Rapid quantification of 3D ultrasound fields with wavefront sensing and Schlieren tomography.

The rapid and precise characterization of three-dimensional (3D) pressure fields inside water is paramount for ultrasound (US) applications in fields as relevant as biomedicine and acoustic trapping. The most conventional way is to scan point-by-point a needle hydrophone across the field of interest, which is an intrinsically invasive and slow process. With typical acquisition times of hours and even days, this method remains impractical in many realistic scenarios. Alternatively, optical techniques can be used to non-invasively and rapidly measure the changes in light intensity or phase induced by pressure differences. However, these techniques remain largely qualitative: extracting precise pressure values can require extensive calibration, and complex processing, or can be limited to low-pressure ranges. Here, we report how combining wavefront sensing and Schlieren tomography enables rapid and direct quantification of 3D pressure fields while obviating any calibration steps. By simultaneously capturing optical phase and intensity information of the US-perturbed fluid using a Wavefront Sensor and Schlieren projections, respectively, 3D pressure fields over several millimeters cubic can be reconstructed after a few seconds. We present a detailed description of the approach and prove its feasibility by characterizing the US field after an acoustic lens, which is in excellent agreement with calibrated hydrophone measurements and simulations. These results are a significant step forward toward the precise and real-time characterization of ultrasound patterns.

Sizing the Depth and Width of Narrow Cracks in Real Parts by Laser-Spot Lock-In Thermography.

We present a complete characterization of the width and depth of a very narrow fatigue crack developed in an Al-alloy dog bone plate using laser-spot lock-in thermography. Unlike visible micrographs, which show many surface scratches, the thermographic image clearly identifies the presence of a single crack about 1.5 mm long. Once detected, we focus a modulated laser beam close to the crack and we record the temperature amplitude. By fitting the numerical model to the temperature profile across the crack, we obtain both the width and depth simultaneously, at the location of the laser spot. Repeating the process for different positions of the laser spot along the crack length, we obtain the distribution of the crack width and depth. We show that the crack has an almost constant depth (0.7 mm) and width (1.5 µm) along 0.7 mm and features a fast reduction in both quantities until the crack vanishes. The results prove the ability of laser-spot lock-in thermography to fully characterize quantitatively narrow cracks, even below 1 µm.