Issue 37

A. Shanyavskiy et alii, Frattura ed Integrità Strutturale, 37 (2016) 22-27; DOI: 10.3221/IGF-ESIS.37.04 27 From Eq. (3) we found the equivalent stress  e  58 MPa for the crack length a 0 = 6 mm. One can see that loading conditions remained the same and no positive or negative deviation occurred from the equivalent-stress level all over the fatigue-striation stage. All these calculations indicate that the fatigue fracture of the longeron was initiated as a result of high stress concentration in the corrosion-cracking site and not because of an overloading event. One can see that, regarding the levels of equivalent stresses, the longeron sections at the R __ -value 0.7 and 0.5 are close to one another; this finding is consistent with the data on crack-growth durations and stress levels calculated for longerons. In the discussed longeron, initial corrosion cracking (intergranular) reached 0.4-mm depth. Then in 5-mm distance predominantly corrosion cracking (transgranular and intergranular) transformed to purely fatigue cracking (transgranular). Next to this zone, fatigue crack propagated for quite a long period. Here, the value of equivalent stress did not exceed the designed level. Therefore, fatigue crack would not nucleate without stress concentration, caused by the corrosion cracking deeper than 0.3 mm from the outer surface of the longeron, as long as the surface layer of such thickness experiences the residual compression stresses greater than the equivalent tensile stress. The monitoring system employed to watch the longeron impermeability of the Mi-family helicopters is efficient. C ONCLUSION – In cases of nucleation and propagation of fatigue cracks in the longerons of Mi-4 and Mi-8 helicopters in-service stresses never exceeded the designed levels at any relative radius of the propeller blades – the longest crack-growth periods are typical of the basement part of the blades – through fatigue cracks show the growth periods of tens flights for a whichever relative radius of a propeller blade; hence, such cracks can be revealed in good time with the pressure gages, installed in the blades to signal about the loss of their impermeability. R EFERENCES [1] Shanyavskiy, A.A., Tolerance fatigue cracking of aircraft structures. Synergetics in engineering applications . Monograph, Ufa (Russia), (2003) [2] Shanyavskiy A.A., Quantitative fractographic analyses of fatigue crack growth in longerons of in-service helicopter rotor-blades. Fatigue Fract. Engng Mater Struc., 19 (1996) 1129-1141. [3] Shanyavskiy A.A., Toushentsov A.L., Effectiveness of safety flight guaranty for helicopters using introduced in longeron rotor-blade device for cracks detecting. Science Works for Investigators Society of Crashed Aircrafts, Russia, 11 (1999) 110-128. [4] Shanyavskiy A.A., Orlov E.F., Koronov M.Z., Fractographic analyses of fatigue crack growth in D16T alloy subjected to biaxial cyclic loads at various R-ratios. Fatigue Fract. Engng Mater Struc., 18 (1995) 1263-1276. [5] Shanyavskiy A.A., Orlov E.F., Grigoriev V.M., Fatigue crack growth in D16 Al-alloy sheet subjected to biaxial out-of- phase loading. Fatigue Fract. Engng Mater Struc., 20 (1997) 975-983. [6] Chan K.S., Hack L.E., Leverat G.R. Fatigue crack propagation in Ni-base superalloy single crystals under multiaxial cyclic loads. Met. Trans. 17A (1986) 1739-1750. [7] Murakami, Y. (Ed.) Stress Intensity Factors Handbook (in 3 volumes), Pergamon Press, Oxford, (1987).