Guideline for penetrant testing on welds by indirect visual test

A study of guidelines for penetrant testing on welds by indirect visual test using image processing is presented. Due to penetrant testing regulations by direct visual test, a porosity (a round defect) whose diameter is under 1.0 mm is allowed, but no crack (a line-shaped defect) is allowed, and the allowable defect size is not clear. It was found in the investigation that the smallest line-shaped indication which the inspectors could recognize was 0.15 mm in width and 0.5 mm in length. This means that the limit of line pair which all inspectors could discriminate was 3.3. A TV camera was used to take pictures of the NSIT (National Institute of Standards and Technology) (NBS: National Bureau of Standard) resolution chart and the smallest indication above, and the limit of line pair which all inspectors could discriminate was 3.6. Therefore, it is recommended as a guideline for penetrant testing through a TV camera by indirect visual test that the resolution of 3.6 line pair per mm is required and a reference test piece be defined to assure the resolution. (author)