FRACTURE INITIATION AND PROGRESS IN WOOD SPECIMENS STRESSED IN TENSION - PART III - CLEAR WOOD SPECIMENS WITH VARIOUS SLOPES OF GRAIN

Clear spruce wood specimens with 30, 20, 10, and 5 degrees slope of gr ain were subjected to in-situ tensile failure tests in order to study fracture initiation and propagation. Fracturing of the specimens was c arried out inside a SEM with a bending and a tension device, respectiv ely. The bending and the tension specimens consisted of two to three g rowth rings. Fracture process in 30 degrees-, 20 degrees-, and 10 degr ees-specimens was quite uniform but differed from that of 5 degrees-sp ecimens. There was no difference between bending and tension specimens . With one exception. the specimens fractured in a stepwise manner. Cr acks initiated at wood rays in the latewood and were arrested in the a djoining earlywood. This is attributed to the greater stiffness of the latewood tracheids and the capacity of the thin-walled earlywood trac heids to deform thereby arresting cracks. In 30 degrees-, 20 degrees-, and 10 degrees-specimens, the main fracture mode was cleavage along t he grain due to tensile stress perpendicular to the grain. Latewood tr acheids showed intrawall fracture while earlywood tracheids showed lon gitudinal transwall failure. In the 5 degrees-specimens, tensile stres s parallel to the grain prevailed.