PSI - Issue 28

L.R. Botvina et al. / Procedia Structural Integrity 28 (2020) 2118–2125 L.R. Botvina et al./ Structural Integrity Procedia 00 (2019) 000–000

2120

3

Table 1. Chemical composition and mechanical properties of low-carbon and 15Cr2MnMoV steels

Steel

Mass fraction of elements, %

Mechanical properties

σ 0,2 , MPa

σ В , MPa

δ, %

Mo

Al

Со

V

C

Si

Mn

Ni

Cu

Cr

0,17

0,21

0,36

0,04

0,07

0,05

-

-

-

-

37,0

283

435,3

Low-carbon steel

15Cr2MnMoV

0,16

0,28

1,12

0,30

0,075

2,12

0,194

0.014

0,024

0,085

15,0

890

1040

Fig. 1. Specimen geometry for fatigue and residual strength tests with AE readings sensor

Fatigue tests until fracture at N f cycles were performed on specimens with continuous radius between ends (Fig. 1) on an Instron 8801 testing machine (maximum load 100 kN) with asymmetry coefficient R = 0.1. Preliminary cyclic loading of specimens from structural steels with different structures was conducted to a relative lifetimes ( N/N f ) of 0.3, 0.5 and 0.7 with stress amplitude equal to 0.9 of the yield strength at testing steel 15Cr2MnMoV and 1,1 at testing mild steel. After preloading, tensile tests were performed with the AE registration using the Express-8 AE system (PAC, USA) and WD 100-900 kHz wideband differential piezoelectric sensors. The recording range was 125-1000 kHz, the sampling frequency was 2.5 MHz; amplitude threshold of signal registration - 32 -35 dB; the number of specimens of the waveform - 2048; registration threshold of digitized waveforms - 40 dB. Based on the test results, the effect of preliminary cycling on acoustic emission parameters, including the intensity (d N AE / dt ), the cumulated number (Σ N AE ) and energy ( Σ E AE ) of acoustic emission signals, and the b AE -value (the slope coefficient of cumulative distributions of the AE signals amplitudes) was estimated. The slope coefficient b AE -value connects the total number of signals with the amplitude of signals (Botvina et al., 2018, 2019): A decrease in the b А E parameter during the damage development characterizes an increase in the proportion of high-amplitude signals. According to the test results, the dependencies of the AE parameters on the relative deformation ( ε * ) were plotted. During loading stops on various stages of tensile tests of polished steel specimens in the initial state at N/N f = 0, t he microcracks patterns were obtained by the means of an optical microscope equipped with a digital video camera. Based on the results of a quantitative analysis of the microcracks patterns, the relative area of the damaged surface ( S *, %) at different loading stages, equal to the ratio of the surface area occupied by microcracks to the area of the frame was estimated. The minimum length of microcracks was at least 0.5 μm. Measurements of microcracks were carried out in the same place of the specimen. 20 log AE AE AE N const b A      (1)