PSI - Issue 36

G. Pysarenko et al. / Procedia Structural Integrity 36 (2022) 30–35 Pysarenko G., Voynalovich O., Maylo A. et al. / Structural Integrity Procedia 00 (2021) 000 – 000

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The surface structure formation of the metal polycrystal in a deformed state is accompanied by evolution of microplastic deformations, Moiseenko et al. (2013). The variation in energy intensity of the light beam reflected from the deformed surface of the polycrystalline material occurs due to the change under loading in the spatial position of the test specimen reflection surface (Fig. 2).

Fig. 2. 3D-correlation diagram of the distribution of the amplitudes of the accumulated deformation of the surface of St 20 steel specimen: (a) reference state; (b) threshold state of the specimen.

This paper contains images of discrete formation specklograms of the specimen’s surface, which represent the basic system of descriptive characteristics of accumulated damage to St 20 steel. The experimental data are used to analyze properties of metal specimens’ surface by methods of mathematical statistics. To identify the correlation dependences of damage, digital images of structural elements (pixels) are used in the form of specklograms (Fig. 3).

Fig. 3. Digital images of specklograms of the specimen surface under loading: (a) reference state; (b) 10 4 cycles; (c) 10 5 cycles; (d) 10 6 cycles; (e) 10 7 cycles.

Statistical characteristics of the parameters of specklograms of St 20 steel specimens are obtained at a frequency of 100 Hz under symmetric cyclic loading at fatigue stages. Table 1 lists the results of the analysis of the statistical characteristics of the specklograms. In the initial state of the specimen, the distribution function of specimen surface specklogram parameters is represented by a graph that corresponds to the normal distribution density. An increase in the number of loading cycles changes the kurtosis and asymmetry characteristics of the histogram of the discrete distribution of the surface deformation relief (Table 1).It causes the normal distribution function to change the asymmetric one. As the amplitude of cyclic stresses decreases, the dispersion increases, and the slope angle of characteristics decreases in the coordinates shown in Fig. 4. It is defined by the decrease in reliability of approximation R 2 of damageability characteristics (Fig. 5). The reason for this evolution may be a decrease in the amplitude of cyclic stresses.