Characterization of load transfer in wood-based composites

The mechanical performance of composite materials is determined by the mechanical properties of their individual components and the effective load transfer between these components. Binders are commonly used to bond composite components together and to provide effective load transfer between them. In wood-based composites the binder-component load transfer occurs at the micro-scale (1-100 μm), where complex morphologies are in play. The bonding of these materials is relatively well understood on the molecular level with respect to the chemical compatibility of the binder and components, but the load transfer and the relative contribution of surface adhesion forcesof wood tissue at the micron- and sub-micron scale remains a subject of limited understanding. Understanding of the load transfer at this scale requires experimental observation of the responses of complex bond morphologies to external loads. It should be noted however, that load transfer in bonds in terms of internal force distribution and stress cannot be measured directly. Instead, methodologies are needed that allow back calculating complex stress distributions in the bond interphase from experimental measurements of displacements and strains. Recently, a conceptual framework for an integrated method for multi-scale/multi-modal investigation of micro-mechanical interaction in bond interphases has been proposed where measurements of properties associated with the load transfer across composite bonds were integrated with predictive numerical modeling tools (Muszynski et al. 2013) (reproduced in Appendix C). This general framework and the modeling tools used are generic and can be applied to a variety of composite materials. The general goal of this dissertation was to develop the crucial non-generic components for this approach, which are the specialized, experimental measurement tools, methods and procedures that are paired with modeling tools. Consequently, measurement methodologies have been developed to assess the deformations and strain patterns across the particle-matrix bonds in WPCs and across adhesive bonds in layered composites. The measurements in these methodologies are valuable as input data and are spatially tied to coordinate systems making them easily compatible with three-dimensional numerical modeling tools. A measurement methodology has also been developed to investigate local surface free energies of wood tissue at a micron and sub-micron scale. These measurements are able to further inform and enrich the predictions made by numerical models with respect to the bonding morphology. Currently these methods are being used to further develop, refine and ultimately validate the numerical model used to quantify and analyze load transfer in wood composite bonds

Evaluating the quality of surface carbonized woods modified with a contact charring or a gas flame charring technique

Surface carbonization, or charring, of wooden exterior cladding boards is a modification method that creates a fully organic barrier layer in resemblance to a coating. The process effectively degrades the wood and transforms it into a carbonaceous residue that protects the underlying unmodified wood from environmental stresses. The surface quality of wood modified in this manner is a combination of several factors and depends on the manufacturing method and wood species. To assess the quality of spruce and birch modified with contact and flame charring techniques, several experiments were set up from the nanoscale to macroscopic evaluation of surface resistance to different stresses. The changes in elemental composition are scaled with the modification severity with little differences between wood species. The carbon structures analyzed by high-resolution transmission electron microscopy (HR-TEM) were found to be amorphous, but the electron energy-loss spectroscopy (EELS) revealed higher ordering with what is assumed to be random graphitic stacking of carbon sheets. These carbon-carbon bonds are stable, so a higher ordering is hypothesized to induce improved resistance to exterior stresses. The scanning electron microscopy (SEM) revealed a clear difference between contact-charred and flame-charred woods. The selected contact charring temperature was not high enough to induce the transformation of cell walls from anisotropic into an isotropic material but provided other benefits such as a relatively crack-free, smooth and scratch resistant surface. Surface roughness was able to adequately predict the surface quality of the contact-charred samples, and scratch tests were found to be suitable for evaluating the mechanical stress resistance of the surface instead of abrasion. In terms of overall quality, birch instead of spruce was concluded to better respond to both charring methods, although contact charring eliminates some species-specific characteristics, resulting in more homogeneous surfaces.OA-hybri

Strain distribution and load transfer in the polymer-wood particle bond in wood plastic composites

The load transfer between wood particles and the matrix was analyzed by observation of the strain patterns in thin films of high density polyethylene (HDPE) with embedded wood particles subjected to tensile loading. Optical measurement techniques based on the digital image correlation (DIC) principle were employed for quantitative measurement of strain distributions on the surfaces of the specimens. Interpretation of these measurements in terms of load transfer between the particle and the matrix below the surface proved challenging and required a structured approach. In this paper, quantitative descriptors were selected as synthesized metrics to support the quantitative interpretation of the measured strains. X-ray computed tomography (XCT) scans were used to assess the effect of the position of the particles in the film specimens on the strains patterns observed on the surface.This is the publisher’s final pdf. The published article is copyrighted by Walter de Gruyter GmbH and can be found at: http://www.degruyter.com/view/j/hfsg.Keywords: optical measurement, wood plastic composites, micromechanics, digital image correlation, short fiber composites theory, load transfe

Stereomicroscopic optical method for the assessment of load transfer patterns across the wood-adhesive bond interphase

The mechanical performance of wood-based composites is determined by the mechanical properties of their individual components and the effective load transfer between these components. In laminated wood composites, this load transfer is facilitated by the adhesive bond. The experimental methodology developed in this study measures and analyzes the full-field deformation and strain distributions across the loaded wood-adhesive interphase at a micromechanical level. Optical measurements were performed based on the principles of digital image correlation by a stereomicroscopic camera system. This system allows the monitoring of in-plane deformations as well as out-of-plane displacements, providing full-field 3D surface strain maps across the adhesive bond. These measurements can be used to improve the understanding of the load transfer between the adherents and the contribution of the adhesive to the mechanical properties of the bulk composite and serve as a quantitative input for numerical modeling and simulations aimed at the improvement of the products.Keywords: digital image correlation (DIC), optical measurement, micromechanics, load transfer, adhesive interphas