Abstract
Internal bond strength testing is a widely used approach for testing quality traits of wood based panels. Generally, failure of internal bond specimens is due to adhesion and/or wood failure in the specimen. It has been reported that a composite product with a large variation in the vertical density profile fails in the center part of the board which is either the middle of the core layer or the transition zone between core layer and face layer. The density in the failure zone is typically 50% lower than the maximum density in the face layers. The aim of this study was to analyze the strain distribution in a specimen under tension perpendicular to the panel plane. The results showed that a high variety of strain magnitude occurred in the specimen. The strain is either aligned with the tension direction or a tension zone is built in one of the edge zones leading to failure. Vector graphics of the specimen show the problematic test setup of internal bond strength measurement. Strain spots in the edges lead to the assumption of an uneven stress distribution due to the momentum which results from non-perfect alignment or irregularities in the test setup.
Highlights
Wood is a highly heterogeneous material with randomly distributed weak zones which act as sources of failure
Reduced internal bond (IB) values were determined for high moisture content [11] and low resin content [12], which predestines this test method as a quality control (QC) tool in the wood panel industry
This study demonstrated the applicability of electronic laser speckle interferometry (ESPI) to study the strain distribution of internal bond specimens
Summary
Wood is a highly heterogeneous material with randomly distributed weak zones which act as sources of failure. To overcome this drawback, wood can be fragmented into strands, particles, as well as fibers which are reassembled again by means of adhesives, pressure and heat to gain homogeneous wood based panels with customized properties. The standard mechanical characterization techniques for wood based panels according to European standards include bending (EN 310), and internal bond (EN 319) [10] tests. Panel producers use these methods for quality assurance of the whole production chain. Reduced IB values were determined for high moisture content [11] and low resin content [12], which predestines this test method as a quality control (QC) tool in the wood panel industry
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