PSI - Issue 28

Yifan Li et al. / Procedia Structural Integrity 28 (2020) 1140–1147 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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researchers for mixed mode fracture experiments, such as compact tension-shear specimen (Richard and Benitz 1983; Zipf and Bieniawski 1986) and asymmetric four-point bending specimen (Fett et al. 1995; Li and Sakai 1996; Ayatollahi and Aliha 2011). Among these specimens, circular shaped samples are one of the most popular specimens used to research mixed mode brittle fracture because of easy manufacturing and simple loading fixtures. Centre-Cracked Brazilian disk (CCBD) is a suggested method by International Society for Rock Mechanics (ISRM) for mode I fracture toughness measurements (Ulusay 2014; Fowell et al. 1995). By changing the inclination angle between the central crack and loading force, mixed mode fracture or pure mode II fracture can be obtained. Dong et al. have also obtained the analytical formulas of stress intensity factors and T-stress for CCBD using the weight function method (Dong et al. 2004; Hua et al. 2015). The CCBD specimen was also used to study fracture properties of different brittle materials like PMMA (Polymethyl methacrylate) and ceramics (Zhou et al. 2006a; Atkinson et al. 1982; Banks-Sills and Schwartz 2002; Aliha and Ayatollahi 2012; Mirsayar et al. 2016), and experimental results about the crack initiation angle and fracture load consistent with measurements by other methods and also agree well with the theoretical predictions. The CCBD specimen was also widely used to measure the dynamic fracture properties of brittle and quasi brittle materials (Dong et al. 2006; Zhou et al. 2006b). In addition, the Brazilian disk without a preset crack can be used to measure the tensile strength of brittle materials (Ulusay 2014). The edge cracked Semi-Circular Bending (SCB) specimen proposed by Chong and Kuruppu was later developed into a standard specimen recommended by ISRM (Chong 1984; Kuruppu et al. 2014). The simple geometry of specimen and easy test set-up procedure with common testing apparatus can be considered as major advantages of the SCB specimen (Ayatollahi and Aliha 2006). This specimen can perform all mixed mode range fracture tests including pure mode I and pure mode II by changing the orientation angle of the crack line relative to the loading direction. Aliha et al. (2010, 2012) used SCB specimen to research the size effects on fracture properties in limestone under pure mode and mixed mode loading. Recently, Ayatollahi et al. (2011) proposed an improved SCB specimen with a vertical crack subjected to asymmetric three-point bend loading (ASCB) and Saghafi and Monemian (2010) presented a modified SCB specimen with an inclined edge crack subjected to asymmetric three-point bend loading (IASCB). An edge notched disc bend specimen (ENDB) containing an inclined edge crack thorough the diameter subjected to three-point bend loading was proposed by Aliha et al. (2015) to investigate I/III mixed mode fracture behavior. The ENDB specimen is able to introduce full and complete mode I/III mixity including pure mode I and pure mode III fracture by rotating the crack plane. Numerous studies have shown that this specimen is able to successfully produce and characterize mixed mode I/III fracture behavior of engineering materials like rock, graphite, PMMA, polyurethane foams and hot mix asphalt concretes (Aliha and Bahmani 2017; Bahmani et al. 2017; Aliha et al. 2016; Aliha et al. 2019; Pirmohammad and Bayat 2016). The manufacturing process of the circular shaped specimens can be summarized as the following steps: The drilling machine is first used to obtain the rock core from a rock block; then the cylindrical rock core is sliced into several circular disks or semi-circular discs by a high-speed electric saw. In the last step, a preset crack is introduced into circular shaped samples by a very narrow rotary diamond saw and the crack tip can be sharpened by a diamond wire saw with a very small diameter. Fig. 1 illustrates the manufacturing process of circular shaped samples and the related loading form. As can be seen from Fig. 1, when the diameter of cylinder is larger than half of square length and no good packed arrangement can be utilized, most of the remaining rock is wasted except the drilling core used to make specimens. It is therefore unsuitable for some rocks that are difficult to mine. The present work proposes a new specimen that can make use of the remaining rock after removal of circular samples. The test configuration and specimen geometry will be described firstly in the next section. Then finite element method will be used to obtain the mode I and mode II stress intensity factors in order to verify the feasibility of this specimen to conduct a full ranges mixed mode fracture experiment. The fracture parameters of this specimen including the stress intensity factors and the T-stress are computed numerically in the next sections for different crack lengths and various diameters of preset hole. It is worth mentioning that this specimen can also be used as a substitute to measure fracture properties of other brittle and quasi-brittle materials.