Radiographic examination

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Radiographic image of the ceremonial sword from the Essen Cathedral Treasury , which reveals a damascene (grid pattern) hidden under rust .

The radiographic examination is an imaging method of non-destructive material testing (NDT) for the display of material differences. With X-rays or gamma radiation (one from a suitable source X-ray tube , an electron accelerator with X-ray target or a gamma-emitting radionuclide ), the density of a component on an X-ray film ready. A projection image of the component appears there. The different material thickness or density can be recognized by the degree of blackening. The thicker or denser a component, the less radiation it can penetrate and the brighter the area appears on the X-ray image.

application

X-ray and gamma radiation is a non-destructive material test for the detection of defects in the interior of components, especially on weld seams of sheet metal, pipes and containers. For testing safety-relevant components, e.g. weld seams (DIN EN ISO 10675-1) as well as safety-relevant cast parts (DIN EN 12681: 2003-06 and DIN EN ISO 5579: 2014-04) e.g. B. in power plants it is a standard procedure.

The most common defects are cavities, pores, segregations and cracks. In order for these to be easily recognizable, the radiation intensity, wavelength of the rays, thickness of the component and exposure time must be coordinated with one another. The radiographic test (abbreviation RT according to DIN EN ISO 9712) is suitable for the detection of volumetric errors. The defect can be proven through differences in density between the defect and the base material. Even fine cracks can be found at a suitable angle of incidence. Contrast and resolution influence the recognition of such details. The contrast depends on the material thickness, the density, the material, the emitter quality / energy intensity as well as the resolution and the type of film.

To assess the image quality, cards (so-called wire web image quality test pieces according to DIN EN ISO 19232-1: 2013-12) with seven wire webs of different diameters are placed on the exposed component. The diameters of the wires range from 0.05 mm to 3.2 mm and are divided into 19 individual diameters according to the standard. The smallest detectable defect size can be deduced from the thinnest wire that can still be recognized.

properties

X-rays and gamma rays are electromagnetic waves. Physically they are similar to light, but have much smaller wavelengths and therefore higher frequencies. The ability to penetrate between the atoms of matter and also to penetrate with sufficiently high energy (frequency) is based on the small wavelengths (condition: the wavelength must be smaller than the distance between the atoms in the crystal lattice). When they penetrate, they are weakened to different degrees by errors and the emerging radiation shows differences in intensity. They penetrate steel up to about 300 mm, light metal up to 400 mm and copper up to 50 mm. The penetration capacity of X-rays and gamma rays is higher, the lower the density of the component, the wavelength of the rays and the higher the frequency. Gamma rays have i. A. greater penetration depths because they are shorter-wave.

Generation of the rays

The gamma emitters for material testing are, for example, 60 cobalt , 192 iridium , 75 selenium and 137 cesium .

The isotopes are available in specimens about 0.5 - 6 mm in size. The preparations are mechanically moved out of the shielding when they are supposed to emit radiation. They are sealed in the source capsule with tungsten - shield (s) and lead - or uranium -Immunity (outside) is included so that the radiation can not escape all sides. Since the radiation source is much smaller than an X-ray tube, it can be brought closer to the test object, e.g. B. the isotope pig , a device that is pulled through pipes to test welds on construction sites. The fact that these test procedures work without a power connection is also an advantage.

X-rays are generated with X-ray tubes or, at high energies, with electron accelerators , see X-rays . The quantum energies are much higher than in the medical field, so the shielding and safety measures are considerable.

execution

The radiation source projects the shadow image of the test piece onto a radiation-sensitive layer. The arrangement consists of a radiation source, irradiated sample and recording device. If there is a cavity or an inclusion of reduced density, the intensity of the radiation behind it is higher than in other places; behind an inclusion with a higher density it is lower. Comprehensive radiation protection rules (DIN 54113, DIN 54115) must be observed.

Imaging process

Checking a pipe construction for reduced wall thickness

X-ray film

The rays emerging from the material hit a double-coated film foil, which is covered on the back with lead foil to keep stray rays away. Differences in intensity translate into differences in the blackness of the film. The different degrees of blackening of the film show the geometric shape and the location of the defect. Film recording is possible with X-rays and gamma rays.

Application: Control of weld seams and cast parts with thicknesses up to 100 mm (steel) and 400 mm (Al); Revision investigations in boiler, bridge and aircraft construction.

Luminescent screen

X-rays stimulate certain crystals to emit green-yellow light. A plate coated with an appropriate powder serves as a fluorescent screen. A shadow image of the test object appears on this, but with a low light intensity. Low density defects are lighter on the silhouette, and higher density defects are darker. The observer must be protected from the scattered radiation by lead glass.

X-ray fluoroscopy is applicable for steel thicknesses up to 20 mm, for light metals and plastics.

Image intensifier

With the help of electronic cameras or residual light intensifier tubes, the X-ray fluorescent screen image can be reduced and amplified in order to reduce the radiation exposure of the examination subject. The camera image can be transmitted by cable so that the observer can sit in a radiation-protected room.

Application: Testing of longitudinally and spiral-welded pipes

Sensor film

Instead of a film, sensor or memory foils can be used, which record the image like a film, save it and read it out digitally on the computer.

Digital X-ray detector

Solid-state detectors with analog-digital conversion (e.g. area detectors ) can also be used for radiographic testing. In this context, one also speaks of digital radioscopy .

Radiographic Testing Standards

German Institute for Standardization (DIN)
  • DIN EN ISO 5579: 2014-04 Non-destructive testing - Radiographic testing of metallic materials with film and X-rays or gamma rays - Fundamentals
  • DIN EN ISO 19232-1: 2013-12 Non-destructive testing - Image quality of radiographs - Part 1: Determination of the image quality number with wire-type image quality test body
  • DIN EN ISO 19232-2: 2013-12 Non-destructive testing - Image quality of radiographs - Part 2: Determination of the image quality number with step / hole type image quality test specimen
  • DIN EN ISO 19232-3: 2014-02 Non- destructive testing - Image quality of radiographs - Part 3: Image quality classes
  • DIN EN ISO 19232-4: 2013-12 Non-destructive testing - Image quality of radiographs - Part 4: Experimental determination of image quality figures and image quality tables
  • DIN EN ISO 19232-5: 2018-12 Non- destructive testing - Image quality of radiographs - Part 5: Determination of the image blurring number with double-wire type image quality test pieces
  • DIN EN ISO 11699-1: 2012-01 Non-destructive testing - Industrial films for radiographic testing - Part 1: Classification of film systems for industrial radiographic testing
  • DIN EN ISO 11699-2: 2018-12 Non- destructive testing - Industrial films for radiographic testing - Part 2: Control of film processing using reference values
  • DIN EN 1330-1: 2015-05 Non- destructive testing - Terminology - Part 1: General terms
  • DIN EN 1330-2: 1998-12 Non- destructive testing - Terminology - Part 2: Terms used by all non-destructive testing methods
  • DIN EN 1330-3: 1997-10 Non- destructive testing - Terminology - Part 3: Terms used in industrial radiographic testing
  • DIN EN 1330-9: 2017-10 Non- destructive testing - Terminology - Part 9: Terms used in acoustic emission testing
  • DIN EN 1330-10: 2003-05 Non-destructive testing - Terminology - Part 10: Terms for visual testing
  • DIN EN 1330-11: 2007-09 Non-destructive testing - Terminology - Part 11: X-ray diffraction terms of polycrystalline and amorphous metals
  • DIN EN ISO 17636-1: 2013-05 Non- destructive testing of welded joints - Radiographic testing - Part 1: X-ray and gamma-ray techniques with films
  • DIN EN ISO 17636-2: 2013-05 Non-destructive testing of welds - Radiographic testing - Part 2: X-ray and gamma-ray techniques with digital detectors
  • DIN EN ISO 10675-1: 2017-04 Non- destructive testing of welded joints - Acceptance limits for radiographic testing - Part 1: Steel, nickel, titanium and their alloys
  • DIN EN ISO 10675-2: 2018-02 Non- destructive testing of welded joints - Acceptance limits for radiographic testing - Part 1: Aluminum and its alloys
  • DIN EN 12681-1: 2018-02, Foundry - Radiographic testing - Part 1: Film techniques
  • DIN EN 12681-2: 2018-02, Foundry - Radiographic testing - Part 2: Technology with digital detectors

Web links

Individual evidence

  1. a b https://www.zfp-muenchen.de/werkstoffpruefung/gammagraphie/ wbsite of the company ZFP-Munich - non-destructive materials testing GmbH on the method of radiographic testing, accessed on July 27, 2018
  2. a b https://www.gfr-hattingen.de/de/technologie-normen/lösungenources website of the company Gesellschaft für Radiographie mbH on radiation sources, accessed on July 27, 2018