In situ observations of fracture mechanisms for radial cracks in wood

This paper presents the findings of work carried out to describe the microm echanisms of radial crack growth in wood. TR and TL cracks are both radial cracks but TR grows radially and TL longitudinally. TR cracks are known to show higher fracture toughness than TL cracks. The TR fracture surfaces als o indicate a more tortuous crack path. Since the reason for this is unclear , details of the TR crack growth mechanisms in green Pinus sylvestris L wer e studied. This was done by in-situ optical microscopy as the crack was cut ting through alternating layers of soft earlywood and stiff latewood. At th e scale of individual cells, the crack tip advanced by separating cell wall s at the middle lamella in a splitting or peeling mode. At the scale of gro wth rings, stick-slip type of crack growth was observed and new crack plane s were often formed. The stress distribution in a material with alternating stiff and soft layers is causing this. This stress distribution also contr ibutes to the tendency for inclined cracks to deviate in the radial directi on. For interpretation of fracture mechanisms, the importance of scale inte raction and the combined influences of microstructure and stress state are emphasized. (C) 2000 Kluwer Academic Publishers.