Seth Eliot wrote:
What are "S, B, or A" in non-military metallurgical terms?
Lee Gearhart
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
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