Assessment of voids and inclusions in thin metal plates using Bragg imaging

For thin metal plates which are used for aerospace pressure vessels, an assessment of voids and inclusions is essential for fatigue analysis. It is known that flaws of these types with sizes larger than 127 μm (5 mils) in diameter can appreciably shorten the fatigue life below the design limits. To improve the current capabilities of detecting flaws of the above size in thin metal plates the application of Bragg imaging techniques has been attempted. Using ultrasonic frequencies near 30 MHz and a helium-neon laser, the spatial resolution has been determined to be close to the above flaw size based on the image of holes in thin Inconel-718 plates. While the system was designed to optimize spatial resolution, it distorts the obtained image in the vertical direction. This is corrected by a simple numerical processing technique. Details of the Bragg imaging system design and analysis of the results are discussed. Also discussed are the elements to determine the sensitivity for flaw detection