Issue 24

A. A. Shanyavskiy, Frattura ed Integrità Strutturale, 24 (2013) 13-25; DOI: 10.3221/IGF-ESIS.24.03 20 All of these fractures exhibited fatigue striations with the striation spacing not less than 0.34  m by the edge of the hole, Fig. 7. The crack-growth rate was noticed to increase along the minor axis a (in-depth of a disk hole). This acceleration appeared the smaller the greater operation time to agree with the idea that the periods of a crack initiation and propagation change in the same proportion with a stress level and stressed state of the structure element [4]. In all examined cases, the value of striation spacing grew linearly long the minor axis of the semi-elliptic crack (see Fig. 7). Such a trend is typical of a structure-element behavior in a biaxial stressed state under LCF conditions with a constant strain range [16]. Figure 7 : The changing trend of the value  of fatigue-striation spacing along the crack-growth direction (minor axis a of the crack front shape) for the group of examined disks (disk data are given in Tab. 1). Nos. 3 and 4 disks were close to the broken disks as concerns their operation times, and the opened fractures revealed the cracking depths of 1.8 and 2.0 mm, respectively. Deep as 1.2 mm, the value of striation spacing was 1.2  m (close to that in the broken disks) and the disks experienced a change of fracture mechanisms: now the facet-type-pattern fracture morphology dominated born by transgranular slip. This just was Zone II, revealed by the fracture of the broken disks. In all the other examined fractures the crack did not reach its critical depth of 1.3 mm, i.e., transition to Zone II of crack growth did not occur. Figure 8 : (a) Block of loading waveform (schematic) of a Stage III turbine disk applied when bench-testing an NK8-2u engine and (b, c) fatigue-striation blocks, h i , illustrating the fracture response of the disk loaded in the engine.