Radiography

Radiography is the most frequently used nondestructive test method. X rays and gamma rays passing through a structure are absorbed distinctively by flaws or inconsistencies in the material, so that cracks, voids, porosity, dimensional changes, and inclusions can be viewed on the resulting radiograph. 

X-ray radiography is similar to a chest X-ray because it measures X-ray absorption, which is a function of density and composition. Pores, cracks, inclusions, and undensified regions cause variations in density. The X-ray absorption of fully dense ceramics is significantly less than that of metals. Metallic inclusions in ceramics can therefore be seen easily, but the contrast of pores and cracks is low. As a result, X-radiography can image metallic inclusions in ceramics down to 25 Xm and cracks and pores down to 100 p,m. 

Resolution of cracks and pores exposed to the surface can be enhanced using radiation opaque penetrant dies. Opaque dies can also be used to differentiate between low-density inclusions and surface-connected pores. Suppose a surface-connected feature is observed that has lower X-ray absorption than the rest of the material. After it is soaked in dye, a pore will have a higher X-ray absorption than the background, while the absorption of a low-density inclusion will not change. 

100 % radiography for welded joints should be asked for when the heat exchanger is to be used for high-pressure, high-temperature or dangerous fluids, (e.g., petroleum 

The principle behind radiography involves the measurement of attenuation of radiation as it passes through the material under investigation. The radiation is provided by a focussed source directed at one side of the structure which is detected /recorded on the other side by photographic film or a detection plate. 

The resulting images can be produced relatively quickly to allow preliminary interpretation on site. 

As the radiation is transmitted through the concrete it interacts with the atoms in the material. This interaction causes some of the radiation to be absorbed by the concrete and some to be scattered. The result is that the amount of radiation detected varies spatially depending on the density and nature (atomic number) of the material(s) of the structure under investigation. Therefore, variations in the concrete (such as cracks, voiding and compaction variation) will be caused by variations in density and the location and extents of inclusions (such as ducts and steel reinforcement) will be produced by variations in material types. 

Multiple layers of reinforcement will be superimposed and may limit interpretation. 

Although interpretation can be straight forward the specialized equipment and safety considerations require expert operators. 

As an alternative to ultrasonics, radiography may be used to examine the internal condition of process pipework and supplement the information on fluid corrosivity received from other monitoring methods. It is particularly useful for the examination of preferential corrosion at weldments and erosion at bends, but the limited accuracy renders it suitable only for the detection of significant changes in 

Due to the absorption of the incident radiation by liquids, as well as the pipe wall, its use on-line is limited to small-diameter process streams containing vapors or gases. Under normal circumstances radiography is impractical for examining pipework larger than 8 inches in diameter. 

Low energy radiation can penetrate reasonable thicknesses of a polymer matrix composite and 1 MeV X-rays are able to penetrate 500 mm of polymer. The efrp have low X-ray absorption and consequently, low voltage, low energy X-rays have to be used in conjunction with high contrast recording techniques, which can then detect transverse cracks and inclusions. Unfortunately, carbon fibers and resin matrices have similar mass absorption coefficients and cannot be differentiated. Glass, on the other hand, can be readily detected and can be used in composite construction as a radio-opaque tracer, showing the direction of fiber lay-up. 

Flgnre 15 CT scan images of CFRP. (a) A woven J-shaped structure and (b) porosity in laminate. 

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Radiography is less effective for detecting arbitrarily oriented planar defects (such as tight cracks or lack of fusion) in thick metal sections unless the likely location and orientation of cracking is known in advance. 

The principles of gamma-ray radiography are the same as X-ray radiography except that a radioisotope is used as the radiation source rather than an X-ray tube. 

It takes time to train a film interpreter. In addition, human interpretation of weld quality based on film radiography is very subjective, inconsistent, labour intensive, and sometimes biased. It is thus desirable to develop some forms of computer-aided systems. 

Sandborg, M. and G. Alm-Carlsson, Influence of x-ray energy spectrum, contrasting detail and detector on the signal-to-noise ratio (SNR) and detective quantum efficiency (DQE) in projection radiography. Phys. Med. Biol., 1992. 37(6) p. 1245-1263. 

The visual information provided by radiography images is complex, considering of the great number of disruptive factors. Processed images ate characterized by three particular phenomena ... 

F.Y. BRIAND al. Segmentation de defauts dans des images de radiographies industrielles, 1988, Revue du Traitement du signal. Vol5 N°4.. 

The last ASTM Standard E94 which classified Industrial Radiography films was published in 1984. Since that time inspection contracts in the U. S. and worldwide have continued to specify Type 1 and Type 2 films which no longer existed. Film manufacturers continued to classify their film using the same nomenclature. 

For wall thicknesses of typically 2-30 mm of steel Selenium has meanwhile become well accepted in international radiography with the clearly legible trend to be the preferential choice in the near future. 

Table 1 Characteristic attributes of the isotopes used in gamma radiography of weldments.

A further advantage is the Selenium halflife of 120 days, which is 60% more when compared to iridium and a factor of approx. 4 when compared to Ytterbium. These differences turn out to be an important economical aspect when comparing the different sources, as they are a direct measure of the useful life of sources. The short halflife and the very high costs for Ytterbium sources have been the main factors for the rather low importance of Ytterbium in the full range of gamma radiography. 

Practical experience has shown that, depending on the field of application, a considerable reduction in inspection costs can be had when opting for radioscopy rather than radiography. By comparison with film technique, the inspection time of turbine blades for aircraft jet propulsion engines is reduced by 45% to 60%. When adding film costs, approximately DM 450.000,- can be saved per year /3/. As far as... 

However, in various industrial sectors the application of the radioscopic inspection technique is aggravated by a lack of the respective standards, contrary to radiography. This leads to complicated approval bureaucracy by the respective supervisory or certificatiomn authorities. 

Contrast Sensitivity For monitoring purposes, a step wedge is to be used that consists of the same material as the workpiece. In addition, IQIs according to EN 462-1 or EN 462-2 - DIN wire penetrameters or step-hole IQIs - can be applied to ensure a comparability with radiography. 

The standard will make a substantial contribution toward the achievement of the goal "Radioscopy - as much as possible radiography - as much as necessary . 

Suitability of Gadolinium Based Systems for X-Ray Real Time Radiography. 

Real Time Radiography (RTR) is an advanced method of radiography in which the image is formed while the job is exposed to ionising radiation. RTR is often applied to objects on assembly lines for rapid inspection. Accept-or-reject decisions may be made immediately without the delay or expense of film development. The main advantages of RTR are thus, reduction in inspection cost and processing time. 

Compared with now used industrial film radiography our computerized X-ray TV introscope has higher sensitivity. The efficiency of our X-ray TV testing method is 15-2(1 times... 

GOST 23055-78. Russian standard. "Classification of welded joints by results of radiography testing and corresponding production forms and records". 

Industrial Application of Computed Radiography with Luminescence Imaging Plates. 

Computed radiography with Luminescence Imaging Plates (IP) has become a routine method in medical applications. It is a new medium for filmless radiography. Since the last five years several tests were performed to check this method for industrial NDT [1-3]. ASTM already issued a proposal for a standard. 

Luminescence IP s are fundamentally and theoretically utilizable for nondestructive testing if all of the recommended guidelines and operational recipes and appropriate systems are used. They are quite capable of producing image quality on a par with industrial radiography. 

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