**Sec. 25.613 - Material strength properties and
design values.**
(a) Material strength properties must be
based on enough tests of material meeting approved specifications to
establish design values on a statistical basis.

(b) Design values must be chosen to
minimize the probability of structural failures due to material
variability. Except as provided in paragraph (e) of this section,
compliance with this paragraph must be shown by selecting design values
which assure material strength with the following probability:

(1) Where applied loads are eventually
distributed through a single member within an assembly, the failure of
which would result in loss of structural integrity of the component, 99
percent probability with 95 percent confidence.

(2) For redundant structure, in which
the failure of individual elements would result in applied loads being
safely distributed to other load carrying members, 90 percent probability
with 95 percent confidence.

(c) The effects of temperature on
allowable stresses used for design in an essential component or structure
must be considered where thermal effects are significant under normal
operating conditions.

(d) The strength, detail design, and
fabrication of the structure must minimize the probability of disastrous
fatigue failure, particularly at points of stress concentration.

(e) Greater design values may be used if
a "premium selection" of the material is made in which a specimen of each
individual item is tested before use to determine that the actual strength
properties of that particular item will equal or exceed those used in
design.

[Doc. No. 5066, 29 FR 18291, Dec. 24,
1964, as amended by Amdt. 25-46, 43 FR 50595, Oct. 30, 1978; Amdt. 25-72,
55 FR 29776, July 20, 1990]