CDS wedges are specialized plastic wedges used for crack detection and quick determination of whether a crack is ID connected. This technique can further indicate if the crack is located in the inner, middle or outer third of the part thickness. CDS stands for Creeping-Direct-Shear, also known as the 30-30-70 technique. CDS wedges use single element transducers to simultaneously generate three waves in the test piece.
An OD creeping wave creates a 31.5 degree indirect shear wave (red path in drawing above), which mode converts to an ID creeping wave. This will produce a reflected signal from all ID connected cracks.
A 30 degree shear wave (orange path) will reflect off the material ID at the critical angle to create a 70 degree longitudinal wave. A signal will be received by the transducer from cracks that extend into the midwall region or deeper.
A 70 degree direct longitudinal wave (blue path) that will create a tip diffraction signal from very deep cracks.