Research On Several Key Issues Of Ultrasonic Phased Array Technology

Ultrasonic phased array (UPA) could m ake the beam s deflect or focus bycontrolling the delay tim e of the transm itting acoustic waves of each e lement. Thus,it’s able to increas e the coverage of detection area without movi ng the probe. It hasobvious advantages on defect detection in co mplex structures. Som e issues such asthe sound field characteristics, defect imaging, qualitative and quantitative analysis ofdefects are researched in this dissertation.The analytical expression of deflecting sound field and formula of directivity ofUPAare derived based on the Rayleigh integr al in the far-field condition. The effectsof param eters such as elem ent width to m inimize the main lobe width, elim inategrating lobes, and suppress side lobes are discussed. In the far-field experiment, thephenomenon of pseudo defects which is ge nerated by the grating lobes is analyzedand the sheme for eliminating grating lobes is proposed.Acalcula tion m odel of discr ete po int so urces for the sound field of UP Aisdeveloped. The simulations of sound field and defect resp onse experiments based onthe CIVAsimulation platf orm show that th e influence of near-field will be reduc edwhen the steering angle increased, and the effect of focusing will get stronger with theincrease of interference ef fect in the near-field region. The im pact of the near-fieldcould be alleviated by reducing the array aperture or focusing at multi-points.The expressions of the sound field and the displacement field of UPAare deducedfrom the radiation field of single ele ment in the liquid-solid interface condition. Theinfluences of wedge, steering angle, wave pattern, and near-field to the deflection andfocusing of beam s are discussed by sim ulation. Com bined with experim ents, thephenomena of pseudo-defects caused by wave mode conversion and the possibility ofreducing the influence of grating lobes by refraction are discussed.The Wavelet Packet Transform (WPT) is proposed to extract features of flawechoes such as crack, inclusion and dela mination in carbon fiber composite, whichprovides a basis for automatic identification of defects. Other researches show that thefocusing feature of beams and the size of de fects have impacts on feature extraction,and the wave type has little influence on feature extraction of defects.The correction algorithms of images of linear-scan and sector-scan are proposed.Experimental results validate that the method of using longitudinal wave andtransverse wave to correct im age can e liminate the pseud o-defects caused by wave mode conversion.The-6dB quantitive m ethod is compared with the boundary tracing m ethod fordefects quantifying. The result s show that-6dB m ethod has an advantage when theoriginal data could be accessed, but the boundary traci ng method is the only choicewhen the processing object is an image.Aprinciple to determ ine the center of defect based on-6dB method is given tolocate the defect. T wo groups of defect-l ocation experim ents are carried out. Thelocation errors of via holes in the ship-block and factors affecting positioning error areanalyzed.