Non-Destructive Controls – (NDC) or Non-Destructive Tests (NDT) are a set of procedure and techniques intended to evaluate the integrity of materials or manufactured goods, do not require the destruction or the removal of some samples from its structure.
Non-destructive controls are carried out for the purpose of obtaining the information necessary for the evaluation of the acceptability of a material or manufactured good.
They are intended to facilitate the search and identification of defects that may be present inside the structure itself; hence, they can be applied during the entire production process.
The acquisition and evaluation of this information must be carried out by expert personnel, suitably trained and qualified to evaluate the degree of reliability and the limitations of the test method used and the results obtained. The documents relative to the inspection procedure must report the level of competence required for the personnel.
The term defect is intended as an anomaly in the material, relevant for the purposes of a potential breakage of the manufactured good. Defects may have metallurgic or processing causes, may exist prior to commissioning of the component or be the consequence of usage. The number, type, and form of defects may be indicated.
Non-destructive controls can be carried out for the purpose of detecting the presence of potential defects in materials or in manufactured goods, to identify the type and define the dimensions.
After having detected the presence of a defect, attempts are made to define its nature. It is the task of the operator to characterise the type of the defect.
The Non-Destructive Controls are one of the main inspection techniques to verify the status of the welded points. In the case of defects in welded points, in order to characterise the defect, other information relative to the welded joint is used, for example, the type of base material, the filler material, the welding procedure used, the type of joint preparation, thermal treatments, etc.
In some cases, the characterisation is sufficient to establish if a defect is acceptable or not. In the case of welding points that are critical or subject to high stress, defects like cracks or sizing are never judged as being acceptable by the standard, regardless of the dimensions. In other cases, and for other types of defects, the acceptability depends on the dimensions of the defects themselves. In this case, it is necessary to move on to the dimension of the defect.
The prescription of the controls on the welding points is cited in different standards, including UNI EN 12062, Non-destructive examination of welds – General rules for metallic materials. This standard suggests the methods to be applied based on the material, the thickness of the metal, and the type of joint. The types of tests carried out and the entity of the potential defects detected are connected to the acceptability criteria indicated in UNI EN ISO 5817 “Quality levels for imperfections in welded joints”, and in standard ISO 3834, on the “Quality requirements for fusion welding of metallic materials”.
There are different types of tests and each one presents advantages and limits. Depending on the type of material and tests to be carried out, it is worth evaluating the type of system to be used in comparison to another.
It is often necessary to use additional and different non-destructive tests to maximise the possibility of detecting potential welding defects.
The methods indicated:
Each has a reference standard that provides indications on the correct execution and conditions for conducting the test.