SAIW’s NDT Training Manager Mark Digby talks to African Fusion about the increasingly important role of Visual Testing (VT) and its place as an NDT method in own right.
“Visual Testing is now being taken on as the ideal starting point for NDT inspection programmes. In the past, many professionals did not formally recognise VT as an inspection method at all, arguing that a visual check should always be followed by Penetrant (PT) or Magnetic (MT) Testing methods,” says Digby.
“There are formal VT qualifications and these are accepted and referenced by ISO 9712: Non-destructive testing – Qualification and certification of NDT personnel,” he continues, adding that SAIW now offers VT qualifications at Level 1, Level 2 and Level 3.
“VT is the first line of defence for any non-destructive inspection testing programme. Generally speaking, if a weld is regular, the right size and clean, and the welder has obviously taken care to avoid or remove any slag or spatter, there is a much better chance that the weld quality will be acceptable,” Digby notes
Since 85% of all weld flaws can be visually detected on the surface, Digby says that considerable amounts of NDT testing requirements are surface-based. “Any good NDT testing programme should start with surface techniques and VT is one of the easiest of these to access, since it requires relatively simple tools such as magnifying glasses, torches and lights, reflectors and, most critically, an inspectors eyes that are tuned to detect all types of surface flaws,” he says.
“Surface breaking flaws are much more dangerous than embedded flaws,” Digby explains. “If you look at the fracture mechanics calculations of a surface breaking flaw, they always pose a bigger risk because of the higher stresses at the surface of the material, which become concentrated at the tips of a surface breaking crack or flaw.
“Flaws in the body of a material are usually exposed to less stress and crack propagation is likely to be slower. But it is important to separate embedded flaws into volumetric flaws such as slag inclusions and porosity, which may be less dangerous, and planar flaws – cracks, lack of fusion or lack of penetration – which are very dangerous,” he warns.
“So when doing NDT, we never simply conduct surface or volumetric testing, we conduct both. Starting with the surface inspection and then, once that is cleared, we look for volumetric/embedded flaws using a method such as ultrasonic or radiographic testing (UT or RT),” he notes adding, “but the most dangerous of all welding flaws are planar defects that break the surface, and the presence of these can almost always be detected using one of the surface techniques, VT, PT or MT.”
SAIW’s VT training courses deal with the method on an equal footing to any other NDT qualification on offer. “We train candidates how to conduct VT thoroughly and systematically: using the appropriate tools; drawing sketches and/or taking photographs using high resolutions cameras, video cameras or borescopes; and keeping accurate records. These days the resolution of cameras is excellent and very short focal distances are possible. Borescopes can be used to look inside tubes and these can be manipulated to enable the penetration seam of a weld to be thoroughly examined, for example,” Digby notes.