Volume 21, Issue 3 (May 1993)
The Gel Electrode Revisited: Fatigue Crack Detection with a Polymer Film
The gel electrode provides a very seasitive technique for detecting and imaging fatigue cracks in metals. In the past, its utility has been restricted by the requirement that, prior to the formation of a crack, the metal surface must be associated to form a thin insulating film. This film is ruptured as a crack forms, so that an impressed current from a gel electrode flows preferentially to the crack and forms an image. In this study, the anodic film has been replaced by a cathodically deposited polymer film. This film is much easier to apply, is applicable to any metal, and can be applied either before or after crack formation. If applied prior to fatigue loading. it is ruptured in the same manner as an anodic film, but it provides much better insulation, so that freshly formed cracks can now be detected in materials such as cast irons. If applied after fatigue loading, the film does not cover up the previously formed cracks so that the gel electrode current still flows preferentially to the crack. Both sequences of application are illustrated by images of fatigue cracks in low-carbon steel, nodular iron, and austempered ductile iron. The only difficulty experienced has been with previously formed cracks held tightly closed by retained compressive stresses. Otherwise, this combination of a polymer film and the gel electrode will detect cracks as small as 0.3 mm in length.