Single Joint Sample Group The first group of 14 single-joint samples failed at forces ranging from 593 to 1,685 Newtons. The eleven biscuited single joint samples broke in the same area, each endgrain crack within 25 mm of the joint. (Figure 5) All the plate-joined samples failed by splitting the wood inward from the endgrain.
Figure 5. Single Joint - Compression initiates splitting in endgrain.
Crack expands past inside corner (fulcrum) and snaps off, causing total failure.
Total failure occurred when the crack progressed past the inside corner of the joint the effective fulcrum and the split wood snapped off. Not indicated on the chart because of the difficulty of quantification are the dimensions of the pieces torn away from the long-grained member. (Table 1)
The authors found that the strength of the joints within categories varied by roughly 20 percent from sample to sample (taking liberties with the rules of statistical inference). The first set of samples tested showed strength proportionate to the width of the strip torn away from the long grain - more wood engaged by joinery creates more strength.
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