Question

A component made of steel subjected to completely reversed bending stress. Ultimate stress = 700 MPa, yield stress = 450 MPa and fatigue strength is 200 MPa.

If the component is subjected to completely reversed stress of 400 MPa the expected life is

• A. 35,000 cycles
• B. 80,000 cycles
• C. 10,000 cycles
• D. 15,000 cycles

Answer 1

The correct option is D 15,000 cycles

Given, $S_{ut}=700MPa$,

$S_y=450MPa$,

Se = 200 MPa

$0.9S_{ut}=700\times 0.9=630MPa$

Plotting on log-log curve :


Straight line equation is

$\Rightarrow \text{log}\sigma -\text{log}630=\frac{\text{log}630-\text{log}200}{3-6}(\text{log}N-3)$

Putting $\sigma =400MPa$

$N=15432\approx 15000cycles$