Failure in wood related to decay weight losses

Abstract

The progressive effects of fungal decay on wood failure morphology were studied, using Populus tremuloides Michx. as the fungal sustrate. The degree of decay was indexed by loss of wood weight. Decayed and control specimens were stressed to failure prior to macroscopic and ultrastructural analyses of the fracture surfaces. Results showed that fungal decay affected fracture morphology in wood. The nature of the fracture surface reflects the degree of degradation of wood cells. Decayed wood (> 2% weight loss) failed differently than sound wood. High- energy fracturing, like splintering and production of rough shear planes, typified the failure characteristics up to 2 percent weight loss. The associated fractographic features included transwall failure of fibers at angles close to the S2 microfibrillar orientation, intra wall failures at the S1IS2 interfaces, and cellular unwinding due to pullout of the S2 wall layer from the whole fiber. These features diminished gradually as weight loss increased. At ~ 10 percent weight loss, failures were often brash and accompanied by abrupt trans wall failure of fibers along the transverse plane. The S2 microfibrillar angle did not control the plane of fiber failure in decayed cells. Longitudinal split failures along the parenchyma cells were frequently noted on specimens decayed by white rot fungi. Such failures were initiated at weight losses lower than those required to produce middle lamella separation.