Abstract:

A nonlinear ultrasonic array imaging method used for the defect loacation is developed. The propagation characteristics of the focusing acoustic field in time-space domain and the conditions to generate the diffusion acoustic field are investigated, then the relationship between the diffused acoustic energy in two different phased mode, parallel and sequential focusing, and the nonlinear effects is analyzed, and a nonlinear ultrasonic imaging method is proposed, in which the difference of acoustic energy of two phased modes within the excitation bandwidth(fundamental wave)is considered as characteristic parameter for imaging. The influence of time delay and time window for diffuse field sound energy and nonlinear imaging are studied, and the optimal parameters for nonlinear imaging are obtained. The time delay has great influence on the nonlinear imaging, however the width of window has little effect on the nonlinear imaging. Over a range of time delay, the diffusion acoustic field is stable, therefore the micro-crack can be detected effectively. It is demonstrated that the proposed nonlinear imaging technique is an effective method for micro-crack detection.

Key words: diffusion acoustic field, phased mode, phased-array, ultrasonic array, nonlinear imaging