A Study on the Properties of TFM Ultrasonic Imaging Using Synthetic Data
Novel developments in the area of ultrasonic inspection techniques are predominantly based on the phased array technology, which allows for focusing and steering the ultrasonic beam fields. As an alternative to classical beam forming methods the "Full-Matrix-Capture" (FMC) approach steps through all pairs of transmitter-receiver element combinations for data acquisition, thus recording the full data matrix. Imaging is then performed using the Total Focusing Method (TFM) by applying the respective delay laws to focus via algorithmic data processing. Such imaging techniques gain increasing relevance, since not only the detection and localization of inadmissible defects is important, but also their sizing and characterization in view of type, geometry and orientation is desired. The efficiency of the applied reconstruction techniques with respect to a specific inspection situation depends on the material and component parameters on the one hand as well as on the defect configuration and geometry on the other. In this contribution, we report on a study concerned with the imaging capabilities of the Total Focusing Method. Addressing side-drilled holes (SDH) as model defects we have simulated the FMC data acquisition for a commercial phased array probe (16 elements, 5 MHz frequency) using a 2D Fraunhofer-approximation. For the scattering of the ultrasonic waves at the SDH we have used two model approaches - a Kirchhoff type far-field approximation and the Separation of Variables method as an exact technique. On this basis we have simulated time-domain ultrasonic data for various scenarios where the defect parameters SDH diameter and distance between two SDHs have been varied. We have used the generated data to perform TFM imaging, also addressing the influence of noise on the image quality. Such a-priori investigations, also for competing techniques, help in evaluating whether the - in comparison to conventional inspection techniques - use of more sophisticated equipment and the higher effort in data acquisition are beneficial for a specific inspection situation.