Creation of non-virtual acoustic images in three dimensions and applications in NDT

摘要

One of the major forth-comings of modern NDT is the availability to image certain areas of interest of a work-piece to the most detailed extend achievable within the resolution limit. The major efforts have been undertaken on the receiver side in order to correct the spatial and temporal characteristics of the reflection of ultrasonic signals. Only few approaches have been realized on the transmitter side, one of which is the Time Reverse Mirror method [1, 2, 3]. Within this approach, analog excitation functions are recorded from the reflections created by discontinuities from an incident plane wave, which is similar to the recording process of an optical hologram. With the upcoming of 2D arrays, this approach is getting significant interest, in particular, because with those it is possible to generate 3-dimensional acoustic images. In this paper we present the approach of a computer holographic generation of the excitation functions. Assuming a real acoustic image shall be created, this method can be used to calculate the excitation functions, which are needed to be imposed onto the transmitters in order to generate this image. This method therefore allows tailoring the ultrasonic transmission to the testing scenario considering in particular the geometric and acoustic conditions. If applied in conjunction with the Paint-Brush technology[4], or any type of image reconstructing algorithm on the receiver side (e.g. DDF/DDA[5]), this transmission method can be flexibly applied throughout the NDT-applications.