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C. Ye et al.
distributed between 20 and 500 kHz. Waveforms can be divided into two main types:
the first stage signal waveform is a short signal with a short duration. The signal
waveforms collected by the two channels are similar, and the peak frequencies are
mainly distributed at 50–150 kHz; the second-stage signal waveforms are mainly
resonant signals with longer durations. Among them, the peak-to-valley spacing of
the 1-channel signal waveform is small, and the peak frequency is mainly distributed
at 150–350 kHz. The peak-to-valley spacing of the 2-channel signal waveform is
relatively large, accompanied by dispersion phenomenon, and the peak frequency is
mainly distributed at 40–100 kHz.
Two types of waveform signals recorded during metal pitting have also been
reported in previous studies [2, 5]. Fregonese et al. [2] studied the AE characteristics
of 316L stainless steel in 3% NaCl solution, and recorded the short-wavelength and
resonance-type waveforms with short duration and resonance waveforms using the
R15α type sensor. They are distinguished by characteristic parameter rise time and
count, the short signal waveform is corresponding to the formation and expansion of
corrosion products on the surface of the sample, and the long-lasting resonance signal
corresponds to hydrogen bubble formation and evolution. Two types of sensors were
used in this test. The R15α sensor used the 1-channel was of the same type as in [2],
and the two types of AE signal waveforms recorded were identical to those reported
in the literature. The 2 channel uses R6α type sensor. The waveform recorded in the
first stage is consistent with the literature report. There are some differences in the
waveform of the second stage, which is mainly caused by the different frequency
response characteristics of the sensor. The activity and number of the second-stage
signals of the 2-channel are much higher than that of the 1-channel, indicating that
the two-stage signal frequency is mainly distributed in the low-frequency response
frequency band. In addition, different types of signal waveforms appear in different
sequences in the experiment.
To further understand the waveform characteristics, a continuous wavelet is
applied to the waveform signal to be transformed and superimposed with the material dispersion curve. The result is shown in Fig. 22.5. It can be seen from the figure
that the distribution characteristics of the signal energy in the time domain and the
frequency domain are basically consistent with the signal waveform and power spectrum analysis results. It can be seen from the superposition of the dispersion curve
and the signal energy that the first-stage signal waveform is mainly expansion wave,
the two-stage one-channel signal waveform mode is mainly expansion wave, and the
two-channel waveform mode is mainly flexural wave. The wavelet transform result
graph and the dispersion curve are not superimposed at the maximum energy of the
signal, which may be related to the fact that the rectangular sample size is small and
the wave propagation process is greatly affected by the boundary.
Based on modal AE theory and pitting corrosion AE source mechanism [9, 10],
thin-plate metal pitting will mainly produce expansion wave and flexural wave. The
expansion wave has the characteristics of fast propagation speed, low amplitude
and high frequency (>100 kHz); while the flexural wave has slower propagation
speed, larger amplitude, lower frequency (<100 kHz) and dispersion phenomenon.
Comparing the signal waveforms collected by the two channels and the frequency
C. Ye et al.
distributed between 20 and 500 kHz. Waveforms can be divided into two main types:
the first stage signal waveform is a short signal with a short duration. The signal
waveforms collected by the two channels are similar, and the peak frequencies are
mainly distributed at 50–150 kHz; the second-stage signal waveforms are mainly
resonant signals with longer durations. Among them, the peak-to-valley spacing of
the 1-channel signal waveform is small, and the peak frequency is mainly distributed
at 150–350 kHz. The peak-to-valley spacing of the 2-channel signal waveform is
relatively large, accompanied by dispersion phenomenon, and the peak frequency is
mainly distributed at 40–100 kHz.
Two types of waveform signals recorded during metal pitting have also been
reported in previous studies [2, 5]. Fregonese et al. [2] studied the AE characteristics
of 316L stainless steel in 3% NaCl solution, and recorded the short-wavelength and
resonance-type waveforms with short duration and resonance waveforms using the
R15α type sensor. They are distinguished by characteristic parameter rise time and
count, the short signal waveform is corresponding to the formation and expansion of
corrosion products on the surface of the sample, and the long-lasting resonance signal
corresponds to hydrogen bubble formation and evolution. Two types of sensors were
used in this test. The R15α sensor used the 1-channel was of the same type as in [2],
and the two types of AE signal waveforms recorded were identical to those reported
in the literature. The 2 channel uses R6α type sensor. The waveform recorded in the
first stage is consistent with the literature report. There are some differences in the
waveform of the second stage, which is mainly caused by the different frequency
response characteristics of the sensor. The activity and number of the second-stage
signals of the 2-channel are much higher than that of the 1-channel, indicating that
the two-stage signal frequency is mainly distributed in the low-frequency response
frequency band. In addition, different types of signal waveforms appear in different
sequences in the experiment.
To further understand the waveform characteristics, a continuous wavelet is
applied to the waveform signal to be transformed and superimposed with the material dispersion curve. The result is shown in Fig. 22.5. It can be seen from the figure
that the distribution characteristics of the signal energy in the time domain and the
frequency domain are basically consistent with the signal waveform and power spectrum analysis results. It can be seen from the superposition of the dispersion curve
and the signal energy that the first-stage signal waveform is mainly expansion wave,
the two-stage one-channel signal waveform mode is mainly expansion wave, and the
two-channel waveform mode is mainly flexural wave. The wavelet transform result
graph and the dispersion curve are not superimposed at the maximum energy of the
signal, which may be related to the fact that the rectangular sample size is small and
the wave propagation process is greatly affected by the boundary.
Based on modal AE theory and pitting corrosion AE source mechanism [9, 10],
thin-plate metal pitting will mainly produce expansion wave and flexural wave. The
expansion wave has the characteristics of fast propagation speed, low amplitude
and high frequency (>100 kHz); while the flexural wave has slower propagation
speed, larger amplitude, lower frequency (<100 kHz) and dispersion phenomenon.
Comparing the signal waveforms collected by the two channels and the frequency