13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- grain size is 2 μm, porosity 7%, bulk density of 1850 kg/m3, mean tensile strength of 28.5 MPa, Young’s modulus of 8.05 GPa and shear modulus of 3.350 GPa. The compressive strength is equal to 110 MPa, whereas the flexural strength is 49 MPa. All tests were performed under load control on a servocontrolled MTS axial testing device (±100 kN/± 110 Nm, ± 75mm/± 55°). The load was measured by a MTS cell with ± 0.5 % error at full scale. Table 1. Mechanical properties Material Property Value Elastic Modulus E [MPa] 8050 Shear Modulus G [MPa] 3354 Poisson's Ratio ν 0.2 Ultimate Torsion Strength [MPa] 30 Ultimate Compression Strength [MPa] 110 Ultimate Tensile Strength [MPa] 46 Fracture toughness [MPa m0.5] 1.06 Hardness [Shore] 58 Density [Kg/dm3] 1.85 Porosity [%] 7 Resistivity [μohm·m] 11 Thermal Conductivity [W/(m·K)] 110 2.2 Geometry of the specimens As shown schematically in Fig. 1, the specimen used in the present investigation is a plate containing a central key-hole crack with a notch radius ρ and the distance between the hole centres a. The specimen is subjected to tensile loading. By changing the angle β, different combinations of mode I and mode II (or tension and shear deformation) can be produced for key-holes. When the load is applied along the notch bisector (i.e. β= 0), the key-holes are subjected to pure mode I deformation. By increasing the angle β from zero, the loading condition changes from pure mode I towards mixed mode I+II. For all the tested graphite specimens, the width (W), the notch depth (a) and the thickness t were 50 mm, 10 mm and 10 mm, respectively. Five values of notch radius ρ=0.25, 0.5, 1, 2, 4 mm were considered for manufacturing the test specimens so that the effects of the notch tip radius on mixed mode fracture of the graphite specimens are studied. With the aim to obtain different mode mixities, four values of the angle β were considered (β=0, β=30°, β=45° and β=60°). In order to prepare the graphite test specimens, first several plates of 10 mm thick were cut from a graphite block. Then, the specimens were precisely fabricated by using a 2-D CNC water-jet cutting machine. Before conducting the experiments, the cut surfaces of the graphite specimens were polished by using a fine abrasive paper to remove any possible local stress concentrations due to surface roughness. A total number of 70 mixed mode I/II fracture tests were performed for various notch geometry parameters. For each geometry shape and loading angle, three separate fracture
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