| Ultrasonic frequency modulated(FM) technique for inspection is a method combined frequency modulated signals exciting and pulse compression technique. this technique has opened new prospects in ultrasonic inspection and evaluation of welding joint, due to its comprehensive inspection ability of long distance detecting, high signal-to-noise ratio(SNR) and high time resolution. In order to improve the reliability, applicability and flexibility of ultrasonic FM technique, and make it solve the problems of low SNR and defect recognition in ultrasonic inspection for coarse-crystalline austenitic stainless steel welds, further investigations as regards parameter selection, waveform design and progatation characteristics still need to be carried out.In the thesis the interaction between FM pulse and ultrasonic system has been analyzed, and then the appropriate bandwidth selction method and the nonlinear frequency modulated(NLFM) wavefrom design method using the spectrum fitting of the probe has been proposed. Futhermore the progatation characteristics of FM signals in this weld has been investigated, which provides theoretical guidance for evaluate the inspection capability and application prospect of this technique. Based on the results above, ultrasonic FM imaging inspection with single probe for coarse-crystalline anisotropic stainless steel weld has been completed,In terms of the fundamental of the ultrasonic FM technique, the systems were constructed for requirements of different inspections, including the system hardware of signals exciting, oscilloscope acquisition, and ultrasonic scanning imaging and so on. In order to improve the efficiency of detection, an oscilloscope automatic testing program based on Lab VIEW was developed to carry out multifunctional fast signal acquisition and storage.Based on the system platform above, the influence of exciting bandwidth on the detection performance of ultrasonic system was studied, and an optimal excitation bandwidth selection method was proposed. The bandwidth test for the probes with different frequencies was performed by through transmission technique, and the matched fiter outputs of the relevant transmission waves received were analyzed. The results show that the variations of time resolution and SNR of these transdcers were almost consistent, and the variation of the time resolution with an increase in the bandwidth is conformed to an exponential function: firstly the time resolution increases rapidly, then becomes slow; the variation of the SNR with an increase in the bandwidth is conformed to a Gaussion function, firstly the SNR gradually increases, when the bandwidth reaches a certain value, it has the maximum, then gradually decreases. It indicates that increasing the exciting bandwidth could enhance the inspection performance of the system effectively. The applicability of the results above are confirmed by FM TOFD testing for a 2mm flaw. The investigations reveal that the excitaiton bandwidth and the spectrum of transdcers have a definite relation, when the FM exciting bandwidths are approximately equal to the-12 d B bandwidth of the probe spectrum, the maximum SNR and a high time resolution can be obtained.The interaction mechanism of the FM excitation signals and the ultrasonic system was analyzed. The amplitude modulated(AM) waveform design was performed to suppress the sidelobe, and the nonlinear frequency modulated(NLFM) waveform design method based on sepctral characteristic of the probe was proposed to further enhance the output characteristics of the system.In the AM design, the waveform design with Hamming window is a simple and effective method that can suppress the sidelobe, whereas it is unavoidable to make the mainlobe widen, thereby reduce the inspection resolution. The investigations reveal that when the spectrum of the input signal and the specturm of the probes in system has a good consistency, the relatively ideal performance of the system can be obtained, thereby the NLFM waveform design method based on the spectrum fitting of the probe was proposed. The waveform designed with this method has a rectangular envelope, an S-shape frequency modulated curve and a spectrum simillar to the spectrum of the probe, and it can match the probe well, and can obtain a relatively small waveform distortion and bandwidth loss. The results show that the NLFM pulse can improve effectively the problem of the mainlobe widenin g, and can achieve the time resolution of 0.22μs and be more 30% than the AM pulse. It can also suppress effectively the sidelobe to 25.8d B and be more 50% than the LFM pulse.The ultrasonic imaging inspection for the austenitic stainless steel welds with the different structures was researched using the NLFM signal with higher synthetic detection performance. The results of TOFD imaging testing for the defects in the butt weld show that the NLFM technique compared with the conventional TOFD testing can improve the quality of image and signal, make the SNR increase by 8.2dB and the relative measurement error is one third of the error of conventional TOFD testing. Based on the large time width and bandwidht characteristics of the FM signal, a broadband double crystal probe with high damping and delay which can independently transmit and receive the FM pulse was designed to carry out the FM exciting imaging inspection with single probe, thereby the adaptability of the FM inspeciton technique was enhanced. It was well applied to the defect inspection for 70 mm thick-walled stainless steel weld with buttering. This method can increase the SNR by about 6.3d B, and the time resolution by 36% compared with the conventional ultrasonic exciting.Finally, the propagation characteristics of the NLFM signals in coarse crystalline anisotropic weld was investigated, and the variation of its relative attenuation cofficient with transmission angles along y and z direction in different areas of the weld was revealed. The investigations show that the NLFM signal has high energy and propotion of low frequency component which determines its lower attenuation and higher SNR during the propagation in the weld, and also has a certain high frequency component, therefore it can also obtain high time resolu tion by pulse compression technique. The relative attenuation coefficient of the NLFM signal is less 1.0d B/mm on average than the conventional ultrasonic signal during the propagation in austenitic stainless steel weld with buttering, and it can achieve a high performace in defect inspection. The study results shows that the NLFM inspection technique can effectively solve the problems of high attenuation, low SNR and low time resolution in the ultrasonic inspeciton of the coarse grain material. |
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