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

Near-vertical flaws in thick-section narrow-gap welds are notoriously difficult to detect and size using traditional Phased Array Ultrasonic Testing (PAUT) techniques. Self-tandem shear inspection has been considered for detection but lacks appropriate full weld sensitivity. The dual-tandem phased array inspection method is proposed to enable multi-mode Total Focusing Method (TFM) pulse-echo imaging with longitudinal through-transmission inspection, by introducing a second, opposite-facing probe on the far weld side. Pulse-echo and through-transmission datasets are obtained in a single Full Matrix Capture (FMC) acquisition, for each weld side. Analysis of the optimum Probe Centre Spacing (PCS) based upon image sensitivity suggests that a spacing corresponding to a longitudinal beam crossover at 2/3 sample thickness provides greatest weld coverage. Testing on near-vertical EDM notches in a 120 mm thick mock narrow-gap carbon steel sample has shown high sensitivity TFM imaging, at notch depths of 27.5 mm, 60 mm and 92.5 mm. Pitch-catch through-weld transmission imaging has exhibited tip-diffraction images with a Signal-to-Noise Ratio (SNR) of up to 17.3 dB. Pulse-echo imaging using multi-mode TFM has shown tip-diffraction and reflection with up to 19.2 dB and 34.8 dB SNR respectively. The possibility of image mixing by considering image defect response type has provided full 2D notch reconstruction. These results demonstrate the strength of the dual-tandem method in enhancing the detection reliability and detection of near-vertical defects in thick-section narrow-gap welds.