L’humanité a-t-elle toujours « travaillé »?

Contrairement à ce que l’on peut penser, l’idéologie économique ne se limite pas à l’affirmation du dogme de l’homo oeconomicus, énoncé par les premiers théoriciens de l’économie politique classique puis repris par les tenants de la pensée néo-classique contemporaine. Avec Marx, les sciences sociales ont fait la critique de ce dogme voulant que le travail, sous sa forme marchande, existe en toute société. Mais ceci ne les aura pas empêchés d’affirmer que toute société connaît le travail en tant qu’activité visant à satisfaire les besoins matériels de l’humanité. Cette conception matérialiste du travail représente la quintessence de l’idéologie économique. Je voudrais montrer ici que l’humanité n’a pas toujours travaillé, que la constitution d’une « société de travail » est étroitement liée à l’émergence des sociétés modernes

Simulation of Vibro-Impact Systems Using Reciprocal Mass Matrices

Computation cost of explicit time integration can be reduced substantially using the reciprocal mass matrices. General variational derivation of the method, its verification by an eigenvalue benchmark and comparison on a transient example are presented in this contribution

Advanced approaches for analysis and form finding of membrane structures with finite elements

Part I deals with material modelling of woven fabric membranes. Due to their structure of crossed yarns embedded in coating, woven fabric membranes are characterised by a highly nonlinear stress-strain behaviour. In order to determine an accurate structural response of membrane structures, a suitable description of the material behaviour is required. A linear elastic orthotropic model approach, which is current practice, only allows a relative coarse approximation of the material behaviour. The present work focuses on two different material approaches: A first approach becomes evident by focusing on the meso-scale. The inhomogeneous, however periodic structure of woven fabrics motivates for microstructural modelling. An established microstructural model is considered and enhanced with regard to the coating stiffness. Secondly, an anisotropic hyperelastic material model for woven fabric membranes is considered. By performing inverse processes of parameter identification, fits of the two different material models w.r.t. measured data from a common biaxial test are shown. The results of the inversely parametrised material models are compared and discussed. Part II presents an extended approach for a simultaneous form finding and cutting patterning computation of membrane structures. The approach is formulated as an optimisation problem in which both the geometries of the equilibrium and cutting patterning configuration are initially unknown. The design objectives are minimum deviations from prescribed stresses in warp and fill direction along with minimum shear deformation. The equilibrium equations are introduced into the optimisation problem as constraints. Additional design criteria can be formulated (for the geometry of seam lines etc.). Similar to the motivation for the Updated Reference Strategy [4] the described problem is singular in the tangent plane. In both the equilibrium and the cutting patterning configuration finite element nodes can move without changing stresses. Therefore, several approaches are presented to stabilise the algorithm. The overall result of the computation is a stressed equilibrium and an unstressed cutting patterning geometry. The interaction of both configurations is described in Total Lagrangian formulation. The microstructural model, which is focused in Part I, is applied. Based on this approach, information about fibre orientation as well as the ending of fibres at cutting edges are available. As a result, more accurate results can be computed compared to simpler approaches commonly used in practice

Intrinsically Selective Mass Scaling with Hierarchic Structural Element Formulations

[EN] Hierarchic shear deformable structural element formulations possess the advantage of being intrinsically free from transverse shear locking, that is they avoid transverse shear locking a priori through reparametrization of the kinematic variables. This reparametrization results in shear deformable beam, plate and shell formulations with distinct transverse shear degrees of freedom. The basic idea of selective mass scaling within explicit dynamic analyses is to scale down the highest frequencies in order to increase the critical time step size, while keeping the low frequency modes mostly unaffected. In most concepts, this comes at the cost of nondiagonal mass matrices. In this contribution, we present first investigations on selective mass scaling for hierarchic formulations. Since hierarchic structural formulations possess distinct transverse shear degrees of freedom, they offer the intrinsic ability for selective scaling of the high frequency shear modes, while keeping the bending dominated low frequency modes mostly unaffected. The proposed instrinsically selective mass scaling concept achieves high accuracy, which is typical for selective mass scaling schemes, but in contrast to existing concepts it retains the simplicity of a conventianl mass scaling method and preserves the diagonal structure of a lumped mass matrix. As model problem, we study frequency spectra of different isogeometric Timoshenko beam formulations for a simply supported beam. We discuss the effects of transverse shear parametrization, locking and mass lumping on the accuracy of results.This work has been partially supported by the Deutsche Forschungsgemeinschaft (DFG) under grant OE 728/1-1. This support is gratefully acknowledged.Oesterle, B.; Trippmacher, J.; Tkachuk, A.; Bischoff, M. (2022). Intrinsically Selective Mass Scaling with Hierarchic Structural Element Formulations. En Proceedings of the YIC 2021 - VI ECCOMAS Young Investigators Conference. Editorial Universitat Politècnica de València. 99-108. https://doi.org/10.4995/YIC2021.2021.12418OCS9910

Improving efficiency and robustness of enhanced assumed strain elements for nonlinear problems

The enhanced assumed strain (EAS) method is one of the most frequently used methods to avoid locking in solid and structural finite elements. One issue of EAS elements in the context of geometrically nonlinear analyses is their lack of robustness in the Newton–Raphson scheme, which is characterized by the necessity of small load increments and large number of iterations. In the present work we extend the recently proposed mixed integration point (MIP) method to EAS elements in order to overcome this drawback in numerous applications. Furthermore, the MIP method is generalized to generic material models, which makes this simple method easily applicable for a broad class of problems. In the numerical simulations in this work, we compare standard strain‐based EAS elements and their MIP improved versions to elements based on the assumed stress method in order to explain when and why the MIP method allows to improve robustness. A further novelty in the present work is an inverse stress‐strain relation for a Neo‐Hookean material model

Environmental Effects over the First 2 1/2 Rotation Periods of a Fertilised Poplar Short Rotation Coppice

A short rotation coppice (SRC) with poplar was established in a randomised fertilisation experiment on sandy loam soil in Potsdam (Northeast Germany). The main objective of this study was to assess if negative environmental effects as nitrogen leaching and greenhouse gas emissions are enhanced by mineral nitrogen (N) fertiliser applied to poplar at rates of 0, 50 and 75 kg N ha−1 year−1 and how these effects are influenced by tree age with increasing number of rotation periods and cycles of organic matter decomposition and tree growth after each harvesting event. Between 2008 and 2012, the leaching of nitrate (NO3 −) was monitored with self-integrating accumulators over 6-month periods and the emissions of the greenhouse gases (GHG) nitrous oxide (N2O) and carbon dioxide (CO2) were determined in closed gas chambers. During the first 4 years of the poplar SRC, most nitrogen was lost through NO3 − leaching from the main root zone; however, there was no significant relationship to the rate of N fertilisation. On average, 5.8 kg N ha−1 year−1 (13.0 kg CO2equ) was leached from the root zone. Nitrogen leaching rates decreased in the course of the 4-year study parallel to an increase of the fine root biomass and the degree of mycorrhization. In contrast to N leaching, the loss of nitrogen by N2O emissions from the soil was very low with an average of 0.61 kg N ha−1 year−1 (182 kg CO2equ) and were also not affected by N fertilisation over the whole study period. Real CO2 emissions from the poplar soil were two orders of magnitude higher ranging between 15,122 and 19,091 kg CO2 ha−1 year−1 and followed the rotation period with enhanced emission rates in the years of harvest. As key-factors for NO3 − leaching and N2O emissions, the time after planting and after harvest and the rotation period have been identified by a mixed effects model