Bending fatigue life of two-pin dowel joints in furniture grade pine plywood

The fatigue life of T-shaped, end-to-side, two-pin dowel joints was investigated by subjecting them to one-sided constant and stepped cyclic bending loads. Frame- 1 furniture grade, 3/4-inch-thick (19.05-mm) 5-ply southern yellow pine plywood was tested in the construction of joints with four rail width groups. Dowels were of white birch wood with spiral grooves and a nominal diameter of 3/8 inch (9.53 mm). Results of one-sided constant load tests indicated that the fatigue life of dowel joints averaged 131,253; 78,122; 31,617; 11,023; 4,161; and 329 cycles for load levels of 40; 50; 60; 70; 80; and 90 percent of their mean ultimate bending strengths, respectively. Regression of M-N data (moment versus log number of cycles to failure) indicated a linear relationship existed between the fatigue bending moment applied to joints and the log number of cycles to failure. Joints with higher static bending strengths tended to have more resistance to fatigue damage. A simplified method of deriving the fatigue life estimation equation based on known information such as the joint static bending strength was proposed. Cyclic stepped load tests verified that the Palmgren-Miner rule was an effective method in estimating fatigue life of two-pin dowel joints subjected to cyclic stepped bending moments based on their M-N curves. Fatigue life comparisons among joint groups with different static bending strengths indicated that a significant increase in static bending strength might not yield a significant fatigue life increase when a joint was subjected to cyclic stepped loads. Joint resistance to fatigue failure should be taken into account in strength design of furniture frames that are subjected to repeated loading.