A New Method for Testing Wood Adhesives: II. Test Apparatus

For testing properties of assembly adhesives, dowel joints, screws, and other fastenings a simple machine and test procedure was designed based upon the use of a diaphram to deliver compressive loads and provide the necessary measures of equalization. By interchanging fixtures the machine may be used as a press for making cross-lap specimens or as a testing machine. The diaphram, which delivers the compressive loads, is a sheet of rubber, circularly restrained by a plywood ring bolted to sheet metal. Air pressure is the actuating medium, although water or other fluid pressure may be used. The force developed by the diaphram is delivered to a circular plate which carries the fixtures for holding the specimens. Since this plate floats on the diaphram, it has freedom to adjust to the resistance pattern offered from above. Also, the diaphram is not appreciably stretched because the total upward travel of the plate is only 1/2 inch. For making testing blocks, the specimens are assembled, five per jig (imposing adhesive variables as desired) and placed in the machine to receive bonding pressure. Thumbnuts are then tightened on the rods of the assembly jig, and the stack of specimens is set aside for the adhesive to harden. When the specimens are ready to be tested, the appropriate fixtures are placed in the machine. The fixture carrying the specimens is self-locating, and force is applied automatically to all edges of the bond. Various adhesives were tested by this method. Moisture contents of wood prior to gluing were 5 and 15 percent. The results show that this procedure possesses ability to reveal differences between various adhesives and differences in their reaction to wood moisture content. The machine cannot maintain absolute control of rate of loading, tending to decrease as the point of rupture is approached. In order to find out the applicability of this machine within practical limits likely to be encountered, further study was made. For this study, self-aligning grips were used in a Baldwin-Tate-Emery testing machine. The results indicate that rather large differences in rate of loading (400 to 6000 pounds per minute) may occur without impairing the validity of a strength observation.

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