Wood Failure–Within Species and Between Species

During the past 30 years, estimates of glue bond quality based on wood failure have gradually been accepted for quality control in the plywood industry and for research purposes. However, differences in the strength of wood species limit the use of wood failure as a universal estimate of bond quality. Problems arise when a species which is weaker than the glue and a species which is stronger than the glue are bonded with a single adhesive. By plotting wood failure against bond strength it is possible to explain this phenomenon. For a large number of species it should be possible to prepare regression lines showing for each specific glue the point at which all species weaker than the glue will yield 100 percent wood failure, and all species stronger than the glue will yield less than 100 percent wood failure. Barring strength defects in the wood–loose fiber, lathe checks, seasoning checks–the point at which wood failure is zero will be determined by the strength of the weakest wood element (springwood) and the point at which wood failure is 100 percent will be determined by the strongest wood element (summerwood). Wood failure itself is not sufficient to describe bond quality. Using this method, the percentage of wood failure can be qualified by the strength of the species in relation to any given glue.

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