Abstract
The effect of the microscopic structure and the moisture content (MC) of wood on its fracture behaviour has been investigated. Green and air-dried spruce (Picea abies [L.] Karst.) and birch (Betula pendula Roth.) wood were subjected to pure mode I loading in the radial- tangential (RT) crack propagation system. Tests were carried out in situ in an environmental scanning electron microscope to observe crack propagation at the cellular level. Crack-tip displacement fields were computed by digital image correlation, and crack propagation was observed from the images captured during testing. Both the MC and the microscopic structure were found to affect the fracture process. In the air-dried birch and spruce, only microcracking caused large displacements ahead of the crack-tip. In spruce, the microcracking zone was larger than in birch. In green birch and spruce, microcracking was less evident than in the air-dried specimens, and in some cases, there were notable deformations in a few cells ahead of the crack-tip before crack extension. Microcracking is considered to be the main toughening mechanism in spruce and birch in the RT crack propagation system.
Acknowledgments
This work formed part of “E-Wood”, a project supported by the Multidisciplinary Institute of Digitalisation and Energy (MIDE, http://mide.aalto.fi). The work was also supported by the Academy of Finland (Decision number 1127759). The ESEM tests were carried out at the Max Planck Institute of Colloids and Interfaces, Department of Biomaterials. The authors would like to thank Dr. Ingo Burgert, Dr. Michaela Eder, and Ms. Susann Weichold for their kind co-operation. P. Tukiainen’s short-term scientific mission to the Max Planck Institute was funded under the auspices of COST Action E35.
References
Davis, P.M., Gupta, R., Sinha, A. (2012) Revisiting the neutral axis in wood beams. Holzforschung 66:497–503.10.1515/hf.2011.180Search in Google Scholar
Dill-Langer, G., Lütze, S., Aicher, S. (2002) Microfracture in wood monitored by confocal laser scanning microscopy. Wood Sci. Technol. 36:487–499.Search in Google Scholar
Eder, M., Stanzl-Tschegg, S., Burgert, I. (2008) The fracture behaviour of single wood fibres is governed by geometrical constraints: in situ ESEM studies on three fibre types. Wood Sci. Technol. 42:679–689.10.1007/s00226-008-0214-5Search in Google Scholar
Frühmann, K., Burgert, I., Stanzl-Tschegg, S.E., Tschegg, E.K. (2003) Mode I fracture behaviour on the growth ring scale and cellular level of spruce (Picea abies [L.] Karst.) and beech (Fagus sylvatica L.) loaded in the TR crack propagation system. Holzforschung 57:653–660.10.1515/HF.2003.098Search in Google Scholar
Garab, J., Keunecke, D., Hering, S., Szalai, J., Niemz, P. (2010) Measurement of standard and off-axis elastic moduli and Poisson’s ratios of spruce and yew wood in the transverse plane. Wood Sci. Technol. 44:451–464.10.1007/s00226-010-0362-2Search in Google Scholar
Kelley, S.S., Rials, T.G., Glasser, W.G. (1987) Relaxation behaviour of the amorphous components of wood. J. Mater. Sci. 22:617–624.Search in Google Scholar
Keunecke, D., Stanzl-Tschegg, S., Niemz, P. (2007) Fracture characterisation of yew (Taxus baccata L.) and spruce (Picea abies [L.] Karst.) in the radial-tangential and tangential-radial crack propagation system by a micro wedge splitting test. Holzforschung 61:582–588.10.1515/HF.2007.089Search in Google Scholar
Landis, E.N., Navi, P. (2009) Modeling crack propagation in wood and wood composites. A review COST Action E35 2004–2008: wood machining – micromechanics and fracture. Holzforschung 63:150–156.10.1515/HF.2009.010Search in Google Scholar
Lecompte, D., Smits, A., Bossuyt, S., Sol, H., Vantomme, J., Van Hemelrijck, D., Habraken, A. (2006) Quality assessment of speckle patterns for digital image correlation. Optics Lasers Eng. 44:1132–1145.10.1016/j.optlaseng.2005.10.004Search in Google Scholar
Majano-Majano, A., Hughes, M., Fernandez-Cabo, J.L. (2012) The fracture toughness and properties of thermally modified beech and ash at different moisture contents. Wood Sci. Technol. 46:5–21.Search in Google Scholar
Navi, P., Stanzl-Tschegg, S. (2009) Micromechanics of creep and relaxation of wood. A review COST Action E35 2004–2008: wood machining – micromechanics and fracture. Holzforschung 63:186–195.10.1515/HF.2009.013Search in Google Scholar
Olejniczak, P., Gustafsson, P. (1994) Rate effect in tangential tension fracture softening performance, Cost 508 – wood mechanics. In: Workshop on Service Life Assessment of Wooden Structures with Special Emphasis on the Effect of Load Duration in Various Environments. Technical Research Centre of Finland, Espoo, Finland, May 18–19, 1994. Ed. S. Gowda. pp. 137–147.Search in Google Scholar
Peng, M., Ho, Y., Wang, W., Chui, Y.H., Gong, M. (2012) Measurement of wood shrinkage in jack pine using three dimensional digital image correlation (DIC). Holzforschung 66:639–643.10.1515/hf-2011-0124Search in Google Scholar
Reiterer, A., Tschegg, S. (2002) The influence of moisture content on the mode I fracture behaviour of sprucewood. J. Mater. Sci. 37:4487–4491.Search in Google Scholar
Samarasinghe, S., Kulasiri, G. (2000a) Displacement fields of wood in tension based on image processing: part 1. Silva Fenn. 34:251–259.10.14214/sf.629Search in Google Scholar
Samarasinghe, S., Kulasiri, G. (2000b) Displacement fields of wood in tension based on image processing: part 2. Crack-tip displacements in mode-I and mixed-mode fracture. Silva Fenn. 34:261–274.10.14214/sf.630Search in Google Scholar
Samarasinghe, S., Kulasiri, D. (2004) Stress intensity factor of wood from crack-tip displacement fields obtained from digital image processing. Silva Fenn. 38:267–278.10.14214/sf.415Search in Google Scholar
Samarasinghe, S., Kulasiri, D., Nicolle, K. (1996) Study of mode-I and mixed-mode fracture in wood using digital image correlation method. Research Report No. 96/09, Lincoln University, Canterbury, New Zealand.Search in Google Scholar
Sebera, V., Muszyński, L. (2011) Determination of local material properties of OSB sample by coupling advanced imaging techniques and morphology-based FEM simulation. Holzforschung 65:811–818.10.1515/HF.2011.110Search in Google Scholar
Sippola, M., Koponen, S. (1999) Fracture behaviour of clear softwood—tests and FEM models. In: COST Action E8: Damage in Wood. Eds. Morlier, P., Valentin, G. Bordeaux, France. pp. 207–221.Search in Google Scholar
Smith, I., Landis, E., Gong, M. Fracture and Fatigue in Wood. Wiley, Chichester, 2003.Search in Google Scholar
Stanzl-Tschegg, S.E., Navi, P. (2009) Fracture behaviour of wood and its composites. A review COST Action E35 2004–2008: wood machining – micromechanics and fracture. Holzforschung 63:139–149.10.1515/HF.2009.012Search in Google Scholar
Stanzl-Tschegg, S.E., Tschegg, E.K., Teischinger, A. (1994) Fracture energy of spruce wood after different drying procedures. Wood Fiber Sci. 26:467–478.Search in Google Scholar
Stanzl-Tschegg, S.E., Tan, D.M., Tschegg, E.K. (1995) New splitting method for wood fracture characterization. Wood Sci. Technol. 29:31–50.Search in Google Scholar
Sveen, J.K. (2004) An introduction to MatPIV 1.6.1, Eprint no. 2. Department of Mathematics, University of Oslo, Oslo, Norway. Availablet at: http://folk.uio.no/jks/matpiv/html/MatPIVtut.pdf.Search in Google Scholar
Taguchi, A., Murata, K., Nakamura, M., Nakano, T. (2011) Scale effect in the anisotropic deformation change of tracheid cells during water adsorption. Holzforschung 65:253–256.10.1515/hf.2011.017Search in Google Scholar
Thuvander, F., Sjödahl, M., Berglund, L. (2000) Measurements of crack-tip strain field in wood at the scale of growth rings. J. Mater. Sci. 35:6267–6275.Search in Google Scholar
Tukiainen, P., Hughes, M. (2008) Fracture of spruce and birch in the RT crack propagation direction. In: Proceedings of the 10th World Conference on Timber Engineering, Miyazaki, Japan, June 2–5, 2008.Search in Google Scholar
Tukiainen, P., Hughes, M. (2010) Fracture of spruce and birch in the RT crack propagation direction based on digital image correlation analysis of ESEM images. In: Proceedings of the 11th World Conference on Timber Engineering, Riva del Garda, Italy.Search in Google Scholar
Tukiainen, P., Hughes, M. (2013) The fracture behavior of birch and spruce in the radial-tangential crack propagation direction at the scale of the growth ring. Holzforschung 67:673–681.10.1515/hf-2012-0139Search in Google Scholar
Vasic, S., Smith, I. (2002) Bridging crack model for fracture of spruce. Eng. Fract. Mech. 69:745–760.Search in Google Scholar
Vasic, S., Stanzl-Tschegg, S. (2007) Experimental and numerical investigation of wood fracture mechanisms at different humidity levels. Holzforschung 61:367–374.10.1515/HF.2007.056Search in Google Scholar
Vasic, S., Smith, I., Landis, E. (2002) Fracture zone characterization –micro-mechanical study. Wood Fiber Sci. 34:42–56.Search in Google Scholar
Zink, A.G., Davidson, R.W., Hanna, R.B. (1995) Strain measurement in wood using a digital image correlation technique. Wood Fiber Sci. 27:346–359.Search in Google Scholar
©2016 by De Gruyter
