DISTRIBUTION A. Approved for public release; distribution unlimited.
EN-SB-08-002, Rev A, Page 1 of 5
Number: EN-SB-08-002, Revision A
Date: 18 March 2011
Subject: Revised Damage Tolerance Requirements and Determination of
Operational Life Limits for Slow Crack Growth Metallic Structures
Background:
The Air Force formally introduced damage tolerance requirements with the release of
MIL-A-83444 in July of 1974. While this specification allowed the use of either fail-safe
or slow crack growth design concepts, the primary focus was on the slow crack growth
concept since most combat aircraft were designed with many single load path
structures. With the slow crack growth concept, it is mandatory that material,
manufacturing and/or service induced defects not be allowed to reach their critical crack
sizes before they are detected and repaired. Initial crack sizes were specified in MIL-A-
83444, and later in Joint Services Specification Guide JSSG-2006, for use in design and
in establishing initial inspection intervals. These assumed initial flaw sizes were selected
as surrogates for the myriad of manufacturing, material and in-service defects (i.e.,
rogue flaws) that can and occasionally do exist in aircraft.
Since the inception of damage tolerance, the slow crack growth inspection approach
has greatly diminished the incidence of catastrophic structural failure. However,
inspection reliability has become a significant issue both due to frailties of the
nondestructive inspection (NDI) systems and concern over inspectors becoming
complacent as a result of performing numerous repeat inspections looking for rogue
flaws without any finds. The inspection issue becomes more acute as weapon systems
age and approach the onset of widespread fatigue damage (WFD). The inspection
burden and aircraft down-times will tend to overwhelm the aircraft depots and
jeopardize both safety and operational readiness. Clearly, there is a need to develop
operational life limits, beyond which the structure should be modified or replaced, or the
aircraft retired.
Structures Bulletin
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