Abstract
Most approaches to the assessment of fatique damage have been aimed at detecting fatigue cracks in the metal. This paper describes techniques for measuring the early accumulation of surface have stemmed from our investigations of exoelectron emisson (i.e. enhanced photoemission), which ocurs when the surface deformation produces microcracks in the natural the surface oxide film. Direct observations in a photoelectron microscope have shown that the oxide is ruptured by crystallagraphic slip in the underlying metal, so that the intensity of exoelectron emission is intimately related to the amount of fatigue deformation. An ultraviolet laser scanning system provides quantitative information on the distribution and extent of the deformation. An ultraviolet laser scanning system provides quantitative information on the distribution and extent of the deformation. The remaining fatigue life may be predicted from a quantitative assessment of damage. An electrochemical method, now being developed, provides a simpler means of measuring the development of microcracks in the surface oxide. The basic principle is to measure the flow of charge during electrochemical reoxidation of these microcracks. The distribution of the fatique damage is again revealed by a simple scanning procedure. Fatigue cracks only ∠50 μm in length have been detected in aluminum and the electrochemical reoxidation currents again provide a method of predicting fatigue life.
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