Nondestructive Examination of Inside Surfaces of Small Holes in a Steel Structure Using a Laser Scan Technique

This paper describes the application of a commercial off-the-shelf laser-based inspection system to nondestructively examine the internal surfaces of small holes 6 mm in diameter and 19 mm in length embedded in a tubular section made of high strength steel. Methods are developed to analyze the data after correcting for laser probe wander while scanning and applying signal processing techniques to extract the Abbott-Firestone curves of the surface features. The tubular sections were subjected to a safety and reliability test simulating storage conditions to withstand exposure to repeated cycles of extreme temperatures and humidity. The temperature extremes were +71 and -54C, and a relative humidity of 95% was used at +71C. Under these conditions, corrosion was induced in the steel and the formation of pits was suggested by the increase in the surface heights with the number of days of treatment. Further analysis of the pit distributions showed that (1) the pit density decreases monotonically with increasing pit depth, (2) the maximum pit depth beyond which the pit density is negligible increases with the number of treatment days, and (3) for a given pit depth below the maximum, the pit density increases with the number of treatment days. No surface defects such as cracks were observed in any of the sections even after the maximum number of 84 days of treatment.