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Ultrasonic Flaw Detection Tutorial

9.2 CDS/Creeping Waves

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.

Fiberglass and composites

  1. 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.

  2. 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.

  3. A 70 degree direct longitudinal wave (blue path) that will create a tip diffraction signal from very deep cracks.

The presence or absence of these signals are the key to interpreting test results. For a detailed discussion of typical test setup and interpretation, see Detection and Sizing Techniques of ID Connected Cracking.

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