The Early Detection of Fatigue Damage by Exoelectron Emission and Acoustic Emission Available to Purchase

The paper describes the development of nondestructive test methods capable of determining the extent of fatigue damage and providing a means of predicting the future safe life of aerospace materials and structures. The fatigue process of 1100-0 aluminum was studied by means of exoelectron emission and acoustic emission measurements. The exoelectron observations were made in both vacuum and air. The exoelectron emission measurement in vacuum was accomplished by counting the emission events for selected time intervals. The measurements in air were performed using a specially developed current-measuring system with a resolution of 10^-15 A. The acoustic emission measurement in air was based on the number of acoustic events occurring in selected time intervals during fatigue. The exoelectron emission curves exhibited an early rapid rise and fall of emission intensity, and again a rapid rise near the end of the test. The initial emission decrease coincided with metallurgically observed changes of surface slipbands. There was also a trend indicative of a stress-independent relationship between the change of exoelectron emission current and the percentage of life after a selected number of fatigue cycles. Analysis of the acoustic emission data showed a marked increase in emission rate in less than 50 percent of the fatigue life of the material.