Radiographic Inspection (RT)

A previous article pointed out that a visual inspection and surface NDE performed by MPI or LPI,�� are effective ways of inspecting welds. However, generally speaking, these particular techniques cannot detect discontinuities that are formed below the surface of a weld. For many applications, therefore, volumetric non-destructive testing is required and one of the traditional methods of performing this task is radiography.��

Since the discovery of X-rays and gamma rays in the 1890’s, Radiographic Inspection (RT) has been an invaluable aid in “seeing inside” structures like welds. To accomplish this, a source of radiation is placed on one side of a weld and a radiographic film, enclosed within a lightproof container, is placed on the opposite side of the weld in relation to the radiation source as shown in��Figure 1. After a calculated period of time, the radiation source is removed and the film is chemically processed, revealing the status of the weld.

An illustration showing radiographic inspection — Figure 1 The Film behind the Weld will darken more under the Void and the least under the Material that is Sound.

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Radiographic inspection can be performed with either X-rays or gamma rays which are both electromagnetic waves with short wavelengths. Short wavelengths enable the rays to travel through various materials such as steel and other metals.

X-rays are generated in a vacuum tube by propelling a stream of electrons through this vacuum against a target constructed of materials with high atomic numbers and high melting point. (Tungsten is a common targeting material.) The electron stream interacts with the atomic structure of the target material, temporarily dislodging electrons. Energy is generated from this dislodgement action, 99% of which is heat and 1% of which are X-rays. The heat is dissipated by the copper anode and cooling media in the tube housing. The X-rays are projected from the target material against the weld under examination as depicted in��Figure 2

The stream of electrons can be controlled by the electrical input and, as such, the wavelengths and penetrating power of x-rays can be changed to suit the application being tested.

A labeled illustration of an x-ray machine — Figure 2. Schematic of an X-Ray Machine for Industrial Radiography

Gamma rays emanate from a radioisotope commonly called a “source”. Small amounts of materials, such as cobalt-59 (Co-59) or iridium-191 (Ir-191), are subjected to high density neutron radiation in a nuclear reactor. During this time the nucleus of the materials capture a neutron and they become the isotopes cobalt-60 (Co-60) or iridium-192 (Ir-192). These isotopes are in an unstable condition. Thus, in the natural order of things, the material constantly strives to return to a stable condition and in so doing releases energy in the form of gamma rays.��

The wavelengths of the gamma rays are fixed by the isotope that emits them. Co-60 has wavelengths that are much shorter than Ir-192 and, therefore, Co-60 has more penetrating ability. These sources are constantly emitting radiation and cannot be shut off like x-rays. Therefore, they are shielded in a protective casing manufactured from dense material such as lead or tungsten. The material absorbs the radiation and protects personnel from exposure.

When the source is to be used, it is remotely handled. A drive cable is connected to the source pigtail and projected through a hose called a “nose tube” to a position from which the radiation passes through the object and onto a film. The drive cable is long enough to allow the technician to stay a safe distance away from the source while it is out of the shielded position.

An illustration showing a typical set-up for gamma radiography — Figure 3. Schematic of Isotope Camera, Nose tube, Drive Cable and Crank.

Figure 3��above illustrates a typical set-up for gamma radiography. The source is “cranked” out of the safe position and is deployed through the nose tube to a position inside the weldment. The “source” remains in position for a period of time calculated to produce an acceptable image on the film

As radiation is directed at the weld a certain amount will be absorbed by the structure of the metal and the remainder will pass through onto the film. A radiograph measures differences in thickness and density and expresses that difference as a gray scale image on a film. These differences in the amount of radiation passing through the weld appear on the developed film as light or dark shadows. The radiographic film is interpreted by evaluating the shape, density and location of the images created by slag, porosity, tungsten inclusions, etc.��

To be detectable the discontinuity must cause enough change in film density to be discernable by the naked eye. Certain flaws, depending on the plane in which they lie with respect to the direction of the radiation, can present so little difference in the amount of radiation absorbed that no detectable image will appear on the film.��

We can illustrate this by reference to��Figure 4 below which illustrates two discontinuities which are exactly the same but which lie in different planes. The left-hand void will be readily discernible in a film as it presents a 3mm round cylinder which is 25 mm long through which radiation will travel and darken the film below. The same defect on the right presents itself to the radiation as only a 3mm round tube, not 25mm as on the left.

An illustration showing how different defects can look under radiographic testing — Figure 4. Illustration of the same defect that will appear differently in a Radiograph due how it lines up with the incident radiation.

Where the beam of radiation is not directed into the plane of the defect, as shown in��Figure 4, the defect can be missed. Significant defects such as tight cracks and incomplete fusion may be missed if the defect does not lie in the right plane. This is the main limitation of radiography.

In recent years, due to the digital revolution, the need to expose and develop film in the traditional fashion is lessening and is being replaced by Digital Radiography

Digital radiography (DR) is an innovative form of radiography using radiation-sensitive plates to capture data for many fields. With DR, data immediately gets transferred to a computer and the technology allows for real-time digital transfers, making the images and information available for analysis within seconds.

Digital radiography cassettes use state-of-the-art photo-simulated screens to capture radiographic imagery instead of traditional films, which can be time-consuming and expensive

Some of the advantages of DR are:

Faster processing of data
No chemicals
Enhanced image storage and archiving
Increased productivity
Reduced safety hazard
Improved discontinuity evaluation in a majority of cases

Finally it must be noted that Industrial radiographers must be certified by the Canadian Nuclear Safety Commission (CNSC) to operate exposure devices and work with radioactive sources. They also must be certified under CAN/CGSB-48.9712, ISO 9712 to perform radiography.��

Both X and gamma radiation are dangerous and, the barriers put out by radiation workers to keep one away from the locus of operation must be strictly obeyed.

Please take the opportunity to view the referenced YouTube video for a short movie on the above subject.

Mick J Pates IWE

President

PPC and A

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