The discovery of X-rays and the phenomenon of radioactivity and their applications to the examination of objects provided the starting point for the advancement of industrial radiography. This technique is one of the most widely used for the detection of internal defects such as voids and porosity. Planar defects can also be detected by radiography with some proper orientation. Radiography is also suitable for detecting changes in material composition, thickness measurements and locating unwanted or defective components hidden from view in an assembled part.
Radiography makes use of the ability of short wavelength electromagnetic radiations such as x-rays or gamma rays to penetrate objects. The shorter the wavelength, the greater is the penetrating power. The amount of radiation that is absorbed by the material is a function of the density and thickness of the material. In the event of the presence of a cavity or discontinuity the beam of radiation will have less material to pass through than in solid material. As a consequence, there will be variation in the absorption of rays by the material in the defective area. This variation is measured and recorded on a film sensitive to x-rays or gamma rays. An image is produced on the film that indicates the presence of the defect. The image obtained is an X-ray shadow of the interior of the material under examination. In summary, radiography is based on the principle of shadow projection and such a shadow picture is known as a radiograph. The basic setup consists of a source of radiation, the object to be radiographed and a detector, which is normally a sheet of photographic film.
The choice of the appropriate technique is made on the basis of geometry, size, sensitivity requirements and available and appropriate testing space. The techniques for various engineering components for radiographic inspection are given below.
Single Wall Single Image Technique: This technique is used when both sides of the specimen are accessible. This is used for plates, cylinders, shells and large diameter pipes. The source is kept outside and the film inside or vice versa and the weld is exposed.
Double Wall Penetration Technique: This technique is used when the inside surface of the pipe is not accessible. The source of radiation and the film are kept outside. The radiation penetrates both the walls of the pipe.
Fluoroscopic Techniques: Fluoroscopy involves the real time viewing of X-rayed parts. The quality of the inspection technique depends on the resolution of the system and skill of the operator/interpreter. The X-ray fluoroscope is a fixed machine operating under typical dark room lighting conditions. To obtain satisfactory results geometry, scatter radiation, and source kilo-voltage must be carefully controlled. Advantages of fluoroscopic inspection include the immediate viewing of the subject from a multitude of angles, the illusion of a three dimensional presentation and the ability to study moving parts in action.
Radiography can be used to inspect most types of solid materials both ferrous and non-ferrous alloys as well as nonmetallic materials and composites.
Radiography is well suited for the inspection of semiconductor devices for detection of cracks, broken wires, unsoldered connections, foreign material and misplaced components. Other NDT methods are limited in their abilities to inspect semiconductor devices.
Radiography is extensively used for checking castings, weldments and need to be free from internal flaws.
Radiography is used for liquid level measurement in sealed components.
Merits and demerits of radiography
The basic advantage of radiography is that objects that range in size from a miniscule electronic part to a mammoth missile or a power plant structures can be examined. Further the method can be used on a variety of materials. No prior preparation of the specimen surface is necessary unlike other NDT methods.
Radiographic methods however have certain limitations.
Certain types of flaws cannot be detected. For example cracks cannot be detected unless they are parallel to the radiation beam.
Tight cracks in thick sections usually cannot be detected at all even when the radiation beam is properly oriented.
Minute discontinuities such as flakes, micro porosity and micro fissures cannot be detected unless they are sufficiently large in size.
The defect or discontinuity must be parallel to the radiation beam or sufficiently large to register on the radiograph. TO be detected, a defect must be at least 2% of the thickness of the material before it can register on a radiograph with sufficient contrast.
Certain areas in many items cannot be radiographed because of the geometric considerations involved. Often it is difficult if not impossible to position the film and source of radiation to obtain a radiograph of the area desired.
Protection of personnel, not only for those engaged in radiographic work, but also those in the vicinity of the radiographic inspection site, is of major importance. Safety requirements impose both economic and operational constraints on the use of radiography for inspection.
Compared to other methods of NDT radiography is expensive. Capital costs can be low only when potable X-ray or gamma ray source is used.
An inspection of thick sections is a time consuming process.
Radioactive sources also limit the thickness that can be inspected, primarily because high activity sources require heavy shielding for protection of personnel.
Methods for exposure control
There are three basic methods to control exposure when working with radiography sources:
Time: Limit personnel time. Proper training, adopting faster time efficient work techniques, mock up practices and efficient administrative control, can reduce time of handling a radiation source.
Distance: Maximize personnel distance. The radiation intensity decreases with distance on the basis of inverse square law. Therefore, equipment design and operating procedure must take into account this factor.
Shielding: A third important way to reduce exposure is to place a shield between the operator and the source of radiation. The more dense the shielding materials, the more effective they are in bringing down the levels of exposure to X-rays and gamma rays. Commonly used shielding materials in radiographic installations and equipment are steel, lead, concrete and depleted uranium.