Issue 31

R. Citarella et alii, Frattura ed Integrità Strutturale, 31 (2015) 138-147; DOI: 10.3221/IGF-ESIS.31.11 142 blocks, again to reduce distortion in the mesh. Of course there is no single method of crack modelling and remeshing that can be used for all crack geometries. However, the combination of standard and large crack-blocks in conjunction with mesh manipulation algorithms, allows many difficult problems to be handled. T EST MATERIAL AND SPECIMENS he test material is carbon steel R2M. Its main mechanical properties including constants of the Paris equation (Eq. 4) are listed in Tab. 1 where E is the Young’s modulus,  b is the nominal ultimate tensile strength,  0 is the monotonic tensile yield strength,  u is the true ultimate tensile strength, n is the strain hardening exponent, C (the numerical value is consistent with a Young modulus and remote applied stress expressed in MPa and distances expressed in mm) and m are the Paris constants. m da C K dN   (4) E [MPa]  b [MPa]  0 [MPa]  u [MPa] n C m 209000 810.3 540.8 890 6.134 4.1  10 -13 2.818 Table 1 : Mechanical properties of steel R2M. The specimen geometry configuration is shown in Fig. 5: the nominal diameter is equal to 10 mm in the test section and the length, including the clamped part, is equal to 100 mm. Using linear cutting machine, surface edge cracks were cut with initial flaw depths b 0 =1.0 mm. Figure 5 : Details of the specimen geometry. The geometric parameters of specimen test section and of growing crack are shown in Fig. 6a-b: b is the current crack depth, with the crack front approximated by an elliptical curve with major axis 2c and minor axis 2 b . The crack length a is obtained by measuring the distance between the advancing crack break through point and the notch break through point, as shown in Fig. 6a. The depth of the initial straight edge notch is denoted by h and the initial notch length by L . The crack opening displacement is measured on the free specimen cylindrical surface, in the central plane of symmetry as shown in Fig. 6b. For the simple cyclic axial fatigue tests, the specimens are tested with an applied maximum remote stress equal to 250 MPa and with a stress ratio R=0.1. The combined tension/torsion tests are performed with the same stress ratio, applying synchronous and in-phase tensile and shear stresses whose maximum values are respectively equal to 250 and 100 MPa. Two different frequency values (10 and 7 Hz) were applied to the specimens in order to highlight the crack front geometry during propagation: during each test, beach marks were produced on each specimen by reducing the applied frequency from 10 to 7 Hz, when the surface crack length was approximately equal to a  1mm. The typical beach marks on the post mortem cross section of different specimens are shown in Fig. 7a-b. T