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.