||The guided wave method can inspect pipelines very quickly and widely. For instance, it can inspect the overall pipelines by digging several detection pits or removing part of coating material to set the array ring. However, it will make the guided wave attenuate more seriously and make the signals hard to identify when setting the array ring on the general corrosion.|
In this study, the wave propagation will be discussed when the general corrosion is under the array ring and the severe localized corrosion is inside the general corrosion via experiment and finite element method. In finite element method, time-domain signal analysis, wave propagation and the two-dimensional Fourier transform are used to explore the guided wave incident, reflection, transmission and other wave propagation phenomena. Furthermore, the relationship between wave propagation, attenuation and localized corrosion on the general corrosion were discussed by different detection frequencies.
The results showed that the excitation energy will be lower when the array ring set on the pipe surface with the general corrosion. By two-dimensional Fourier transform analysis, its non-uniform contact surface will increase asymmetric modal and mix signals. The energy attenuation will increase when the corrosion depth is deepened or the inspection frequency is risen. For example, the 2 mm deep general corrosion will attenuate -1.09 dB/m at 20 kHz and attenuate -3.01 dB/m at 40 kHz; the 4 mm deep general corrosion will attenuation -5.76 dB/m at 20 kHz and attenuation -23.19 dB/m at 40 kHz. However, the coherent signals which were caused by the general corrosion will decay with increasing frequency. For example, the coherent signals of 2 mm deep general corrosion are -23.67 dB at 20 kHz and -35.44 dB at 40 kHz; then, the 20 mm long and 3.5 mm deep localized corrosion which signal is -26.34 dB at 20 kHz and -26.94 dB at 40 kHz will be detected easily at high frequency. It can provide detectors to understand the impact when the array ring set on the area of general corrosion and the way to distinguish the localized corrosion which is inside the area of general corrosion.