A New Approach to Quality Control in the Manufacture of Exterior Softwood Plywood

A present day technique of using average wood failure to predict the long term durability of exterior plywood can be simplified through the use of a relationship between average wood failure and the frequency distribution of wood failure in shear specimens. Thus in a group of shear specimens having, an average wood failure of 80 percent, about half of the specimens are found to have a wood failure between 90 and 100 percent. This makes possible a glueline evaluation system in which average wood failure is replaced by a count of the number of specimens having wood failure >90 percent. Such a system simplifies the labor of reading, wood failure, since the only decision which has to be made on each specimen is a go no-go decision whether it lies above or below 90 percent wood failure. Another useful parameter can be derived from the portion of the frequency distribution below 30 percent wood failure. If the average wood failure of a group of plywood shear specimens is 80, the percentage of specimens falling in the range from 0 to 30 percent wood failure is about 10. The fundamental difference between these two approaches is that the one based on high wood failure is estimating the proportion of very well glued plywood, while the one based on low wood failure is estimating the proportion of badly glued plywood. Either approach could simplify the procedure for controlling glue-bond quality in exterior plywood.

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