Low cycle fatigue, Coffin's law

[Name:riley]

Homework Statement

Hi, I require some assistance using coffin's law.
I have the strain range and cycles to failure but need coffins slope- 'c'.
C2 can then be found after the other values are known.

Homework Equations

Δε^pl=C2/N^c
Where Δε^pl is plastic strain range, N is the cycles to failure and C2 and c are constants.

The Attempt at a Solution

I have the strain range and cycles to failure data in excel. No problems here.
It seems from the above graph that coffins slope-c is the slope of the Log(stress amplitude) and Log(cycles to failure) regression line.
So I use excels slope function and get -0.05

The textbook and other sources say this value is usually +0.5.

using the 'c' I attained in the equation results in lower cycles to failure for lower strains, the opposite of what is expected.

Thanks for any help you can offer.

 

[KataKoniK]

Hi there,

Thank you for reaching out for assistance with using Coffin's law. Coffin's law is a widely used empirical relationship in materials science and engineering to predict the fatigue life of materials. It relates the plastic strain range (Δε^pl) to the number of cycles to failure (N) through the equation: Δε^pl=C2/N^c, where C2 and c are constants.

To find the value of c, you can use the slope of the log(stress amplitude) and log(cycles to failure) regression line. However, it is important to note that the value of c can vary depending on the material and testing conditions. In general, a value of +0.5 is commonly used for c, but it is not uncommon to see values ranging from 0.3 to 0.7.

If you are getting a negative value for c, it could indicate that there may be some issues with your data or the regression analysis. I would recommend double-checking your data and making sure that you are using the appropriate regression method and analysis. You may also want to consult with a materials science expert to ensure that you are using the correct value for c for your specific material and testing conditions.

I hope this helps and good luck with your research!