Fillet rolling

If you regularly read the RET-Monitor articles or Race Engine Technology magazine, the value of residual compressive stress will not be lost on you. When considering the design of a critical part, it is common to specify a surface treatment that leaves the surface of the part in a state of compressive stress. Such processes have often been used in a remedial sense in the past, as a quick-fix for a fatigue problem. With the increasing use of stress analysis software, however, engineers are more aware of points of high stress before a breakage happens, and it is now common to see such processes considered before a part is manufactured.

The surface treatment may be applied generally to all surfaces of the component, as is common with some surface hardening techniques such as nitriding or nitrocarburising. Equally, the surface treatments may be applied locally, only to critical surfaces. In many race engine components, fillets are a weak point. Those with mechanical design textbooks to hand can browse tables and charts to find stress concentration factors for fillets in all manner of components. Books such as Peterson's "Stress Concentration Factors"* even deal with specific cases such as fasteners and crankshafts.

There are a number of mechanical surface treatments that can be used to improve the endurance limit of a component. Not only can we increase fatigue life for a given component by using the correct treatment, we can also optimise the component to be lighter, but still have acceptable service life. Fillets, as mentioned, cause us particular problems. Many components that have failed due to fatigue will have cracks that started in a fillet. Race engine components are usually generously filleted, but space constraints often prevent us from reaching a level of stress in the part that will give us the operating life we want.

Where we have components with critical fillets at the junction of a cylindrical surface and a perpendicular surface, or undercut fillet, we can use fillet rolling to increase fatigue life in these areas of high stress concentration. The process cold-works the surface of the material, plastically deforming it and placing the top layer of the component into a state of compressive stress. When the compressive stress is superimposed on the service stress, the actual state of stress at the surface - where most fatigue cracks begin - is lowered, thereby increasing fatigue life.

Fillet rolling has some advantages over other mechanical or thermal processes in that it is a 'clean' process that can be done in a conventional machine shop, albeit usually in a special machine. By contrast, peening using small media requires a special booth, although many people will send such work out to a specialist company.

The fillet-rolling process is particularly suited to parts such as critical fasteners or crankshafts. Con rod bolts, for example, often have the underhead fillet rolled in racing applications. In a well-designed bolt, this part of the shank is often the point of lowest fatigue life, as it experiences high nominal stresses that are then subject to a high stress concentration factor. When discussing surface treatments with racing fastener manufacturers in the past, a number used fillet rolling for their most highly stressed products.

Reference
Peterson, R.E., "Stress Concentration Factors", Wiley, 1974, ISBN 0-4716-8329-9

Written by Wayne Ward