PROCESS CONTROL TESTS
Whether equipment has the capacity to detect and show critical discontinuities is an important consideration in RTR. The American Society for Testing Materials (ASTM) E1255 indicates that the system shall be checked each day, a log maintained and a system requalification required when image quality is diminished. Equipment is usually much more sophisticated than that used for film radiography, and it must be ascertained that equipment is performing as it should. ASTM E1411 is a standard practice that can be used initially to qualify and later to requalify a system by determining how it performs in a static mode-this means that the test object and the radiation beam are stationary. Many different equipment configurations and arrangements are possible with RTR, and ASTM E1411, addresses the elements of an RTR system operating together, rather than evaluating them individually. The procedures intent is to obtain benchmark performance data at certain settings and a standardized test part. Because inspection of actual parts involves different variables, results differ. Operations differ in their ability to obtain optimum results from the system. Also, as a system ages, its performance diminishes because various materials degrade, and the cesium iodide input screen is particularly prone to fading in the image intensifier. Obtaining an image of quality indicator when the system is new (a so-called golden image) is important because this image can be used for later comparison.
Measuring the focal spot of the X-ray tube can be done in accordance with ASTM E1165. The caveat is that this procedure may not produce useful results when the nominal focal spot is smaller than 0.3 mm (300 microns); with minifocus and microfocus X-ray tubes, this procedure is not recommended, for geometric unsharpness greatly influences image quality. ASTM E1161 indicates that the film must be sufficiently fine-grained to resolve discontinuities down to 0.025 mm (25 microns), or .001 inches. When RTR is evaluated for use in locating small discontinuities, it is necessary to determine whether image magnification is necessary. If magnification is used, the potential for geometric unsharpness increases greatly unless extremely small focal spots (microfocus X-ray tubes) are used. To determine X-ray tube resolution, the LP gauge is used, with values expressed in a certain number of LP/mm. Another test device, made by Victoreen and consisting of a grid pattern also can be used. As indicated in ASTM E1255 section 6.24, spatial resolution and contrast sensitivity can be checked with LP test pattern and step, respectively. The step wedge needs to be made from the same material as a test part and in various thicknesses. (Refer to ASTM E1255 section 6.24.)