Issue 38

M.V. Karuskevich et alii, Frattura ed Integrità Strutturale, 38 (2016Y) 205-214; DOI: 10.3221/IGF-ESIS.38.28 206 I NTRODUCTION here is a number of physical backgrounds that explain the nature of an ordered surface relief formation including ones based on the principle of self-ordering [1-3]. One of the most well-known concepts is underlined by the effect of "chessboard" (ordered pattern) of normal compressive/tensile stresses and strains distribution in surface layers and interfaces of solids to occur under the impact of external loading (mechanical, thermal, electrical, etc.). This physical phenomenon is related to the strain incompatibility of two interfaced media and occurs in various multilevel systems: at the "surface layer - bulk material" interfaces, in "coating - substrate" compositions, in multilayered or thin film materials, at grain boundaries (triple joints) in polycrystals [4], etc. It is known that accumulation of defects in surface layers takes place under the cyclic loading that is followed by the material self-organization there that is exhibited in the form of dissipative structure and microextrusions [5-7]. It should be noticed that depending on the mechanical impact on the material the structure of surface damages will be varied. This numerously experimentally proved evidence testifies for the fact that surface layer of the material is the most "sensitive" in order to be used for characterization the kinetics of damage accumulation at various loading modes [8, 9]. These above mentioned properties of material self-organization are widely used at development of instrumental techniques for fatigue fracture diagnostics of aircraft structures [10-12]. A big number of automated methods for monitoring parts made of cladded aluminum alloys are available. In particular, a non- destructive computerized optical-digital inspection technique is based on determination of damage parameter of strain– induced surface relief of fatigue sensors for aircraft structures was described in [13-14]. In doing so the peculiarity of using the strain induced relief as a measure of damage accumulation is related to the fact that the latter is already formed after a few loading cycles and keeps developing till fatigue crack nucleation. This makes possible to run the diagnostic and prediction of the mechanical state and residual life-time of structure elements from the early damaging stage till approaching their limit state. Currently, various parameters which are used for describing strain induced surface relief in structural aluminum cladded alloys being formed under cyclic loading are employed. They typically depend on the nature and accumulation mechanisms of deformation structures at the material surface. Recently, various irregular structures of natural origin are effectively quantitatively described with the use of the following parameters: fractal dimension, Shannon entropy, etc. [7, 10, 12]. In our opinion, the accuracy of the assessment of the critical state approaching in the optical inspection application may be attained by attracting modern techniques of strain induced relief analysis based on calculation of several informative parameters. The aim of the study is establishing the basic principles of damage accumulation on the fatigue sensor surface under cyclic bending and “bending + torsion” loading schemes. T ECHNIQUES AND MATERIAL FOR FATIGUE SENSOR INVESTIGATION 16AT aluminum alloy which is widely used in aviation was selected for the research. This alloy has a cladded layer made of pure aluminum for anti-corrosion protection. In doing so, the strain induced relief is formed [7] under the fatigue process. Flat specimens with dimension of 140 × 10 × 1.0 mm were employed for cyclic cantilever bending as well as "bending + torsion" tests. The stress concentrator in the form of a central hole with 1.0 mm diameter was drilled. These allowed us to reveal the basic regularities of strain induced relief evolution under various schemes of the loading. Experiments were conducted with the help of testing machine (home made in the National Aviation University in Kiev) at the stress level of σ max = 107.8 MPa under the cyclic bending. When complex loading pattern "bending + tension" was used the tensile stress component made σ max = 108 MPa while the shear one –  = 85 MPa. The parameters described were constant at the region where surface relief was examined. Surface relief images were captured after certain operating times at the left and at the right from the stress concentrator. In doing so, the optical magnification made × 300. The calculation and comparative analysis of the surface damaging parameters in the regions under observation were performed with the help of automated image analysis. Algorithm for image analysis According to data of our recent studies on fatigue sensors behavior the image of regions with accumulated microdefects differ from non-damaged ones by brightness intensity [6, 8]. This is related to the changing sensor surface relief to take place under the loading. The former is responsible for the changing optical properties of the polished surface. In so doing, with the increasing the number of loading cycles, the number and area occupied with plastically deformed surface regions are enlarged [15]. In this response, the following potential informative parameters that might be used for the assessment T D