Issue 42

D. V. Orlova et alii, Frattura ed Integrità Strutturale, 42 (2017) 293-302; DOI: 10.3221/IGF-ESIS.42.31 296 E XPERIMENTAL RESULTS AND DISCUSSIONS Stress-strain curves and propagation of elastic surface Rayleigh waves ig. 1 and 2 show the stress-strain curves of the test materials. It is seen that these curves are parabolic and can be used to determine the stages in accordance with the law of strain hardening. The fracture is ductile, and there is well visible necking in the samples. The mechanical characteristics and length of the section with the n < 0.5 are given in Tab. 2. The exception is the σ-ε curve for carbon steel G10080 (Fig.2). The feature of this curve is the presence of a tooth and a yield plateau typical for carbon steels [38]. The length of the yield plateau is 0.013 < e < 0.047 (3.4%). The strain within the yield plateau is known to be in the form of the Luders band [39]. Steels σ y , MPа σ 0.2 , МPа δ,% Pre-fracture stage, n < 0.5 AISI 420 1183 420 6.0 0.033 < e < 0.05 (1.7 %) 321H 178 512 55 0.45 < e < 0.55 (10 %) G10080 310 210 40 0.25 < e < 0.35 (10 %) Fe-Si alloy 590 320 25 0.064 < e < 0.22 (15.6 %) Table 2 : Mechanical properties of the samples and the length of the pre-fracture stage. Figure 1 : Stress-strain curves of the test materials. Figure 2 : Stress and relative sound velocity versus the strain of steel (G10080). The measurement of acoustic characteristics within the yield plateau is of great interest. The position of the Luders bands and the change in the propagation velocity of ultrasonic Rayleigh waves in steel (G10080) within the yield plateau were recorded simultaneously with the deformation of the sample. In Fig. 3, the σ-ε loading diagram is plotted by a light gray line, and the yield plateau is accompanied by the formation of the Luders band front near the lower testing machine head. The position of the Luders band fronts is plotted in the form of black circles (black lines). The deformation is accompanied by the propagation of the two Luders band fronts at a velocity of 0.052 mm/s and 0.038 mm/s. The region of the sample in which the change in the velocity of ultrasonic waves is recorded is indicated by two horizontal dashed lines. Red markers indicate the entry of the Luders band fronts in the ultrasonic velocity measurement area. It should be noted that the formation of the Luders band out of the acoustic measurement area is accompanied by the increase in the velocity of ultrasonic waves. When the fronts move to the area of acoustic measurements, a maximum is formed on the V( ε ) curve and the ultrasound velocity starts decreasing, since the front moves inside the acoustic measurement area. During plastic deformation, two types of stresses has an effect on the propagation velocity of ultrasonic waves: macrostresses caused by an external applied load [40] and microstresses caused by the evolution of the dislocation structure [27-29, 41]. The formation of the Luders band leads to the fact that the stresses in the sample are localized in the Luders band, and the stresses in the remaining volume of the sample are decreased. This leads to an increase in the ultrasound velocity. The propagation of the localized deformation front is accompanied by the increase in the dislocation 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0 200 400 600 800 1000 1200  , MPa  Fe-Si alloy AISI 420 321H 0.0 0.1 0.2 0.3 0.4 0.988 0.990 0.992 0.994 0.996 0.998 1.000 1.002 1.004 0 100 200 300 400 V/V 0  , MPa  F