Issue 30

A. Shanyavskiy et alii, Frattura ed Integrità Strutturale, 30 (2014) 340-348; DOI: 10.3221/IGF-ESIS.30.41 341 a) b) c) Figure 1 : (a) The AN-24 aircraft wing (general view) in the braking zone of the propeller shaft; (b) a fraction divided by braking from the propeller shaft; (c) the view of the propeller shaft with a fatigue crack (pointed to by arrow). We mentioned above that, in a PS case, one should relate estimations of the N p /N f magnitude to the HCF-to-UHCF transition range and to N p /N f varying as 5 to 10%. Such an approach appears realistic as confirmed by the N p /N f estimations done for the cracks that grew from the surface damage sites in gear wheels [5]. Yet one still wants to ascertain which of the two cases -unsatisfactorily tightened bolts or locally overstressed (beyond a limiting state in above-mentioned zones) joint-structure components- dominates as the cause of the spline joints failure. Below we critically assess the data (of more than 40-year period research) on the patterns and various causes of the in- service propagation of fatigue cracks in the airscrew shafts of AI-24 and AI24-VT engines on IL-18, AN-24, and AN-26 aircrafts [2–4]. I NFORMATION ON THE CRACKS AND FAILURES OF PROPELLER SHAFTS hey use a spline-bolted joint to have a propeller hub fasten to the propeller shaft of an AI-24 engine (Fig. 2). The splined flange portion of an PS experiences dynamically applied loads that cause fretting over the spline surface. Cracks became visible in the PS of AI-24 engines almost as early as at the very beginning period of running the engine. The first two PS exhibited many a cracked spline on the first 500 and 300 hours of running. Those cracks were of fatigue nature, started propagating from fretting-corrosion zones and grew as far as for 2/3 spline length at the smaller-modulus side. Nevertheless, even having inspections repeatedly and frequently performed (see Fig. 2), with the magnetic inspection T