Issue 33

V. Shlyannikov et alii, Frattura ed Integrità Strutturale, 33 (2015) 335-344; DOI: 10.3221/IGF-ESIS.33.37 336 loading conditions on fatigue life of cylindrical specimens is discussed. The relations of crack opening displacement and crack length on the free surface of specimens are obtained and it is shown that the growth of the crack fronts is dependent on the initial notch form. Using the aforementioned relations, the crack front shape and crack growth rate in the depth direction can be predicted. The simulations for the crack path assessment are based on the constrain parameters behaviour. The computational 3D fracture analyses deliver a governing parameter of elastic-plastic stress field distributions along the crack front. On this base crack growth interpretation is performed using the traditional elastic and new plastic stress intensity factors [5-7]. Different crack growth rate is observed in the direction of the deepest point of the crack front with respect to the free surface of the hollow cylindrical specimen. S PECIMENS AND MATERIAL PROPERTIES he test materials are aluminum alloys D16T and B95 which main mechanical 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,  is the elongation,  is the reduction of area, n is the strain hardening exponent and α is the strain hardening coefficient. Aluminum alloy  0.2 MPa  b MPa  %  %  u MPa E GPa n α D16T 439 590 9 9 645 75.922 5.88 1.5 438 598 12 13 686 77.191 5.85 1.58 B95AT 442 604 11 11 658 77.734 5.79 1.66 470 637 10 15 731 74.135 6.62 1.62 Table 1 : Main mechanical properties of aluminum alloys. Figure 1 : Details of the hollow specimen geometry and initial notches. The hollow cylindrical specimen geometry configuration is shown in Fig. 1. The diameter is equal to 28 mm in the test section and the length is equal to 130 mm. Using linear cutting machine surface edge cracks were cut with initial flaw depths b 0 3.0 mm for both a circular arc and elliptical-arc initial edge notch. The geometric parameters of the specimen test section and of the growing crack are shown in Figs. 1 and 2. In these figures, b is the current crack depth, with the crack front approximated by an elliptical curve with major axis 2c and minor axis 2a. The crack length b is obtained by measuring the distance between the advancing crack break through point and the notch break through point, as shown in Fig. 2. The depth of the initial curvilinear edge notch is denoted by a and the initial notch length by h . The crack opening displacement is measured on the free hollow specimen cylindrical surface, in the central plane of symmetry as shown in Fig. 2. The Axial Torsion Test System Bi-00-701 is used for axial-torsional fatigue and fracture testing of the hollow cylindrical specimens. This system is equipped with: fatigue rated axial-torsional dynamic load cell with axial capacity 100 kN and torsional capacity 2 kN-m; Bi-06-3XX series axial extensometers and torsional strain measurement fixture. The crack length on the specimen lateral surface were monitored using the optical instrumental zoom microscope whereas, to fix the crack opening displacement of specimen at the gauge length, a pulley arrangement with an externally axial encoder T