Who can help me with S-N Curve equations for bending fatigue? 2

[Henryzhang]

Mr.LCUBED has supplied the S-N equations for rotating beam test, I doubt very much if anyone can supply S-N curve equations based on bending fatigue data for any steel or alloy.I can't find the Reference: Shigley, Joseph Edward, and Mischke, Charles R., Mechanical Engineering Design, Fifth Edition, McGraw-Hill Book Co., Inc., 1989, Section 7.

Thanks million

 

[hacksaw]

try

http://www.amazon.com/exec/obidos/tg/detail/-/0070568995/104-2528612-0223159?v=glance

or your local university library.

you might also contact the Linda Hall Lib. as they have a good information retrieval service

 

[metengr]

As an option, the following equation for describing S/N curves is from "Fracture Mechanics of Engineering Materials" Hertzberg. For reversed bending, the equation below is for elastic strain controlled fatigue (stress amplitude);

stress amplitude = fatigue strength coefficient* (2 N(f))^b

N(f) is the cycles to failure
2N(f) is the number of load reversals to failure

b is the fatigue strength exponent

For an AISI Type 1015 steel
b= -0.11
fatigue strength coefficient where 2N(f) =1, 120 Ksi

For an AISI Type 1045 carbon steel (quenched and tempered-225 BHN)

b= -0.095

fatigue stregnth coefficient, 178 Ksi

 

[Henryzhang]

Hello Metngr

Thank you for your rapid reply,and I really appreciate your valuable information.

Lcubed's post is as follows:

"A reasonably accurate approximate S – N curve for any steel can be constructed using the following equations given in this reference.

Log Sf = Log a + b Log N = Log (1.62 Sut) + Log N-0.0851

a = (0.9 Sut)2 / Se = 1.62 Sut

b = -(Log (0.9 Sut / Se)) / 3 (= -0.0851 when Se = .5 Sut)

Se = 0.5 Ftu = Endurance Limit = Stress corresponding to “infinite” life of 1,000,000 or more cycles.

Sf = Stress corresponding to a fatigue life, N, of 1000 to 1,000,000 cycles inclusive.

These equations are based on rotating beam test data, and must be adjusted for stress ratios other than R = -1."

I mean I want to predict bending S-N curves of some steel for design use,just like the formula given above by Lcubed.That is,We give a series of bending cycles,and get a series of bending fatigue strengths,with the alreday known static strngth as ultimate tensile srength or hardness.based on these data,we can draw a S-N curve for any steel.

In your supplied formula,I wonder the difference between N(f) and 2N(f).and do you mean I can write the formular as follows:
For 1015: Sf=a*1120^b
where b=-0.11
If I give a N(f)=10000 cycles, How can I calculate the Bending strength Sf?

Thanks again.

 

[metengr]

The formula that I posted above is known as Basquins Rule. The cycles to failure is converted to load reversals for purposes of plotting the data. Basically, 1 cycle to failure = 2 load reversals.

The stress amplitude, which is stress range/2, is plotted versus 2N(f) or load reversals, where R=-1. This is common for S/N fatigue data curves.

Thus, if you have N(f) = 10,000 cycles, the 2N(f) = 20,000 reversals

Therefore using Basquins Rule;

stress amplitude = 120 Ksi * (20,000)^-0.11

stress amplitude = ~ 40.4 Ksi where R=-1

 

[Henryzhang]

Hello Metengr

Thanks again for your valuable post!

Can I use this equation for predicting bending fatigue S-N curve of any steel? for example, low alloy carburizing steels(8620),quench and tempered steels(4140),nitriding steels,low carbon steel sheets or bars? What fatigue stregnth coefficient should I use?
I want to predict bending S-N curves of any steel for design use with no test data avilable.

Can I use the present known static strngth as ultimate tensile srength or hardness to predict S-N curve for any steel?

Forgive me for so much questions.

Best regards

 

[metengr]

Henryzhang;
The equation can be used for most carbon and low alloy steels. You will need to determine two important parameters; the fatigue strength coefficient and the fatigue strength exponent. I believe that the reference book which I cited earlier ("Fracture Mechanics of Engineering Materials" Hertzberg) contains these parameters for many steels and other materials, as well. I would recommend you obtain a copy of this reference book.

 

[Henryzhang]

Metengr

Thanks a lot!
In China,I can't get the information of this book.I have contacted the Author Dr.Hertzberg,He said He Had retired and no longer worked in this field. He can't give me any help. Just as you said,Can you recommend me where I can get this book,or if you don't mind,send me the equations for any metal material.Thanks again.

Henryzhang

 

[metengr]

There are various web sites where you can purchase this book (used or new);

http://www.newtonlibrary.com/si/A001710.html

http://www.brook-tech.fsnet.co.uk/stress_books/0471012149.htm