Adri Ludick, NDT manager for WearCheck, outlines the benefits and details of various non-destructive testing techniques.
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Advanced field services, transformer chemistry testing, reliability solutions – these are some of the extra services offered by condition monitoring specialists, WearCheck, in addition to traditional used oil analysis following recent expansion.
“The NDT approach features a variety of testing techniques through which the properties and condition of a component or system are evaluated without causing any permanent damage to it. NDT is typically used in critical component assessments, machine condition assessments and inspection of ancillary equipment such as main vent fans, compressors, mills, pumps and conveyors,” says Ludick.
“Our NDT team delivers quality assurance and quality control of new as well as refurbished components. Our core NDT offering includes eddy current-, magnetic particle-, liquid penetrant-, radiographic- and ultrasonic-testing, along with visual inspection,” he says, adding that NDT’s non-destructive nature means that both money and time are saved in a condition monitoring programme.
Describing the advanced techniques on offer, Ludick begins with eddy current testing (ET), which is ideal for detecting surface defects such as early-stage cracks on metallic machine components. The process is now used across a wide range of industries, from aerospace to beer brewing.
During the testing process, a high frequency electric current (an eddy current) is induced into the material and the response of the associated eddy current field is measured. This information is processed to yield a profile of the component.
Defect-free material has a very specific ‘fingerprint’, so when the test results are compared to this, the presence of defects can be easily picked up and assessed. “When it comes to cracks, the earlier they can be detected, the less potential damage they can will cause to the component. Eddy-current testing can detect crack initiation at extremely early stages,” he notes. “A more advanced version of this NDT technique is phased-array eddy current testing, which creates a 3D picture of the component, giving a clearer visual insight into anomalies,” he adds.
Magnetic particle testing (MT) has a similar purpose to eddy current testing, in that both techniques detect surface cracks on magnetic materials. This is used across a wide range of industries. The process in this test is to magnetise the component and then saturate it with very fine magnetised ink or powder.
Any anomaly in the surface being tested causes a concentration of the magnetic field around it, therefore drawing the magnetic ink to the crack and making the defect visually detectable. A key advantage of magnetic particle testing is that it can be used to test very large surface areas, very quickly.
Liquid penetrant testing (PT) is typically performed on non-magnetic materials, such as copper or aluminium, and is essentially a non-magnetic version of magnetic particle testing. Once the surface is cleaned, it is saturated with a non-harmful penetrating ink. The penetrant is then wiped off the surface of the material before a developing chemical is applied over the test area. “This developer draws the penetrant from any cracks to form a visible indication, which is visually examined by a specialist and the results recorded. This method is used to test such items as vehicle components and ventilation fan blades,” he says.
Radiographic testing (RT) is similar to having X-rays done on humans. It uses different types of X-ray sources that penetrate the material and pass onto an X-ray plate behind the component. It is a widely used technique that detects sub-surface defects which cannot be detected visually. “For thinner materials, such as a metal plate, a weak X-ray source is used, while thicker components such as ventilation fans require a stronger dose. The X-ray sources are contained in a ‘bomb’, which is aimed at the target,” Ludick explains.
“Radiographic testing is often used on welds to test the integrity of the bond between the weld and the parent metal. Called digital radiography, using an electronic form plate instead of photographic film, it is the new trend for this technique,” he adds.
He says that ultrasonic testing (UT) works just like a ‘fish finder’ used on a fishing boat to reveal the depth and size of fish. UT allows technicians to see sub-surface defects in both metal and non-metal components. During the process, an ultrasonic flaw detector instrument fires ultrasonic pulses into the material, while simultaneously detecting the reflection. By measuring the time difference between pulse and the reflection – and knowing the speed of sound in the test material – the location of a defect can be pinpointed.
An advanced UT option is phased array ultrasonic testing where, instead of one pulse, 64 pulses are fired into the component. “By fine-tuning the pulse-strength, it is possible to ‘steer’ the beam in different directions. The advanced option enables us to get a 3D picture of the component, instead of merely a pulse on a screen,” he says.
Visual inspection involves WearCheck’s highly-experienced, well-trained technicians who conduct a multitude of visual inspections on a range of components every day. “Their eyes are conditioned to recognising early-stage defects that are visible, and to identify potential ‘hotspots’ that require further testing,” Ludick concludes.
