80 A. F. Fahem et al. 13.3 Determination of Mixed-Mode Fracture Parameters 13.3.1 Specimen Preparation Clear impact-resistant polycarbonate bar (ASTM D3935-PC) of radius r =12.7 mm sourced from McMaster-Carr are used. All experiments are performed at room temperature. A fully spiral v-notch crack is cut on the cylindrical surface of the bar, as shown in Fig. 13.5. The cross-section of the specimen, Fig. 13.5b, shows the crack ligament of a =8.7 mm, crack depth of 4 mm, and a crack tip v-notch angle of 45◦ with a tip radius of less than 100 μm. A stainless-steel small curving blade (item #35435A67 on McMaster-Carr.com) was used to make the final sharp artificial crack. The spiral v-notch has an inclined angle of βsp =67. 5◦ with the central longitudinal axis of the bar for all experiments performed in this study. The specimen was tested under a uniform far-field torsional load. Thus, the crack front was subjected to two components of stress, uniaxial tensile stress σn (Mode-I) and shear stress σt (Mode-III), as shown in Fig. 13.5c. In general, the crack tip is subjected to Mixed-Mode (I/III) conditions. 13.3.2 Fracture Load Measurement A tension-torsion machine manufactured by Test Resources is used for all experiments. Unless the experiment is carefully designed, a compressive load could develop during loading since the specimen extends during twisting. The spiral v-notch specimen is twisted up to fracture at a rotational rate of 1 deg./min, while the axial compression load is controlled and kept equal to zero throughout the experiment test. Thus, the specimen is subjected to a pure torsional load condition. The fracture torque obtained from the measurement is used to numerically calculate Stress Intensity Factor (SIF) for the cylindrical solid specimen (Fig. 13.6). The torque versus angle of twist is shown in Fig. 13.7. As shown in Fig. 13.7, the specimen response is linear elastic up to point A. Beyond point A, a lightly nonlinear response is evident, potentially due to plastic deformation around the crack front, continuing until the specimen fractures at point B with TB =40.1N-m. Fig. 13.5 Schematic of PC specimen with spiral crack and effective stresses
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