Issue 30

A. Shanyavskiy et alii, Frattura ed Integrità Strutturale, 30 (2014) 340-348; DOI: 10.3221/IGF-ESIS.30.41 343 a) b) Figure 3 : PS rejected out of the service in (a) 1977 and (b) 2001, 2002, and 2003 (statistical data). The greatest stress - 175 MPa (average for various frights) - achieved by dynamic loading and measured by experiment in the R = 8 mm fillet part - approaches the bifurcation point-fatigue limit - for a fatigue life of 10 7 loading cycles. This case is closely reproducing an in-service state of the most stressed PS when executing, landed, a nonstandard abrupt turn by 90 o in a two- to three-seconds time. However, only two PS unites broke over the R = 8 mm fillet zone in service (see Fig. 3) after the most extensive cracking after the 3000- to 6000-h operation. Despite the thorough screening out of the cracked PS conducted on repairs or in service, several cases of PS broken away in flight still took place. One of the incidents occurred with an AN-26 No. 26582 AK aircraft, named Irbis, which belonged to Kazakhstan: on April 04, 2004, an PS broke out of the right engine and hit the plane cabin, which caused decompression and forced landing. On the strength of this in-service incident of a broken PS, they took up again regular magnetic inspections of these structure components. Another breakdown incident with an PS (AN-26RV RA-4628 aircraft) inspired, in February of the same year, inspection of 18 PS units (already inspected once earlier) of which six units exhibited cracks. Next to the first inspection, the PS running time continued to 977 h … 1158 h and to 286 h … 811 h for the AN-24 and AN-26 aircrafts, respectively. The second inspection done 286 h past the previous one showed that this running time was long enough for the cracks in the PS to extend by another 12 mm. a) b) c) Figure 4 : (a) The running time and (b) percent of rejected out PS, respectively, for an AN-26 aircraft; (c) a similar histogram for the rejected out PS, with the superimposed curves of their running times for different engines.