The Effect of Stress Concentrations on Fatigue Life
Ignore stress concentrations and surface finish discontinuities at your peril!
Fatigue in metals refers to the development of cracking in component's as a result of cyclically applied loading, often over a long period. Most frequently this fatigue crack initiation is aided by stress concentrating features, which is not always appreciated by the fabricator or machine shop, where fabrication errors, scores, weld strikes or sharp corners can significantly reduce a components fatigue life.
Since fatigue cracks frequently originate from the surface of a component as a result of applied cyclic stresses, it is important that the combination of such cyclic stresses and stress raising features are well below the fatigue limit for the component, or else fatigue cracks will indeed initiate and subsequently propagate. It is important when considered with the applied cyclic stress that the stress intensity, K, (combined effect of both stress and flaw parameters) associated with the stress concentration features is below this fatigue threshold limit, ∆Kth. Such fatigue threshold values are available for specific material/environment combinations or can be measured experimentally to facilitate appropriate fatigue design. This is appropriate on both macro (large changes in geometry) and micro scales (surface finish discontinuities).
Although the effect of stress concentrations is well known a number of cases are often overlooked. These include cases where:
- Chamfers are machined in place of correctly radiused fillets. Although better than nothing, cracks will initiate at the relatively sharp radius at the interface of the chamfer and body of the component.
- Weld caps are not blended. The stress concentration associated with a convex weld cap (often erroneously believed to strengthen the weld) causes fatigue cracks to initiate in the HAZ at the toe of the weld. Improved fatigue performance can be achieved if the weld cap is ground flush with the parent material (obviously taking care not to reduce the material thickness).
- Shrink fit sleeves are employed. Although not a change in the geometry of the parent material shrink fit components introduce stress concentrations of similar magnitude – especially when the central bore of the sleeve is machined perpendicular to the end face of the sleeve (which is typically the case).
- Fatigue resistant bolts with carefully tapered fillet radii are damaged when the bolt is assembled through components that are not well aligned and the holes are not concentric - causing the fillet radius to be damaged during installation.
- Keyways are machined with sharp edged cutters. The failure to include a fillet radius at the base of a shaft keyway can often promote the initiation of a torsional 'peeling' failure. Significant performance improvement can be gained by including a fillet of up to a third of the depth of keyway.
Published in Technical Tips by Origen Engineering Solutions on 1 December 2014