Hilbert Huang Transform for enhancing the Impact - Echo Method of Non Destructive Testing

The impact echo technique, has been used successfully for detecting flaws in many different types of concrete structures, including plate-like structures such as bridge decks, slabs, walls, beams, columns, layered structures and hollow cylindrical structures. In the used technique in this work (impact echo technique), a transducer is the sender and the receiver at the same time. A pulse of ultrasonic energy is sent through the tested material in order to diagnosis and control the quality of this material. Usually the received signals consist of components and additive structural noise. This noise often masks the signal of the defects and creates an embarrassment in its detection. Also, in thin samples the reflected signals are overlapping thus making detection of defects in the sample and accurate measurements impossible. It is thus necessary to enhance the visibility and resolution of the defects echoes by signal processing techniques for characterizing, detecting and locating the imperfections present in the used materials. In this work the ultrasonic signals are treated by Hilbert Huang Transform (HHT). In this paper experimental results have been presented to evaluate the effect of HHT on ultrasonic signal analysis. Using HHT, we can then measure the time of flight (TOF) of the different echoes and the sample thickness. From measuring thickness and the TOF, it is easy to calculate the speed and longitudinal cross respectively, and then determine the elastic constants (Young modulus and Poisson's ratio) that allow us to describe the development of mechanical properties of materials and resolve the problems of material characterization and defects detection. The results have indicated the potential feasibility of the ultrasonic impact echo method and signal processing tool (HHT) for ascertaining structural differences in materials.