Learning Forum

[Jeone]

Hi,

Even though, the cycle is higher than 10^4, the max stress result is higher than the material's yield strength. The low cycle fatigue limit is 10^3, isn't it? When I search on the internet, the limit changes. 

[danielshaw]

By limit, do you mean the cycle value at the transition between high cycle fatigue (HCF) and low cycle fatigue (LCF) where the stresslife method (SN) becomes invalid?  If so, then there is no precise cycle limit.  In theory SN is only valid for stresses below yield which results equates to a allowable cycles of about 10^3 for many ductile metals.  So as a "rule of thumb", 10^3 is often considered to be the transition from HCF to LCF.  However, the true value varies based on the material and some SN curves extend below yield, so SN is often used even if the stress is slightly above yield.

[Jeone]

You are right, but I try to say that the material's yield strength is 480 MPa, and when the fatigue analysis is done in nCode, I see that the result is 520 MPa, but the cycle is higher than 10^4 (almost 15737). In this way, due to 520 MPa, shouldn't the result be less than 10^4?

[danielshaw]

Not necssarily.  It depends on the SN curve.  DesignLife does not use the yield stress in SN fatigue.  It uses the calculated alternating stress and the SN curve to predict the fatigue damage/life.  See the SN curve below for basic structural steel from Engineering Data.  Its YS is 250 MPa, which correlates to a N of about 12,000 cycles.  So, an alternating stress slightly greater than 250 MPa will predict more than 10,000 allowable cycles.  For other materials, the spread maybe larger.  What is the N value on your SN curve for 480 MPa?