Issue 30

A. Shanyavskiy et alii, Frattura ed Integrità Strutturale, 30 (2014) 340-348; DOI: 10.3221/IGF-ESIS.30.41 344 In the following years, inspection for cracking at the repairing plants brought about greater amounts of the sorted out PS articles (from 8.8% in 2001 to 41.8% in 2003). The most numerous of the sorted out were the articles run for 2000 to 4000 h on AI-24VT engines. No data are available regarding a peak percent of the shafts screened out of the engines AI- 24VT, Series 2. One can expect such a peculiarity as long as the type AI-24VT engine differs from type AI-24, Series 2, engine in the strength of a combined effect that the loading conditions have on an PS. If one replaces arbitrarily an PS between the two engine types, he alters in the opposite direction the running duration required to initiate fatigue cracking. So having moved an PS from an AI-24, Series 2 to an AI-24VT engine brings about longer running time (4000…6000 h) to initiate fatigue cracks, whereas the opposite change of the running time (to 2000…4000 h) is the case for having the PS moved in the reversed direction—to an AI-24, Series 2 engine. Having analyzed the screening trends of PS on repairs revealed that the numbers of fatigue cracks increased in the PS from AI-24 engines. Such an increase occurs owing to the more extensive employment of AN-26 aircrafts that, though at the ordinary service regimes, bring the greatest damage in the splined joints. Such a view follows from the statistical data acquired since1996 and compared with those acquired earlier - between the 1991 and 1993 (Fig. 4) - on the screening intensity of PS for the sake of cracks. In the earlier period, the share of the PS screened out at repairs increased from 6.3 to 32.1% despite the prominently reduced total running period of the An-24 and An-26 aircrafts. By 1993, the share of the PS screened out at repairs exceeded 30%; nevertheless, in that year the first case of an airscrew shaft broken out in flight took place. That share also exceeded 30% in 2003 and, again, the airscrews torn out in flight showed themselves on (year 2003) An24RB RA-46828, (year 2004) An-26 RA-26552, and (year 2004) An-26 RA-26582 aircrafts. At last, late in 2004, a through crack showed itself in the fillet part (R = 8 mm) by oil leak and increased shaking of the plane in flight. They only had two flights to perform until the full separation of the shaft! C HARACTERISTIC PATTERNS OF INITIATING AND GROWING CRACKS racking begins from a spline surface in the fretting-corrosion area or, otherwise, form a conjugation edge (corner) between the surfaces of a shorter spline and a pinhole. Crack initiation in a fretting-corrosion site shows that the shaft material behavior is characteristic of a partially closed synergetic system (Fig. 5). Crack initiation at the corner is largely relevant to the behavior of an open synergetic system (Fig. 6). a) b) c) Figure 5 : (a) a spline rim (general view) with the spline sites of the fretting damage; (b) the fractures (open cracks) initiated at the fretting sites (pointed to by arrows); (c) the main fatigue fracture surface with Macro-Beach-Marks of the propeller shaft, with the crack origin site (shown by arrow) at a fretting spot. Nevertheless, fractographically, the crack origin site at the corner showed the crack focus situated at some distance from the metal surface—under a sort of beading—a burr formed due to the pinhole machining. An important finding was that in some instances of breaking PS by growing fatigue cracks the latter grew concurrently from the damage (fretting) sites of spline surface and from the edge corners of the pinholes (Fig. 7). From the origin sites of both kinds, the cracks grew leaving behind the patches of the macroscopic lines (Macro-Beach- Marks) of fatigue fracture. On the fracture surface, each of the single macroscopic fatigue lines is representative of a single flight cycle (FC) [2]. At the early stage—not far from the fracture origin site—the cracks propagate slowly. They grow, cutting the pinhole surface as well as the spline surface until they hit the rounded zone of transition to the internal butt- end surface of an PS. C