Overlook stress concentrations at your peril
The effect of stress concentrating features such as notches, fillets, grooves and threads, has been known for many years and has been documented in many texts. However they are often overlooked and cause failure by locally increasing stresses and aiding fatigue crack imitation.
Stress concentrations are typically evaluated by using graphical data, developed historically from photo elastic studies, to determine a multiplication factor Kt used to increase the nominal stress levels in the component. With the increased reliance on finite element analysis in design the emphasis on these analytical methods has decreased. However, often local details such as small fillets, chamfers and welds will not be modeled accurately in a finite element analysis and local high stress areas in these regions of discontinuity can be discounted as being numerical anomalies and be overlooked - particularly when the analysis is being undertaken by poorly qualified/inexperienced engineers.
In a number of cases, illustrated by the following examples, the effect of stress concentrations has been overlooked. In at least three cases investigated by Origen the fillet radius at the bottom of a keyway has been cut with sharp edged cutters resulting in a sharp notch at the base of the keyway. This oversight resulted in the initiation of small fatigue cracks and the outer layer of the shaft 'peeling away' as the fatigue crack propagates subsurface from the edge of the keyway.
In other cases failure has been caused as a result of the discontinuities formed when worn components are machined down for rebuilding by hard chroming or metal spraying and the notch at the end of the reduced section is not blended. Owing to the change in the properties of the built-up and parent material, this ‘hidden’ discontinuity remains after the rebuilding (even if the built-up material has bonded successfully) and the associated stress concentration increases the local stresses allowing subsurface cracks to initiate at the discontinuity. This is less likely to occur when the component is built up using processes where the material deposited has similar properties to the base material or where the step is blended.
The final example involves cases in which components have been shrunk or pressed over a shaft or where shrink couplings with sharp edges/marked changes in diameter have been employed. These changes in local stiffness increase the effective local stress in the shaft adjacent to the discontinuity by a factor of between 2 and 3 depending on the stiffness of the components, the degree of interference and the size of the discontinuity. This increase in stress obviously has a significant effect on the fatigue initiation and subsequent fatigue life of the components and leads to premature failure.
The lesson is simple - be aware of the effects of geometric discontinuities and take steps to avoid these if you want to prevent premature failure!
Published in Technical Tips by Origen Engineering Solutions on 1 January 2017