Dual-tandem phased array inspection method for imaging near-vertical defects in narrow gap welds

Thick plate welding, particularly in the nuclear industry, is conducted using a narrow gap welding technique, preferred for its reduced deposition and welding times. However, due to the narrowness of opposing bevel walls, lack of sidewall fusion (LOSWF) defects and centreline cracks can be more frequent. Narrow-gap LOSWF defects are often near-vertical, dictated by the weld angle typically in the range 2° - 20°. Single-probe phased array pulse-echo inspection possesses a relatively low sensitivity to these low angle defects, as they require a desirable defect orientation for a favourable reflection. In this case, transmitted waves are predominantly reflected under the array, with little reflected energy returning to the array. Self-tandem methods can be used to achieve a desirable incident angle on the bevel, however these still rely on defect reflection and with a large plate thickness, backwall skips can greatly increase attenuation effects.
To overcome the reliance of near-vertical defect detection on reflections, a dual-tandem setup is proposed. This introduces a second, parallel and opposite facing array on the far weld side. This addition opens the possibility of though-transmission inspection - transmitting on one array on receiving on the second - thus removing reflection dependency and increasing the system sensitivity to diffraction effects. By combining pulse-echo and through-transmission acquisition through the use of full matrix capture (FMC), the ability to obtain defect indications from both reflection and diffraction effects is possible. In turn, sensitivity to near-vertical defects, specifically LOSFW flaws in narrow-gap welds, can be increased. By considering the system as a single array, FMC acquisition provides four sub-datasets; one for each of the two pulse-echo and two through-transmission firing processes. Each of these datasets can be used to form an image using the total focussing method (TFM), and in doing so, four ‘views’ are obtained.
Additionally, flexibility of the system is increased in terms of probe separation and wedge considerations. In order to maintain the high resolution of shear wave pulse-echo imaging, while also combining diffraction-sensitive longitudinal waves for through-transmission, the wedge angle used must be carefully considered. This should be chosen in order to maximise the transmission of each wedge, without limiting the other. On this basis, analysis of wedge angles using numerical simulations led to a wedge angle of ~20° being chosen. This is closer to a standard longitudinal wedge angle, so to avoid the first critical angle - typically surpassed by standard shear wedge angles of ~35°.
The benefit of using FMC and TFM is the flexibility in mode choice, with each view capable of being optimised in terms of mode selection. Using a multi-mode TFM algorithm, mode selection within views, as well as views themselves, can be appropriately fused to obtain a high signal-to-noise ratio (SNR) image of a vertical defect. Initial tests have shown that multi-view TFM tip-diffraction imaging has shown SNR of up to 36dB.