Issue 30

A. Shanyavskiy et alii, Frattura ed Integrità Strutturale, 30 (2014) 340-348; DOI: 10.3221/IGF-ESIS.30.41 347 We also took into account the forced (for various reasons) disconnections of an airscrew and of its reinstallation at other aircraft, which had its effect on the crack-growth behavior. Such a replacement of an airscrew can result in a long-term arrest or deceleration of the crack growth. Therefore, those increments in the operating time of PS supplied us with correction figures to the calculated N p /N f magnitudes. The carefully examined patters formed by the macroscopic lines (Macro-Beach-Marks) of fatigue fracture made it possible to estimate the relative duration of fail-safety operation (durability) of the shafts. In case that, for various objective reasons, necessary information was lacking, we employed the ratio N p /N f of 5 to 10% to have acquired the estimations of the relative durability by calculations. Those estimations showed such a share of a crack growth period in the total running time of an PS as being reasonable and worth using. Fractographically, early crack growth periods appeared to share 7.1 or 8.1 percent of the intermediate running time—up to the first violation of the crack growth pattern, caused by the first PS reinstallation, and the shaft No. 1456e exhibited (also by the first violation point of the crack growth pattern) a relative durability of 9.9%. Our calculations show these ideas to agree completely with the operation history of the PS. At the intermediate (between the PS installation events) stages of running which do involve crack propagation, the relative durability of PS can vary as 22 to 100 per cent, depending on whether the crack retardation effects do (22%) or do not take place. At a single running stage (No. 1109e shaft), which showed both fatigue fracture and a crack retardation effect, the relative durability achieved 7.25%; subsequently, the crack continued growing to cut through. The growth rate of fatigue cracks in PS, calculated based on the spacing of fatigue MBM, was twice as high in the PS of AI-24VT engines as of AI-24, Series 2 engines. This finding indicates that the PS are more extensively loaded on the AI- VT engines and confirms that the above transient effects of crack retardation do show themselves when having the PS reinstalled from an AI-24VT to AI-24, Series 2 engine. Therefore, the earlier recommended 500-h periodicity of inspection appears only efficient for the AI-24, Series 2 engines. The PS of АI-24ВТ engines, however, can fail before its next standard inspection time, be a crack overlooked in a repair or in periodic inspection. Statistical data on screening out PS (see Figs. 3 and 4) indicate that, notwithstanding the three stress raiser phenomena present, one should regard the cracking effects in PS as being a consequence of imperfect designing. The actual operating regimes, materials quality, pinholes geometry, and the contact characteristics of the splined joint—all accepted by the designer—concurrently contribute to the imperfection manifesting insufficient fatigue resistance of the construction. Compared with the acute edges, the fretting areas of the spline surface do not so efficiently shorten the initiation period of fatigue cracks. Only if we completely prevent the energy exchange of the shaft surface and environment, and, thereby, shift the crack origin site to the subsurface regions in the most highly stressed part of the shaft, a significant lengthening of the lifetime can occur. Practically, however, such a reconstruction of the joining, employed in its present design for transmitting a torque, appears hardly possible. For that reason and according to the above discussion, the safety operation of an PS – AI-24VN engine assembly, in its present design, is possible providing a nondestructive inspection occurs after every 250 hours of the operating period. Thereby, a small crack, even if unnoticed by an inspector, will not achieve its critical length but become definitely detectable by the time of the next nondestructive inspection. C ONCLUSION 1. Owing to the special characteristics of their present design, the spline-bolted joints between the propeller and reducer shafts of an AI-24 engine remain unsafe from the in-service initiation and propagation of fatigue cracks. 2. In the components of a spline-bolted joint, a crack nucleates owing to the (1) lost tension and to fatigue failure of the fastening bolts, (2) fretting corrosion of the spline surface, which makes a crack-origin site, and (3) the sharp edge of a pinhole. Cracks can form concurrently at both the fretting spot and the acute edge. 3. The ratio N p /N f for the periods of crack growth and propeller operation (durability) ranges as 5 to 10%; as regards the examined components of spline-bolted joint in that fatigue cracks nucleated in and propagated from various crack-origin sites; this ratio also holds in case that the crack ceases growing for a while on having the propeller assembly replaced to another engine. 4. Having acquired the period of crack growth in the propeller shafts from the number of Macro-Beach-Marks on the fracture surface made it approved to perform nondestructive in-service inspection of the shafts after each of the 250-h running periods.