During optimisation of the PAUT setup, artificial defects were inserted at positions in the thick laminate, both between the shear webs and in the shell-to-shear-web joint at the shell–adhesive and adhesive–shear web interfaces. The results clearly showed the adhesive bond-line between the shell and shear webs (Figure 2).
Testing the optimised setup on the blade section used for the thermography setup, with defects inserted in the balsa/GFRP sandwich sections, revealed features in the shear web/shell bond line (Figure 3 – dashed box).
Although not a project objective, researchers also conducted focused scans of the adhesive bond lines, as this was known to be an area of concern for the blade manufacturer who donated the samples.
For the blade section available during the project, the phased array scanner assembly provided sufficient scan width to capture the whole joint while scanning axially along its length.
Researchers conducted full-length scans of the blade sample used for the phased array optimisation, containing defects in the shear-web-to-shell joint (Figure 4). The artificial defects were clearly visible in the C-scan, with the B-scan showing the defect depth position at either the upper or lower adhesive interface.
The PAUT data showed a relatively even signal amplitude and readily identifiable blade features. The sample was visually observed to have a consistent adhesive bond line, indicating this was the first joint made in the blade assembly. This allowed control of the adhesive layer thickness and coverage used, with any excess possibly being removed prior to closure of the structure.
Scans along the bond line were carried out on the blade sample used for thermography testing, containing defects in the balsa/GFRP sandwich sections (Figure 5), with features visible in the C-scan. Data indicating that adhesive was incomplete in places across the bond line and spot anomalies were also visible.
Physical investigation of the sample showed that thin pre-cured GFRP strips were bolted to the shear webs, presumably to retain the excess adhesive that is squeezed out of the joint during manufacture, hereby termed ‘excess adhesive catch strips’. The self-tapping bolts attaching the excess adhesive catch strips were the cause of the spot amonalies; removing the strips revealed missing adhesive, matching the PAUT C-scan data.