Successful determination of residual fatigue life requires a comprehensive understanding of the fatigue related material deformation mechanism. Neither macroscopic continuum mechanics nor micromechanic observations provide sufficient data to explain subsequent deformation structures occurring during the fatigue life of a metallic structure. Instead mesomechanic deformation on different scaling levels can be studied by applying fractal analysis of various means of nondestructive inspection measurements. The resulting fractal dimension data can be correlated to the actual material damage states, providing an estimation of the remaining residual fatigue life before macroscopic fracture develops. Recent efforts were aimed to apply the fractal concept to aerospace relevant materials AA7075-T6 and Ti-6Al-4V. Proven and newly developed fractal analysis methods were applied to eddy current (EC) measurements of fatigued specimens, with the potential to transition this approach to an aircraft for an in-situ nondestructive inspection. The occurrence of mesomechanic deformation at the material surface of both AA7075-T6 and Ti-6Al-4V specimens could be established via topography images using confocal microscopy (CM). Furthermore, a pulsed eddy current (PEC) approach was developed, combined with a sophisticated new fractal analysis algorithm based on short pulse excitation and evaluation of EC relaxation behavior. This paper presents concept, experimental realization, fractal analysis procedures, and results of this effort.

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.

Ferromagnetic materials are essential for data recording devices. For inductive or magnetoresistive (MR) sensors softmagnetic thin layer systems are used. Optimal performance of these layers requires homogeneous magnetic properties, especially a pronounced uniaxial magnetic anisotropy. Furthermore, microstructural imperfections and residual stresses influence the magnetic structure in the layer system. Barkhausen Noise Microscopy enables the characterization of such thin layers. By cycling the magnetic hysteresis of ferromagnetic material electrical voltages (the Barkhausen noise) are induced in an inductive sensor. Miniaturization of the sensor and the scanning probe technique provides resolution down to few micrometers. Two materials were examined in terms of their structure, thickness, residual stresses and heat treatment condition: Sendust, used in inductive sensors and nanocrystalline NiFe, used in MR-sensors. In quality correlations to Barkhausen noise parameters were found. For representative sample a quantification of residual stress distribution could be established employing X-ray stress analysis.

This paper gives a brief overview of present NDE techniques that use infrared cameras. Starting with physical basics, current state of the art and future trends are discussed. Two new methods for materials characterization that use heat dissipation and hot air for heat stimulation will be presented. A new technique called fan thermography, has high potential for corrosion detection under intact coatings. Using this technique, low-cost uncooled infrared cameras are sufficient for obtaining excellent, highly sensitive imaging results.

Ferromagnetic materials are essential for data recording devices. For inductive or magnetoresistive (MR) sensors softmagnetic thin layer systems are used. Optimal performance of these layers requires homogeneous magnetic properties, especially a pronounced uniaxial magnetic anisotropy. Furthermore, microstructural imperfections and residual stresses influence the magnetic structure in the layer system. Barkhausen Noise Microscopy enables the characterization of such thin layers. By cycling the magnetic hysteresis of ferromagnetic material electrical voltages (the Barkhausen noise) are induced in an inductive sensor. Miniaturization of the sensor and the scanning probe technique provides resolution down to few micrometers. Two materials were examined in terms of their structure, thickness, residual stresses and heat treatment condition: Sendust, used in inductive sensors and nanocrystalline NiFe, used in MR-sensors. In quality correlations to Barkhausen noise parameters were found. For representative sample a quantification of residual stress distribution could be established employing X-ray stress analysis.