## 3 Results  
The processing of the experimental data results contributed to the evaluation of AE parameters depended on the specimens' thickness. To eliminate the noise signals arising  
Figure 4: Photos of the surface (**a**) and normal strain fields on the surface (**b**) for one of the specimens at the moment before failure. When analyzing the AE signals for two specimens, the entire tension process was conditionally divided into several stages and signal parameters were compared for the same crack sizes. The parameters of acoustic signals were compared during the development of cracks up to a size of \(l\approx 1.7\) mm (appearance of a visible crack), \(l\approx 4.4\) mm (stable crack development), the period of acoustic gap \(l\approx 7\)-\(8\) mm, and the period before destruction (\(l\approx 9\)-\(10\) mm). As shown in Figure 6a,b, at the initial loading stage of a \(1\) mm thick specimen (test time \(\tau\leq 60\) s), the amplitude and AE hits rate reached \(u_{m}=68\) dB and \(\dot{N}=2\) hits/s, respectively. The obtained values of the AE parameters were registered with an increase in the crack size to \(l\approx 1.7\) mm. Further growth of the applied testing load (\(60\) s \(<\)\(\tau\leq 104\) s) led to severe plastic deformation of the specimen in the area of the stress concentrator and a rise in the crack size to \(l\approx 4.4\) mm. In the considered time interval, the amplitude and AE hits rate attain \(u_{m}=89\) dB and \(\dot{N}=6\) hits/s, respectively. In the time interval from \(104\) s to \(112\) s, the moment of \"acoustic gap\" registered [27], characterized by the localization of the plastically deformed zone at the crack tip and a decrease in the amplitude and AE hits rate to the values of \(u_{m}=65\) dB and \(\dot{N}=3\) hits/s, respectively. When the specimen reaches the state of pre-destruction (\(l\approx 9.5\) mm, \(\tau=130\) s), there was significant growth in the amplitude and AE hits rate up to the values of \(u_{m}=82\) dB and \(\dot{N}=27\) hits/s, respectively. Similar results for a \(6\) mm thick specimen are presented in Figure 6c,d. When the crack achieved \(1.7\) mm in length (\(\tau\leq 72\) s), the parameters \(u_{m}\) and \(\dot{N}\) reached \(100\) dB and \(26\) hits/s, respectively. Growth in the maximum values of the parameters under consideration was due to an increase in the energy of elastic waves generated in the process of internal stress relaxation in a larger volume material. During time interval \(73\) s \(<\)\(\tau\leq 129\) s, the crack length increased to \(l\approx 4.4\) mm. There was also a decline in the activity of AE signals registration to a value of \(\dot{N}=10\) hits/s. Upon reaching the \"acoustic gap\" (\(130\) s \(<\)\(\tau\leq 140\) s), there was a drop in the amplitude and AE hits rate to the values of \(u_{m}=70\) dB and \(\dot{N}=6\) hits/s, respectively. When the specimen reached the state of pre-destruction (\(l\approx 9.5\) mm, \(\tau=167\) s), there was a significant increase in the amplitude and AE hits rate up to the values of \(u_{m}=100\) dB and \(\dot{N}=38\) hits/s, respectively. For clarity, the obtained results are presented in Table 3. It can be seen from the results that at all loading stages, the deformation of a specimen with a greater thickness was accompanied by higher values of AE hits rate and amplitudes. As can be seen from Table 3, the greatest difference in the AE parameters is observed at the initial stage of testing, which may be due to the different nature of cracks formation and propagation. In a thin specimen, a crack propagates fairly quickly over the entire thickness of the specimen; while a thicker specimen is characterized by asymmetric crack growth in thickness. As a rule, cracks, in this case, are initially formed along the edges of the sample, and then propagate deep into the specimen, merging into a single crack.