Issue 42

D. V. Orlova et alii, Frattura ed Integrità Strutturale, 42 (2017) 293-302; DOI: 10.3221/IGF-ESIS.42.31 295 For example, silicon iron chosen by Amelinckx and Hull [31, 32] is a traditional material for investigating the mechanisms of plastic flow and fracture. The elemental composition of the test steels is given in Tab. 1. The samples were obtained using sheet semi-fabricated products subjected to pressing and recrystallization annealing. In the mechanical tests, flat samples were in the form of a double blade 2  10  50 mm in size. Steels C Cr Ni Si Mn Cu AISI 420 0.35-0.45 12-14 ≤0.6 ≤0.8 ≤0.8 ≤0.3 321H ≤0.12 17-19 9-11 ≤0.8 ≤2.0 ≤0.3 G10080 0.05-0.11 0.1 ≤0.25 0.05-0.17 0.35-0.65 ≤0.25 Fe-Si alloy - - - 2.8-3.8 - - Table 1 : Chemical compositions of experimental steels, in wt.%. The samples were subjected to uniaxial loading at room temperature and a constant velocity of 6.67·10 -5 s -1 by using the one universal testing machine head (Walter + Bai, Switzerland), the second testing machine head was fixed. The initial load curves of all materials     were recalculated into true stresses s and strain e , the relationship between which was described by the Ludwik's equation   0 n s e s Ke   [33], where K is the work hardening coefficient, and n is the hardening exponent. To determine the stages of the plastic flow, the dependence   s e was taken in the logarithmic form. The sections corresponding to the stages of the plastic flow are easily separated when the curve   s e is transformed into the coordinates   0 ln ln s s e   , and represented by the straight lines for K = const and n = const . The hardening exponent n = 0 corresponds to the yield plateau, n = 1 corresponds to the linear stage, and n = 0.5 corresponds to the parabolic stage of strain hardening. The continuous pre-fracture stage, for n < 0.5, was observed for all materials (Tab. 2). The stress-strain state of objects in the real time mode, visualization of deformation and fracture localization zones were investigated by speckle interferometry, described in detail in [34, 35]. The method allows the patterns of plastic deformation localization to be recorded by fixing the displacement fields of points for a deformed sample. The use of a two-exposure speckle photograph and the analysis of the speckle structure for the images of deformed objects are very promising for investigating the features of plastic deformation at the macroscale level. This technique is successfully used in solving similar problems [36, 37]. The method is valid in the field of vision about 100 mm in size and has a measuring accuracy of 1 μm for displacement vectors. A series of sequential specklegrams reflecting the displacement field of the sample points for an increase in the total strain of 0.2% was recorded within the yield plateau for the steel (G10080) and at the pre-fracture stage for all the materials studied. Using the data on the displacement fields and numerical coordinate differentiation, all components for the tensor of the plastic material distortion can be determined for the plane case: local narrowing, local elongation, shift and rotation. Then, the data on the displacement fields for the points of a deformed sample are transformed into the distributions of local elongations  хх , where the localized plasticity domains are clearly separated. The propagation velocity of ultrasonic Rayleigh waves was used as an acoustic informative parameter at a frequency of 5 MHz. Using this type of waves allows the propagation of ultrasound velocity in the sample to be kept constant during experiments. To measure the propagation velocity of Rayleigh waves, a transmit-receive sensor was used, which contained transmitting and receiving piezoelectric transducers installed in the same enclosure and based on piezoceramics (CTS-19) with a resonance frequency of 5 MHz. Piezoelectric transducers are installed at an angle of 56  to the normal to the incidence plane of an acoustic wave, which provides the formation of a surface acoustic wave (Rayleigh wave) in iron based alloys. The distance between the transducers (the length of the acoustic path in the test object) without considering the length of the sensor waveguide was 32 mm. The propagation velocity of Rayleigh waves was determined by the ratio between the path length of a wave in the sample and the delay time for the signal of the receiving transducer relative to the transmitting one. The delay time was measured using an oscillogram recorded with a digital oscilloscope at a sampling frequency of 2 GHz. The measurement accuracy was 10 4 – 10 5 .