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Shanyavskiy A.,State Center for Civil Aviation Flight Safety
Key Engineering Materials | Year: 2011

The fatigued specimens have attested the surface hardening of the 2024-T351 aluminum alloy based on the laser shock peening (LSP). Tests were carried out with simple regular specimens bending. It was shown that each crack started from or near to the specimen-hardened surface of the thickness 4mm. The hardened surface layer influenced material fatigue cracking on the Stage I of crack growth as well as on the Stage II where fatigue striations formation mechanism takes place. Parameters of S-N curves were analyzed for tested specimens and bimodal distribution of the fatigue durability (BDFD) was revealed. The left branch of the BDFD attested fatigue behavior of not hardened material. The right branch of S-N curves attested the material behavior after surfacehardening procedure by the LSP technology. The paper discussed mechanisms of material fracture in the bifurcation area for the stresses interval ranged from 270-295 MPa where transition from the left to the right branch of the discovered BDFD takes place. The subsurface crack origination occurs for the right branch of BDFD because of twisting of material volume under combination of material compression in a local area of crack origination with gases diffusion in the volume of crack origin. © (2011) Trans Tech Publications, Switzerland.

Shaniavskiy A.A.,State Center for Civil Aviation Flight Safety | Toushentsov A.L.,State Center for Civil Aviation Flight Safety
Engineering Failure Analysis | Year: 2010

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.

Shanyavskiy A.,State Center for Civil Aviation Flight Safety | Toushentsov A.,State Center for Civil Aviation Flight Safety
Frattura ed Integrita Strutturale | Year: 2016

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.

Shanyavsky A.A.,State Center for Civil Aviation Flight Safety
Physical Mesomechanics | Year: 2015

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.

Shanyavskiy A.A.,State Center for Civil Aviation Flight Safety
Engineering Fracture Mechanics | Year: 2013

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.

Shanyavskiy A.A.,State Center for Civil Aviation Flight Safety | Burchenkova L.M.,State Center for Civil Aviation Flight Safety
International Journal of Fatigue | Year: 2013

Effect of varying negative R-ratio on the fatigue striation patterns was examined for Al-based V95-T4 (analogue of 7057T6) and D16T (analogue of 2024T3) alloys. Three different sequences of cyclic loads and of the varying maximum and minimum stress levels were applied in air atmosphere to the test alloy panels 10 × 160 × 460 mm in size. The tests were conducted at room temperature with the loading frequencies in the range of 0.5-1.5 Hz. Fractographically, the magnitudes of striation spacing were found increasing for increasingly negative R-ratios. To estimate the effects of negative R-ratio on the crack growth rate correction-function surfaces were constructed for the stress-intensity factor KI. The fractographic analysis suggests that the unloading portions of cycling trajectory are only responsible for the fatigue striation effects. Such a mechanism appears dominating in the positive as in the negative ranges of R-ratio. © 2012 Elsevier Ltd. All rights reserved.

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 | Year: 2010

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.

Shanyavskiy A.,State Center for Civil Aviation Flight Safety | Osipov D.,State Center for Civil Aviation Flight Safety
International Journal of Fatigue | Year: 2015

Regularities of fatigue crack appearance and growth in-service cracked stringers of helicopter Mi-26 tail-boom area were demonstrated. Statistic analyses of cracks distribution in helicopter tail-boom has shown crack appearance not only in stringers manufactured from Al-Li alloy but in frames too. Fatigue tests of complicated sheet-stringer specimens reproducing spot-welding-bonding joint assembly experienced constant cyclic loads amplitude were performed. Result of tests have shown fatigue cracking by the regular change of inter- and transgranular crack propagation. This manner of material cracking is similar case for in-service cracked stringers. The discovered mechanism of Al-Li alloy fatigue cracking is considered based on the results of fractographic investigations in scanning electron microscope. The discussed mechanism of the crack propagation reflects Li diffusion in grain boundary under stressing ahead of a crack tip during fatigue crack propagation that weakened grain boundary and accelerate crack propagation. Crack growth periods for tested specimens were estimated and compared with in-service registered crack growth duration based on regular non-destructive inspections. In-service fatigue cracking period of stringers was established and tolerance in-service duration for helicopter between two inspections has been recommended. ©2015 Published by Elsevier Ltd.

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