NDT: constantly changing methods

Non Destructive Testing (NDT) is used to check for defects in parts, components, materials and equipments without damaging them. The methods are not new but they are steadily changing to keep pace with technological progress.

Non Destructive Testing (NDT) “detects issues that can affect the proper operation or installation of the facility or the elements tested, including problems relating to production or use (fatigue cracking), appearance (corrosion, pitting) and foreign bodies,” says Serge Selles, Business Unit Manager at Actemium NDT Engineering & Services.

In the last century, “Non Destructive Testing initially consists in hitting a part with a hammer to check for consistent sound across the surface of the object (a hollow sound indicating a problem), or using powdered chalk and oil to carry out the first dye penetrant tests,” says Serge Selles. Since then, with technological progress, the methods have obviously changed.

“Leak, ultrasound and acoustic emission testing has made progress at as the processing hardware and software have improved, just as eddy current and ultrasound testing has been further improved with progress in measuring devices and sensors. Conventional x-ray testing is increasingly being replaced by digital radiography in order to avoid film waste, or by ultrasound testing as sensitivity has increased.”

Methods with multiple applications

The method is used depends on the type and nature of the components or structures to be analysed. A combination of methods is also possible. Of the various techniques, radiology, ultrasound, dye penetrant, leak, eddy current, acoustic emission and magnetic testing are the most widely used.

In the aerospace sector, for example, we inspect aircraft engines during production to ensure their integrity by using immersion UT, digital radiology and eddy current testing

Companies across a wide range of sectors employ NDT to test their products. The companies making most use of it operate in sensitive industries producing high-cost components or components that must meet critical safety reliability requirements. “In the aerospace sector, for example, we inspect aircraft engines during production to ensure their integrity by using immersion UT, digital radiology and eddy current testing,” says Brigitte Urbaniak, Business Unit Manager at Actemium NDT-Aerospace. In other sectors, the elements to be analysed vary widely. “During my career, I have been called on to leak test coffee packaging and cat food cans and to x-ray the first wheels for high-speed railway rolling stock. We once even once x-rayed a sarcophagus to see what it contained before it was opened.”

Improving industrial performance and reliability

The primary purpose of ND testing – to ensure safety requirement compliance and risk and quality management – applies at each stage of the part life cycle (manufacturing and operation). “We carried out direct helium tests on an LNG tanker for five months to check for leaks. The technical shutdown was a world first since the primary membrane was to be completely replaced. We used our testing robots, which are normally employed during LNG tanker construction. The end result was conclusive for the shipowner. The tanker has been in use and has had no problems to date.”

The tests are also used to monitor equipment aging and help prevent failure. Eddy currents can for example be used on heat exchangers to measure wear on the tubes and repair them before they fail.

“For the past year, we have been working at an EDF site to provide support and technical expertise during helium testing of a generating unit in operation. The tests check equipment without stopping production. This reduces costs and eliminates the need to remove parts and the risk of causing damage,” says Serge Selles.

Technology makes progress, so does NDT

NDT has come a long way since the sound test using a hammer: visual inspection, then gas and now sensors. Miniaturised, faster sensors are more precise and able to detect smaller and smaller defects. They are used throughout the industry of the future and in particular in maintenance. In another improvement, ultrasound testing can now be carried out using wireless technology, via Bluetooth and on a smartphone or tablet. X-ray tomography, originally developed for use in medicine, can provide a cross-section or 3D image of the interior of an object. “The devices are increasingly sensitive and can detect elements that were previously invisible. More effective methods replace less effective ones, as in the case of x-ray and ultrasound. They are constantly being upgraded complement each other,” says Serge Selles.

We can place two different sensors in a single device, for example combining eddy currents and ultrasound, so as to identify defects that are both on the surface and within the body of the object – says Brigitte Urbaniak. As part of our maintenance work, we tested the High Pressure compressor of the General Electric 90 engine using equipment specially designed to acceptance test the part using eddy currents and ultrasound.

Despite improved processing software and greater hardware sensitivity, “We will always need expertise and the final decision will always be taken by a human technician. Though some of the testing is automated and alarm thresholds are applied during production, the NDT technician will have the last word,” says the Business Unit Manager.