Graduate Thesis Or Dissertation

Check Characterization in Maple Plywood

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  • Small hairline cracks called checks in the decorative face veneer of hardwood plywood are a costly problem for the hardwood plywood industry. Hardwood plywood describes any plywood sheet with a hardwood face veneer glued onto a core substrate. Checks form due to differential shrinkage stresses. Not all hardwood plywood panels will produce checks and those which do, usually produce differing amounts of checks. Authors of technical and trade articles have provided an abundance of recommendations, advice and discussion on many potential causes of checking. These authors’ notes when combined have shown checking to be a very complex issue. Little research has been conducted on checking, and any research conducted has been challenged because check formation is sporadic as well as because check detection and measurement techniques are very time consuming. Further, there have been contradictory findings in past research; as a result, there are no clear ‘best practices’ for industry to follow. This all has created the need for a system which can rapidly identify and measure checks as they develop. Burnard (2012) developed such a system using the principles of Digital Image Correlation (DIC). The present study follows-up this work by refining Burnard’s system and testing check development in hard maple veneer due to: veneer and core moisture content before layup (6, 12, and 20%), adhesive type (soy-based and urea formaldehyde) and core type (veneer core and medium density fiberboard). One-foot by one-foot panels of maple plywood conditioned at 90% relative humidity (RH) and 30 C for at least a week were tested at conditions between 5-10% RH and 35 C for approximately five hours. DIC techniques were employed for initial check detection and measurement. Strain in the x-direction (perpendicular to grain) calculated by DIC was placed into a custom check characterization software program which provided eleven different output variables related to checking. Checks formed in only 21 of the 288 test panels, 17 of which were on a veneer core platform. Shrinkage strain, a measure of how much the veneer itself moves, was also used to describe test panels. Analysis of shrinkage strain showed core, adhesive, veneer moisture content before layup and the interaction between core and adhesive to be statistically significant. These phenomena show manufacturers should use composite materials rather than veneer for core whenever possible and potentially make sure veneer is below 12% moisture content before layup. Results also suggest that shrinkage strain may also be used to predict checking in hardwood plywood, although further research is needed.
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