Non-destructive Testing (NDT)

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Non-destructive testing

Non-destructive testing (NDT) or analysis is to be understood as an umbrella term for non-destructive materials and polymer testing, as it also encompasses the examination of archaeological [1] or artistic objects, as well as books or images [2], in the broadest sense. A key aspect of this is that cultural-historical or criminological objects, such as mummies, pictures or books, can be examined for possible overpainting, forgeries or their internal structure and composition without causing any damage.

In a narrower sense, however, the term refers to non-destructive materials testing, which, in contrast to destructive materials testing, does not affect the integrity of the test specimen or the component. Materials testing procedures are therefore divided into destructive methods (e.g. tensile, bending, compression or torsion testing and impact testing), low-destruction testing methods, such as micro- or nanohardness testing, and non-destructive testing.

In non-destructive materials testing, the oldest known testing methods – which do not cause damage to the test specimen – are density testing, visual inspection for cracks or surface damage, and sound testing, which is used to assess the characteristic sound of an intact or damaged object.

A modern definition of non-destructive testing generally encompasses the examination of materials, components and structures for quality defects such as defects or imperfections, or the visualisation of those that could impair or prevent their integrity or serviceability before and during use, whereby the application of the non-destructive testing method to the test specimen must under no circumstances impair its functionality [3]. The fundamental aim of NDT is therefore damage prevention through reproducible and repeatable testing procedures, in order to avoid risks to people, property and the environment. It thus produces a static representation of macroscopic and microscopic defects, taking into account the damage tolerance of these imperfections with regard to the integrity and functionality of the structure or component [4].

In contrast to non-destructive materials testing, materials diagnostics utilises NDT methods by combining various hybrid methods whilst simultaneously applying different types of mechanical, thermal or environmental stresses to test specimens or components. This coupling serves primarily to determine damage kinetics and to elucidate relevant damage mechanisms, as well as to identify material-specific limit states, and is carried out with the following objectives:

to increase the information content of conventional testing methods,
to elucidate relationships between microstructure and properties,
to evaluate structure or morphology–property correlations,
to establish physically based functionalities,
to determine local material properties in order to enhance design reliability (see: plastic component),
to interpret deformation phases in terms of events and structure,
to determine material damage and failure kinetics, and
to represent material limit states and diagnostic functions.

In principle, any NDT method or sensor technology is suitable for use as a hybrid method of materials diagnostics, provided it exhibits sufficient sensitivity to defects and meets the material-specific requirements.

Modern non-destructive materials testing makes use of various physical measurement principles, whereby the applicability of such measurement methods is often characterised by the sensitivity and suitability of the test method. A basic classification of non-destructive materials testing can be made according to the underlying physical principle or the field of application (macrostructural or microstructural testing), whereby the following classification principle has proved suitable:

radiographic testing methods (X-ray and gamma-ray flaw detection),
acoustic testing methods (sound, ultrasonic and acoustic emission testing),
thermographic testing methods (heat flux, thermal wave analysis and videothermography),
electromagnetic testing methods (magnetic crack testing, coating thickness measurement and eddy current testing),
crack testing (visual inspection, endoscopy, penetration testing, leak testing, magnetic particle and fluorescence testing),
microwave testing (transmission and reflection methods) and
optical testing methods (holography, laser speckle interferometry, shearography, laser extensometry, etc.).

Specialised testing methods such as eddy current testing, experimental strain analysis and vibration analysis round off this list, as do beta-ray testing and terahertz testing, without claiming to be exhaustive. It can generally be observed that NDT has become an indispensable tool for efficient quality assurance in almost all branches of industry and serves to detect intolerable defects, monitor the condition of machinery and plant, and carry out failure analysis.

The umbrella organisation for NDT in Germany is the German Society for Non-Destructive Testing (DGZfP e.V.), whose main objectives are the research, development, application and dissemination of non-destructive testing methods [5]. The DGZfP’s training division (Ausbildung und Trainings GmbH) centrally organises the initial and continuing training, as well as the qualification of NDT inspection personnel, in accordance with DIN EN ISO 9712 (formerly DIN EN 473) and DIN 54161, in the form of a course system comprising three different training levels for the following areas of NDT [6–8]:

AT– Sound emission testing
DR – Digital radiology
ET – Eddy current testing
HT – Mobile hardness measurement
LT – Leak testing
MT – Magnetic particle testing
PT – Penetration testing
RT– Radiographic testing
TT – Thermography
UT – Ultrasound testing
VT – Visual inspection

References

[1]	Claridge, T. D. W.: High-Resolution NMR Techniques in Organic Chemistry. Tetrahedron Organic Chemistry Series 19 (1999) DOI: https://doi.org/10.1016/C2015-0-04654-8
[2]	Hahn, O., Bretz, S., Hagnau, C., Ranz, H.-J., Wolff, T.: Das Schwarzlot in der Hinterglasmalerei − Zerstörungsfreie Untersuchung von Kunst- und Kulturgut. ZfP-Zeitung 113 (2009) 2 Download as pdf
[3]	Erhard, A.: Aufgaben und Abgrenzung der Zerstörungsfreien Prüfung. DGZfP-Jahrestagung, Fürth (2007), V11 Download as pdf
[4]	Blumenauer, H.: Werkstoffprüfung. Deutscher Verlag für Grundstoffindustrie Stuttgart, 6th Edition (1994), (ISBN 978-3-342-00547-6; see AMK-Library under M 3)
[5]	Homepage der DGZfP: https://www.dgzfp.de/ (access on Juli 13, 2026)
[6]	ISO 9712 (2021-12): Non-destructive Testing – Qualification and Certification of NDT Personnel
[7]	DIN EN 473 (2008-09): Non-destructive Testing – Qualification and Certification of NDT Personnel – General Principles (withdrawn, replaced by DIN EN ISO 9712:2022 Download as pdf)
[8]	DIN 54161 (2021-03): Non-destructive Testing – Qualification of Non-destructive Testing Assistants

See also

References