Issue 30

A. Shanyavskiy et alii, Frattura ed Integrità Strutturale, 30 (2014) 340-348; DOI: 10.3221/IGF-ESIS.30.41 340 Focussed on: Fracture and Structural Integrity related Issues In-service fatigue cracking of the propeller shafts joined by a spline-pinned construction to the engines of AN-24, AN-26, and IL-18 aircrafts A. Shanyavskiy, A. Toushentsov State Centre for Flights Safety, Sheremetievo-1 Airport, PO Box 54, Moscow region, Khimki State, 141426, Russia shananta@mailfrom.ru , 103otdel@mail.ru A BSTRACT . The paper delivers a critical review of the research data on the crack initiation and crack growth patterns characteristic of the components of the spline-bolted joints between the propeller shaft and reducer shaft at An-24, An-26, and Il-18 aircrafts. Cracks in the shafts nucleated because of reduced bolt-fastening force. Actually, the bolt (bolts) failed first (also by fatigue) and then fatigue cracks nucleated and grew in the shafts, the spline surface fretting zones and/or sharp edges of the attachment (bolt-conducting) holes making the crack origin sites. The crack growth history shows itself through the regular Macro-Beach Marks, each mark sequentially pointing to the next loading event of the propeller shaft, i.e., to each next flight. The cracks cease growing for some while in the airscrews and their shafts just replaced to another aircraft. For the airscrew shafts, the critically assessed data show the crack growth period Np ranging as five to ten percent of a total running period N f . We recommend performing nondestructive inspection of the airscrew shafts on every 250- hour running period to ensure the safety flights. K EYWORDS . Propeller shaft; Fatigue; Spline-bolted joint; Fractography; Crack-growth period. I NTRODUCTION t the stresses reduced to the level typical of high-cycle fatigue (HCF) behavior, the structure components exhibit lowered ratios N p / N f between their crack growth period Np and total duration N f of cyclic loading [1]. So, near the stress levels low as the so-called fatigue limit, the ( N p / N f ) magnitudes can range as 0.05 to 0.1 for a smooth- surface test body. The ( N p /N f ) data scatter primarily in response to the stress concentration effects peculiar to the free surface of a structure component. An N p /N f magnitude can increase, typical of the contacting components of structure joints that additionally suffer surface damage from wear, and, thus, exceed the magnitude peculiar to a stationary loading regime free of a surface damage. As regards the propeller shafts (PS) of AI-24 and AI24VT engines of AN-24, AN-26, AND IL-18 aircrafts, cracks nucleated and grew—at the HCF or ultra-high cycle fatigue (UHCF) regimes (for the durations of 10 8 – 10 9 loading cycles [2–4])—from spline fretted sites, from sharp edges, or from material defects in a shorter spline area. In some cases, crack propagation terminated at the complete separation of the PS from a still operating engine, Fig. 1. Though the aircraft did land successfully this time, an urgent problem arose to reveal why the cracks generation occurred and to program reasonably periodic in-service inspection of the PS items to prevent, thereby, the in-flight cases of broken away PS. A