A, B, and S basis

[Gearhart, Lee (corp) US]

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 lgearh...@moog.com

[J.A.Morris]

The devil is in the details.

The specification acceptance values are in EVERY procurement
specification. These values may or may not be statistically based. If
there are there are statistical A or B values for Ftu and Fty then the
statistical variation of these values are used to establish the other
minimum design properties such as bearing strength. The set of design
properties are then termed A or B Basis. If proper statistical
requirements cannot be met, then the design minimum properties may be
established as a function of the specification acceptance values. These
sets of design properties are termed S Basis. This situation commonly
occurs when a vendor has a unique material hence only one source. A and
B basis require three sources.

Jet engines like any other machine have primary and secondary structure.
What is acceptable for clips and brackets may not be statistically
acceptable for rotating components. The application must ALWAYS be
considered.

J.A. Morris
Senior Specialist Metallurgical Engineer
The opinions expressed are my own.

[mr.m...@gmail.com]

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?

Madhukar
India

[vikas....@gmail.com]

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?
>
>

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)