Issue 41

S.M.J. Razavi et alii, Frattura ed Integrità Strutturale, 41 (2017) 424-431; DOI: 10.3221/IGF-ESIS.41.53 426 As shown in Fig. 1, different round bar specimens were used for multiaxial (tension and torsion) static tests: unnotched specimens (Fig. 1a) and cylindrical specimens with V-notches (Fig. 1b). This allows us to explore the influence of a large variety of notch shapes in the experiments. In more detail:  For V-notched graphite specimens with a notch opening angle 2 α = 120° (Fig. 1b), notches with four different notch root radii were tested; ρ = 0.3, 0.5, 1 and 2.0 mm. The effect of net section area was studied by changing the notch depth p. Two values were used, p = 3 and 5 mm, while keeping the gross diameter constant (20 mm).  For V-notched graphite specimens with a notch opening angle 2 α = 60° (Fig. 1b), four different notch root radii were considered in the experiments: ρ = 0.3, 0.5, 1.0 and 2.0 mm. With a constant gross diameter (20 mm), also the net section area was kept constant, such that p = 5 mm.  For V-notched graphite specimens with a notch opening angle 2 α = 30° (Fig. 1b), three different notch root radii were considered in the experiments: ρ = 0.5, 1.0 and 2.0 mm keeping constant the notch depth p = 5 mm. At least three samples were prepared for each of the 15 specimen geometries described above, with a total number of 45 specimens. In order to prepare the specimens, first several thick plates were cut from a graphite block. Then, the specimens were precisely manufactured by using a 2-D CNC 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. The tests were conducted under three different combinations of tensile and torsional stresses, with the nominal mode mixity ratios σ nom / τ nom = 0.4, 0.5 and 1. Different nominal mode mixity ratios have been achieved by properly setting the torsional loading rate with respect to the tensile loading rate. In particular the tensile loading rate was varied keeping constant the rotation control conditions with a loading rate of 1°/min. The load-angle curves recorded during the tests always exhibited an approximately linear trend up to the final failure, which occurred suddenly. Therefore, the use of a fracture criterion based on a linear elastic hypothesis for the material law is realistic. The same trend has been observed for the tensile curves plotting the load as a function of the axial displacement. All loads to failure (tensile load and torque) are reported in Tabs. 1-3 for each notch configuration and loading conditions. In particular Tab. 1 reports the data for σ nom / τ nom = 1 while Tabs. 2 and 3 summarize the data for the two ratios 0.4 and 0.5, respectively. Specimen code Notch opening angle 2 α (°) Notch radius ρ (mm) Tensile Load (N) Torque (N mm) σ nom (MPa) τ nom (MPa) σ nom / τ nom 1-01 120° 0.3 1193 2659 15.19 13.54 1.12 1-02 1025 2280 13.05 11.61 1.12 1-03 1114 2500 14.18 12.73 1.11 2-01 0.5 1190 2690 15.15 13.69 1.10 2-02 1234 2631 15.71 13.40 1.17 2-03 1200 2694 15.28 13.72 1.11 3-01 1 1302 2873 16.58 14.63 1.13 3-02 1251 2673 15.93 13.61 1.17 3-03 1283 2845 16.34 14.49 1.13 4-01 2 1497 3798 19.06 19.35 0.98 4-02 1451 3634 18.47 18.51 1.00 4-03 1532 3710 19.51 18.89 1.03 5-01 60° 0.3 1073 2632 13.66 13.40 1.02 5-02 1037 2867 13.20 14.60 0.90 5-03 1125 2883 14.32 14.68 0.98 6-01 30° 0.5 1097 2852 13.97 14.53 0.96 6-02 1157 2704 14.73 13.75 1.07 6-03 1213 2917 15.44 14.86 1.04 7-01 1 1178 3038 15.00 15.47 0.97 7-02 1112 2972 14.16 15.14 0.94 7-03 1214 3248 15.46 16.54 0.93 8-01 2 1302 3102 16.55 15.80 1.05 8-02 1319 3386 16.79 17.24 0.97 8-03 1486 3489 18.92 17.77 1.06 Table 1 : Experimental results in the case of σ nom / τ nom = 1.0; notch depth p = 5 mm.