Ultrasonic Weld Testing

_{(Non Destructive Testing)}
Ultrasonic Weld Testing

Thickness measurements are performed using a conventional UTTM detector and a compression wave probe, which sends longitudinal waves into the component at normal incidence to the surface. Signals are displayed on the flaw detector screen in the form of an A-scan, in which the horizontal axis represents distance and the vertical axis represents signal amplitude. Since a 0° compression probe is being used, the horizontal axis is equivalent to depth from the scanning surface. When the probe is placed on the surface of the component, a reflection appears at a range corresponding to the thickness of the component at that point. The use of an A-scan display allows the operator to distinguish more easily between signals originating from embedded plate flaws and the nominal back wall response. Also, the dynamics of the back wall echo can be observed on the A-scan display to detect the presence of pitting.

How does it work?

A strong specular reflection is required to resolve a flaw response from the background noise level with pulse echo ultrasonics. For planar flaws (cracks, lack of fusion, etc.) a specular reflection will only result if the ultrasonic beam is normal (or near normal) to the plane of the flaw. Angled beam shear wave probes are commonly used for the manual ultrasonic inspection of welds in ferritic steels, as these provide the only way of directing ultrasound into the weld body when the cap reinforcement is still present. Where a weld cap restricts probe movement, the sound can be reflected off the bottom surface and directed into the weld body under the cap.

For a typical girth weld, a 45° probe is used for inspecting the root region, and 60°/70° probes for the sidewall fusion faces and weld body. The behaviour of the echo-dynamic pattern and shape of the flaw response (with respect to probe movement) can be used to identify the type of flaw, estimate the length and, in some cases, the through-wall height of the flaw.

The type of material to be inspected affects the choice of angle probe. Shear wave probes are commonly used for examining welds in fine grained materials such as ferritic steels and aluminium. Welds in coarse grained materials such as stainless steels, duplex stainless, copper and composites have a severe attenuating effect on shear waves and can cause beam skewing effects at fusion faces. For welds in these types of materials, angled compression waves are used. However, these have a longer wavelength than shear waves, so there is a reduction in their resolving power.

What will it find

Most manufacturing flaws (lack of sidewall fusion, lack of root fusion, lack of root penetration, porosity, solidification cracking, etc.) and in-service flaws (fatigue cracking, stress corrosion cracking, etc.).