A New Method for Testing Glue Joints of Laminated Timber in Service

A means of testing glue joints in laminated timbers in service by removing and shearing 1-inch diameter plugs was studied with promising results. Comparative shear tests were made on glue joints in matched cylindrical plugs and block shear specimens. Casein and phenol-resorcinol glues were used on both Douglas-fir and southern pine because of their wide use in commercial laminating. Average shear strengths for groups of plug specimens varied from 86 to 100 percent of the values obtained with standard block shear specimens. Average shear strengths were based on 20 specimens for each species, each glue, and for each of three closed assembly times. Percent wood failure was judged to be in reasonably good agreement for both types of specimens. The author concluded, therefore, that the test method, based on cylindrical plugs, provided an acceptable means of testing glue joints of laminated timbers in use. The modified shear tool built to test cylindrical specimens is illustrated as well as the extended plug cutter for cutting plugs up to 5 inches long. A cutting speed of 295 rpm and feed rate of 0.004 to 0.02 inches per revolution gave the smoothest plugs with the cutter and drill press used in the study but speeds of 475, 715, and 870 rpm gave reasonably smooth plugs. Plug diameters ranged from 1.013 to 1.015 in the 120 pine plugs and from 1.012 to 1.01 for the 120 Douglas-fir plugs.

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