Linear and nonlinear ultrasound time reversal using a condensing raster operation

One of the weaknesses of nonlinear methods is the excitation power required to generate nonlinear effects such as higher order harmonics (single frequency) and sidebands (dual excitation). In conjunction with the high-power prerequisite, is the required sensitivity of sensors to capture nonlinear effects which can be multiple orders of magnitude lower than the fundamental/linear response of the system. In this work a semi-air coupled nonlinear ultrasound modulation method combining phase symmetry analysis and time reversal is developed, in the aim of alleviating some of these issues. Phase symmetry analysis is used to characterise the second order and modulated nonlinear responses of a dual frequency excitation signal containing a single frequency (f₁) and a sweep burst (f₂). Time reversal allows for optimal focusing, in this case at locations on composites panels with barely visible impact damage. Time reversed signals typically focus at single locations at a time, in this work, a raster time reversal methodology is proposed which focuses on multiple locations simultaneously. The raster methodology reduces distortions which may affect air-coupled techniques, reduces inspection time, and shows clear enhancement of damage imaging when compared against the raw fundamental response and standard time reversal.