Mode I fracture behaviour of Norway spruce and silver birch in the radial-tangential and tangential-radial crack orientations

by Pekka Tukiainen

Institution: Aalto University
Year: 2016
Posted: 02/05/2017
Record ID: 2070524
Full text PDF: https://aaltodoc.aalto.fi/handle/123456789/21549


Wood has a very high strength to the weight ratio, but only in the grain direction. Wood always contains cracks, which reduces the already low tensile strength perpendicular to grain and in addition induces stress concentrations. In design, failures due to cracks can be avoided by employing fracture mechanics, but for this, the fracture behaviour of wood has to be known. In wood, a fracture process zone (FPZ) is observed around the crack-tip. By studying the FPZ, the macroscopic toughness of the material can be related to its microstructure. However, relatively little is known about the FPZ in wood. Furthermore, the effect of moisture content (MC) and temperature on the fracture behaviour of unmodified and thermally modified (TM) wood should be understood better. The main aim of this study was to investigate the effect of the cellular structure, the temperature and MC on the fracture behaviour of birch, spruce and TM spruce in the radial-tangential (RT) and tangential-radial (TR) orientations. It was found that in birch and spruce, MC and elevated temperature significantly changes the fracture behaviour of both species and under all conditions, both the calculated fracture mechanics parameter values were higher in the RT than in the TR system. Elevated temperature and MC did not affect the failure mode, except in the case of spruce in the RT system. The fracture behaviour of spruce was altered by TM: the material becomes more brittle as the severity of the TM increased. Spruce was altered more in the RT than in the TR system. The failure mode changed due to TM in the TR system but in the RT system the failure mode of unmodified and TM material differed only at high MC and at elevated temperature. When, displacements near the crack-tip were analysed at the scale of the growth ring, it was discovered that MC as well as the cell structure affected the size of FPZ. In both spruce and birch in RT system, the fictitious crack model overestimates the size of the FPZ whereas the linear elastic fracture mechanics model underestimates the size of the FPZ. When the near crack-tip displacement fields were analysed in greater detail it was observed that only micro-cracking caused large displacements ahead of the crack-tip in the RT system in air-dried birch and spruce. In green material, micro-cracking was less evident. Based on these observations, micro-cracking is considered to be the main toughening mechanism in spruce and birch in the RT orientation. This dissertation provides new knowledge about the fracture behaviour of birch, spruce and TM spruce and how elevated temperature and high MC affects the fracture behaviour.; Suhteessa painoonsa puu on hyvin lujaa, mutta vain syyn suunnassa. Puussa on aina säröjä, jotka alentavat jo ennestään alhaista vetolujuutta kohtisuoraan syyn suuntaa vastaan. Lisäksi säröjen kärkiin syntyy jännityshuippuja. Rakenteita suunniteltaessa säröjen aiheuttama murtuminen voidaan välttää soveltamalla murtumismekaniikkaa, mutta tällöin puun murtokäyttäytyminen on tunnettava. Puussa särön kärjen… Advisors/Committee Members: Hughes, Mark, Prof., Aalto University, Department of Forest Products Technology, Finland (advisor).