Issue 33

A. Shanyavskiy, Frattura ed Integrità Strutturale, 33 (2015) 8-16; DOI: 10.3221/IGF-ESIS.33.02 8 Focussed on characterization of crack tip fields Spherical particles formation under biaxial cyclic loading due to mesotunneling effect A. Shanyavskiy State Centre for Civil Aviation Flights Safety, Airport Sheremetievo-1, PO Box 54, Moscow region, Chimkinskiy State, 141426, Russia shanantal@mailfrom.ru A BSTRACT . Fatigue fracture surfaces of Al-based alloys with fatigue striations pattern and such wear debris pattern as spherical particles were investigated fractographically, on the bases of the OG’e spectroscopic analysis. The sequence of events during fatigue crack edges opening was discovered when the elliptical or spherical shapes of wear debris build up on the fracture surface in crosspieces between mesotunnels under mode III of mode I fatigue crack opening because of volume rotation. The cause of black colour of places with fretting patterns on the fracture surfaces of Al-based alloys is discussed. K EYWORDS . Mesoscopic fatigue fracture; Fractography; Spherical particles; OG’e spectroscopy; Rotation plastic deformation. I NTRODUCTION metal with a growing crack represents an open dynamic system, which is far from equilibrium [1]; the system is exercising a series of sequential transitions from one to another stability state and the continued energy exchange with the environment. An open system evolves by passing through the critical states, referred to as the bifurcation points, to, alternatively, a stability or instability condition [2]. As longer as the system experiences fluctuations, it cannot avoid instability immediately before a bifurcation point. The newly activated processes of damage accumulation develop or, alternatively, die out, depending on whether the system is able to the self-organized absorption of energy in the ways that shift the construction element toward a greater stability, i.e. longer life-time. The evolution of an open system is commonly discussed in the terms of microscopic, mesoscopic, or macroscopic scale levels [3]. The first is relevant to the effects on the atomic spacing; the second, to the behaviour of atomic ensembles, and the third, to the creation of bulky space structures. As cyclic loading of a construction element is continuing, mechanisms of damage accumulation replace one another sequentially for the three scale levels, each starting and keeping on for a certain time [4]. The microscopic (Stage I) and mesoscopic (Stage II) scale levels of crack growth are in common with one another as concerns the subjects of forming shear lips at the free surface of the work-piece [5], mesoscopic tunnelling (or holes) of crack, or combining shear and cleavage in the metal when subjected to uniaxial tension, Fig.1. Isolated regions of the failed metal, formed all along the crack front, are stretched in the crack-growth direction and separated by unbroken crosspieces. Mesotunnels (or holes, see Fig.1) are formed by shear during Stage I, and the crosspieces can fail, during Stage I, by the type-III shear or by growing a crack from one to other tunnel just like growing the tunnels themselves. However, rotational instability of deformation and fracture of the inter tunnel crosspieces may A