Pipe wall damage detection by electromagnetic acoustic transducer generated guided waves in absence of defect signals.

Most investigators emphasize the importance of detecting the reflected signal from the defect to determine if the pipe wall has any damage and to predict the damage location. However, often the small signal from the defect is hidden behind the other arriving wave modes and signal noise. To overcome the difficulties associated with the identification of the small defect signal in the time history plots, in this paper the time history is analyzed well after the arrival of the first defect signal, and after different wave modes have propagated multiple times through the pipe. It is shown that the defective pipe can be clearly identified by analyzing these late arriving diffuse ultrasonic signals. Multiple reflections and scattering of the propagating wave modes by the defect and pipe ends do not hamper the defect detection capability; on the contrary, it apparently stabilizes the signal and makes it easier to distinguish the defective pipe from the defect-free pipe. This paper also highlights difficulties associated with the interpretation of the recorded time histories due to mode conversion by the defect. The design of electro-magnetic acoustic transducers used to generate and receive the guided waves in the pipe is briefly described in the paper.

Combined surface-focused acoustic microscopy in transmission and scanning ultrasonic holography.

Employment of ultrasound techniques in nondestructive testing may require identification of the acoustic modes contributing to imaging. Such identification can be achieved, with some restrictions, by time-of-flight analysis. Another approach is acoustic holography that reveals the propagation properties of any selected mode. In anisotropic media, the propagation features are distinct and allow for a reliable classification of the selected mode. Both techniques were applied for classification of bonded, disbonded, and weakly bonded areas in directly bonded semiconductor wafers.

Application of spatially and temporally apodized non-confocal acoustic transmission microscopy to imaging of directly bonded wafers.

Application of a line-shaped point spread function (PSF) to imaging of void defects in directly bonded wafers is considered. Two non-confocally adjusted spherical transducers are employed to implement an acoustic microscope operating in transmission with a time dependent point spread function, whose shape is optimized by both temporal apodization of the received signal and spatial apodization of the transducer aperture. Strong imaging artifacts resulting from the generation and detection of edge waves are eliminated in this way. It is shown by several examples that only a broadband system can be utilized in order to obtain a line-shaped PSF suitable for imaging.