A product must be covered by an industry specification prior to being considered for inclusion into MIL-HDBK-5. Within a specification, one of the basic requirements is to provide minimum properties (S-basis) which includes tension yield, tension ultimate, elongation and compression yield (when specified). The statistical significance to the S-basis properties is typically not known. However, since ~ 1975, the minimum mechanical properties in the SAE/AMS specifications have been statistically justified with a procedure described in their documents. With that in mind, a procedure has been established to provide a level of statistical significance to S-basis properties contained within the Handbook.
A material being submitted for inclusion into MIL-HDBK-5 must include the basis of the specification properties as part of the substantiation package. This substantiation package should include the number of test samples, the number of lots, and the method used to determine any property covered in the specification, even if it will not be reported in MIL-HDBK-5. This could include the development of minimum as well as maximum properties. Consideration will be given to the specified sizes, product forms, heat treatments and other variables affecting the physical and mechanical properties. It is also expected that the test material chemistry be in the nominal specification range and not tailored to the chemistry extremes.
It is recommended that the substantiation of properties be based on a procedure similar to SAE/AMS in which the analysis of data or other appropriate documentation supports a statistical S-basis value, where at least 99 percent of the population of values is expected to equal or exceed the minimum value with a confidence of 95 percent. The data requirements for an S-basis value are described in Section 9.2.4.1. The S-basis value may be computed by assuming the distribution of the sample population to be normal and using the following equation:
Minimum
| X̄ | = | sample mean |
| s | = | standard deviation |
| k99 | = | one-sided tolerance-limit factor corresponding to a proportion at least 0.99 of a normal distribution and a confidence coefficient of 0.95 based on the number of specimens (See Table 9.10.1). |
All data analyses must to be performed in English units. Strength data recorded in metric units should be converted to English units, to the nearest 0.01 ksi, before data analyses are undertaken. If desired by the data supplier, metric equivalent tables and figures can be included as part of the working data submitted with a data proposal, but the tables and/or figures proposed for inclusion in MIL-HDBK-5 will contain only English units.
When the tensile and compressive properties vary significantly with thickness, regression analysis should be used.
Although the establishment of an S-basis value should be based upon the statistically computed value, the S-basis value may be slightly lower, based on experience and judgement.