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Shanyavskiy A.A.,State Center for Civil Aviation Flight Safety
Engineering Fracture Mechanics

In terms of a synergetic system at its sequentially increased scale levels, evolution of the fracture-behavior patterns in various cyclically loaded metallic alloys is analyzed together with the alternatives of subsurface initiation of fatigue cracking. When free of the non-homogeneities like lamination sites, inclusions, etc., subsurface cracks arise due to the loss of plastic stability at the micro- or nanometer-scale level, i.e., in the local flat areas up to 500. nm in depth, normal to the load axis. Two mechanisms are controlling the formation of such a region, which is due to the instability of rotational plastic flow and fracture of the material in the state of three-dimensional compression and twisting; thereby, an even facet or a nano-structured zone forms, the latter comprising tiny particles of irregular, ellipsoid and/or spherical shapes. On further cycling, the fracture surface develops on the particle boundaries. The data of numerous investigations are shown to confirm the validity of the above-proposed models on the subsurface nanostructures in metal. © 2013 Elsevier Ltd. Source

Shanyavsky A.A.,State Center for Civil Aviation Flight Safety
Physical Mesomechanics

A new paradigm is proposed for considering metal fatigue cracking based on the principles of synergetics and physical mesomechanics. Fatigue cracking is described as a three-stage process. Metal evolution is studied with stress growth from the micro- (ultrahigh cycle fatigue) to meso- (high cycle fatigue) and then macroscale (low cycle fatigue). The notion of two effective stress concentration factors on the metal surface and in its bulk is introduced; their variation pattern with stress growth is discussed. In the general case, the propagation of through-the-thickness cracks is shown to also occur in three stages—on the micro- (shear), meso- (rotation with the formation of triangular fatigue striations) and macroscale (rotation plus shear which lead to the formation of fatigue striations of complex shape), consecutively. © 2015, Pleiades Publishing, Ltd. Source

Shanyavskiy A.,State Center for Civil Aviation Flight Safety | Toushentsov A.,State Center for Civil Aviation Flight Safety
Frattura ed Integrita Strutturale

Fatigue cracking of longerons manufactured from Al-alloy AVT-1 for helicopter in-service rotorblades was considered and crack growth period and equivalent of tensile stress for different blade sections were estimated. Complicated case of in-service blades multiaxial cyclically bending-rotating and tension can be considered based on introduced earlier master curve constructed for aluminum alloys in the simple case of uniaxial tension with stress R-ratio near to zero. Calculated equivalent tensile stress was compared for different blade sections and it was shown that in-service blades experienced not principle difference in this value in the crack growth direction by the investigated sections. It is not above the designed equivalent stress level. Crack growth period estimation in longerons based on fatigue striation spacing or meso-beach-marks measurements has shown that monitoring system introduced designer in longerons can be effectively used for in-time crack detecting independently on the failed section when can appeared because of various type of material faults or in-service damages. © 2016, Gruppo Italiano Frattura. All rights reserved. Source

Shaniavskiy A.A.,State Center for Civil Aviation Flight Safety | Toushentsov A.L.,State Center for Civil Aviation Flight Safety
Engineering Failure Analysis

The paper demonstrates results of quantitative fractographic analyses of in-service fatigued cast AL5 Al-based alloy of hydropumps NP-89D. A model based on Vickers hardness of metal matrix and the sqrt(area) of defects (the "sqrt(area) model") has been introduced. It describes microcracks threshold originating from cast porosities. Crack growth period estimated by the result of meso-beach marks spacing measurements and stress equivalent calculated by the "sqrt(area) model" are used to discuss causes of the in-service cracking of the pump casing. It is shown that damping of self-oscillations with simultaneous decrease in an equivalent stress level of the hydrounit will allow increasing the operating time up to pump probable failure corresponding to more than 10 10 cycles. It will lead to an increase in the period of operation of casings without their fatigue cracking by more than of two orders. © 2009 Elsevier Ltd. All rights reserved. Source

Shanyavskiy A.A.,State Center for Civil Aviation Flight Safety | Burchenkova L.M.,State Center for Civil Aviation Flight Safety
18th European Conference on Fracture: Fracture of Materials and Structures from Micro to Macro Scale

Al-based alloys V95-T4 (analogue of 7057T6 D16T (analogue of 2024T3were investigated in the case of negative R-ratio influence on the fatigue striation formation process. Three different sequences of cyclic loads with variation of maximum and minimum stress levels were used to test panels in thickness 10mm, width 160mm and high of 460mm. Fractographic analyses have revealed striation spacing increasing for negative R-ratio increasing. Surfaces of functional correction for the stress intensity factor KI were constructed that used to estimate R-ratio influence on the crack growth rate. Fractographic analysis has shown the shape of fatigue striations formation during unloading portion of cyclic loads. This mechanism is dominant for both regions of positive and negative R-ratio. KEYWORD Fatigue tests; AL-Alloys; negative R-ratios; crack growth; quantitative fractography; mechanism; fatigue striations. Source

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