PSI - Issue 41

M.Sh. Nikhamkin et al. / Procedia Structural Integrity 41 (2022) 759–765 Author name / Structural Integrity Procedia 00 (2019) 000–000

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and also the thermal state of the specimen were constantly recorded. The deformation fields in the samples were evaluated using the digital image correlation (DIC) method implemented in the VIC-3D system from Correlated Solutions. To determine the deformation fields a contrasting finely dispersed coating (speckle pattern) is applied to the surface of the test object before loading. When the sample is loaded, the speckle moves along with the surface. Cross-correlation processing of images obtained before and after loading makes it possible to qualitatively and quantitatively evaluate the displacement field [18].

Fig. 2. Experimental setup consisting of: 1 – test sample; 2 – high-frequency electromagnetic pulsator Zwick HFP 5100; 3 – thermal imaging system NEC TH9100 WR; 4 – acoustic emission sensors; 5 – AE registration system AMSY-6; 6 – digital monochrome cameras; 7 –digital image correlation system Vic 3D.

To record the process of sample destruction, an multichannel acoustic emission system AMSY-6 with two piezoelectric microphones AE204A was used. The sensors are connected to the signal processing unit through standard preamplifiers from Vallen systeme, model AEP4, with a gain of 34 dB. The temperature of the samples was controlled using an infrared thermal imaging camera NEC TH9100 WR Pro New with a sensitivity of 0.06/0.08 °C. 3. Analysis of the experimental results For the group of studied specimens, two characteristic variants of the change in the resonance frequency were revealed during the fatigue tests, depending on the level of load. At high load, when the maximum stress in the loading cycle σmax is higher than 58,7% of the static strength of the specimen σ0, the resonant frequency gradually decreases monotonously from the very beginning of cyclic loading (Fig. 3, a). On the graphs (Figure 3, a, b), the abscissa shows the relative operating time, which is equal to the ratio of the current operating number of cycles of the sample N to the operating number of cycles of the sample at the time of testing stop Nmax. At a lower load, when the maximum stress in the loading cycle σmax was 12,9% of the limit of the static strength of the specimen σ0, the resonant frequency and, accordingly, the stiffness of the specimen drops abruptly as a certain level of operating time is reached (Fig. 3, b). In doing so each jump corresponds to the moment of crack appearance or its fast growth, and at the initial stage of loading the resonant frequency practically does not change. As a rule, cracks in both the first and second cases are delaminations near the specimen fixation zone (Fig. 4). The abrupt decrease in the resonant frequency in this case is 0.2 - 1% of the initial value.