Fatigue Failure: How Can You Tell?
Fatigue was not always well understood in the engineering world. It was long thought that if a material was deformed within its elastic range it would not carry any of the residual stresses or deformation when the stress was removed.
How do I know if I have a fatigue failure?
Let’s assume that your product has experienced a failure of some sort. What might indicate the nature of the failure? If we look at the broken component, we can first look for a brittle or ductile failure mode on the fracture surface. In metals, a ductile failure is generally indicative of static failure. Conversely, fatigue in metals often causes a brittle fracture. In the simplest terms, in a ductile failure there will often be a large deformation or deflection within the part. This deformation gives an advance warning of eminent failure.
Fatigue failure occurs based on crack formation and crack propagation. As a crack initiates, it may be hard to notice much change until the crack size reaches a critical limit. By then it may be too late. A quick analysis of the fracture surface of a fatigue failure will often show features casually referred to as “beach marks”. These indicate the propagation of the failure from the initial cracks. Once the crack size has reached a critical level, it will propagate very rapidly until the fracture is complete.
How do I design against fatigue failure?
A few of the most important factors are material selection, stress concentration, surface finish, and material discontinuities.
- Material selection is paramount to all design considerations. Material selection may be limited by any of a number of factors including economic, environmental, and service restrictions. Selecting a material with a high endurance limit is good practice.
- Stress concentration is another key factor. Essentially, all sharp corners should be made into a radius if at all possible. Sharp corners provide stress concentration and are often responsible for the initial crack.
- Surface finish is another critical component. Strength of Materials classes teach a very important lesson: in many loading configurations, like bending and torsion, the critical stress is located on the surface. Therefore, a blemish-free surface will generally lend itself to a good fatigue life.
- Lastly, material discontinuities are inevitable on the microscopic level, but a good forming process will help to reduce them.
It is important to keep in mind the factors that affect fatigue failure the most and, if necessary or economically feasible, produce a conservative design. Conservative designing usually takes into consideration a “worst-case scenario”. In this instance designers usually assume fully reversed loading and often consider the Modified Goodman design criterion with a modest factor of safety when determining a theoretical endurance limit. Once the theoretical service life is determined and the part comes to fruition, mechanical fatigue testing is the next step for product verification in the design cycle.
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