For steel products which are subject to moving loads, such as automobile springs, crankshafts, and power takeoff shafts, it is important to know the endurance, or fatigue limit.
Specimens are prepared for testing and are subjected to bending as they revolve. In every revolution of the test machine the stress is reversed twice-under load.
With the initial specimen, stress is applied which is far beyond the breaking point. Testing is repeated with different specimens and the applied stress is gradually reduced until a specimen will undergo ten million reversals without breaking. This stress is referred to as the fatigue limit.
The Fatigue Limit is the maximum stress that a material can endure for an infinite number of cycles without breaking. It is also referred to as the Endurance Limit.
Ten million cycles is the engineering community’s testing approximation for infinite.
Image source: Steelways chart dated 1955 on steel testing from my personal archive.
Stress and stress risers are words that we often hear in our shops. Usually when bad things have happened to our work. Here are 5 ideas to reduce stress in precision machined products and a brief tutorial on what it is when the engineers say “stress.”
Stress (when used by designers, engineers, and metallurgists) refers to the measurement of load on a part or test specimen related to the area under that load. Stress can be considered to be have three modes, axial (in line) , bending (you know what that means), or torsional (twisting or torque). The following graphic illustrates some stress states our parts may encounter.
Residual stress can be considered to be a kind of “internal pressure” in the material which may act in the same direction as the stress applied to the part. Because of this, it can actually reduce the load carrying ability of the part. This is what usually results in failures. Characteristics of the part may also contribute to the concentration of these internal stresses, leading to premature failure of the parts once in service and subjected to load.
Here are 5 ideas to reduce stress in precision machined parts.
Assure a smooth surface.
Use a larger not smaller diameter for threading.
Always maximize the fillet or radius between section or diameter changes.
Provide both pads and relief areas on parts where applicable.
Be alert to the fact that some materials are particularly notch sensitive, especially in the transverse direction.
Assure a smooth surface. The creation of a smooth surface prevents the concentration of internal stresses at sharp changes in surface. Parts with smooth surface finish are much less likely to fail than parts where deep grooves, tool marks or pits can allow stresses to build up. Use a larger not smaller diameter for threading. This is both related to the strength of the additional material as well as to the geometry and radii between change of dimensions. The more generous radius possible with the larger diameter for threading can improve the endurance limit of the part substantially. In heat treated 4340, the increase in radius from 0.015″ to 0.090″ increase the endurance limit from 34,000 to 65,000 psi. Always maximize the fillet or radius between section or diameter changes. Any design which allows stress to concentrate locally will promote fatigue failure. Generous radii and fillets are inexpensive insurance against premature failure. Make sure that the designer has provided both pad and releif areas on parts joining perpendicularly. Instead of having a single point or locus for the change in forces to be distributed through the part, pads and relief areas diffuse the stresses that would otherwise be concentrated, improving the performance of the part. Be alert to the fact that some materials are particularly notch sensitive, especially in the transverse direction. Many of the materials that we prefer to machine are resulfurized, and in these steels, the manganese sulfides can in fact lower the steel’s transverse mechanical properties. Also, cold drawing and or forging prior to machining can influence grain flow which can enhance the ability of the material to carry the load. The material the designer selected could be a large reason for the material’s ability to handle stress, or not.
There you have it. Stress = Load. Don’t give it places to concentrate on your precision machined parts.