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Non-destructive tests (N.D.T.)
Non-destructive tests (N.D.T.) are inspections, checks and surveys carried out by means of methods that do not alter the material and do not require the destruction or removal of test samples from the concerned structure. The main feature of this kind of tests is the possibility to check the concerned parts without interfering with the tested material. Non-destructive tests are thus a crucial tool for the product final check. As for safety parts, the check by means of non-destructive test also ensures the product conformity.
TEC Eurolab S.r.l. is equipped to carry out the following non-destructive checks:
» visual inspection
» penetrating fluids
» magnetic-particle test
» ultrasound
» radiography
The checks are carried out by qualified technicians:
level 2 and 3 ASNT (SNT-TC-1A qualification according to code ASME V)
level 2 and 3 CICPND according to standard UNI EN 473 (qualification according to European standards)
Principle of the method
With visual inspection we mean the examination of the objects to the naked eye or with the only aid of lenses or low-magnifying endoscopes.
Actually, differently from what seems, the visual inspection not only highlights surface macroscopic defects, but also allows to immediately determine which are the most suitable surfaces to sound (e.g. ultrasounds). As non-destructive test, the visual inspection plays a fundamental role and, despite it is the most natural method, it requires a great predisposition to observation and above all a great experience in order to get all the potentially available information, thus actual results according to specific acceptability ranges for the particular imperfection of the part being tested.
Application
Check used to point out specific surface and dimensional characteristics such as: alignments, shape and dimensions of components of machines, systems and manufactures, surface condition etc. in welding, casting, precision casting and mechanical components.
Principle of the method
This non-destructive test exploits the capability of some fluids of penetrating inside surface defects (cracks, cavities, etc) by capillarity and not by gravity.
The low surface tension and the good wettability of these fluids ensure their penetration inside the thinnest irregularities. After the fluid application and penetration (the fluid is thus called “penetrant”) requiring a variable time depending on the kind of product used, of material to be inspected and of irregularity to find out, the excess of penetrating fluid is removed from the surface by running cold water.
Since water has a higher surface tension and a worse wettability than the penetrant, this is not removed from the cracks in which it is penetrated by capillarity. After washing - once again exploiting the capillarity principle - the penetrating fluid still inside the defect is extracted; to do this, a properly thick layer of white powder (detector) is laid on the piece surface. The penetrating fluid “comes up” and leaves in the detector a signal bigger than the defect generating it. Depending on the technique used, the defect can be highlighted in different ways: as a red or fluorescent spot that can be easily detected by irradiation with Wood’s light, in the dark. This method involves some difficulties in the defect evaluation as well: sometimes, mechanical machinings give some defect “indications” that can be solved by a particularly experienced operator, only.
Application
The inspection by penetrating fluids is a method particularly suitable for highlighting and locating surface irregularities such as cracks, porosity and pinching, in a rapid, cheap and accurate way. Contrary to magnetic checks, penetrating fluids can be successfully applied to any component, irrespective of the component geometry or material (except for porous pieces, e.g. sintered pieces, cast iron or pieces with very rough surfaces). To carry out this test, TEC Eurolab S.r.l. has a tank bank ideal for checking aluminium and titanium pieces. The check at the customers’ is carried out by means of aerosol bombs.
Principle of the method
This technique makes use of a particular characteristic of ferrous alloys: the ferromagnetism, that is the capability of concentrating the field to highlight the defects of the magnetic field flux lines near a surface defect.
Near an irregularity such as a microcrack, the flux lines of the magnetic field locally divert and create a magnetic field anomaly at the edges of the defect. When the defect reaches the surface, some of the magnetic field flux lines are scattered beyond the surface; to highlight the defect, just spray the surfaces with suitable coloured or fluorescent ferromagnetic powder suspensions. The particles that concentrate and line up along the magnetic field flux line are made visible by Wood lamp.
Application
The magnetic-particle test is particularly suitable for pointing out surface or subcortical defects: such as cracks, inclusions, pinching, etc., while it is completely useless in case of inner defects. Also the sensitivity to round defects is rather low. The minimum dimension of the detectable defect depend above all on its distance from the surface and the detecting limit is anyway better than the one of penetrating fluids. It is a sensitive method that can be applied for semi-finished or finished parts, sheets, forgings, butt-to-butt welding, fillet welding, structural works and mechanical parts. TEC Eurolab S.r.l. is able to carry out this check using a table magnetoscope or working at the customers’ by a portable yoke or a pillar-magnetoscope.
Principle of the method
The ultrasound defect-detecting method is the technological application of the echo phenomenon. The echo results from the reflection of sound waves that bounce against the surface of an obstacle and come back to the listener ear.
During the ultrasound check, the waves (ultrasonic waves not audible by human ears) are generated making use of the piezoelectric properties of some materials; these properties are no other than the capability of these materials to shrink and expand again when subjected to the action of an alternated electric field. If the alternated electric field has the right frequency, the material vibrations generate elastic waves with ultrasonic frequency. This phenomenon is reversible, in other words, the material capable of generating ultrasounds is also capable of generating an electric signal when hit by a beam of elastic waves. Ultrasonic waves can be sent into the material to be tested; once there, the waves spread with the same generator frequency and with a speed depending on the material crossed.
When the beam meets an obstacle, it is reflected, absorbed, diverted or broken according to the common rules of all wave propagation phenomena. The reflected waves have the same frequency as the incidence waves, but are out of phase to them, also according to the distance covered, that is to the distance from the transducer to the different points of the obstacle surface. The same happens to broken waves. The energy absorbed by the defect hit by incidence waves makes the defect vibrate generating variously out-of-phase elastic waves with a frequency typical for its resonance. As a matter of fact, the signal coming back to the transducer is very complex because it results from the sum of many waves with the same frequency but out of phase, and of other waves with different frequency and out of phase as well. This signal contains all information about the dimension, geometry and nature of the obstacle hit by the incident ultrasound beam. As said before, the piezoelectricity physical phenomenon is reversible; therefore, when the wave reflected or emitted by the obstacle comes back to the sound generating it, it gives an electric signal which, if suitably amplified and filtered, can be viewed on the oscilloscope monitor, fitted in any ultrasound detecting instruments.
Application
As for tests carried out by TEC Eurolab S.r.l., ultrasounds are highly used to check welding, spheroidal cast iron, thicknesses and to check for inclusions in rolling products. It is a techniques suitable for testing rolled-section components (rolls, shafts, press columns, etc.) power equipments (turbines, rotors, pressure ducts and components, reactor components, etc), vehicle components, machine material, car and railway parts, material thicknesses and parts subject to wear. The check can be carried out both in our lab and directly at the customers’.
Principle of the method
When X rays, electromagnetic waves, go through the object to be examined, they are absorbed according to the thickness and density of the crossed material.
The X rays succeeding in going through the material expose a photoplate located behind the object to be examined. After the photographic development, the different plate areas are more or less black according to the radiation dose absorbed. In practice, if the examined object shows defects such as cavities, cracks, big and less-absorbing matrix inclusions or thicker more-absorbing material parts, lighter or darker spots form on the plate, with an intensity proportional to the thickness of the defect, which appears delimited by its perspective projection.
Application
This method allows to point out a number of irregularities of industrial manufactures, casting or welding such as: porosity, inclusions, blowholes, cracks, tiny holes, slag inclusions, missed penetration. This method is ideal for examining steel tube welding but also for pieces manufactured by companies working in the aerospace and automotive industries. In TEC Eurolab S.r.l., this technique is used for checking welding, casting and precision casting.
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