On the Mechanics of Fracture in Wood

Approximately 740 board feet of kiln-dried western white pine (Pinus monticola) was selected for the experiment. The experimental procedure consisted of finding a suitable specimen design for measuring the opening mode, strain-energy release rate (Gic) for wood. It was found that the edged-notched, double-beam specimen was the most favorable design. To measure the strain-energy release rate, the procedure used was a modification of a method suggested by Irwin based on the change of stiffness of the specimen with varying crack length. An initial crack 3/4-inch in length was formed in the desired plane to induce crack propagation in the specimens. End pieces 3 by l/4 by 3/4 inch were bonded to each “cantilever” of the specimen and served to fasten it in the grips of an Instron testing machine. A rate of crosshead motion of 0.05 inch per minute was used in all the tests performed. The crack length was measured by steel rulers. Values of load and deflection were noted at l-inch increments in crack length. The effect of changes in specimen length, thickness, and height on Gic of wood was studied, and the effect of temperature and moisture content on Gic of wood was determined. The results indicated that the strain-energy release rate (Gic) for western white pine wood is independent of specimen length, thickness, and height in both LT and LR planes, and it is not a function of crack length. Increased temperature and moisture content in wood increase energy requirement. This is due to the viscoelastic behavior of wood under such environmental conditions. Based on experimental evidence that these flaws grow in size with the passage of time, there is a critical crack length for any given stress system, above which rapid crack propagation occurs and results in the catastrophic failure of the body. Knowledge of Gic permits the prediction of the critical crack length for a Gic imposed stress system and gives a basis for comparing the fracture resistance of wood to other materials.

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