Research On Phased Array Ultrasonic Testing Of High Density Polyethylene Pipe Butt Fusion Joint In Nuclear Power Plant

High density polyethylene(HDPE)pipe has excellent corrosion resistance and good toughness,and it has found its application in gas transportation and water supply system for several decades.Recent years,HDPE pipe has been gradually applied to the cooling water transportation pipelines in nuclear power plants(NPP).For considerations of operation efficiency and safety,the diameter and the wall thickness of HDPE pipes in NPP are large.In addition to the complex conditions on site,embedded defects will produce in the joints of HDPE pipes during welding.Therefore,it's necessary to conduct safety testing with the help of nondestructive examination.The large wall thickness,the complex structure,the acoustic attenuation and dispersion of HDPE also bring great challenges to the PAUT of HDPE pipe joints in NPP.This paper is supported by the Fundamental Research Funds for the Central Universities,Grant No.2017FZA4012)and the National Natural Science Foundation of China(Grant No.51575480,51842501).The phased array ultrasonic testing(PAUT)for HDPE pipe butt fusion joint in NPP is studied through material acoustic properties testing,acoustic field simulation in high attenuation material and PAUT experiments of thick wall HDPE pipe.The main works are as follows,(1)Acoustic field simulation and defect imaging simulation of HDPE pipes are realized by Field II.The validity of the simulation is verified by comparing the experimental results of the ultrasonic microscope.An exclusive delay law for ultrasonic testing of HDPE pipe is proposed on the basis of peak shift.Comparing the simulation results and experimental results of the exclusive delay law and the general delay law under different deflection angles,it is found that the exclusive delay law can increase the acoustic pressure before the focus by 5.3%,and then improve the intensity of defect reflection pulse,the exclusive delay law also has a larger scan range.(2)Considering the structure of HDPE pipe butt fusion joints in NPP,an exclusive scan mode for PAUT is proposed to makes the focus of many scan lines locate in the bisect plane and increase acoustic pressure.Defect imaging simulation and experiment of the exclusive scan mode and sectoral scan mode are compared.The simulation is consistent with the experiments.The exclusive scan mode can detect the defects in the deeper position of the joint,and the defect image is closer to the real shape.The exclusive scan mode can significantly improve the sensitivity and resolution of PAUT for HDPE pipe butt fusion joints,and the practical operation is simpler than sectoral scan mode.(3)The PAUT parameters of HDPE butt fusion joints in NPP are optimized by acoustic field simulation.And the effects of frequency,element distance and element number on the focusing performance are discussed.Considering the detection depth and accuracy,three PAUT schemes are proposed to detect the HDPE pipe butt fusion joints in NPP.Taking hole and crack as research objects,a batch of HDPE pipe butt fusion joints and blocks with defects are processed to test the detection ability of PAUT system.By using the proposed detection schemes,the holes and cracks in the range of 110 mm can be detected,and the cracks length in the range of 75 mm can be obtained.The PAUT results also verify that the simulation can be used as the theoretical support for the development of PAUT technique and parameters optimization.