27 Radiographic Testing (RT)
X-rays! Gama-rays!
- Similar concept to getting an x-ray at the doctors or dentist
- Moderately expensive
- Requires lots of training
The test material absorbs radiation, but less absorption takes place where there is a void, leading to darker areas on the processed radiograph.
Radiographs
A radiograph is a permanent, visible image on a recording medium produced by penetrating radiation passing through a material being tested. Image distortion occurs when the plane of the part and the plane of the film are not parallel. To minimize image distortion, the radiation beam must be directed in a direction perpendicular to the plane of the film. If distortion of the film image is unavoidable, the radiographer must take into consideration that all parts of the image are distorted; otherwise, the radiograph may be incorrectly interpreted.
Radioisotopes
Radioisotopes have different ranges of energy, making them suitable for different thicknesses of metals.
Gamma rays are produced from portable sources and are used extensively for field-testing of welds. The gamma ray source is made as small as possible in the shape of a cylinder whose diameter and length are approximately equal. The cylindrical shape permits the use of any surface of the source as the focal spot since all surfaces, as viewed from the test specimen, are equal in area. The wavelength of the gamma rays (energy level) is determined by the nature of the source. Gamma rays have different ranges of energy and different thickness limitations for materials examined.
X-Rays
The penetrability of X rays from the X-ray machine into the part depends on the voltage applied across the elements of the X-ray tube. Maximum voltages are established based on the thickness of the metal to be tested. The wavelengths of X radiation are determined by the voltage applied between the elements of an X‑ray tube. Higher voltages produce X rays of shorter wavelengths and increased intensities, resulting in deeper penetration capability. The penetrating ability of X rays depends on the X‑ray absorption properties of the particular metal.
Image Quality Indicator
An image quality indicator (IQI) is a device or combination of devices whose demonstrated image determines radiographic quality and sensitivity. The image or images demonstrated by an IQI provide visual data, quantitative data, or both to determine the radiographic quality. An IQI is not intended for use in judging size of, or acceptable limits for, discontinuities. An IQI is also called a penetrometer, or penny. Each IQI is identified by an identification number that gives the maximum thickness of material for which the IQI is normally used. Each IQI, or penetrometer, shows a lead number that identifies the thickness of the IQI in thousandths of an inch. Additionally, an IQI has three holes drilled through it, each to a specified diameter.
Shim stock may be used to compensate for the additional thickness of a weld compared with the base metal. Shim stock is sometimes used in RT of welds because the area of interest (the weld) is thicker than the part thickness. Shims are selected so that the thickness of the shim(s) equals the thickness added to the specimen by the weld in the area of interest. Shim stock is placed underneath the IQI, between it and the part. In this way, the image of the IQI is projected through a thickness of material equal to the thickness in the area of interest. The shim stock length and width are greater than those of the IQI.
Single-Wall RT
Single-wall RT for plate, pipe, or tubing welds is relatively simple to achieve because the critical areas of the weld are clearly defined in terms of their length, width, and thickness. The film is placed on the side opposite the source with an exposure angle of 90°. Single-wall RT should be used whenever possible for flat or circular objects. Subject contrast is small and exposure calculation is relatively simple.