For comparison, the data of AA 7075 FSW joint obtained from the full specimen in our previous work are also presented , in which the fatigue failures occurred at the TMAZ in high cycle fatigue regime and at the NZ in the VHCF. It is clear that the fatigue strength keeps going down with cyclic load increasing.
Similarly, the special morphology around the crack initiation site is also observed in the VHCF study of AA 6061 FSW joint , and it was reported that the formation of these zones consumes over 98% of total fatigue life in the VHCF range.
These small defects have limited influence on tensile and fracture resistance but are the crucial factor to control fatigue crack initiation process in the VHCF range, because the VHCF failure became more sensitive to the small impurities and microstructural heterogeneities.
For high strength steels, some researchers had studied the effects of interior inclusions on the VHCF behavior in recent years [36-39].
Some errors of setting or hidden defects of the fuel control system affect the long-term working of compressor near the unstable limits (Figure 8) (stall, flutter, or surge during acceleration and deceleration, disturbance of the temperature field in compressor, combustion chamber and turbine), as well as high level of rotor unbalancing and alignment are the source of generating hidden fatigue problems of compressor blades (HCF and VHCF problems).
(ii) The indicator of the dangerous vibration amplitude is not detecting the HCF and VHCF threat of blades;
Taking the VHCF risk into account requires using the widened S-N curve, appointed on the base of the bimodal theory of the metal fatigue (containing other bifurcations).
Influence of the local plastic strain of material (LCF, HCF and VHCF problems--Table 2) the best is visible in the weak magnetic field .