Simultaneous Velocity, Thickness and Profile Imaging by Ultrasonic Scan

Ultrasonic velocity is widely used in the investigation of material properties. Physical dimensions such as sample thickness and surface profile can also be the information sought after in nondestructive testing. In a conventional ultrasonic measurement, however, the measured quantity is usually the time-of-flight, not velocity or thickness. When either velocity or thickness is precisely known, the other can be easily deduced. Unfortunately in many cases, neither the velocity nor the thickness is known to the required precision, or they can both vary spatially. A method is therefore needed to map out the spatial distribution of the velocity and thickness simultaneously. The ability to image the surface profiles that give rise to the thickness variation is also desirable. Simultaneous velocity and thickness scan can be implemented in a number of configurations and have been reported by a number of authors in the literature. This paper focuses on the ability of this technique to image very small changes in velocity and thickness simultaneously. This technique is also extended to map out the surface elevation contours and cross-sectional profiles of a sample. Special considerations for achieving accurate and reliable velocity, thickness and profile images are discussed in detail. This technique for nondestructive evaluation and material characterization is then demonstrated using three industrially relevant materials: (1) plasma sprayed thermal barrier coating, (2) composite laminates containing foreign objects and anomalies, and (3) creep/rupture tested superalloy samples.