Non-destructive testing of welds

A weld bead can look perfect on the outside and still hide a crack or air pocket inside the melt bead itself. Non-destructive testing, often abbreviated NDT, is a collective term for methods that can detect such defects without destroying the workpiece, and the methods are a key part of quality control in structural welding.

§Why non-destructive testing

With a destructive test, the workpiece is cut or broken into pieces to reveal internal defects — it works for testing a welding procedure or a test weld but not for inspecting the finished product the customer should receive. NDT makes it possible to inspect the actual delivered structure without it becoming unusable afterwards.

§Visual control

The simplest and most widespread method is visual inspection. You look for visible defects such as pores, overlaps, incomplete penetration, surface cracks, incorrect weld bead form and spatter. Visual inspection requires no expensive instruments but good lighting, good eyesight and a trained eye for what a sound weld looks like.

§Penetrant testing

With penetrant testing a colored or fluorescent liquid is applied to the surface. The liquid penetrates even very fine surface cracks by capillary action. After a dwell time excess liquid is wiped off and a developer is applied which draws the penetrant out of the crack again and makes it visible as a clear line. The method only finds defects that have a connection to the surface.

§Ultrasound.

Ultrasonic testing sends high-frequency sound waves into the material through a probe. When the waves hit an internal flaw—for example a crack a slag inclusion or lack of fusion—some of the sound is reflected back and this gives a signal that shows the flaw's location and approximate size. The method is suitable for finding flaws deep inside thick material where visual inspection and penetrant testing cannot reach.

§X-ray (radiography)

Radiographic testing uses X-ray or gamma radiation, which is passed through the component and captured on a film or detector on the other side. Internal defects such as pores, slag inclusions and certain cracks appear as shadows on the image, because they affect how much radiation passes through. The method produces an image that can be saved and assessed again later, but requires radiation protection and area isolation while being performed.