PSI - Issue 12

Venanzio Giannella et al. / Procedia Structural Integrity 12 (2018) 499–506 V. Giannella/ Structural Integrity Procedia 00 (2018) 000 – 000

501

3

To study such phenomena, cruciform specimens were tested (Dhondt et al., 2018), undergoing static and dynamic loads as illustrated in Fig. 2 (courtesy of Christian Kontermann, TU Darmstadt, Germany). The LCF loading was simulated by a static load applied along the X direction whereas, the HCF load was simulated by a loading in the Y direction (5 Hz frequency). A 45° notch was machined in the center of the specimens with a geometry depicted in Fig. 2. Then, two cracks propagated from the two notch tips and their paths were monitored during the tests. The total length of both the initial notch and the two cracks was equal to 2 mm after a precracking phase.

Fig. 1. Complex loading conditions on a cracked turbine assembly of rotor disk and blade.

Tested specimens were made of Ti6246 with main mechanical and fracture properties listed in Tab. 1.

Table 1: Main mechanical properties and Walker law parameters of Ti6246, as used for the numerical analyses.

υ [-] 0.32

E [GPa]

C [mm/cycle/(MPa mm0.5)^m]]

m [-]

w [-] 0.67

120.4

4.83828E-13

3

In each test different ratios of HCF and LCF loading conditions were considered, providing corresponding varying crack paths. In a first experimental test, useful to calibrate the crack growth law, a static load equal to 24 kN was considered together with a dynamic load ranging between -8 kN and +8 kN. Further experimental tests were then performed varying both the dynamic load amplitude and the static load magnitude in order to obtain different crack paths for different static/dynamic load ratios. The numerical simulations, performed with the DBEM code BEASY, considered a mission profile defined by means of two load cases built as in the following: A. static tensile load in X direction plus HCF tensile load in Y direction (load case 1); B. static tensile load in X direction plus HCF compressive load in Y direction (load case 2). The portion of cruciform specimen highlighted by the red line in Fig. 2 was submodelled in the DBEM environment (Fig. 3). Traction boundary conditions, as provided by the overall FEM analysis, were applied on the cut surfaces, together with springs of negligible stiffness applied on few elements in order to prevent rigid body motion. 3. Numerical calculations