Recommandations pour une récolte durable de biomasse forestière pour l'énergie - Focus sur les menus bois et les souches

Les modalités d'exploitation du bois pour la production d'énergie ont fortement évolué ces dix dernières années, notamment dans le contexte de la transition énergétique. Des compartiments de bois laissés en forêt (menus bois notamment), sont ainsi de plus en plus mobilisés. Le prélèvement accru de bois pour l'énergie peut engendrer, dans certaines conditions, des perturbations de l'environnement (sols, biodiversité, eaux). Ce document de recommandations s'adresse aux acteurs de la filière bois-énergie et fait état des précautions à prendre lors de la récolte des menus bois (diamètre < 7 cm) et des souches, et pour réduire au maximum l'exportation de feuillage. Les autres formes de bois énergie, en particulier le bois bûche, ne sont pas concernés. Les recommandations se fondent sur le diagnostic de la sensibilité des sols à faire face à l'exportation de nutriments et sur la situation éventuelle de chantier envisagé au sein d'une zone protégée vis-à-vis de la biodiversité

Adhesive bond testing between composite laminates by laser shockwave loading

"Assembling materials by adhesive bonding has several advantages compared to other joining methods such as the use of fasteners or welding. Fasteners require drilling holes in the parts to be joined and both fastening and welding require significant investment in machinery. For metals, welded joints also generally produce a mechanically weaker heat affected zone. Adhesive bonding also has significant advantages for polymer-matrix composites. Drilling through composites has the drawback of cutting load-bearing fibers with adverse effects of possible delamination and excessive tool wear. It may also be economically advantageous to bond several small parts to make a large structure instead of having it co-cured. However, for all materials, the use of adhesive bonding for loadbearing structures is impeded by the absence of reliable nondestructive methods that can guarantee the strength of the joint, and in particular are able to very reliably identify the presence of near zerostrength \u201ckissing bonds\u201d [1]. Kissing bonds are undetectable by conventional ultrasonic inspection since the return echo from the interface in the pulse-echo technique does not depend upon the bond strength and only requires mechanical contact between the adherends. This is also the case for the transmitted echo. Although there have been many attempts to develop other ultrasonic approaches, such as using waves that propagate essentially along the bond line, none of these approaches has succeeded in detecting a weak bond other than those that are weakened by defects such disbonds or porosity [1-3]. These defects can be detected by the well established ultrasonic inspection technique and in the case of porosity, also by x-ray radiography. Among possible causes of weak bonds are contamination of surfaces prior to bonding, inadequate surface preparation, degradation of the adhesive from improper storage, and inadequate mixing ratio for two-part adhesives. In all these cases, there can be good mechanical contact without defects, combined with poor mechanical strength, undetectable by established ultrasonic inspection techniques. Ultrasonic techniques only apply weak stresses to the bond line and such weak stresses cannot reveal characteristics that are only apparent by applying significant stresses, like in destructive tests. Therefore, a reliable technique to identify such weak bonds requires application of a strong tensile stress across the bond line. A convenient approach that has been previously studied for evaluating the dhesion of coatings to their substrate and fibers to their matrix uses a pulsed laser to generate a large amplitude wave (shockwave) that propagates throughout the material [4-9]. This wave, being initially in compression, is converted by reflection on the back surface of the sample into a strong tensile wave that can pry apart the sample and disbond the assembly. This approach has been more recently extended to proof testing of adhesive bonds between carbon-epoxy laminates [10,11]. To probe bond strength, higher and higher tensile stress loading is applied by increasing the laser pulse energy step by step. A \u201cgood\u201d joint will be unaffected under a given stress level whereas a weaker one will be damaged, allowing this method to evaluate the bond strength. The principle of the method is described next in more detail. We then describe how the ethod is implemented, the instrumentation that has been developed and the fabrication of weak bond test specimens. Finally we present some results and indicate erspectives and future developments.Peer reviewed: YesNRC publication: Ye