Issue 37

F. Berto et alii, Frattura ed Integrità Strutturale, 37 (2017) 69-79; DOI: 10.3221/IGF-ESIS.37.10 76  1A = 950 MPa and  3A = 776 MPa, and on the mean values of the NSIFs,  K 1A and  K 3A , with reference to a given number of cycles, N A = 2·10 6 : 1 1 1 A1 A1 1 1 K e2 R            (9a) 3 1 1 A3 A3 3 3 K 1 e R             (9b) R 1 2  R 3 Figure 4. Control volumes for V-shaped notches under tension and torsion loadings. On the basis of Eqs. (9a) and (9b), the control radii result to be R 1 = 0.051 mm and R 3 = 0.837 mm, respectively. Accordingly, the control radius R 1 has been adopted to compute the averaged strain energy contribution tied to tension loading, whereas the control radius R 3 has been employed to evaluate the averaged SED contribution due to torsion loading. It should be noted that the control radius R 3 is highly affected by the presence of larger plasticity under torsion loading as compared to tension loading, and by friction and rubbing between the crack surfaces, as previously discussed also for different materials [23,30]. Under these conditions, the averaged SED is called also ‘apparent linear elastic SED’ to highlight that the strain energy density calculation over two different control volumes (under tension and torsion, respectively, as determined from experimental data) allows us to overcome the problem tied to shielding mechanisms, keeping a linear elastic criterion. To summarise in the same scatter band experimental results obtained by adopting different nominal load ratios R, a coefficient c w , defined on the basis of simple algebraic considerations (reported in detail in Ref. [24, 31]), must be introduced. The coefficient c w results to be dependent on the nominal load ratio R according to the following expression:                      1R0 for R1 R1 0 R for 1 0R for R1 R1 c 2 2 2 2 W (10)