Detectability of small true-t hole image quality indicators by digital radiographic techniques

Image quality indicators (IQIs) are used to ensure quality in radiography. The requirements for hole-type IQIs have changed from MIL-STD-453-C (US DOD 1984) to ASTM E1742 and E1025 (ASTM 2018a, 2018b). For materials thinner than 0.50 in., the 2-2T hole does not represent 2% sensitivity. E1025 introduced true-T hole IQIs, where the 2-2T hole represents 2% sensitivity. Stainless steel true-T hole IQIs were fabricated using drills and a plasma-focused ion beam instrument. They were imaged using digital radiography (DR), computed radiography, and X-ray film to determine the limit of their visibility. DR was able to detect holes on the order of 2 to 3 pixels in diameter. All three techniques were able to detect the 2T hole in the number 4 IQI while DR was able to detect it in the number 3 IQI. These hole sizes are near the limit where geometric magnification would be required. The better sensitivity of DR is probably a result of being able to minimize fixed structural noise. Presently, for materials thinner than 0.50 in., there is the option to use IQIs defined by either E1742 Annex A1 or E1025. It is recommended that inspection procedures require a particular sensitivity percent that would necessitate the use of a specific IQI.