Radiographic testing

The radiographic examination is an imaging method of non-destructive material testing ( NDT) to display material differences. With a suitable reflector ( an X-ray tube, a particle accelerator or a gamma-emitting radionuclide such as cobalt 60, iridium -192 or selenium -75), the density of a component is imaged on an x-ray film. A projection image of the component displayed on the radiographic film. The different material thickness or density can be seen at different darkness. The thicker or denser a component, the less radiation can penetrate it and the brighter the X-ray film appears.

Application

The X-ray and gamma radiography is a non-destructive material testing. It is used for error detection in the interior of components. In particular, at welds of metal sheets, pipes and vessels. The most common errors are cavities, pores, cracks and segregation. This error can be easily recognized, radiation intensity, wavelength of the beams, the thickness of the component and exposure time must be coordinated. Radiographic testing ( RT abbreviation acc. EN 473 ) is suitable for the detection of volume adhere errors. If there are differences in density between defect and base material of the error is detected. Also, hairline cracks can be found at a suitable angle of incidence. Contrast and resolution affect the recognition of such details. The contrast depends on the material thickness, density, mass and atomic number of the material of the radiators quality / level of energy, as well as the resolution power and the type of the film.

Particular, it is a standard method for testing of safety -related components, for example, welds ( DIN EN 12517-1 ), as well as safety-related castings ( EN 12681 and EN 444), for example in power stations.

To evaluate the image quality maps (DIN EN 462-1 ) are placed with seven wire webs of different widths on the exposed part. The wire diameter is stepped to 1.25mm. Based on the thinnest yet to be recognized wire can be concluded that the smallest detectable flaw size.

Properties

X-rays and gamma rays are electromagnetic waves. Physically they are like the light, but have much smaller wavelengths and therefore higher frequencies. To the small wavelengths, the ability to penetrate between the atoms of the material and to penetrate with a sufficiently high energy (frequency) and is based (condition: the wavelength must be less than the distance between the atoms in the crystal lattice to be). Penetrating they are then attenuated to varying degrees by mistake and thus the radiation emerging shows intensity differences. They penetrate steel up to about 300 mm, 400 mm and light alloy copper up to 50 mm. The penetration of X-rays and gamma rays, the better the smaller the density of the component, the wavelength of the beam and the greater the frequency. Gamma rays have higher penetration depths, because they are short-wavelength.

Generation of radiation

Gamma rays: gamma rays resulting from the nuclear decay of radioactive elements. The principal gamma emitters for material testing are Natural ( radium, radon, mesothorium ) and Artificial ( cobalt, tantalum, cesium, thulium ). Radiation source is the actual isotope ( elements with the same number of protons but a different number of neutrons) the cylindrical and about 0.5 - 6 mm. Because gamma rays can not be " turned off " it is a gas-tight in the source capsule with W - shield (inside) and Pb shielding ( outside) included, so that the radiation can not escape on all sides. Since it is much smaller than an X-ray tube, it can also bring closer to the DUT, such as for the isotopic Newt, a device that is pulled to weld inspection on construction sites by pipes. X-rays: X-rays are produced in X-ray tubes. There, electrons are emitted from incandescent tungsten wires ( hot cathode ) and accelerated by a high voltage electric field in highly evacuated tubes. There they meet a tungsten plate and are slowed down by the electron shell of the W atoms. This gives rise to X-rays. X- rays are produced when fast electrons impinge on solid bodies and are braked. The wavelength is derived from the anode material (copper) and from the applied voltage. It is with increasing voltage shorter and therefore more penetrating ( = harder ). Soft and hard X-rays are categorized according to their penetrating power.

Implementation

From the radiation source, the silhouette of the test piece is thrown on a photographic layer. The arrangement consists of source, by -irradiated sample and recording device. For example, if a void or an inclusion is present in the sample with a lower density than that of the sample of the underlying film is blackened stronger. However, if an inclusion of higher density material is present, then the film is behind less blackened. The quality of the film is controlled in accordance with DIN EN 462-1:1994 with wires of different thickness. The wires are placed on the sample and will be featured on the film as bright stripes. On the film, the wires must the thickness of the test piece were assigned to be visible. In carrying out the X-ray or gamma extensive examination Radiation protection regulations (DIN 54113, DIN 54115 ) are observed.

Error detection capabilities

Movies: Meet The material emerging from the rays on a double- coated film film that is covered on the back with lead screens to keep out stray radiation. Intensity differences translate into the blackness of the film. By varying degrees of blackening of the film we see the geometrical shape and the position of the error. The movie recording is possible with X-rays and gamma rays. Application: control of welds and castings with thicknesses up to 100 mm (steel) and 400 mm (Al ); Revision studies in boilers, bridges and aircraft.

Fluorescent screen: X- rays excite certain crystals to deliver visible green - yellow rays on. A coated by these crystals plate forms the fluorescent screen. A shadow image of the specimen appears on him, but with less intensity. Error with low density are darker on the shadow image brighter, errors with a higher density. The observer must be protected by lead glass front of the scattered radiation. Application: The screening with X-rays, up to 20 mm is applicable for steel thicknesses. In addition, for light metals and plastics.

X-ray image - intensifier tube: With the help of electronics, the X-ray - are reduced and enhanced phosphor screen image. It then increased a 10 ³ times the brightness on the monitor and can be transmitted television technology. Observers can then sit in a radiation-proof room. Application: examination of longitudinal and spiral welded pipes.

Standards for radiographic examination

  • DIN EN 444, Non destructive testing - fundamentals for radiographic examination of metallic materials by X-and gamma rays
  • DIN EN 462, Non-destructive testing - Image quality of radiographs
  • DIN EN 584, Non destructive testing - Industrial radiographic film for radiographic examination
  • DIN EN 1330-4, Non-destructive testing - Terminology - Part 3: Terms used in industrial radiographic testing
  • DIN EN 1435, Non-destructive examination of welds - Radiographic examination of welded joints (expected in early 2011 replaced by DIN EN ISO 17636-1 and EN ISO 17636-2 )
  • DIN EN 12681, Founding - Radiographic testing
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