Abstract
Clear spruce wood specimens with 30, 20, 10, and 5° slope of grain were subjected to in-situ tensile failure tests 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. Fracture process in 30°-, 20°-. and 10°-specimen; was quite uniform but differed from that of 5°-specimens. There was no difference between bending and tension specimens. With one exception, the specimens fractured in a stepwise manner. Cracks initiated at wood rays in the latewood and were arrested in the adjoining earlywood. This is attributed to the greater stiffness of the latewood tracheids and the capacity of the thin-walled earlywood tracheids to deform thereby arresting cracks. In 30°-, 20°-, and 10°-specimens, the main fracture mode was cleavage along the grain due to tensile stress perpendicular to the grain. Latewood tracheids showed intrawall fracture while earlywood tracheids showed longitudinal transwall failure. In the 5°-specimens, tensile stress parallel to the grain prevailed.
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