R-zones are important part of the skin-stiffener joint in composite panels. Owing to their complex geometric shapes, defect behaviour, and poor accessibility, it is difficult to perform non-destructive testing and evaluation of their defects. A new method based on reflection behaviour generated by mono-pulse ultrasonic (MU) waves in R-zones for characterising and evaluating their defects was developed. Specimens with artificial and actual delaminations were used. The MU reflection behaviours, time-domain signals, and image characteristics originating from the R-zones were studied. Optical microscopy experiments were conducted to verify the MU results. Methods for discriminating the delamination and resin layer as well as determining their accurate depth were established using the MU signals from the R-zones. The experimental results show that delamination, resin layer, and pockets in the R-zones can be characterised and evaluated in detail using the MU method. Its resolution and dead zone reached 1 prepreg ply thickness. The minimum deviation of the layup depth of the detected delamination and resin layer in the R-zone is less than one prepreg ply thickness. Delaminations of ∅ 3 mm at different depths were detected reliably.
Keywords: Material characterization using ultrasound; Polymer-matrix composites; Ultrasonic wave propagation; Ultrasound NDE.
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