The fracture properties of spruce (Picea abies) and birch (Betula spp.) wood under pure Mode I loading in the RT-direction were studied under “green†and air dried conditions (moisture content circa 12-15%), using small Compact Tension (CT) specimens. Testing was performed on a purpose built loading device mounted beneath an optical microscope. Load and Crack Mouth Opening Displacement (CMOD) were measured electronically. Crack propagation and the behaviour of the process zone were observed using a light microscope fitted with a CCD camera. Local deformations of the specimen and crack propagation were then calculated from the acquired images by Digital Image Correlation (DIC) using MatPIV, an open source MatLab toolbox, suitably adapted for this purpose. Based on DIC analyses, large radial deformations were detected about 1-2 mm ahead of macro-crack tip. It is concluded that since specimen size, the characteristic length of the specimen and the ligament length were small compared to the process zone size, Linear Elastic Fracture Mechanics (LEFM) principles cannot be applied and a clear size effect on fracture energy is expected.
Keywords: Spruce, birch, fracture, RT-direction, digital image correlation, process zone
Authors
Tukiainen P.
Laboratory of Wood Technology, Department of Forest Product Technology, TKK Helsinki University of Technology, TKK
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