Issue 38

A. Shanyavskiy et alii, Frattura ed Integrità Strutturale, 49 (2016) 399-404; DOI: 10.3221/IGF-ESIS.38.49 400 introduced earlier master curve constructed for aluminum alloys in the simple case of uniaxial tension with stress R-ratio near to zero [1]. Calculated equivalent tensile stress was compared for different blade sections and it was shown that in- service blades experienced not principle difference in this value in the crack growth direction by the investigated sections. It is not above the designed equivalent stress level. Crack growth period estimation in longerons based on fatigue striation spacing or meso-beach-marks measurements has shown that monitoring system introduced designer in longerons can be effectively used for in-time crack detecting independently on the failed section when can appeared because of various type of material faults or in-service damages. a) b) Figure 1 : Scheme (a) of rotor-blade section with numbering “1”-“5” areas for crack origination and pointed out its external loading and (b) cells position for a blade. The pressure gage, installed in the basement portion of the rotor blade, is designed to send a signal of lost excessive pressure to a monitor (a window in that a watcher can see the red cup to appear) once discontinuity arose in the longeron in any section of the propeller blade. According to the service norms, such inspection is to be performed each time before the flight. In-service fatigue cracks nucleated in the longerons in the defect sites that were revealed all over the blade length, from R = 0.085 to R = 0.71, where max ( ) / ( ) d i d R R R  (see Fig.1b). In the paper we shall discuss in details the fracture trends of the longerons made of aluminium AVT-1 alloy. I NSPECTION RESULTS OF FATIGUE - CRACKING BEHAVIOR OF THE LONGERONS atigue cracks only nucleated in operating longerons of the rotor-blades if their material was somehow damaged [2, 3]. Despite the fact that the defects differed in the sites of location in the cross-section area of the longeron (see Fig. 1), all the defects caused failure of the fatigue nature. We can formalize these failure cases according to a general scheme of the fracture surface, Fig. 2, as composed of: - a fracture-origin exhibits a fracture relief typical of the damage kind and may be located anywhere on the inner or outer surface of the longeron walls; - a zone (1) of stable crack growth typically exhibits a smooth fracture surface with distinct mesoscopic or macroscopic (depending of the fracture location) beach-marks of fatigue fracture; - a zone (2) of accelerated crack propagation typically shows a wavy fracture surface of the “Christmas-tree” or “chevron- like” type; - a zone (3) of fast crack propagation when the plastic-shear lips form entirely over the wall-thickness of the longeron. In the zone 1, pseudo-striations pattern forms first (P-region), peculiar to low-rate crack growth in the near-threshold range of the kinetic diagram. The material here experiences extensive shear. As the crack length increases, a pattern of fatigue striations forms or, alternatively, mesoscopic fatigue beach-marks dominate, depending on the blade section in that the crack propagates. Formation of fatigue striations alternating with dimpled fracture is typical of the zone 2: here, the crack growth becomes accelerated. Purely dimpled relief is typical of the zone 3 of final fracture; here the plastic-shear lips form throughout an entire thickness of the blade wall. Having fatigue meso-beach-marks and fatigue striations distinctly visible made it possible to analyze the growth trends of fatigue cracks in the longerons and to determine the growth durations of the cracks and estimate the in-service stress level. F