Issue 48

V. Giannella et alii, Frattura ed Integrità Strutturale, 48 (2019) 639-647; DOI: 10.3221/IGF-ESIS.48.61 640 include low-cycle fatigue/high-cycle fatigue (HCF/LCF) interactions, Mixed Non-Proportional Loading (MNPL), fracture mode asymmetry and both elastic and fracture resistance anisotropy. Legacy crack-growth criteria, such as the Maximum Tangential Stress criterion [17], consider proportional loading (K II /K I = constant throughout one cycle), and define the crack-growth along a K II ≈ 0 path. When the loading condition is not proportional, the relative ratios of K I , K II , and K III vary over time during the cycle, making the setup of effective crack path criteria more difficult. MNPL condition occurs when static and cyclic loads simultaneously act along different directions, as for example in a turbine blade where the steady state centrifugal load couples with the vibrations of the blades (Fig. 1). The blisks, i.e. turbomachine components that integrate rotor disk and blades in a single piece, are of common use in aero engines for all compressor stages due to the significant weight savings compared to traditional configurations with blades mounted in disk slots. The latter design could be affected by catastrophic failures if a crack in the blade slots, propagating because of HCF-loading, turned into the disk due to centrifugal LCF-loading condition. Thus, 45° centrally notched cruciform specimens made of Ti6246 plate, were proposed as a potential simplification to analyze this phenomenon [18]. A static load was considered for simulating the LCF loading whilst a cyclic load, applied perpendicularly to the static load at a frequency of 5 Hz, for the HCF loading condition. Several experimental tests were performed for different ratios between static and cyclic loads and the resulting crack paths and crack propagation rates were analyzed and compared with numerical results. This work can be seen as an extension of [18] which presented the experimental outcomes here reported and the related numerical results performed with the FEM code CRACKTRACER3D. Here we present our numerical results performed with the FEM code FRANC3D and the DBEM code BEASY. Figure 1 : Complex loading conditions on a cracked turbine assembly of rotor disk and blade. E XPERIMENTAL TESTS xperimental tests contained in this work have been taken from literature [18] and are here reported for more clarity. Different ratios of HCF and LCF loading conditions were considered for the experimental tests, providing corresponding crack propagation directions. A list of the tests considered in this work is summarized in Tab. 1. Main mechanical and fracture properties of the specimens’ material are listed in Tab. 2. In a first experimental test (Test I), a static load equal to 24 kN was considered together with a second perpendicular load cycling between -8 kN and +8 kN. A second test (Test II) was carried out with a different stepwise increment of the static load from 3 kN to 12 kN, 18 kN and finally to 12 kN, keeping the cyclic load amplitude at the fixed value of 24 kN with R=-1. Only the first two load steps were considered in this work since cracks bifurcated during load case three [18]. A pre-cracking procedure was realized in order to produce two initial cracks from the two notch tips. To this aim, an equal cyclic load (R = 0.1, f = 5 Hz) was applied to both axes and, starting from an initial magnitude of 8 kN, the cyclic load was increased stepwise, based on the fact whether a crack was initiated or not. After successfully producing cracks from the notch tips, the load was stepwise reduced to its initial value in order to let the crack grow out of the major part of the plastic zone generated during pre-cracking. E