A steel rotatingbeam test specimen has an ultimate strength

A steel rotating-beam test specimen has an ultimate strength of 80kpsi and a yield strength of 60kpsi. It is desired to test low-cycle fatigue at approximately 500 cycles. Cheek if this is possible without yielding by determining the necessary reversed stress amplitude.

Solution

Sol:

Given

Sut = 80 kpsi

Syt = 60 kpsi

N = 500 cycles

From graph of strength fraction (f) vs Sut

we can know that for Sut = 80 kpsi , fraction (f) =0.878

We note that below 10³ cycles on the S-N diagram constitutes the low cycle region

Sf = Sut *N^{(log f)/3}

Sf = 80 * 500^(log0.878)/3

Sf = 71.16 kpsi

The testing should be done at a completely reversed stress of 71.16 kpsi, which is below the yield strength, so it is possible.