• #### A, B, and S basis

From mr.marur@gmail.com@21:1/5 to All on Sat Apr 6 22:06:17 2019
On Wednesday, 13 August 1997 12:30:00 UTC+5:30, Gearhart, Lee (corp) US wrote:
Seth Eliot wrote:

What are "S, B, or A" in non-military metallurgical terms?

Lee Gearhart

Dear Gearhart,

Thanks for your answer. Now I have better understanding of confidence level.

I tried to go through Handbook to learn how to compute A basis property. I find difficult to understand. Can you please suggest a Tutorial in this regard?

India

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• From vikas.shrivas@gmail.com@21:1/5 to All on Tue Dec 31 23:06:17 2019
On Wednesday, August 13, 1997 at 12:30:00 PM UTC+5:30, Gearhart, Lee (corp) US wrote:
Seth Eliot wrote:

What are "S, B, or A" in non-military metallurgical terms?

To elaborate on Christopher Wright's answer:
An S basis value is taken from a specification. For example, if I buy hot finished, annealed, 304 stainless steel bars using ASTM A276 as my purchasing specification, the bars have a yield strength of 30 ksi and a tensile strength of 75 ksi. If I test them and they show lower values, I can reject the material. So my S basis yield strength is 30 ksi, and tensile strength is 75 ksi.

Yet I do not want to use these values if I am designing something for which failure would be catastrophic: a jet engine, for example. Strength values that are based on a statistical analysis of many tests are preferred for critical designs, and this is where A and B values are used.

A basis means 95% confidence of 99% exceedance, which is best explained as IF a test program of 100 tests were run, and
IF this test program were repeated 100 times,
THEN at most 1 out of 100 test results will be below the A basis value in at least 95 of the 100 test programs.

The B basis values mean 95% confidence of 90% exceedence, and is similar. Section 9 of Military Handbook 5, Metallic Materials and Elements for Aerospace Vehicle Structures contains the statistical formulas for determining basis.

Sorry to drone on, but I ve had a number of discussions , usually with new design engineers, on why they can t use the typical strength value found in a manufacturers brochure.

Lee Gearhart lgearhart.inc@moog.com

Thanks Gearhart for enlighting us. Can you suggest very basic reading material about A-basis, B-basis or S-basis ?? Which will be more normally.
Vikas
(India)

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