Abstract—Investigation of fracture of wood components can prevent from catastrophic failures. Created fracture process zone (FPZ) in crack tip vicinity has important effect on failure of cracked composite materials. In this paper, a failure criterion for fracture investigation of cracked wood specimens under mixed mode I/II loading is presented. This criterion is based on maximum strain energy release rate and material nonlinearity in the vicinity of crack tip due to presence of microcracks. Verification of results with available experimental data proves the coincidence of the proposed criterion with the nature of fracture of wood. To simplify the estimation of nonlinear properties of FPZ, a damage factor is also introduced for engineering and application purposes. Keywords—Fracture criterion, mixed mode loading, damage zone, microcracks. I. INTRODUCTION OOD is a natural, heterogeneous, anisotropic, hygroscopic composite material with cellular structure. This natural structure dictates strongly directional dependent properties. Wood is extremely mechanically efficient compared to most other materials, due to the high strength and stiffness relating to its weight [1]. Nowadays, large dimension wood structures are developed in different modern civil structures [2]. Also, wood can be produced from natural renewable sources without any complicated manufacturing process. Cracks in wood structures are usually available due to inherent defects or production process [1]. These cracked structures often are subjected to various loading conditions. Most probable load situation which may lead to fracture of wood structures is mixed mode I/II loading. Fracture phenomenon of wood is associated with a significant process zone [3]. The created damage zone is often in the form of bridging or microcracks and can prevent from catastrophic fracture of wooden structures due to consumption of energy. Although the presence of microcracks in the vicinity of crack tip will improve fracture properties of wood, it also causes to some difficulties and complicacies in the analysis and investigation of fracture phenomenon. To estimate a good approximation and applicable model for wood fracture process, a calculation of the microcrack distribution and a Mahdi Fakoor is with the Faculty of New Sciences and Technologies, University of Tehran, Tehran, Iran (phone: +98 21 61118597; fax: +98 21 88057915; e-mail: mfakoor@ut.ac.ir). Seyed Mohammad Navid Ghoreishi is with the Faculty of New Sciences and Technologies, University of Tehran, Tehran, Iran (e-mail: navid_ghoreishi@ut.ac.ir). change in the compliance of material in FPZ are necessary. Since the mechanisms responsible for wood fracture are numerous and complex, formulating of a strict and practical model appears to be very difficult and has not been published yet. Because of these difficulties, curve fitting approach has been employed by some researches, which needs significant number of experimental data. A mixed mode fracture criterion is proposed by Mall and Murphy [4]. Significant researches have been done by Wu on Balsa wood center notch specimens [5]. Wu has an interactive experimental mixed mode criterion which is widely used for composite structures. This empirical criterion contains two experimental coefficients, which makes it difficult to use. Another approach for the presentation of mixed mode I/II fracture criterion is the extension of well-known available fracture criteria in to composite materials. Jernkvist was the first researcher who has extended Maximum Strain Energy Release Rate (MSER) [6] and Minimum Strain Energy Density (MSED) criteria [7], in to wooden structures [8]. He used these common isotropic fracture criteria and this experimental observation where cracks in wood components are propagated along the fibers, for any crack-fiber inclinations. He suggested two criteria for orthotropic materials based on MSER and MSED criteria with assumption of self-similar crack propagation. His results are too conservative and are not verified with experimental data [9]. Neglecting the effect of wasted energy in FPZ may be the main reason for this deficiency. Maximum Principal Stress (MPS) criterion is also extended for cracked composite materials which contained better results [9]. Nature of FPZ and nonlinear behaviour of this region are also studied by some researchers separately without paying attention to the presentation of a fracture criterion. Vasic and Smith [3] found the significance of FPZ effects on fracture behavior of wood specimens. They proposed bridging crack [10] and lattice fracture approach [10] based on Hillerborg’s model [11]. In this paper, we will first show that the stress based criteria which are material independent (always Poisson’s ratio) are not able to predict the material nonlinearity effects in FPZ. Therefore, one of the main objectives of this article is to modify the energy based mixed mode criteria, which are material dependent, in order to consider the effects of energy wasted by micro crack formation in FPZ surrounded around the crack tip. By extraction of mechanical properties of damage zone, modified maximum strain energy release rate criterion is introduced with real elastic properties of FPZ. Scots pine and Norway spruce mixed mode experimental Failure Criterion for Mixed Mode Fracture of Cracked Wood Specimens Mahdi Fakoor, Seyed Mohammad Navid Ghoreishi W World Academy of Science, Engineering and Technology International Journal of Mechanical and Mechatronics Engineering Vol:11, No:7, 2017 1364 International Scholarly and Scientific Research & Innovation 11(7) 2017 scholar.waset.org/1307-6892/10007571 International Science Index, Mechanical and Mechatronics Engineering Vol:11, No:7, 2017 waset.org/Publication/10007571