FRACTURE INITIATION AND PROGRESS IN WOOD SPECIMENS STRESSED IN TENSION - PART-II - COMPRESSION WOOD SPECIMENS STRESSED PARALLEL TO THE GRAIN

Spruce compression wood specimens were subjected to in-situ tensile fa ilure tests parallel to the grain in order to study fracture initiatio n and propagation. Fracturing was carried out inside a SEM with a bend ing and a tension device, respectively. The specimens comprised two to three growth rings. In the bending specimens fracture proceeded in a stepwise manner. Fracture initiated as a long straight crack perpendic ular to the grain in a wood ray thereby crossing a growth ring border. Subsequent cracks also propagated within wood rays. In the tracheids with helical gaps, loading led to the formation of numerous checks bet ween the gaps of the S2. Cracks propagated along these checks. The fra cture process in the first-formed tracheids which lacked helical gaps but are still considered to be compression wood tracheids, indicates t hat matrix degradation occurs prior to microfibril rupture. The tensio n specimens broke abruptly with a straight crack perpendicular to the grain. Their tracheids showed the same fracture mode like those in the bending specimens.