Abstract:The coded ultrasonic testing has the problem of long duration of mainlobe and high level of sidelobe after pulse compression. To solve this issue, a composite coded ultrasonic excitation technique based on the nonlinear frequency modulation Barker is proposed, which is applied in the plate weld flaw detection. The generation method of the proposed composite code is presented and its mathematical model is deduced. The timefrequency performance, the transducer response, and the pulse compression performance have been researched and simulated, which are compared with other coded signal excitation method, such as Barker, nonlinear frequency modulation signal, and linear frequency modulation Barker. Compared with the nonlinear frequency modulation signal and linear frequency modulation Barker, experimental results show that the power ratio of mainlobe to sidelobe is increased about 11 and 5 dB, respectively. Compared with Barker code, nonlinear frequency modulation signal, and linear frequency modulation Barker code, the level of peak sidelobe is reduced about 78, 7, and 36 dB, respectively. The composite coded ultrasonic signal can effectively detect the common five types of defects on a steel plate weld sample.