Materials Science, Vol. 47, No. 5, March, 2012 (Ukrainian Original Vol. 47, No. 5, September–October, 2011) MONITORING OF THE FATIGUE OF D16АТ ALLOY ACCORDING TO THE CHARACTERISTICS OF DEFORMATION SURFACE PATTERN S. R. Ignatovich 1,2 and S. S. Yutskevich 1 It is shown that the deformation pattern is formed under cyclic loads and develops on the surface of the cladding layer of a structural aluminum alloy. The quantitative parameters of the deformation pattern, namely, the saturation, roughness, and plastic strains on the surface, serve as indicators of damage to the material. The experimental dependences of the analyzed parameters on the operating time (number of loading cycles) can be used for the creation of new methods aimed at the prediction of the limit state of structural elements of the aircraft structures. Keywords: deformation pattern, interference profilometry, fatigue, parameters of the deformation pat- tern. The processes of fatigue damage in metals connected with the formation and evolution of the dislocation structure, plastic loosening, and microfracture are localized in the surface layer. More intense plastic deforma- tion of the surface as compared with the bulk of the material enables us to interpret the surface layer as an inde- pendent functional subsystem determining the specific features of the local plastic deformation and fracture of the material as a whole [1]. Thus, the surface can play the role of a specific indicator of the degree of accumu- lated damage, and the assessment of its state can be used as a method of diagnostics of the fatigue damage to the entire structure and the exhaustion of its load-bearing capacity. As one of promising directions in the investigation of plastic deformation of the surface, we can mention the development of a procedure of monitoring of the engineering state of structures in the course of operation. The sensors (reference specimens) in the form of thin plates or films made of readily deformable materials (e.g., of single-crystal aluminum) are pasted to the most loaded regions of the structure and bear all in-service force ac- tions together with the structure [2]. The history of in-service loading of the monitored structural element speci- fying the degree of its damage manifests itself in the form of typical traces of the local plastic deformation of the surface of a sensor, namely, slip lines, slip strips, extrusions, intrusions, etc. This means that the deformation pattern (DP) forms on the surface. In aircraft structures, sheet aluminum alloys coated with a thin cladding layer of commercially pure alumi- num are used as structural materials. Under loading, the DP is formed on the surface of the cladding layer and can readily be recorded by optical devices. In this case, the procedure of monitoring of the exhaustion of service life of structural elements can be realized by using the criterion of saturation of the DP (equal to the relative area of the analyzed region covered with traces of plastic deformation) [3]. The specific features of this method are described in [4, 5]. The quantitative estimation of the DP according to its area, i.e., in the two-dimensional case, is not exhaustive. It is necessary to study the quantitative characteristics of DP in the three-dimensional case [6, 7]. 1 National Aviation University, Kiev, Ukraine. 2 Corresponding author; e-mail: ignatovich@nau.edu.ua. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 47, No. 5, pp. 60–65, September–October, 2011. Original article submit- ted March 17, 2011. 636 1068-820X/12/4705–0636 © 2012 Springer Science+Business Media, Inc.