Application Research Of Guided Wave Detection For Laser Ultrasonic Transducer

Ultrasonic guided waves are sensitive to small damages in structures,have the advantages of long propagation distance and high spatial resolution of detection,and have high practical value in industrial non-destructive testing(NDT).The use of laser-excited ultrasonic guided waves for NDT testing is a new type of technology developed in recent years,which have characteristics of non-contact excitation,low structural requirements and high efficiency.In the current research on NDT applications,laser-excited ultrasonic guided waves are realized by directly laser irradiation on the surface of the object to be tested.In the process of exciting the high signal-to-noise ratio guided wave,there are some problem that high laser energy required,low photo-acoustic conversion efficiency,and poor flexibility of specific frequency and modal excitation.Aiming at the above problems,an originally flexible ultrasonic transducer having high photo-acoustic conversion efficiency is proposed by using candle soot nanoparticles(CSNPs)with high light absorption and polydimethylsiloxane(PDMS)with high thermal expansion properties.Experiments results demonstrate that the developed transducer can generate a 0.28 us narrow band longitudinal wave in time domain.Meanwhile,the amplitude of the excited longitudinal wave by the proposed transducer is 10 times to that of the signal generated by direct exciting in the aluminium block without using the transducer.A high signal-to-noise ratio signal is obtained at low laser energy excitation conditions.The amplitude of the longitudinal wave signal is approximately proportional to the excitation level of laser energy.Lamb waves,made up of a superposition of longitudinal wave and shear wave,are available in a thin plate.Thus the excited longitudinal wave of the proposed transducer is utilized to excite Lamb wave by the merits of the longitudinal wave.The specific A0 and S0 modes of the Lamb wave with the central frequency of 647 kHz were successfully excited in the aluminum plate of 1.5mm thickness by changing the angle of the wedge according to the Snell’s law.By comparing the characteristics of the two special modal,A0 mode is applied to the defect detection in the aluminum plate.A single signal source is used to transmit signal,and a one-dimensional array of PZT piezoelectric sheets receive the signal reflected from the defect.According to the principle of the synthetic aperture focusing imaging technology,a delay-and-sum signal processing method is adopted for damage location in plate.A 3.5 mm defect was well imaged and the results demonstrate that the developed flexible photo-acoustic transducer can be a promising method for the NDT.