MechChem Africa March-April 2021

⎪ Back page ⎪

Advanced NDT offered by WearCheck AFS

Adri Ludick, NDT manager for WearCheck, outlines the benefits and details of various non-destructive testing techniques.

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 indi- cation, 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 compo- nent. It is a widely used technique that detects sub-surface defectswhich can- not be detected visually. “For thinner materials, such as ametal plate, a weak X-ray source is used, while thicker com- ponents suchas 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 be- tween the weld and the parent metal. Called digital radiography, using an electronic form plate insteadof photographicfilm, it is thenew 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-surfacedefects inboth metal andnon-metal components. During the process, an ultrasonic flaw detector instru - ment fires ultrasonic pulses into thematerial, while simultaneouslydetecting the reflection. By measuring the time difference between pulse and the reflection – and knowing the speed of sound in the testmaterial – the loca- tion of a defect can be pinpointed. An advanced UT option is phased array ultrasonic testingwhere, insteadof onepulse, 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 amultitudeof 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 test- ing,” Ludick concludes. www.wearcheck.co.za

Jaco Venter, senior machinery inspector at WearCheck, conducts ultrasonic testing on a winder’s brake components during a routine NDT assignment.

A dvancedfield services, transformer chemistry testing, reliability solu- tions – these are some of the extra services offeredby conditionmoni- toring specialists, WearCheck, in addition to traditional used oil analysis following recent expansion. “The NDT approach features a variety of testing techniques throughwhich theproper- ties and condition of a component or system areevaluatedwithout causing anypermanent damage to it. NDT is typically used in critical component assessments, machine condition assessmentsand inspectionof ancillaryequip- ment such as main vent fans, compressors, mills, pumps and conveyors,” says Ludick. “OurNDT teamdelivers quality assurance and quality control of new as well as refur- bished components. Our core NDT offering includes eddy current-, magnetic particle-, liquid penetrant-, radiographic- and ultra- sonic-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 test- ing (ET), which is ideal for detecting surface defects such as early-stage cracks on metal- lic machine components. The process is now used across a wide range of industries, from aerospace to beer brewing. During the testing process, a high fre- quency electric current (an eddy current) is induced into thematerial and the response of the associatededdy current field ismeasured.

This information is processed toyieldaprofile of the component. Defect-free material has a very specific ‘fingerprint’, sowhen the test results are com - pared to this, the presence of defects can be easilypickedupandassessed. “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 3Dpicture of the component, giving a clearer visual insight into anomalies,” he adds. Magneticparticle testing (MT) hasasimilar purpose to eddy current testing, in that both techniques detect surface cracks onmagnetic 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 visu- ally 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 par- ticle testing. Once the surface is cleaned, it is saturatedwithanon-harmful penetrating ink. The penetrant is then wiped off the surface

44 ¦ MechChem Africa • March-April 2021