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Using contact dynamics simulations, we compare the effect of rolling resistance at the contacts in granular systems composed of disks with the effect of angularity in granular systems composed of regular polygonal particles. In simple shear conditions, we consider four aspects of the mechanical behavior of these systems in the steady state: shear strength, solid fraction, force and fabric anisotropies, and probability distribution of contact forces. Our main finding is that, based on the energy dissipation associated with relative rotation between two particles in contact, the effect of rolling resistance can explicitly be identified with that of the number of sides in a regular polygonal particle. This finding supports the use of rolling resistance as a shape parameter accounting for particle angularity and shows unambiguously that one of the main influencing factors behind the mechanical behavior of granular systems composed of noncircular particles is the partial hindrance of rotations as a result of angular particle shape.

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RESUMEN

This is the first of two papers investigating the mechanical response of cemented granular materials by means of contact dynamics simulations. In this paper, a two-dimensional polydisperse sample with high-void ratio is constructed and then sheared in a simple shear numerical device at different confinement levels. We study the macroscopic response of the material in terms of mean and deviatoric stresses and strains. We show that the introduction of a local force scale, i.e., the tensile strength of the cemented bonds, causes the material to behave in a rigid-plastic fashion, so that a yield surface can be easily determined. This yield surface has a concave-down shape in the mean:deviatoric stress plane and it approaches a straight line, i.e., a Coulomb strength envelope, in the limit of a very dense granular material. Beyond yielding, the cemented structure gradually degrades until the material eventually behaves as a cohesionless granular material. Strain localization is also investigated, showing that the strains concentrate in a shear band whose thickness increases with the confining stress. The void ratio inside the shear band at the steady state is shown to be a material property that depends only on contact parameters.

RESUMEN

This is the second of two papers investigating the mechanical response of cemented granular materials by means of contact dynamics simulations. In this paper, a two-dimensional polydisperse sample with high void ratio is sheared in a load-controlled simple shear numerical device until the stress state of the sample reaches the yield stress. We first study the stress transmission properties of the granular material in terms of the fabric of different subsets of contacts characterized by the magnitude of their normal forces. This analysis highlights the existence of a peculiar force carrying structure in the cemented material, which is reminiscent of the bimodal stress transmission reported for cohesionless granular media. Then, the evolution of contact forces and torques is investigated trying to identify the micromechanical conditions that trigger macroscopic yielding. It is shown that global failure can be associated to the apparition of a group of particles whose contacts fulfill at least one of the local rupture conditions. In particular, these particles form a large region that percolates through the sample at the moment of failure, evidencing the relationship between macroscopic yielding and the emergence of large-scale correlations in the system.

RESUMEN

La yucuyahui (zoapatle-Montanoa tomentosa) es una planta silvestre con propiedades oxitócicas. Tomaron infusiones de esta planta durante el trabajo de parto ocho embarazadas y sus recién nacidos evolucionaron con signos de depresión cardio-respiratoria que requirió de apoyo vital básico o avanzado para su recuperación. La efectividad de estas maniobras se midió de acuerdo al puntaje de Apgar que promediado para el primer minuto fue de 4.5 y se incrementó a los 10 minutos por efecto de las maniobras citadas, a 7.4 (p< 0.05). El tiempo para la recuperación total de los neonatos fue de 11.9 horas (10 minutos - 36 horas). Se encontró una correlación negativa entre el número de infusiones administradas a las madres y la calificación de Apgar al minuto (r=- 0.65), sugiriendo que a mayor número de infusiones menor puntaje y por tanto mayor depresión. La administración simultánea de bebidas alcohólicas a tres de las embarazadas pareció no influir en la gravedad de la depresión; la recuperación de los neonatos expuestos al etanol fue más rápida que en aquellos no expuestos: 2.4 vs. 9.7 horas, p< 0.05. Se desconoce cuál es el mecanismo íntimo de acción de Montanoa tomentosa, se especula que pudiera ser similar al de otros oxitócicos como la oxitocina y los alcaloides del cornezuelo de centeno, con los que guarda semejanzas en sus efectos uterotónicos y sistémicos

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