Hole Coining To Prevent Fatigue Failures

Author:  Phillips, Austin
Source:  Douglas Aircraft engrg. Paper No. 1068
Doc ID:  1962002
Year of Publication:  1962
Abstract:  
Functional needs of modern aircraft and missile components demand that metals work harder and more effectively than ever before. Metals react to stresses imposed upon them whether by manufacturing processes or by service, and when the stresses are repeatedly applied, the metal may reach its fatigue limit and fall. How can we give metals a built-in resistance to fatigue? The Douglas Company has used for many years a process designated as "coining." Very simply, it is the impressing of a groove around an opening such as a bolt hole, slot, or window frame. Essentially, coining produces compressive residual stress adjacent to these high stress points. It is known that serious fatigue troubles have generally been the result of unusually high stress concentration factors existing in significant structural members. One of the primary origins of fatigue failure in structure is associated with holes and cut-outs. These holes may be bolt and rivet attachment holes; passage holes for control line, plumbing or other equipment; or access doors and windows. These locations represent a stress concentration and the effect of the concentration can be severe. A major step toward improving the overall fatigue life of any structure, therefore, can be achieved by improving the detailed fatigue resistance of the hole itself. Coining is offered as a means of significantly improving fatigue life without adding additional weight and without necessitating major design changes. Failures may not occur at the same locations in coined structural specimens as in comparable uncoined specimens, since coining often strengthens the vital areas so successfully that failures are ultimately found in regions of lower stress. Coining is the addition of an impressed groove around a hole or other open configuration. A die or rolling wheel is used under pressure to form the groove. The formation of the groove causes displacement of the metal toward the center of the opening. The metal adjacent to the opening is simply forced inwardly so that it is usually necessary to ream out the hole to its original diameter if a fastener is to be inserted. Both photoelastic and x-ray diffraction studies have verified that a favorable compressive residual stress at the hole edge is the factor in coining which is responsible for the fatigue life improvement. Under cyclic tension loading of structure it is a well recognized fact that control of residual stress is an important factor in the control of fatigue strength, i.e., a residual compressive stress lowers the applied mean stress and hence the fatigue life is improved. Designers are becoming increasingly aware that residual tensile stresses sometimes rob them of some of the potential strength of load-carrying members. There are several other mechanical means known for putting compressive stresses in the surface--the most well-known being cold working by surface rolling, abrasive barrel tumbling, and shot peening. Heat treating operations, carburizing, nitriding may also introduce some beneficial stresses into the surface. Further processing of the metal part, such as machining or grinding unbalances these internal stresses and can result in harmful tensile stresses, seriously reducing fatigue strength. Residual stress, therefore, is seen as an agent which can work for you or against you.