Endurance Life of Aging Aircraft Components Predicted by Conductivity Changes in Aluminium 2024

the technical literature 3, we have described in detail our method of using eddy currents to compare precisely the electrical conductivity in a shot-peened metal surface to that of a similar, unpeened, surface. Encouraged by these results we began a research program to see if the surface conductivity of an ordinary aluminum plate was affected by the fatigue effects of repeated heavy loading. Since fatigue produces work hardening and other metallurgical effects we expected these effects to be observable in the conductivity. Further, in a bent specimen the stress is largest right at the surface where our technique is most sensitive. We constructed a machine which cyclically deformed a plate of 2024 aluminum to a maximum surface stress of about 25KSI(170 megapascals) tension and about 15KSI(103 megapascaIs) compression. The driving motor was turned off after each 30,000 cycles to permit us to make the surface conductivity measurements. The specimen was notched on two edges to insure that the peak stress and ultimate failure would occur in the area under observation. A photograph of the apparatus is shown in Figure 1. Indeed, we did find a significant variation in surface conductivity. The first twenty or thirty thousand cycles showed a progressive decrease in surface conductivity, presumably due to a