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

When considering narrow-gap welding processes, common in the nuclear industry, wherein the weld angle is in the range of 2° – 10°, lack-of-fusion defects can appear at near-vertical angles. Traditional single-probe phased array weld inspection, operated in pulse-echo mode, have shown a relatively low sensitivity to vertical and near-vertical planar defects. This is largely due to the dependency of pulse-echo inspection on defect orientation for a favourable reflection angle. Using a single-probe setup may not provide suitable assurances for detection of such defects as the majority of sound energy is reflected away from the transducer by the defect. Multi-mode self-tandem inspection has been used to improve sensitivity to defects of this type, but this method still relies on reflection orientation from the defect. Furthermore, in the case of narrow gap welding, material thicknesses are often large in the hundreds of mm’s and a growing number of required skips can greatly increase ray paths, and in turn attenuation effects.
To overcome the aforementioned traditional phased array inspection challenges, introducing a second opposite facing phased array probe on the far side of the weld to perform simultaneous through-transmission and pulse-echo inspection was investigated. The addition of through-transmission acquisition allows for enhanced detection capability enabled by the combined indications obtained from defect reflections and diffraction effects. For near-vertical defects, this will allow responses from tip-diffraction to be included, in addition to body reflections obtained from pulse-echo. Moreover, Full Matric Capture (FMC) acquisition was deployed to obtain four distinct sub-datasets - one for each of the two pulse-echo and two through-transmission acquisitions performed.
In addition, the added adaptability of the inspection system due to the inclusion of a second probe, including mode choice, probe separation and wedge considerations was investigated through numerical simulations and experiments. One such wedge consideration is the balance between beamforming and transmissibility for shear and longitudinal modes. Longitudinal waves exhibit greater diffraction effects, and are therefore the desirable mode choice for through-transmission. Conversely, shear wave modes provide better resolution and amplitude, so are desirable for pulse-echo. It is therefore advantageous to maximise the transmission of each of these modes without completely limiting the other. Analysis of different wedge angles was conducted and a conclusion drawn from the results of an optimal wedge angle for aluminium of ~20° . This is closer to a standard longitudinal wedge angle to avoid the first critical angle, which is typically surpassed by standard shear wedge angles of ~35°.
It has been shown using ray-tracing simulations that using a combined pulse-echo and through-transmission setup can produce high SNR TFM images of vertical and near-vertical defects. Care must be taken in choosing the correct modes for each FMC sub-dataset, so as to maximise defect detection sensitivity. Data fusion techniques can also be applied for multi-mode and multi-view imaging, further increasing available image parameters. Initial multi-view data fusion tests have shown tip diffraction TFM imaging with a SNR up to 36dB .