. Feasibility was analyzed from the viewpoint of allowable stress verification. Two test cases are then studied: a morphing airfoil (for aeronautical applications) and a geometric inverter (for mechanics and bio- medical applications). For both, a hyperelastic Neo-Hookean material was chosen. Finally a complementary study on the effects of constraints and the input force intensity is also presented

On some properties of the compliance-volume fraction Pareto front in topology optimization useful for material selection

Selecting the optimal material for a part designed through topology optimization is a complex problem. The shape and properties of the Pareto front plays an important role in this selection. In this paper we show that the compliance-volume fraction Pareto fronts of some topology optimization problems in linear elasticity share some useful properties. These properties provide an interesting point of view on the efficiency of topology optimization compared to other design approaches such as parametric structural optimization. We construct a simple meta-model which requires only one full topology optimization to fit the whole Pareto fronts. Precise Pareto fronts are obtained independently. The fast meta-model constructed has a maximum error of 6.4% with respect to these precise Pareto fronts, on the different problems tested. The selection of the optimal material is then successfully tested on the mass minimization of an MBB beam with an illustrative choice of 4 materials.Comment: including proof

When are females dominant over males in rats (Rattus norvegicus)?

Funding was provided to MAP-E (scholarship program 783–2017 of the Ministry of Science, Technology and Innovation – Minciencias, formerly Colciencias), and to CKH (Lucie Burgers Foundation grant, for the research project titled “Sex and competitive dynamics in groups of rats: is there resemblance to primates?”).In group-living animals, males are assumed to be dominant over females when they are larger than females. Despite this, females have sometimes been proven to be dominant over some males possibly via the winner-loser effect, which becomes clearer when the intensity of aggression in the group is higher. To test whether the winner-loser effect can lead to (partial) female dominance in a species with a pronounced sexual dimorphism, we studied the hierarchy in 12 rat colonies (Rattus norvegicus) in which the rats could freely interact with their group members within a spacious area. To investigate the underlying mechanisms, we compared the empirical data to hypotheses generated by the agent-based model ‘DomWorld’. We show that females dominated on average 55% of the males, and occupied the alpha position in four colonies, in three of them they shared it with one or several males. Moreover, in line with the predictions of the computational model, females dominated a higher percentage of males when the intensity of aggression of the colony was higher. This shows that although females are only half as heavy as males, they dominate part of the males probably through the winner-loser effect. We suggest that this effect may be widespread in many other species and can be tested experimentally.Peer reviewe

The benefit of a tough skin: Bullet holes, weathering and the preservation of heritage

© 2017 The Authors. Projectile damage to building stone is a widespread phenomenon. Sites damaged 100 years ago during the First World War still see daily use, while in a more contemporary setting numerous reports show the damage to buildings in Babylon, Mosul and Palmyra. While research has been carried out on the long-term effects of conflict such as fire damage, little is known about the protracted damage sustained through the impact of bullets, shrapnel and other metal projectiles outside of the field of engineering focused on ceramics and metals. To investigate alterations to mineral structure caused by projectile damage, impacts were created in medium-grained, well-compacted, mesoporous sandstone samples using 0.22 calibre lead bullets shot at a distance of 20 m. Half these samples were treated with a surface consolidant (Wacker OH 100), to mimic natural cementation of the rock surface. These samples were then tested for changes to surface hardness and moisture movement during temperature cycles of 15–65°C. Petrographic thin section analysis was carried out to investigate the micro-scale deformation associated with high-speed impact. The results surprisingly show that stress build-up behind pre-existing cementation of the surface, as found in heritage sites that have been exposed to moisture and temperature fluctuations for longer periods of time, can be alleviated with a bullet impact

Structural wingbox optimization in a coupled FSI problem of a flexible wing: FEA sol200 versus surrogate models

This work presents a two-step approach that was adopted in a collaborative multi- disciplinary work named OSYCAF for wing-box design optimization. This approach encompasses: 1) the initialization of a full parametric PCL flexible wing optimization with analytical design, and 2) the comparison of the sol200 optimization (mass of the wing) with a simple surrogate model (also known as Reduced Order Model due to the quadratic form in the regression). Our main objective is to optimize the global structure weight while respecting all structural criteria and constraints, and using the spars and skin thickness as design variables. We show that after the optimization the importance of upper and lower skins is minimized and almost all efforts are concentrated on spars, specially the rear spar. It is also shown that the strain criterion is stronger than the stress one, which considers shear and buckling deformations as the critical design points, although fatigue is also relevant when designing the lower Wing-Box Skin. We show the results obtained for the local optimization of several considered NACA-4 profiles by using an automated process.This work is developed such that an association with an aerodynamic approach using CFD would make possible to create a variation of the required profile to construct the real wing that, when deformed, would assume its best shape in terms of aerodynamics, still respecting all structural constraints and minimum weight possible