FRACTURE INITIATION AND PROGRESS IN WOOD SPECIMENS STRESSED IN TENSION .1. CLEAR WOOD SPECIMENS STRESSED PARALLEL TO THE GRAIN

Clear spruce wood specimens were subjected to in situ tensile failure tests parallel to the grain in order to study fracture initiation and propagation. Fracturing of the specimens was carried out inside a SEM with a bending and a tension device, respectively. The bending and the tension specimens consisted of two to three growth rings. In all spec imens, fracture propagated in a stepwise manner and the earlywood zone s fractured prior to the latewood zones. The fracture initials compris ed parallel-to-grain cracks at the earlywood-latewood borders and perp endicular to-grain cracks in wood rays at a growth ring border. The pa rallel-to-grain cracks were probably due to transverse tensile strains and were frequently arrested or became discontinuous at wood rays. Wo od rays were shown to be structural points of weakness as well as rein forcements depending on the direction of load. The role of the bordere d pits remained ambiguous.