Non destructive testing

See also: NDT

The Non destructive testing ( C.N.D. ) is a whole of methods which makes it possible to characterize the state of integrity of industrial structures, without degrading them, is during the production (the parts which leave the foundries are not never free from defect), that is to say in the course of use (appearance of defect). It is thus necessary to determine (with breakage, in an empirical way) which size of defect is acceptable and then to be able to detect them, without breaking the part, and replacing it if need be. One also speaks about E know NR one D estructifs or of E xamen NR one D estructifs ( E.N.D. ).

These methods are very much used in:

the Auto industry (control of the driving blocks);
the Oil industry (pipelines, tubes, bars, weldings, tanks);
the Shipbuilding industry (control of the hulls);
the Aeronautical (beams, wings of plane, many parts engines, landing gears, etc);
the Aerospace and the Armed (whose totality of the programs are confidential or secret defense);
the industry of energy (engines, boilers, piping, turbines, etc);
the railway one in manufacture and maintenance in particular for the concerned with safety units (axles, wheels, bogies);
others (manufacture of snap hooks of Climbing, works of art, etc);

And in general in all the sectors producing:

of the parts with production costs raised in small quantity (nuclear, petrochemical…) ;
of the parts of which the reliability of operation is critical (BTP, nuclear power, pipeline gas…).

In France, the majority of the agents which carry out these controls are certified by the CO nfederation FR ançaise for the E know NR one D estructifs (COFREND) according to a principle of third party. The methods of certification are controls by the French and European standard NF IN 473 entitled: " Non-destructive tests, qualification and certification of personnel END. This standard was written in April 1993 and revised in December 2000.

For the parts intended for Aerospace industry (Aeronautical and space), the C.N.D. are regarded as special Procédés within the meaning of the standard IN 4179 and must be qualified like such. In France these certifications are managed by the Aerospace Sectoral Committee of Certification (COSAC) () on behalf of the COFREND. IN 4179 is identical to American standard NAS 410.

For railway maintenance, the Railway Committee of Certification for Maintenance (CFCM) ensures certification COFREND of the operators of sector (the SNCF, the RATP, coaches,…) according to IN 473.

General principles

Whatever the adopted method, one can represent the implementation of a system NDT following the synoptic following:

The target is characterized by a whole of parameters which one will seek to estimate in order to form a diagnosis of integrity. The implementation of an adequate system NDT will make it possible to produce a certain number of signals which are function of the required parameters. A stage “of inversion”, more or less complicated, is very often necessary in order to find the initial parameters of the part.

Various control methods

The standard IN 473 defines a certain number of symbols for the usual methods (cf table).
These symbols generally correspond to the abbreviation of the English designation of the method, for example, symbol Pt for sweating comes from P enetrant T esting.

The Dye penetrant inspection (Pt)

It is a method intended to reveal the presence of open discontinuities on the surface of metal parts, primarily, but also out of ceramics. It consists in whitewashing (by immersion or electrostatic pulverization, sometimes but seldom, with the brush) the target with a liquid Fluorescent or coloured in red, which penetrates in discontinuities. After cleaning of the target, revealing is applied and, while making “sweat” the liquid remained in the cracks, will reveal them.

This method seems very simple to implement and it is sensitive to open discontinuities. One can highlight discontinuities of 1 µm of opening, 100 times finer than a hair. On the other hand, it is not automatizable and the results remain with the appreciation of the operator. Moreover, it requires the use of nonrecoverable products, even contaminated after use (e.g.: nuclear plant: one tries to reduce the volume of waste), but this method is irreplaceable for the whatever their site, description of emerging discontinuities, whatever their orientation.

The Radiography (RT)

This method has the advantage of providing directly exploitable images, the stage of inversion can be extremely reduced and the excellent space resolution. However, the interpretation of the images asks for a strong level of expertise on behalf of the operator. Moreover, this method is extremely expensive and imposes security conditions for the operator and the environment, who return his use in difficult industry. Control using the X-rays is particularly used to control the weldings in Micro-électronique.

The ultrasounds (UT)

The ultrasonics inspection is based on the transmission, the reflection and the absorption of an ultrasonic wave being propagated in the part to control. The wave train emitted is reflected in the content of the part and the defects then reconsiders towards the transducer (which often plays the part of transmitter and receiver). The interpretation of the signals makes it possible to position the defect. This method presents a high space resolution and the possibility of finding defects in-depth. The stage of inversion is simple, at least for the geometrically and materially simple parts. On the other hand, it is a slow method because it is necessary to make an exhaustive mechanical sweeping of the part. It is often necessary besides to control several surfaces of the part to be able to make a three-dimensional representation of the defects.

Sealing

The methods of contôle of echancheity call upon tracer gases, coupled to instruments very sensitive to tiny concentrations of these tracer gases. More current are helium, hydrogen (nitrogenizes hydrogenated), possibly the SF6.

Thermography

The control methods thermal consist in exciting a material or a structure by a contribution of energy (mechanical, photonic, Chauffage by induction, hot air…). The diffusion of heat in material and the impact which it has on the distribution of temperature of surface inform about the thermophysical properties of materials and possible defects. The main advantages of the whole of these methods lie in the possibility of carrying out a control without contact and automatizable. The disadvantages are related on the slowness of control, the capital cost and the difficulty of implementation of the stages of inversion to establish the diagnosis.

The Video tape recording (MT)

Video tape recording is a technique of non destructive testing which consists in creating an intense magnetic flux inside a ferromagnetic material.

At the time of the presence of a defect on its way, the magnetic flux is deviated and creates an escape which, by attracting the particles (coloured or fluorescent) of a tracer substance, provides a particular signature characteristic of the defect.

Coupled methods

Recently, of the coupled methods were born, and for some are still in the field of research: methods electromagnetic acoustic, electromagnetic ultrasonic or magneto-optical.

See too

Occlusion
Corrosion
Crack
Delamination
Foundry
Non destructive testings on castings out of aluminum

External bonds

Soudeur.com: The French-speaking site gate of the welder, the welding and welding

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