Radiographic testing (or famously known as R.T.) is a process where penetrating radiation beam passes through a test object. The transmitted radiation is then collected by a form of sensor that is capable of measuring the relative intensities of the penetrating radiations imposing upon it. This sensor is called the radiographic film.
Nowadays, radiographic testing implies modern sensors such as the real-time radiography (like the one you see in the security check area at airports) and computer radiography that digitally stores the radiographs films.
Although the use of these devices is on the rise, radiographic film is still a popular choice in the industry.
The Penetrating Radiation
The sources listed below generate penetrating radiation;
- The high energy electron beams and X-rays, or
- Nuclear disintegrations (atomic fission), or popularly known as gamma rays.
However, there exist other forms of penetrating radiation but they are not used in weld radiography.
Generally, X-rays that are used in the industrial radiography of welds have photon energies in the range of 30keV to 20MeV. Dependent upon the output, a conventional X-ray tubes which generate up to 400keV may be suitable for a portable or fixed installation. However, X-ray tubes which generate more than 400keV will be less portable because it utilizes large and heavy devices such as betatrons and linear accelerators.
All sources of X-rays produce a continuous spectrum of radiation. They reflect the spread of kinetic energies of electrons within the electron beam. Low energy radiation is easily absorbed and its presence within the X-ray beam gives a better radiographic contrast. Therefore, it will produce better radiographic sensitivity compare to using gamma-rays.
Conventional X-ray units are capable of performing high-quality radiography on steel thickness up to 60mm and up to 300mm for betatrons and linear accelerators.
The early sources of gamma rays used in industrial radiography were naturally occurring radium. The sources activity was low but they were widely accepted by modern standards. However, the radiographs produced were not of a particularly high standard.
The naturally occurring Radium fission reaction produces radioactive radon gas which is extremely hazardous to the user. Nowadays, it is possible to artificially produce much higher specific activity isotopes that do not produce hazardous fission products.
Gamma sources do not produce a continuous distribution of quantum energies when compared to X-ray sources. For instance, gamma sources produce a number of specific quantum energies unique for any particular isotope.
The four common isotopes use for the radiography of welds (in ascending order of radiation energy) are:
- Thulium 90
- Ytterbium 169
- Iridium 192 and
- Cobalt 60.
Thulium 90 is useful in penetrating steel up 7mm thickness. Its energy is similar to 90keV X-rays. Due to its high specific activity, useful sources can be produced with a physical dimension of less than 0.5mm.
Ytterbium 169 only recently become available as an isotope for industrial use. Its energy, which is similar to 120keV X-rays is useful for the radiography of steel up to approximately 12mm thickness.
Iridium 192 is probably the most frequently used isotopic source of radiation in the radiographic examination of welds. It has a relatively high specific activity and output sources with a physical dimension of 2-3mm in common usage. Its energy is approximate which is equivalent to 500keV X-rays is useful for the radiography of steel of 10-75mm thickness.
Cobalt 60 has an energy similar to 1.2MeV X-rays and they are heavy with large suitable source containers. Therefore, Cobalt 60 sources are not fully portable. They are useful for the radiography of steel in 40-150mm of thickness.
Isotopes Vs. X-rays
The major advantages of using isotopic sources over X-rays are:
- Increased portability.
- No need for a power source.
- Lower initial equipment costs.
Meanwhile, the major disadvantages are:
- The quality of radiographs produced by gamma-ray techniques is inferior to those produced by X-ray
- Hazards to personnel may be increased (if the equipment is not properly maintained or if the operating personnel have insufficient training)
- Due to their limited useful lifespan, new isotopes have to be purchased on a regular basis so that the operating costs may exceed those of an X-ray source.
The Radiography of Welds
Radiographic techniques work by detecting the differences in absorption of the beam. This means that the defective areas will be revealed when the effective thickness of the test object is changed.
The radiographic techniques can easily detect volumetric weld defects such as slag inclusions and various forms of gas porosity. This is due to the large negative absorption difference between the parent metal and the slag or gas. Although there are certain cases when the radiographic technique can’t detect slag as it absorbs more radiation than the weld metal.
Radiography can’t detect planar defects such as cracks, lack of sidewall or inter-run fusion. This is because the defects may cause little or no change in the penetrated thickness. This lack of sensitivity to planar defects makes radiography unsuitable where a fitness-for-purpose approach is taken when assessing the acceptability of a weld. In this case, NDE techniques such as ultrasonic testing are preferable.
Film radiography produces a permanent record of the weld condition which can be archived for future reference. It also provides an excellent means of assessing the welder’s performance so it is often still the preferred method for new construction.
Radiographic Testing Advantages and Limitation
|Permanent record||Health hazard. Safety (important)|
|Good for sizing non-planar defects/flaws||Classified workers, medicals required|
|Can be used on all materials||Sensitive to defect orientation|
|Direct image of defect/flaws||Not good for planar defect detection|
|Real-time imaging||Limited ability to detect fine cracks|
|Can be positioned inside a pipe (productivity)||Access to both sides required|
|Very good thickness penetration||Skilled interpretation required|
|No power required with gamma||Relatively slow|
|High capital outlay and running costs||Isotopes have a half-life (cost)|
There you have it, a short info about radiographic testing. Do you have anything to add? Please let us know in in the comment section below.
And please share if you liked this article.