Experimental Study On Damage Detection In Pipes Using Ultrasonic Guided Waves Technique

Pipeline transportation business plays a much important role in the economy construction and national defence industry, but the leakage of the pipe defects, corrosion and cracks can not only make the huge damage to the economy, but also pollute the environment and affect the balance of the ecology, so it is very important to detect the pipes. Ultrasonic guided waves technique has not only good command of efficiency and low cost, but also can inspect the whole wall thickness of pipe. Therefore, ultrasonic guided wave technique is paid more attention in pipe NDT area.Based on previous study on the guided wave propagated in the pipeline, the main of this paper is to do experimental study on damage detection in pipes using ultrasonic guided waves technique. First of all, based on the analysis of the excitation signal, it's found that the more the number of cycles, the band becomes narrower, more conducive to control the frequency, but the signal lasts longer in the time domain and it is easier to superipose, so select the sinusoidal signal of 20 cycles which modulated by window of HANNING as the excitation signal in the experiment. And then to 3-meter-long steel pipe to simulate the crack damage, the 5-meter-long steel pipe to simulate the corrosion damage was found that increase with the excitation frequency, the energy amplitude of the excitation signal increased, but easier the energy of high-frequency loss, so select 70KHz as the excitation frequency of the experiment. The results showed that the longitudinal ultrasonic guided waves can be very accurately locate a single crack and double cracks, and can successfully detect the small defect which account for 3.7% of cross-sectional area of pipe, and with the expansion of defects, the reflectivity of guided wave energy increase monotonically, while the transmittance of guided wave energy decrease monotonically. When the guided wave propagate through the end of the pipe and defects, in addition to reflection occurs, but also a certain degree of mode conversion, and the amplitude of conversion mode has something with the frequency of the excitating guided wave. Selecting the appropriate excitation frequency can well inhibit the conversion mode amplitude. Finally the rules of ultrasonic guided waves propagate in the 4-meter-long PVC-U pipe were explored and found that the energy of longitudinal guided wave was greatly attenuated, spreading about 1 meter distance, far less than the transmission distance in steel pipe, it indicates that the nature of the pipe material will significantly affect the guided wave propagation distance.