Research On Mechanism Of Surface Crack Defect Detection In Pipeline Based On Laser Ultrasonic

Pipelines are widely used in many important fields, which are long-term in harsh environments, such as high temperature, high pressure, corrosion, and with the increase of service time, can easily cause fatigue failures that lead to the occurrence of cracks and then trigger accidents such as pipelines media leakage Laser ultrasonic testing technology make up the shortage of conventional ultrasonic technique, which Sets on the merits of complete non-contact, no coupling agent, suitable for long distance detection, large ultrasonic bandwidth, high spatial resolution and other advantages, can be utilized in high temperature, corrosion, radioactivity and other harsh environment. Considering the urgent needs of surface crack defects detection of pipeline, finite element simulation method based on thermosetting coupling and detection experiments based on laser ultrasonic was combined in paper to research the mechanism of interaction between laser-induced ultrasonic surface wave and surface crack of pipeline.Process of laser excitation of ultrasonic wave in pipe based on thermal solid coupling simulation was researched, power density of pulsed laser source and transient temperature gradient distribution were explored, ultrasonic vibration characteristics with and without pipeline surface crack defect was researched, influence of crack depth on ultrasonic vibration signals, first ultrasonic flaw echo spectrum and propagation characteristics of ultrasonic stress field were analyzed. Finite element simulation provided theoretical basis for the study of technical research experiments.Laser ultrasonic pipeline inspection system was established based on the theory of modular design, using pulse laser to excite ultrasonic and Interferometer based on principle of double-wave mixing interference to receive ultrasonic, parameters of each module was quantitatively analyzed to get best signal-to-noise. Surface wave flaw echo signal was extracted base on fractional differential method, effects of pulsed laser power density on ultrasonic wave signal was researched, ultrasonic signal characteristics with and without defects and reaction of laser ultrasonic surface wave to surfacedefects was analyzed, B scan results before and after extracting defect echo features were explored, relationship between surface crack depth and time interval of two defect echo are analyzed.Experimental results and finite element simulation results were comprehensively compared, research process was evaluated, mechanism of exciting ultrasonic using laser in pipeline and interactions with surface crack defect were obtained.