The samples were built of #2 ponderosa pine (P. ponderosa, cabinet grade, not lumber grade) selected at random and assembled without regard to the placement of grain patterns, knots or other structural features. Unconscious positioning of these features could have affected joint strength. The samples measured 44 mm thick, 128 mm wide and 450 mm from outside corner to outside corner. The samples were joined with #20 or S6 beechwood biscuits, or tenons of pine or particle board and glued with either Titebond Type II a "yellow exterior aliphatic resin glue"or "Excel" a one-part urethane glue.
#2 ponderosa pine is a mild,even-grained wood of average specific gravity. It has randomly positioned knots of 1/2"or less, averaging about 1 per 300 mm length of 125 mm wide door frame. It adheres well with suitable adhesives. [1]
The First Sample Set The authors built the first sample set in the form of two arms, one measuring 450 mm with the second butted into the first using the type of joint to be tested. With the addition of the 322 mm arm to the 128mm wide stock, the second side of the sample also had a length of 450 mm.(Figure 3)
All samples had a 25 mm by 44 mm bearing surface cut onto two opposing diagonal corners to distribute the force of the press without crushing or splitting.
The Second Sample Set The authors observed on the first set of samples that the eleven single-joint biscuited samples all broke within a 25 mm distance from the edge of the joint and that failure progressed in a similar way - from the outer endgrain past the inside corner of the joint. See RESULTS.
The authors followed up by building three, four-sided samples (Figure 4) to better simulate the inter-related strengths of the four-plus corner joints of a door. The continuum of possible outcomes ranged from the frame having strength only equal to its weakest joint or that with two joints in compression and two in tension (a potentially stronger joint) the four-joint samples could be more than four times as strong as the single joint samples.
Figure 4. Sample configuration
and press orientation
for testing the second set of samples.
for testing the second set of samples.
Procedures The authors compressed the samples in an Instron Model 1125 press set to a crosshead speed of 12.5 mm per minute. In some cases the procedure was temporarily halted to visually inspect a sample while other samples were run continuously to total failure.
Sounds of cracking, visible cracking or sudden advances in displacement or drops in pressure (Figure 6.) could be observed either directly or on a CRT wired into the recording equipment.