Non-destructive testing involves a set of methods used to characterise the state of integrity of structures or materials, without degrading them and at different stages in their life cycle.
All techniques and processes providing information on the health of a part or a structure, without causing alterations that could prejudice their subsequent use fall into the category of Non-destructive testing (NDT). Called NDT in most industrial sectors (including in the standards) apart from the nuclear industry, which prefers to use non-destructive examination, or civil engineering’s non-destructive evaluation.
The goal of NDT is therefore to reveal any flaws likely to alter the availability, safe use and/or, more generally, the compliance of a product with its intended use. For this reason, using NDT appears to be a key element in product quality control and risk management, thereby ensuring the safety of people and goods.
Located on the borders of metrology and industrial, scientific and medical instrumentation, NDT is a specific scientific and industrial activity sector with its own professional structures. These include, among others, manufacturing industrialists or specialist distributors, study and training bodies, service companies and specialist departments of a certain number of industrial companies that are major users of these techniques.
Using NDT can sometimes assume a regulatory aspect for the safety of facilities and people, as in the case of ski lifts or pressurised tanks. However, industrialists often have a deliberate approach which is included in their quality policy.
They can be motivated by the potential of technical and economic advances that NDT offers competitive industries not required legally to carry it out (e.g. controlling the manufacturing process to guarantee and improve product quality).
Scope of application
Nowadays, quality and risk management are two parameters that cannot be ignored by industrialists manufacturing, implementing or using all types of material, product or structure. NDT has therefore become essential in just a few decades, which explains why its scope has grown increasingly beyond the traditional use developed by the metallurgical industries and activities where safety is vital, like the nuclear and aeronautical industries.
Although the type of defect sought is also increasingly diverse, non-destructive testing of a product or object can still be carried out at three different stages in its life cycle, thus leading to several types of application distinguished by both the industrial context and by the nature of the test itself.
Before any manufacturing takes place, NDT can be used to analyse a structure, an installation or a batch of parts and thus verify its quality specifications compared with those defined in the technical specifications. Be it the choice of a process, the choice of adjustment settings, calibration or presentation and archiving of results obtained, the most important aspect of the approach is procedure. At this stage, this means detecting defects but also frequently defining their nature and dimensions.
During manufacture, NDT plays the role of a tool for testing a process that is often automated and involves instrumentation installed in the production line. Defects are normally identified clearly in this type of test and defective products are identified or sorted when testing is completed.
When an NDT instrument cannot be installed on the production line, ancillary test benches, significant in both size and investment costs, can be used.
When the product or object is in use, NDT is performed during maintenance or after anomalies are detected in the behaviour of the structure. Given the importance of the consequences of failing to detect a serious defect, it is vital for the test to be extremely reliable. It is important for this type of test to be able to estimate as far as possible the nature and dimensions of defects to assess their potential harm; it must also be possible to reproduce the non-destructive test extensively so that changes in the damage can be monitored over time.
Trends and changes
NDT is included substantially in the quality policy of a company and moves in line with changes in the instrumentation. Today, it is not enough to detect a defect, it has to be characterised and dimensioned. Non-destructive techniques and processes also have to be designed that can highlight complex physical heterogeneities or irregular properties such as, for example, variations in microstructure in a metal, variations in surface texture or roughness or variations in electromagnetic properties in a band.
Increasingly efficient and more reliable instrumentation therefore appears on the market year after year. This instrumentation is above all easier to use in complying with very strict testing procedures, although it is often difficult to achieve all these objectives due to physical or economic reasons.