Issue 35

E. Fessler et alii, Frattura ed Integrità Strutturale, 35 (2016) 223-231; DOI: 10.3221/IGF-ESIS.35.26 226 method, transgranular fracture was also observed on the fracture surface (see Fig. 3) before a fully intergranular fracture observed at higher ΔK. These observations confirm the results of Li [9] for DA Inconel 718. Figure 2 : SEM micrographs showing the fracture surface of a specimen tested at 650°C with 10-300-10 cycles: a) 1 - fatigue pre- cracking, 2 - 10-300-10 propagation and 3 - 10-300-10 propagation in the low ΔK regime during the K-decreasing procedure. b) Transition from fully intergranular to transgranular fracture in the low ΔK regime. The solid line indicates the end of the fatigue pre- cracking step and the dashed line indicates the start of the K-decreasing procedure. c) Transgranular fracture at low ΔK. Figure 3 : Transgranular fracture observed at low ΔK under 10-300-10 cycles at 600°C. The solid curve corresponds to the application of the 10-300-10 cycle, at low ΔK. The dashed curve indicates the fully intergranular fracture obtained at higher ΔK. Figure 4 : Inverse pole figure showing transgranular fracture along the crack path in the low ΔK regime. The nickel coating was removed from EBSD data by filtering data with the index quality of collected Kikuchi diagrams. Crack path observations using EBSD An in depth characterization of the crack path at a microscopic scale was performed using EBSD (Fig. 4). These analyses were performed to confirm the transgranular fracture in the low ΔK regime under hold time conditions, as it does not appear as a typical transgranular fracture mode, usually observed under 2 Hz pure fatigue loading (compare Fig. 1-a and