OBRABOTKAMETALLOV Vol. 23 No. 3 2021 MATERIAL SCIENCE EQUIPMENT. INSTRUMENTS 4 4 2 A grinding wheel is considered to be a solid elastic isotropic body in the form of a disk with a central hole (sometimes as an annular plate) [22, 23, 24]. The proposed disk is a round plate, the thickness of which (wheel height H, mm) is small in relation to the diameter (wheel diameter D, mm), which has a central axial hole (bore diameter d, mm). Each excess kurtosis on the spectrograms (Fig. 5, 6) indicates the physical frequency, at which vibrations appear with a certain mode: the spatial configuration of the vibrations of the grinding wheel points under the impact force (GOST R ISO 2041–2012). The graphs show that each wheel is characterized by the simultaneous appearance of several vibration modes. Each of the modes has its own shape – i.e. spatial configuration. This is when the points on its surfaces are at the maximum distance from the equilibrium (resting) position. Dominant modes are manifested at high sound levels. Due to significant deviations of body microvolumes from the equilibrium state, dominant modes are characterized by a more powerful acoustic signal recorded by the device at a certain frequency. Modes with the lowest natural vibration frequency are called fundamental natural modes. With regard to GW 1, the fundamental natural vibration mode ffun1 = 2.062 Hz; for GW 2, ffun2 = 2.337 Hz. Sound index. An elastic acoustic wave propagating in the material of the grinding wheel and excited by the impact force has several specific characteristics. In our study, the illustrative characteristics are the normalized speed of its propagation CL and the sound index. The sound index is a normalized integrated index, i.e. the value of CL averaged in a certain range, provided for by GOST R 52710–2007 and used in acoustic monitoring [25, 26]. The connection of the SI with the shape factor of the object, Young’s modulus (E-modulus), and the material density makes it a highly informative parameter enabling certain physical and mechanical properties of abrasive materials to be established, in particular the hardness degree. The tests results enabled us to establish SI for each of the objects under study according to its parameters (Table 1). The sound indices for GW 1 and GW 2 took identical values and amounted to 47 units. This corresponds to the hardness degree K, pursuant to GOST R 52587–2006 (or CM1 as per GOST 2424–83). The values of the degree of hardness coincide with the marking as set by the manufacturer. The normalized acoustic wave propagation speed CL recorded by the ICHSK-2 device falls within the range of 4,600–4,800 m/s. Based on the measurement results, the properties of the materials of GW 1 and GW 2 do not differ, which is factually correct. In this case, the change in the shape of the tested GW did not affect the SI value, i.e. the acoustic wave propagation speed CL did not change, since this indicator depends on the physical Fig. 5. Spectral composition of GW 1 natural vibration frequencies
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