Published: Jan 1960
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Minimum-weight, high-strength joints are essential in helicopters designed for long service and low maintenance. In practically all joints subjected to cyclic loading varying degrees of fretting occur. Cold working usually results in substantial increases in the fatigue strength of joints. The magnitude of fatigue-strength increase ranged from 15 to 300 per cent for applications discussed in this paper. The greatest improvements were found for aluminum joints processed by cold rolling. It is concluded that each of the three different cold-working methods that were tested is applicable to particular types of joints, and no one method can be used universally. However, shot peening is the process that is generally to be preferred from the standpoint of cost, reproducibility, availability of equipment, and availability of a large amount of test data to assist the designer. Cold rolling has the advantage of providing maximum depth of cold work for cases where severe fretting is a problem. Roll peening results in close-tolerance holes with excellent surface finish, but it produces only a thin compressive layer and, therefore, should be used in cases of light or moderate fretting. Roll peening and cold rolling are practical production processes, but both have the disadvantage of requiring extensive testing and development prior to use in production.
Waters, K. T.
Boeing Airplane Co., Morton, Pa.