Research On Phased Array Focal Field Of Thick Polyethylene Electrofusion Joint In Nuclear Power Plant

High density polyethylene(HDPE)pipe has been used in nuclear power plant(NPP)to transport cooling water due to the advantages of corrosion resistance,fouling resistance and a life of almost 50 years.Current phased array ultrasonic inspection technique was found to be inable to detect defects in HDPE electrofusion(EF)joints of large size used in NPP with less focal energy,low singal-noise-ratio and low resolution.Lack of effective ultraosonic technique has seriously restricted the application and extension of PE pipe for the nuclear field.Different from metal,PE of viscoelasticity has obvious acoustic attenuation and dispersion,and ultrasound distorts in shape gradually when propagating in PE.Therefore,a superposition of focal beam cannot be realized at large depth as expected,which further has influences on defect recognition and imaging.Consequently,research on phased array focal field of PE considering its viscoelasticity is the basis of developing effective ultrasonic technique for thick PE EF joint in NPP.With the financial support from the Youth Foundation of National Natural Foundation of China"Formation Mechanism of Eigen-Line in Ultrasonic Inspection of Electrofusion Joint for Connecting Polyethylene Pipes"(Project No.51305394)and the General Program of National Natural Foundation of China"Study on the Mechanism of Slow Crack Growth in Polyethylene Pipe Based on the Viscoelastic Damaged Model"(Project No.51575480),focal field of ultrasonic inspection technique for HDPE pipe used in NPP was studied.The main content and conclusions are presented as follows:(1)Based on Szabo acoustic model considering acoustic attenuation and dispersion of PE,a numerical analysis method was proposed to describe the acoustic field of PE.Firstly,time domain finite difference method was applied to discrete both the calculating space and time period.Then,the original equation was modified to remove the hypersingular term,improve the stability,and enhance the calculating accuracy.Besides,the infinite calculating issue was transformed to a finite calculating issue using absorbing boundary conditions.Finally,sound pressure distribution of the whole acoustic field was obtained though numerical analysis.A good agreement between the numerical results and experimental results was achieved both in time-domain and frequency-domain.The model with time-space of 4 order and space approximation of 3-layer was determined to be optimal.(2)A numerical method was presented to predict the focal acoustic field of phased array ultrasonic inspection after a relation between plane wave and radiation wave was established.Through the method,accurate delay time for PE was calculated,and the deviation of testing results using inaccurate delay time was investigated.Besides,focal properties were analyzied,including pressure distribution along the center axis and focal diameter.Finally,focal diamemter was used to characterized the focal properties of inspection system.(3)The influence of aperture,element distance,scan method on focal properties was further investigated,and consequently a set of optimal parameters for HDPE pipe in NPP were provided considering testing space,transducer manufacturing,testing convenience etc.After a successful field inspection in NPP,the presented method and criteria of determining the optimal inspecting parameters were proved effective.