(NASGRO 3 versus NASGRO 4?)
The real question that needs to be asked is: where is the “dadN test data” upon which the NASGRO 3 and NASGRO 4 databases are based?
I’ve heard arguments about the differences in the Forman equation for years now and believe we have become blinded to the central issue … BOTH ARE CURVE FITS.
Somehow the community has been lead to believe that curve fit equations are better than honest test data.
The inaccuracy pointed out in ignoring the double knee indicated by the real test data versus curve fit da/dN is far more significant than the threshold issue, which NASGRO 4 supposedly addresses. Of course it is wrong to ignore either but to make a point.
If one would download http://www.tc.faa.gov/its/worldpac/techrpt/ar05-15.pdf or the reference located on the AFGROW documentation web page, and go to FIGURE B-4a. NASGRO EQUATION FIT FOR 2024-T3 CLAD AND BARE SHEET, L-T (M2EA11AB1) WITH POSITIVE R VALUES on page B-8, we might be able to understand all the tumult by examining what curve fitting does.
For example suppose a spectra which has many R=0 cycles.
And consider the following:
|
Delta K
|
5
|
10
|
20
|
|
Actual dadN
|
2.0E-7
|
6.5E-6
|
2.0E-7
|
|
Curve Fit dadN
|
3.5E-7
|
3.5E-6
|
3.0E-7
|
|
Act/CF
|
0.57
|
1.85
|
0.66
|
The point is:
The life prediction at DK = 5 would be 0.57 times shorter than it should be.
The life prediction at DK = 10 would be 1.85 times longer than it should be.
The life prediction at DK = 20 would be 0.66 times shorter than it should be.
Your guess is a good as mine as to how accurate the cumulative life prediction would be but this certainly throws some doubt on it … right?
I mean with this kind of accuracy I could just about make up some dadN and be just as accurate.