Evaluation of the influence of pit-like defects on the strength of D16T sheet alloy
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EVALUATION OF THE INFLUENCE OF PIT-LIKE DEFECTS ON THE STRENGTH OF D16T SHEET ALLOY A. Yu. Pokhmurs’kyi,1 B. P. Rusyn,1,2 and Yu. V. Obukh1
UDC 620; 519.2
We have analyzed quantitatively parameters of pit-like defects on the surface of corroded D16 sheet alloy by methods of statistical processing of their metallographic images. The depth of penetration of corrosion damages has been determined by 3D-optical profilometry and by measuring sizes on crosssections. We have determined the relation between the geometric parameters of the corrosion damage of the surface and the decrease in the yield strength of the corroded alloy. Keywords: sheet alloy, pitting corrosion, quantitative analysis, processing of metallographic images, mechanical properties, nondestructive testing.
High-strength sheet aluminum alloys are used not only in aeronautical engineering, but also in the carbuilding, automobile, chemical, power, building industries, etc. This is due to their high specific strength and corrosion resistance (particularly, with a clad aluminum layer) under atmospheric conditions and in many corrosive media. Surfaces of aircrafts and other structures are rather often damaged by pits or pit-like defects, which can decrease substantially their strength [1–4]. The existing methods make it possible to determine the size of pits in a two-dimensional coordinate system and their propagation over the surface of the metal [5–7]. However, these methods do not enable one to determine the influence of pitting corrosion on the strength of the alloy. In what follows, based on metallographic analysis, we study the influence of pit-like defects on the strength of the surface of specimens of D16T sheet aluminum alloy. Method of Investigation We tested flat specimens with a working part of size 15 × 4 mm cut off from a sheet of D16T alloy 0.8 mm thick. In choosing the thickness, we were guided by the fact that the influence of surface defects on the strength for these materials is larger than for thick materials due to the larger relative depth of damage. To decrease the influence of the method of preparation on the structure and properties, specimens were cut off from a sheet by the gas-jet method, which does not cause their plastic deformation and heating. Localized corrosion damages on the surface were obtained by holding specimens in a 5% iron chloride solution (up to 72 h) at room temperature (Fig. 1а). We evaluated the geometric parameters of defects from results of metallographic and profilometric analyses and the state of the surface using a special-purpose software. The depth of pits was determined on transverse microsections by the double focusing method in microscopic analysis of the surface. The mechanical characteristics of the specimens were determined at a stretching rate of 3 mm/min. 1 2
Karpenko Physicomechanical Institute, Ukrainian National Academy of Sciences, Lviv, Ukraine. Corresponding author; e-mail: [email protected].
Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 48, No. 1, pp. 95–99, January–February
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