A Fatigue Life Assessment of Aircraft Alloys Using Fractal Analysis in Combination with Eddy Current Testing

For the forecasting of the fatigue life of metallic constructions it is important to understand the mechanism that leads to the failure. Neither, the continuum mechanics nor the micromechanics provide sufficient answers. However, a basic understanding of structural changes on different scaling levels is given by the physical Mesomechanics derived for a medium with local structure. For ferromagnetic steel, these mesomechanic structures were analyzed using surface topography as well as Barkhausen noise signals. The acquired results, described in earlier announcements, allow to assess the actual damage state of the material and to estimate the residual time before fracture appears macroscopically. Currently, attempts were made to transfer the fractal concept to non-magnetic materials, especially the feasibility of applying this method on Al7075 and Ti6Al4V aerospace materials and, in particular, the successful application of Eddy Current (EC) approach, with the potential to transition this inspection to an aircraft for an in-situ solution for Structural Health Monitoring (SHM). With the development of optimal test procedures, it became possible to establish the occurrence of mesomechanic deformations at the material surface by the use of topography images of confocal microscopy (CM). The efforts were concentrated on the development of Pulsed Eddy Current (PEC) approach, combined with sophisticated fractal analysis algorithms based on short pulse excitation and evaluation of the EC relaxation behavior. This article explains the idea, shows the measurement setup, and discusses the outcome of this work, which is the result of the cooperation with Wyle Laboratories Inc. on a U.S. Air Force research project, BAA-0812-PKM.