Effects of prior fatigue-stressing on the impact resistance of chromium-molybdenum aircraft steel

Author / Creator

Kies, J. A., author

Available as

Online

Summary

Fatigue continues to be an important cause of service failures in highly stressed structural members of aircraft. Fatigue cracks of detectable dimensions usually cannot be found until after a relat...

Fatigue continues to be an important cause of service failures in highly stressed structural members of aircraft. Fatigue cracks of detectable dimensions usually cannot be found until after a relatively long period during which suitable preparation for such cracks is made by continued stressing. Once such a crack is started, however, complete failure may be anticipated within a short time, often 10 percent or less of the total service life. Detection of fatigue cracks, even of visible dimensions, is often difficult and is usually practically impossible during operation. There is urgent need for some means of detecting and evaluating the deterioration brough on by fatigue-stressing before cracks appear and of evaluating the damage caused by fatigue cracks after they have reached detectable size.

In attempts to detect and evaluate damage of this kind, the impact behavior of normalized SAE X4130 steel was studied after a variety of repeated stress treatments. Fatigue specimens of several types were used and the effects of surface finish, rest periods, stress amplitude, mean stress, stress concentration, and temperature during repeated stress received consideration. Comparative impact-test results were obtained for several temperatures ranging from room temperature to -78 degrees C.

The results serve to emphasize the seriousness of fatigue cracks, particularly at low temperatures, but are reassuring in the cases in which fatigue cracks are absent or have not developed to a size permitting detection.

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