A STRESS TRANSFORMATION APPROACH TO PREDICTING THE FAILURE MODE OF WOOD

A simple model, based on the use of transformations of second-order te nsors, is presented in this paper to predict the failure mode of wood members stressed in various degrees of parallel- and perpendicular-to- grain tension and parallel-to-grain shear. This type of loading is ind icative of structural wood members with cross grain or grain deviation s in the vicinity of knots subjected to bending or tension. The model is based on the assumptions that failure is dictated by the presence o f any of the aforementioned stresses that exceed the clear wood streng th in that mode and that failure does not result from stress interacti ons. The magnitudes of the applied stresses are normalized relative to the wood strength in that mode. The ratio of applied stress to materi al strength that is greatest at any particular angle of load to grain is presumed to be the failure mode at that angle. To verify model pred ictions, optical and microscopic analyses of surfaces of failed specim ens loaded in uniaxial tension at angles between 0 degrees and 90 degr ees to grain were compared to previously obtained, or otherwise known, surfaces of specimens tested in tension and shear. Specimens tested a t various angles to grain demonstrated failed surfaces very much like those associated with specimens loaded in the modes predicted by the m odel.