The history and the principle of penetration testing
Some creative minds have contributed significantly to the development of the penetration test, a main method of non-destructive testing (NDT) through their efforts and their use.
The beginnings of the penetration test are probably among the railway operators, using used machine oil as a penetrant, solvent as an intermediate cleaner and lime powder as a developer. This (oil-and-whiting) process is mentioned for the first time around 1890 by the US-American railway operators. It is even documented that not only dry powder, but also suspended in water or solvent (suspended / slurried) developer was used.
Improved inspection conditions, that is, a better searchability of the displays against the dark, used machine oil was achieved around 1930 with the use of ultraviolet-ray fluorescing penetration oils under ultraviolet radiation.
The penetration test is a non-destructive test method, which shows material separations which are open to the test surface.
Suitable liquid substances have the property of wetting surfaces and penetrating gaps and cavities. This ability is used to make very narrow, invisible cracks, visible at the surfaces of objects to be tested. If the liquids are dyed and collected from these columns with a kind of "blotter effect", the areas where the cracks lie are marked on the surface.
No other destructive test method is used as frequently as the penetrant test. It is also one of the simplest methods. But for this reason, it is often underestimated.
Process steps and penetration systems
Pre-cleaning - Any surface, which is to be tested with the penetrant test, shall be prepared in such a way as to remove any contaminants that may interfere with the test.
Penetration - Penetrating agent is applied to the prepared surface and left there for as long as it has filled as much material as possible.
Intermediate cleaning - After the end of the penetration time, the excess penetrant is to be removed from the test surface, so that it is clean on the outside, but the liquid remains in the narrow gaps.
Development - In this step, the agent is recovered from the material separations by applying a developer, such as a blotter sheet.
Inspection - A subsequent inspection with the naked eye of the observer reveals a two-dimensional, enlarged image, the opening of the narrow gap.
Cleaning - At the end, the test area shall be cleaned.
Each penetration test therefore runs in the following 6 basic steps:
Penetrant, intermediate cleaner and developer form the penetration system in the penetration test. They must be coordinated by the manufacturer to be optimally effective and should therefore always be from the same manufacturer and be applied only in the combination specified in the sample test. The rules shall prescribe external tests to qualify the intrusion systems of the manufacturers for the applications of such approved ones within a set of rules. The European standard EN 571-1 only permits the use of such approved penetration systems.
The batch tests must be distinguished from these sample tests. The batch test determines, among other things, whether the currently produced product complies with the standard and whether the impurities contained in a production batch are liable to damage the test item. The analytical data in the batch test certificates, used in the testing of certain materials e.g. austenitic steel, is important.
Areas of application and limits of penetration testing
The basic principle of the penetration test also includes a limit of the method. If material separations are not open towards the surface, the liquid cannot penetrate and no indication of this material separation is obtained.
In contrast to other surface test methods, such as the magnetic powder or eddy current test, the penetration test can, however, in principle be applied to almost all test objects irrespective of the material and the geometry. Main applications are metallic materials, but many non-metallic materials such as ceramics, glass, etc. are also testable. There is a certain limitation in plastics.
Some plastics cannot be wetted by the penetrant and others are attacked by the penetrant, the cleaner or the solvent of the developer suspension. In cases of doubt, a wetting test and a compatibility test must be carried out before the application of the penetration test.
If porous materials are used for examination, such as porous sintered metals or low-density ceramics, many indications are obtained from the natural "pores" of the material (depending on the density of the material) which cover the displays of the material separations. Many low-compacted ceramics and most sintered metals are not readily testable with the penetration test.
The material tests are subject to certain guidelines and standards:
DIN EN 1371-1
DIN EN 1371-2
DIN EN 2002-16
DIN EN ISO 3059
DIN EN ISO 3452-1
DIN EN ISO 3452-2
DIN EN ISO 3452-3
DIN EN ISO 3452-4
DIN EN ISO 3452-5
DIN EN ISO 3452-6
DIN EN 10228-2
DIN EN ISO 10893-4
DIN EN ISO 12706
DIN EN ISO 23277
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