Issue34

A. Shanyavskiy, Frattura ed Integrità Strutturale, 34 (2015) 199-207; DOI: 10.3221/IGF-ESIS.34.21 200 Usually, dispersion of test results for constructing S-N curves in HCF regime is considered as material properties dispersion, with difference in a surface layer state from one to another specimen [1]. That is why “run-out” specimens are considered as S-N curve characteristic for HCF regime. But reality can be something different from that. As far as “run-out” specimens are concerned, several possibilities exist in their cracking after interrupting fatigue tests: (1) crack origination takes place in specimens on the surface; (2) crack has not evidence on the surface but it exists in the subsurface; (3) crack is not on the specimen surface and not in the subsurface. In the third case, material behavior has to be considered as in VHCF regime with high probability. The present paper discusses this problem related to titanium alloy Ti-6Al-4Mo (VT3-1), with different technological influence on the material state. The fractographic analysis for cracks path determination is essential in these investigations. M ATERIAL AND TESTING PROCEDURE Tested specimens nvestigation has been performed on the two-phase titanium alloy Ti-6Al-4Mo (VT3-1) which is usually used in aircraft engines for manufacturing compressor disks and blades [6]. Metallographic investigations have shown that in specimen’s material dominates mixed type (  structure – lamellar plus globular. The first group of specimens for tests was manufactured from two billets for compressor disks in their rim area. Smooth and notched bar specimens with stress concentration equal to about 1.46 were prepared in this case, Fig.1. The mechanical properties of the billets are shown in Table 1. Disk № and standard Ultimate tensile, σ u , MPa Elongation, δ % Area reduction , Ψ % Impact toughness, kgl/mm 2 I 1076 13.8 32.1 4.6 II 1083 13.4 27.1 3.3 Standard ≥ 960 ≥9.0 ≥20.0 ≥ 3.0 Table 1 . Mechanical properties of two titanium disks of VT3-1 alloy at 20 o C. Figure 1 . Schema of (a) smooth and (b) notched specimen, used in fatigue tests. Some tested specimens (T) were tempered at 530 o C during 6 hours and then cooled on the air. Some specimens were not tempered (NT). Some tempered specimens (TSP) were subjected to shot-peening procedure with metallic balls (0.05- 0.3mm) up to intensiveness of residual stress 1.1-1.27. Some specimens were not tempered and not peened (NTP). I a) b)