Influence du vieillissement en humidité relative sur les propriétés mécaniques des fibres de chanvre

L'effet du vieillissement des fibres naturelles dans un matériau composite modifie fortement leurs propriétés mécaniques initiales. Ce travail consiste à étudier l'influence de différents facteurs de vieillissement (humidité, température, lumière du jour) sur le comportement mécanique des fibres de chanvre. Ces fibres vierges et vieillies sont caractérisées par des essais de micro traction. Des analyses microscopiques sur le faciès de rupture, la variation de masse et la largeur des fibres permettent de comprendre le mécanisme de la perte des propriétés mécaniques par vieillissement

Acoustic properties of air-saturated porous materials containing dead-end porosity

International audienceThis study examines the acoustic properties of materials with complex micro-geometry containing partially open or dead-end porosity. One of these kinds of materials can be obtained from dissolving salt grains embedded in a solid metal matrix with the help of water. The solid matrix is obtained after the metal in liquid form has invaded the granular material formed by the salt particles at negative pressure and high temperature, and after cooling and solidification of the metal. Comparisons between theoretical and experimental results show that the classical Johnson-Champoux-Allard model does not quite accurately predict the acoustic behavior. These results suggest that the assumptions of the Biot theory may not be all fulfilled and that Helmholtz-type resonators and dead ends can be present in the material. The first part of the study proposes a simple model to account for this geometry. Based on this model, two acoustic transfer matrices are developed: one for non symmetric and one for symmetric dead-end porous elements. This model can be used to study the acoustic absorption and sound transmission properties of the type of material described above. In the second part, a series of simplified samples are proposed and tested with a three-microphone impedance tube to validate the exposed model. Finally, the third part compares predictions of the exposed model to impedance tube results on a real aluminum foam sample containing dead-end pores

Independent director network, agency costs and stock price crash risk

It is of great significance to improve the corporate governance structure to study whether independent directors play the role of ‘vase’ in the governance of listed companies. Based on the social network theory, this article constructs the social network formed by interlocking independent directors and examines the influence of independent director network on stock price crash risk. The mechanism test analyses the mediating effect of principal–agent problem and large shareholder’s tunnelling on stock price crash risk. The empirical research shows that the higher the network centrality of the company’s independent directors, the lower the stock price crash risk. The independent director network can restrain the company’s stock price crash risk by reducing two types of agency costs. Further research finds that the influence of independent director network on stock price crash risk is more pronounced in companies with unreasonable ownership structure, poor internal governance and weak external supervision. The research conclusions have important implications for listed companies to reduce the risk of stock price crash and maintain the stability of the capitalmarket

Secondary Impact of Manganese on the Catalytic Properties of Nitrogen-Doped Graphene in the Hydrogen Evolution Reaction

Catalysts play a key role in hydrogen production as green energy carriers. We show herein for the first time that manganese impurities in graphene can improve the catalytic activity of synthesized N‐doped graphene (NG) for the hydrogen evolution reaction in acid media by influencing the ratio of different N‐functionalities. A 122 mV improvement in the overpotential was found following the Mn impregnation of graphene. Transmission electron microscopy images confirmed the formation of manganese oxide nanoparticles on the NG sheets. X‐ray photoelectron spectroscopy revealed structural alteration in favor of higher quantities of quaternary and pyrrolic nitrogen functionalities, from approximately 37 % in NG to 84 % in the Mn‐inserted‐doped graphene catalyst. This enhanced catalytic performance, based on density functional theory calculations in the literature, was attributed to an increase in the number of active sites with higher activity