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# S/N curves for unnotched 7050-T7451 plate

## S/N curves for unnotched 7050-T7451 plate

(OP)
Hi Guys,
I recently noticed that there is significant difference between the predicted life with S/N curves shown in Figures 3.7.5.2.8(b)and 3.7.5.2.8(e) in MMPDS-09. Both of them are "for unnotched 7050-T7451 plate, long transverse direction, t/4 specimen locations". The second one is giving very conservative results comparing to the first one. I would like to know how we can explain this significant difference? I also noticed that these curves are not shown in MMPDS-15.
Thanks,
A.

### RE: S/N curves for unnotched 7050-T7451 plate

Give us the entire titles please.
Different thicknesses? Test conditions? confidence factor?
There are lots of possibilities.

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P.E. Metallurgy, consulting work welcomed

### RE: S/N curves for unnotched 7050-T7451 plate

(OP)
Figure 3.7.5.2.8(b):
Product Form = 4.25 to 8.50 inches thick
Specimen Details: Unnotched 0.25 inch diameter
Surface Condition: Polished, final surface finish unspecified
Test Parameters: Loading - Axial, Frequency - 20 Hz, Temperature - RT, Environment - Air
No. of Heat/Lots: Unspecified
Sample Size = 57
Std. Error of Estimate = 0.183
Standard Deviation = 0.814
R^2 = 95%

Figure 3.7.5.2.8(e):
Product Form = 1 to 6 inches thick
Specimen Details: Unnotched 0.25 and 0.30 inch diameter
Surface Condition: Not Specified
Test Parameters: Loading - Axial, Frequency - 800 cpm and unspecified, Temperature - RT, Environment - Air
No. of Heat/Lots: 15
Sample Size = 85
Std. Error of Estimate = 0.507
Standard Deviation = 0.794
R^2 = 59%

### RE: S/N curves for unnotched 7050-T7451 plate

Since the product forms overlap, and they are both reasonably sized samples I am left looking at the test freq.
800cpm or 13Hz vs 20,000Hz, that is a big difference and is likely the reason for the different values.
The second one is nearly low cycle fatigue.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

### RE: S/N curves for unnotched 7050-T7451 plate

(OP)
Thank you for your reply. I think that 800 cpm (cylces per minute) is equivalent to 800 /60 = 13.3 Hz (cycles per second) versus 20 Hz (not 20,000 Hz).

### RE: S/N curves for unnotched 7050-T7451 plate

Sorry, yes you are correct.
One thing that puzzles me is the large amount of scatter in the second set of data.
That just does not sit right, and would lead to a much lower value in order to have the same level of confidence.

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P.E. Metallurgy, consulting work welcomed

### RE: S/N curves for unnotched 7050-T7451 plate

(OP)
Yes, I agree with you. I also noticed that there are no S/N curves for 7050-T7451 in MMPDS-15. I sent an email about these S/N Curves to Battelle Memorial Institute, and I didn't receive any answer until now.

### RE: S/N curves for unnotched 7050-T7451 plate

The person from Battel Columbus that attends the AMS specifications meetings is Jana Rubadue.
This is a dotorg email.
Just don't tell here where you got this

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P.E. Metallurgy, consulting work welcomed

### RE: S/N curves for unnotched 7050-T7451 plate

(OP)
Perfect. Thank you.

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