Radiographic inspection is accomplished by passing radiation through an object and onto a recording medium. The result is a two-dimensional image of a three-dimensional object, much like casting a shadow. The final radiograph must meet certain quality and density requirements. Radiographic quality is demonstrated by means of an image quality indicator (IQI), a separate device positioned on each radiograph. IQI’s are also referred to as penetrameters. The density or darkness of the radiograph must be within a specific range to ensure that flaws will be visible to the film reader.
The radiation used is either x-radiation or gamma radiation. X-radiation is produced electrically in an x-ray tube and is familiar to people since they encounter this with medical or dental procedures. Gamma radiation is emitted by radioactive materials and is utilized more commonly in field radiography since it does not require a power source. Radioactive materials and gamma radiation are also used in medical treatments.
X-rays were discovered accidentally in 1895 by Wilhelm Conrad Roentgen. Studies done with Crookes tubes, a predecessor of the cathode ray tube (CRT), caused certain chemicals and photographic plates to be affected when these tubes were energized. This energy or radiation was labeled as x-radiation because it was an unknown form of energy. The name was never changed. About the same time Marie and Pierre Curie discovered a similar form of energy being emitted by radium ore.
These forms of radiation are similar in their effects. One major difference is that x-radiation can be turned off, but gamma radiation cannot because it is being continuously emitted by the radioactive material.
This method of inspection can be used on virtually any material, metallic and nonmetallic, since radiation can penetrate all forms of matter. It can be used to inspect castings, forgings or weldments. The results of radiographic inspection are most commonly viewed on a radiograph or x-ray which becomes a permanent physical record of the inspection. Digital and real-time radiographic techniques are becoming more common more widely used. These allow the images to be viewed and stored electronically.
Radiographic inspection is limited by the overall part configuration and the equipment capabilities available. The relationship between the x-ray beam direction and flaw orientation determines the detectability of flaws. Due to safety considerations, radiography is typically performed in a cabinet or a vault to shield the operator from radiation exposure.
Radiography is one of the more expensive methods of inspection. Contributing factors are the radiographic film which contains silver, chemicals used for processing the radiographs, hazardous waste disposal, and licensing issues related to the radiographic equipment.