Fracture Toughness and Duration of Load Factor I. Six Principal Systems of Crack Propagation and the Duration Factor for Cracks Propagating Parallel to Grain

Critical stress intensity factors for air-dry Douglas fir were found to be 2200, 2450, 281, 373, 323, and 323 psi inch for the LT, LR, TL, RL, TR, and RT systems, respectively, where the first letter indicates the direction normal to the crack plane and the second the direction of crack propagation. Long-term load tests with notched beams showed that for the TL system the load duration factor was similar to that applying to modulus of rupture. Moisture changes significantly reduced the mean time to failure of notched beams under dead loads as compared to tests under constant moisture conditions.