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December 2012 - About the classification of the NDT methods

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Written by Administrator
Saturday, 10 November 2012 13:32

1- Introduction

The nondestructive testing (NDT) methods classification may come in different ways. Very recently, we felt mix-up in the minds of some people.
First, let us remind you of a process common to all the NDT methods.

2- Process common to all the NDT methods

In the early 80s, the Quality Manager of a plant of an aircraft manufacturer, who happened to be also a kind of philosopher, explained us the following process, common to all the NDT methods.

This process involves three steps: excitation, perturbation and revelation.

The excitation source induces in the part or material, without impairing its integrity, a perturbation, locally influenced by the existing discontinuities.

Suitable sensors detect these discontinuities in the form of signals. The revelation step includes an appropriate signal processing which allows for classifying the indications into three categories:
• Discontinuities,
• Defects, i.e. discontinuities exceeding the acceptance criteria,
• Non-relevant indications.

Let us detail the different phases:
• Firstly, some kind of energy must be provided to the part by an external source,
• Discontinuities in the part change the flow/flux and distribution of this energy,
• A sensor detects these changes,
• The data gathered by the sensor (which may be the inspector’s eyes) are then signal processed (for instance, by the inspector’s brain), sometimes leading to a picture, a film, a display on a monitor or on an oscilloscope,
• The results are interpreted by a Level 2-qualified inspector who decides to accept or to reject the part, or that it may be repaired.

Let us give some examples:

In Penetrant Testing (PT), the excitation step is the penetrant application, the perturbation step is the penetrant entering the open-to-surface discontinuities, and the revelation step is the penetrant bleeding out of the discontinuities after developer application. (Note that in some cases, no developer is applied. Then, the indications come from the convex meniscus that appears at the top of the discontinuities.)

In Magnetic Particle Inspection (MT), the excitation step is the part’s magnetization; the perturbation step comes from the magnetic flux leakages due to discontinuities; the revelation step is when discontinuities are made visible, thanks to the application of the detection medium (magnetic powder or ink).

One of us, who worked for the International Council of the French Language (CILF), put the description of this process in the introduction of the (French, English and German) No. 32(1) booklet of the Banque des Mots. This booklet was used as the basic document for drafting the AFNOR A 09-590 standard, superseded by the ISO 12706:2009 standard.

Sometime later, several training organizations also used this process description in their training documents(2)(3).

3- Methods classification

Another way to classify NDT methods is to take into account the type of discontinuities that they are able to detect. Some detect mainly, or only, open-to-surface, or very close to the surface, discontinuities, while others mainly detect discontinuities inside the part, sometimes deep inside. These latter are called "volumetric methods."

The main NDT methods for detecting surface-discontinuities are:
• Visual Testing (VT),
• Penetrant Testing (PT),
• Magnetic Particle Testing (MT),
• Eddy Current Testing (ET).
• Etc.

The main NDT methods for detecting volumic-discontinuities are:
• Ultrasonic Testing (UT),
• Radiographic Testing (RT),
• Etc.

4- The so-called global NDT methods

In many NDT methods, the position and the direction of the excitation flow/flux versus the location and direction of discontinuities play a major role.

Such is the case of:
• Magnetic Particle Testing (MT),
• Ultrasonic Testing (UT),
• Etc.

There are other NDT methods in which these parameters have no influence. These are so-called global NDT methods.

These NDT methods allows for the detecting of surface-discontinuities, with only one test, with a total reliability, regardless of:
• Their direction,
• Their location on the part,
• The size and the shap of the part to be inspected.

Among these global methods, there are:
• Visual Testing (VT),
• Penetrant Testing (PT): very often, people forget it!
• Acoustic Emission Testing (AE),
• Infrared Testing (IRT),
• Leak Testing, also called Leak Tightness Testing (LT),
• Etc.

5- Important remark

"Global method" does not mean that the specified NDT method is enough for an overall testing of the quality of a part.
Indeed, all NDT methods are complementary. It is never possible to detect, using a single NDT method, both surface- and volumetric-discontinuities.

As a final point, keep in mind that these methods must be implemented by qualified personnel (Level 1 operators and Level 2 inspectors.)


References

(1) Fascicule N°32 de la Banque des Mots, Revue de Terminologie Française (Editor’s note: N°32 Booklet of the Bank of Words, Journal of the French Terminology), Conseil International de la Langue Française (CLIF) (Editor’s note: International Council of the French Language), 11 Rue de Navarin, 75009 Paris (France), 1986. CLIF Website.

(2) Alain LAMBERT, Jacques RIVENEZ and Gilbert WACHÉ (†), Les contrôles non destructifs : généralités (Editor’s note: The nondestructive methods: general points), revised and corrected third edition, Centre Technique des Industries Mécaniques (Editor’s note: Technical Center for the Mechanical Industry) (Cetim), 1994. Cetim Website.

(3) VHS video cassette presented on the CETIM booth at the National Congress on Nondestructive Testing of the French Confederation for Nondestructive Testing (COFREND) in Nantes (France), 1997. COFREND Website.


Normative references

• AFNOR NF A 09-500 Standard, Non-destructive testing – Penetrant testing – Vocabulary, 1987.

• ISO 12706:2009 standard, Non-destructive testing – Penetrant testing – Vocabulary, International Organization for Standardization, Geneva, Switzerland, 2009.

• EN 1330-8:1998 standard, Non-destructive testing - Terminology - Part 8: Terms used in leak tightness testing, European Committee for Standardization, Brussels, Belgium, 1998.

• ASTM E1316 – 11b Standard, Terminology for Nondestructive Examinations, ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA, 19428-2959, USA, 2011.

Last Updated ( Saturday, 10 November 2012 14:02 )