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Radiographic Testing

Radiographic Testing - RT

Radiographic Testing is one of the most widely used NDT method to inspect the flaws in engineering components, Castings, Weld Joints and Structures. The method is much suitable for finding volumetric discontinuities.

Radiographic testing usually requires exposing film to X rays or gamma rays that have penetrated a specimen, processing the exposed film, and interpreting the resultant radiograph.

X rays and gamma rays possess the capability of penetrating materials, even those that are opaque to light. In passing through matter, some of these rays are absorbed. The amount of absorption at any point is dependent upon the thickness and density of the matter at that point; therefore, the intensity of the rays emerging fron1 the matter varies. When this variation is detected and recorded, usually on film, a means of seeing within the material is available. Radiography consists of using the penetration and differential absorption characteristics of radiant energy to examine material for internal discontinuities.

X-Rays

X rays and electromagnetic waves of lower energy are generated when rapidly moving (high-energy) electrons interact with matter. When an electron of sufficient energy interacts with an orbital electron of an atom, a characteristic X-ray may be generated. It is called characteristic because its energy is determined by the characteristic composition of the  disturbed atom. When electrons of sufficient energy interacts with the nuclei of atoms bremsstrahlung (continuous X rays) are generated. They are called continuous because their energy spectrum is continuous and is not entirely dependent upon the disturbed atoms' characteristics. To create the conditions required for the generation of X rays, there must be a source of electrons, a target for the electrons to strike, and a n1eans of speeding the electrons in the desired direction.

Digital Radiography

30% of all film radiography could be replaced by today's technologies in the field of digital radiography. Only few of these applications have indeed replaced film. The choice to go digital depends on cost, quality requirement, workflow and throughput. Digital images offer a lot of advantages in terms of image manipulation and workflow. But despite the many advantages, a lot of considerations are needed before someone can decide to convert his organization from conventional to digital radiography. This paper gives an overview of all different modalities that can be used in digital radiography with today's technologies, together with the experiences of the pioneers of digital radiography. Film Scanning, Computed Radiography and Direct Radiography by using of different kinds of flat panel detectors all have their specific application fields and customers. What is the status of the technology today, which advantages brings digital radiography, and which are the limitations radiographers have to consider when replacing film by digital systems.

CR

Computed radiography uses a reusable imaging plate in place of the film. This plate employs a coating of photostimulable storage phosphors to capture images.

When exposed to X-rays, electrons inside the phosphor crystals are excited and trapped in a semi-stable higher-energy state. The CR reader scans the plate by means of a laser beam. The laser energy releases the trapped electrons, causing visible light to be emitted. This light is captured and converted into a digital bit stream which encodes the digital image.

No more retakes

The storage phosphors on the Imaging plate have an extremely wide dynamic range. This gives a high tolerance for varying exposure conditions and more degrees of freedom in selecting the exposure dose. As a consequence, the need for retakes is drastically reduced.

Dose Reduction 

The wide exposure latitude of the Imaging plates allows, in many cases, the visualization of all diagnostic information with only one exposure. In this way, the use of Imaging plates results in a substantial reduction of the dose load. Also the fact that the sensitivity is about 10 times higher than the sensitivity of conventional film results in shorter exposure times and thus significant dose reduction.

Long Lifetime

NDT Imaging plates are protected by an EBC (electron-beam-cured) top coat. EBC top coating is an Agfa-proprietary technology for hardening a prepolymer lacquer coat into a high-density polymer on top of the phosphor layer. This results in plates with superb protection from mechanical wear and extensive immunity to chemical cleaning solutions. Superior durability of the RADView Imaging plate is thus secured.

Image Quality 

The recent efforts done for medical CR systems to be used in mammography applications, have also found their way into the NDT applications. Where until recently the image quality could be compared with D7 or D8, are we now comparing to D5 and even D4 image quality.

Equipment Used

GE – 200KV/320KV X-ray Tube

Portable X-Ray Unit- XRS-3

Applications:

Because of the penetration and absorption capabilities of X and gamma radiation, radiography is used to test a variety of non-metallic products and metallic products such as welds, castings, forgings, and fabrications. Since it is capable of revealing discontinuities (variations in material composition, or density) in a variety of dissimilar materials, radiographic testing is one of the primary nondestructive test methods in use today.

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