Dear Qamar,

The start loading depends on many parameters.

1) Which surface finish method used

2) Which heat treatment used?

3) size of sample 4) Load Factor and Temperature.

Tensile strength of AISI 4140 alloy steel is 655 Mpa

So Endurance Limit will be 0.725*0.81*1*1.018*0.667 *(0.5*665) =132.58Mpa...

You can start from 90 Mpa. Now consider 90 Mpa as a bending stress. Then calculate how much weight you can put to get 90 Mpa as a bending load.

Dear Qamar;

Mr Mahesh Thorat answer is O.K ,please refer ASTM E466 for carrying out low and high cycle fatigue test.As it very clear your component is designed to work under elstic limit portion of stress-strain diagram so your load will be calculated based on yield strength of materialexcluding safety factor say about 75% of yield load.

Shivraj Singh Kasana

Smooth or notched sample?

With 90 MPa I guess you will have run-out (no failure)

Dear Mr. Mahesh,

Thank you for your detailed reply.

1. Sample is prepared by CNC machine and grinding is done to get surface finish of almost 0.8um.

2. Normalizing heat treatment is done and hardness is 255HBW.

3. Sample Diameter is 15mm and gauge length is 60mm.

4. Temperature is 24C.

5. Sample is smooth and of dumble shape.

Pls also guide which formula is used to calculate 132MPa and how 90MPa is selected.

Thanks for your help.

Regards

Qamar

Dear Qamar,

I have attached the picture where you will find the formula. 132 Mpa is the endurance limit below which component will not fail for infinite life. Above 132 Mpa the part will fail for finite life. I told 90 Mpa for experimental purpose.

Regards !

Mahesh

Dear Mahesh,

Thanks very much for this much details. can you please share research paper/book name from where you get above relation. It would be really helpful for me.

Regards

Qamar

ASTM E466 gives you the guidelines

Check on Shigley´s Design of Machine Elements on how to make an estimate for fatigue stress limit for rotatory bending.

Hello,

In attached MSc. Thesis, you may find useful information on parameters of rotating bending fatigue testing.

Force is derived from the formula "δa = 32*F*l/pi*d3". For this study, they have calculated forces according to 50%, 60%, 70% and 80% of yield stess value.

Jump to chapter 3

Dear Qamar the load applied on the test is depends on mechanical properties of the steel used in the test.