Abstract
A study was made of three aspects of fatigue damage, namely, metallographic methods of detection, the nature of damage, and the control of damage. The work was carried out mainly on iron and copper, although a number of other metals ware also used for comparative purposes. The fatigue tests were carried out In reversed bonding at constant stress amplitude. The results of the metallographic work indicated that fatigue damage was in general associated with the formation of microcracks and pits in fatigue bands. Microcracks could be identified by lightly electropolishing the surface whereupon dark groove-like persistent bands were created. In contrast to earlier work, the electropolishing treatment was found to have a detrimental influence on fatigue strength, probably because the resistance of the individual microcracks to an applied stress was thereby reduced. Numerous observations were also made of extrusions, and it seemed clear that none of the existing theories adequately explained all the observations. The observations of fatigue bands, microcracks and extrusions was interpreted in terms of the ability of the material to undergo cross-slip. An attempt was made to control the fatigue processes, using coatings of various types. In particular, the effect of thin coatings of copper and nickel on iron was studied. In this case the mechanism of failure depended on the thickness of the deposit. Deposits of about one micron thickness failed by the penetration of "extrusions" originating in the substrate, through the deposit, whereas thicker deposits failed by a surface rumpling mechanism. A. number of other coatings, including chromised and chromium plated layers were also studied where it was established that the fatigue behaviour was mainly determined by the properties of the coat itself.