An efficient anisotropization technique to transform isotropic nonlinear materials into unidirectional and bidirectional composites

This work presents a procedure to transform an isotropic nonlinear and rate-dependent material model into orthotropic. This transformation relies on a fast computation modifying the material inelastic evolution along any desired direction. With this concept, the constitutive law of the pure isotropic, nonlinear and rate-dependent material is fully preserved. This paper shows a direct application to describe the nonlinear response of unidirectional (UD) or bidirectional (BD) fiber-reinforced polymer composites. Imposing one constraint leads to a UD composite, whilst imposing two orthogonal directions leads to a BD composite. The proposed methodology is implemented as user-defined material for Finite Element solvers. To prove the method, two different visco-plastic material models used to describe isotropic polymer resins are anisotropized to obtain their corresponding composite counterparts. The mechanical response of the simulations are compared qualitatively and quantitatively to experimental tests on a UD coupon under off-axis tensile test under different load directions. Consistency between the UD and BD anisotropization formulations is proved under tension, shear and different loading directions. Additionally, a complete study of computational performance is addressed to assess the anisotropization's feasibility to be applied in modeling and design of new laminated and multilayer-based structures exhibiting nonlinear material response

The role of supply chain strategies and supply chain integration on financial performance. An empirical study in Peru

El objetivo de este estudio es desarrollar y probar un marco para el papel que las estrategias de la cadena de suministro (SCS) y la integración de la cadena de suministro tienen del rendimiento financiero de una empresa y aumentar la comprensión del papel que estos factores tienen en el diseño de la cadena de suministro. Se utilizó la modelización de ecuaciones estructurales para probar estas relaciones a partir de datos tomados de pequeñas y medianas empresas (PYMES) exportadoras peruanas. Nuestro estudio responde a un vacío en la comprensión del papel de la cadena de suministro en las PYME, así como a llenar el vacío de cómo las empresas en América Latina, especialmente en Perú, aplican los conceptos de la cadena de suministro. Los resultados indican que las empresas deben priorizar su esfuerzo de integración dependiendo del tipo de estrategia de la cadena de suministro. Asimismo, los resultados muestran que la integración de los clientes está directamente relacionada con el rendimiento financiero de la empresa. Este estudio responde a la necesidad de encontrar respuestas al desarrollo de estrategias de cadena de suministro y a la generación de ventajas competitivas en las PYMES exportadoras peruanas.The objective of this study is to develop and test a framework for the role that supply chain strategy (SCS) and supply chain integration have in a firm’s financial performance and to increase the understanding of the role that these factors play in supply chain design. Structural equation modeling was used to test these relationships based on data obtained from small and medium exporting enterprises in Peru. This study responds to a gap in understanding the role of supply chains in small and medium enterprises (SMEs) and how firms in Latin America, especially in Peru, apply supply chain concepts. Findings indicate that companies should prioritize their integration efforts depending on the type of supply chain strategy. Likewise, results show that customer integration is directly related to a firm’s financial performance. This study responds to the need to understand the development of supply chain strategies and the generation of competitive advantage in Peruvian export-manufacturing SMEs

Universidad franquista: debate sobre la libertad de enseñanza (1939-1962). [Reseña]

Reseña de: María Ángeles SOTES ELIZALDE, Universidad franquista: debate sobre la libertad de enseñanza (1939-1962), EUNSA, Pamplona 2004, 585 pp

Macro- and micro-modeling of crack propagation in encapsulation-based self-healing materials : application of XFEM and cohesive surface techniques

Encapsulation-based materials are produced introducing some small healing fluid-filled capsules in a matrix. These materials can self-heal when internal cracks intercept and break the capsules. If the healing agent is released, the crack can be sealed. However, this is not always the case. These capsules need to be designed with the adequate shape and material to be properly broken. This paper presents two application models based on the combination of eXtended Finite Element Method (XFEM) elements and Cohesive Surfaces technique (CS) to predict crack propagation. Two types of encapsulated systems are considered: a concrete beam in a three-point bending test, and a micro-scale model of a representative volume element of a polymer subjected to a uniaxial tensile test. Despite both systems rely on different capsule shapes and different constituent materials, the models predict a similar non-linear response of the overall material strength governed by the coupled effect of the interface strength and the capsule radii-to-thickness ratio. Furthermore, even if an inadequate material and geometry combination is used, it is found that the mere presence of capsules might achieve, under certain conditions, an interesting overall reinforcement effect. This effect is discussed in terms of clustering and volume fraction of capsules

Discrete simulations of model, loose cohesive powders: plastic consolidation, fractal microstructure and tensile strength

4 pagesInternational audienceIsotropic packings of cohesive disks in 2D are studied by discrete, stress-controlled numerical simulations. Depending on the assembling process and on whether contacts possess rolling resistance (RR), configurations form under low pressure $P$ with varying solid fraction $\Phi$ and fractal dimension $d_F$ describing small scale correlations below some blob size $\xi$. The gradual collapse observed under growing $P$ is described as a linear relation between $\ln P$ and $1/\Phi$ within some range, once the influence of initial conditions has faded out, and until a maximum density is approached. This corresponds to a decrease of $\xi$ that is compatible with the fractal blob model. The isotropic tensile strength is always considerably smaller than the naive Rumpf estimate, and grows with consolidation. Coordination numbers in systems with small RR change little while density increases by large amounts in consolidation

Computer simulation of model cohesive powders: influence of assembling procedure and contact laws on low consolidation states.

31 pages Only 26 pages in the published PRE paper, but figures look better in THIS version !International audienceThe structure and mechanical properties of a simple two-dimensional model of a cohesive powder are investigated by molecular dynamics simulations. Micromechanical ingredients involve elasticity, friction, a short range attraction and, possibly, rolling resistance (RR) in contacts. The microstructure of the cohesive packing varies according to the assembling procedure, from rather densely packed if isolated particles are directly compressed to much looser if the formation of large aggregates is allowed prior to compression. A crucial parameter is the ratio P*= Pa/F0 of applied pressure P, acting on grains of diameter $a$, to maximum tensile contact force F0. At low P* the final structure depends on the level of velocity fluctuations at the early stages of cluster aggregation. With RR the coordination number approaches 2 in the limit of low initial velocities or large rolling friction. The force network generally comprises small hyperstatic clusters with forces of the order of F0, joined by nearly unstressed, barely rigid arms. As P* grows, it quickly rearranges into force chain-like patterns. Density correlations witness a fractal structure, with dimension Df, up to some density-dependent blob size. WIth RR Df coincides with the ballistic aggregation result, despite a possibly different connectivity. Possible effects of some parameters on material strength are evoked