Issue 41

S.M.J. Razavi et alii, Frattura ed Integrità Strutturale, 41 (2017) 424-431; DOI: 10.3221/IGF-ESIS.41.53 427 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 9-01 60 0.5 636 3923 8.10 19.98 0.41 9-02 600 4010 7.64 20.42 0.38 9-03 630 4009 8.02 20.42 0.39 10-01 1 645 4449 8.21 22.66 0.36 10-02 660 4634 8.40 23.60 0.36 10-03 631 4326 8.03 22.03 0.36 11-01 2 895 5164 11.40 26.30 0.43 11-02 811 5259 10.33 26.78 0.39 11-03 750 4700 9.55 23.94 0.40 Table 2 : Experimental results in the case of σ nom / τ nom = 0.4; notch depth p = 5 mm. 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 12-01 120 0.3 1482 11606 9.63 21.54 0.45 12-02 1324 9657 8.60 17.92 0.48 12-03 1768 12129 11.49 22.51 0.51 13-01 0.5 1701 11768 11.05 21.84 0.51 13-02 1619 11196 10.52 20.78 0.51 13-03 1657 12005 10.76 22.28 0.48 14-01 1 1739 12150 11.30 22.55 0.50 14-02 1788 12756 11.62 23.68 0.49 14-03 1816 12611 11.80 23.41 0.50 15-01 2 2034 13891 13.21 25.78 0.51 15-02 1756 12500 11.41 23.20 0.49 15-03 1931 13452 12.54 24.97 0.50 Table 3 : Experimental results in the case of σ nom / τ nom = 0.5; notch depth p = 3 mm. As visible from the tables the imposed mode mixity ratio is almost fulfilled with a variation of approximately ±10% with respect to the nominal value. The variability of the loads to failure as a function of the notch opening angle is weak although not negligible. For a constant notch radius, the fracture load slightly increases for larger notch opening angles, although this effect is very low. S TRAIN E NERGY D ENSITY AVERAGED OVER A CONTROL VOLUME : THE FRACTURE CRITERION ith the aim to assess the fracture load in notched graphite components, an appropriate fracture criterion is required which has to be based on the mechanical behaviour of material around the notch tip. In this section, a criterion proposed by Lazzarin and co-authors [19] based on the strain energy density (SED) is briefly described. The averaged strain energy density criterion (SED) states that brittle failure occurs when the mean value of the strain energy density over a given control volume is equal to a critical value W c . This critical value varies from material to material but it does not depend on the notch geometry and sharpness. The control volume is considered to be dependent on the ultimate tensile strength σ t and the fracture toughness K Ic in the case of brittle or quasi-brittle materials subjected to static tensile loads. The method based on the averaged SED was formalised and applied first to sharp (zero radius) V-notches under mode I and mixed mode I+II loading [19] and later extended to blunt U- and V-notches [20]. When dealing with cracks, the control volume is a circle of radius R c centred at the crack tip (Fig. 2a). Under plane strain conditions, the radius R c can be evaluated according to the following expression [26]: 2 (1 )(5 8 ) 4 Ic Ic t K R              (1) W