In brief
Non-destructive testing (NDT) covers methods used to examine components for defects such as cracks or porosity without damaging them.
Contents
What is Non-destructive testing?
Non-destructive testing (NDT) methods examine materials and components without impairing their fitness for use. Unlike destructive testing, such as a tensile or notched-bar impact test on a specimen, the tested part can continue to be used afterwards. NDT is used in manufacturing, at goods receipt and in the periodic inspection of plant.
Common methods include visual testing (VT), penetrant testing (PT) for surface cracks, magnetic particle testing (MT) for ferromagnetic materials, ultrasonic testing (UT) for internal defects and wall thicknesses, radiographic testing (RT) with X-rays or gamma rays, and eddy current testing (ET). Which method is suitable depends on material, geometry, type of defect and accessibility. Testing personnel are often qualified and certified to ISO 9712 (DIN EN ISO 9712).
In maintenance, NDT is used to detect damage to shafts, welds, vessels or pipework at an early stage and to decide between repair and replacement. For spare parts, especially castings, forgings or welded parts, NDT can be part of acceptance and is then documented in the inspection certificate.
In practice: maintenance and procurement
If you require NDT for a spare part, specify the method, scope of testing, acceptance standard and required documentation in the enquiry. Without this information, quotations are hardly comparable, and the test report will later not match your acceptance requirement.
Synonyms and abbreviations: NDT · non-destructive examination (NDE) · non-destructive evaluation · non-destructive inspection (NDI)
Frequently asked questions
What non-destructive testing methods are there?
The most important methods are visual testing, penetrant testing, magnetic particle testing, ultrasonic testing, radiographic testing and eddy current testing. Surface methods such as penetrant and magnetic particle testing find open cracks, while volumetric methods such as ultrasonic and radiographic testing also find internal defects. Methods such as thermography or acoustic emission testing are used in addition, depending on the component and the question to be answered.
What is the difference between destructive and non-destructive testing?
In destructive testing, a specimen is loaded until it fails, for example in a tensile, bend or notched-bar impact test. This yields material properties, but the specimen is then unusable. Non-destructive testing, by contrast, examines the component itself for defects without damaging it. The two approaches complement each other: material properties usually come from destructive tests, freedom from defects from NDT.
When is ultrasonic testing used?
Ultrasonic testing is suitable for finding internal defects such as lack of fusion, shrinkage cavities or inclusions in welds, forgings and plates. It is also used to measure wall thicknesses, for example on vessels and pipework, in order to monitor corrosion loss. The method requires a couplant and well-prepared surfaces, and evaluation calls for qualified personnel.