A-Scan Weld Inspection

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.
Where sound is angled directly at the area of interest, this is referred to as "half skip testing". "Full skip" testing occurs when the bottom surface is used to reflect the sound before it enters the weld.
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.
Vertically orientated planar flaws can be a particular problem for detection using an angle probe in pulse-echo mode. However, a variation of angled shear wave ultrasonics is the Tandem technique, which is normally used for the detection of vertical flaws in thick section components. Two 45° shear wave probes are positioned in a jig, one behind the other facing the area of interest. The rear probe is used to transmit ultrasound into the joint area and the front probe receives sound reflected from flaws within the insonified region. By moving the probes relative to each other, it is possible to obtain full-through thickness coverage.
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. A-Scan weld inspection using angled compression wave probes can be very difficult due to the presence of spurious mode converted signals on the flaw detector display. Consequently, such probes are restricted to half skip testing and are preferably used in conjunction with an imaging system.
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.).
Where is it used?
Inspection of welds made in both ferritic and non-ferritic metals in pressure vessels, pipework, storage tanks, bridge structures etc.