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Fatigue is a process of mechanical failure resulting from the application of repeated cyclic stresses. The stresses can be a combination of tensile and compression or fluctuating tensile stresses.
The result is that mechanical failure occurs at lower stresses than predicted by short-term tensile tests, provided there have been a large enough number of ‘stress reversals’.
If the stress level is reduced then a larger number of stress cycles is needed for failure.
Improvements in fatigue performance can therefore be achieved by reductions in service stresses and making a design life allowance for fatigue where parts may be subject to cyclic stresses such as vibration or bending in rotating shaft applications.
Stainless steels exhibit a ‘fatigue limit’ or ‘endurance limit’ during cyclic stressing. This means that there is a stress level, below which fatigue failure should not occur. This is determined from a series of fatigue tests, run to failure at various stress levels.
The fatigue stress limit is reached when failure does not occur after a million, (106), or 10 million, (107), cycles.
Test results are sensitive to test method and the surface finish of the test piece. Test results can show ‘scatter’ due to their complex nature of fatigue failure mechanisms.
However, results published by Avesta/Outokumpu researchers conclude that as a general rule austenitic and duplex stainless steels stainless have fatigue limits in air around their tensile 0.2% proof strength levels. They also note that the fatigue strength is also dependent on stress fluctuation frequency. Lower fatigue strength values occur as frequency increases.
A more conservative conclusion from Alan Haynes, (Nickel Institute), is that duplex types have fatigue limits around 50% of their tensile strength, Rm, (UTS).
The INCO data published by the Nickel Institute, (publication No 2978), supports the Avesta conclusion.
This data shows endurance limits from reverse bending fatigue tests shown in the table. These values have been converted from psi to MPa. and rounded to the nearest 5 MPa.
|AISI Type||Endurance Limit (MPa-N/mm2 )|
Fatigue resistance is dependent on surface finish, smoother un-notched designs and surfaces are helpful.
Materials that are notch sensitive are more prone to fatigue failure, so that as a general rule ferritic and martensitic types cannot be expected to be as fatigue resistant as the austenitic stainless steels. The higher strength of the duplex types may account for better notch sensitivity resistance compared to the ferritics.
Corrosion pits can act as notches. This results in lower fatigue lives in corrosive environments.
Where corrosion fatigue is a concern, materials with inherent corrosion resistance are better choices than those with just high fatigue strengths, but poorer corrosion resistance. Stainless steels can therefore be considered in preference to high strength alloy steels for corrosive environment service.
The good pitting resistance and inherent strength of the duplex stainless steels makes them useful choices where corrosion fatigue is a hazard.
As corrosion is time dependent, high fluctuating stress rates may however result in fatigue failure before corrosion damage occurs.
Laboratory test data on corrosion fatigue is difficult to generalise on due to the added dimension of corrosion environment in addition to stressing regime.
Austenitic stainless steels are sensitive to thermal fatigue due to the unfavourable combination of high thermal expansion rate and low thermal conductivity.
The stress raised during thermal cycling is proportional to thermal expansion coefficient, elastic modulus and temperature differences.
Expansion allowances must be made when designing in austenitic stainless steels for cyclic, (fluctuating), elevated temperature applications.