Issue 41

S.M.J. Razavi et alii, Frattura ed Integrità Strutturale, 41 (2017) 424-431; DOI: 10.3221/IGF-ESIS.41.53 425 combination of tension and torsion with different values of the mode mixity ratios (i.e. the ratio between the tensile stress and the applied stress due to torsion loading): 0.4, 0.5 and 1.0. Considering a large variety of geometrical configurations obtained by varying the notch opening angle, notch radius and notch depth, a complete set of experimental data on cylindrical specimens subjected to combined tension and torsion loads was provided by the authors in previous paper [23]. The notch opening angle has been varied from 30° to 120° and the notch radius from 0.3 to 2 mm. The ASED criterion which is based on the strain energy density averaged over a control volume [19-25] is used for fracture assessment of notched samples subjected to the multiaxial static loading. The ASED criterion allows a sound fracture assessment of the critical load for the specific material under investigation and it can be potentially extended to other types of graphite and brittle materials subjected to different combinations of mode I and mode III loading conditions. In the current paper, first the experimental procedure for testing the notched graphite samples is presented. Afterward, the formulation and application of the ASED criterion is presented on the experimental data. F RACTURE EXPERIMENTS he details of the graphite material, the test specimens and the fracture experiments are presented in this section. The fracture tests were conducted on a grade of isostatic polycrystalline graphite with commercial name of EG022A. The basic material properties of the tested graphite are as follows: the mean grain size is of 300 μm, the porosity of 15%, the bulk density of 1830 kg/m 3 , the mean tensile strength of 30 MPa, the Young’s modulus of 8.00 GPa and the shear modulus of 3.30 GPa. Nonlinear deformation sometimes is observed during fracture tests of graphites, which makes the determination of Young’s modulus rather complicated. However, for simplicity the Young’s modulus was obtained in this research from load-displacement graphs recorded by a universal tension-compression machine. The deviation observed from linear behaviour was less than 0.01% at failure for the specimen used in the test. Young’s modulus has been measured at a load where the deviation from linear behaviour was less than 0.005%. The mean grain size was given in the material certify and measured by using the SEM technique while the density of the material was determined from the buoyancy method, submerging the tested graphite in a liquid of known density. The values have been checked and confirmed by the authors independently. All tests were performed under displacement control on a servo-controlled MTS bi-axial testing device (  100 kN/  1100 Nm,  75 mm/  55°). The load was measured by a MTS cell with ± 0.5 % error at full scale. A MTS strain gauge axial extensometer (MTS 632.85F-14), with a gage length equal to 25 mm was used for measuring the tensile elastic properties on plain specimens while a multi-axis extensometer MTS 632.80F-04 (with a gage length equal to 25 mm) was used for measuring the torsional elastic properties on unnotched specimens. Some load-displacement curves were recorded to obtain the Young’s modulus ( E ) of the graphite using an axial extensometer. The tensile strength ( σ t ) was measured by means of axis-symmetric specimens with a net diameter equal to 12.5 mm and a diameter of 20 mm on the gross section (see Fig. 1a). Due to the presence of a root radius equal to 40 mm, the theoretical stress concentration factor is less than 1.03. The torque-angle graphs recorded by the MTS device were employed together with the bi-axis extensometer to obtain the shear modulus ( G ) and to measure the torsion strength ( τ t ) of the tested graphite. The ultimate shear strength τ t was found to be equal to 37 MPa. 60 60 80 p    2   40  Plain specimens V-notched specimens (a) (b) Figure 1 : Geometry of plain (a) and notched (b) specimens used in the experimental tests. T