In the early 1970’s, the United States Air Force (USAF)
developed a damage tolerance philosophy to help eliminate the type of
structural failures and cracking problems that had been encountered on various
military aircraft. Air Force review of structural failures had revealed that
the safe life philosophy did not protect against designs that were intolerant
to defects that could be introduced during manufacturing or during in-service
use. From the standpoint of flight safety, it was found prudent to assume that
new airframe structures could contain initial damage (e.g. scratches, flaws, burrs, cracks, etc) and that not all
cracks would be found during inspections of older airframes. Accordingly, a
damage tolerance philosophy was formulated based on the demonstration of
structural safety under the assumption that pre-existing damage would be
present at critical locations of all structurally significant details. The
intent was to ensure that the maximum possible initial damage would not grow to
a size that would endanger flight safety during the service life of the
aircraft. Damage tolerance was formally adopted by the Air Force as part of the
Airplane Structural Integrity Program (ASIP) [MIL-STD-1530, 1972] and was
implemented originally through MIL-A-83444, Airplane Damage Tolerance
Requirements. The Air Force now implements damage tolerant design through the
recommended practices of the Department of Defense Joint Services Specification
Guide, JSSG-2006 [1998].
The primary purpose of this handbook document is to provide
guidelines and state-of-the-art analysis methods that should aid engineering
personnel in complying with the intent of the USAF Airplane Damage Tolerant
Guidelines for metallic structures. A
secondary purpose is to provide specific background data and justification for
the detailed guidelines. The handbook has been structured to provide a clear
and concise summary of the Damage Tolerant Requirements and the supporting data
and rationale behind the critical assumptions.
Where appropriate, analysis methods, test techniques, and NDI methods
are provided with suggested and/or recommended practices, limitations, etc. so
stated. In the Handbook, pertinent
paragraphs of JSSG-2006 will be referenced.
The remaining subsections of Section 1 provide:
a) an
historical perspective on the evolution of the Air Force approach to structural
integrity;
b) an
overview of the Air Force Aircraft Structural Integrity Program as implemented
through MIL-HDBK-1530;
c) an
overview of USAF damage tolerance design guidelines as specified in the Joint
Service Specification Guide (JSSG-2006); and,
d) an
overview of sustainment in aging aircraft.
The topics covered in Sections 2 through 11 are given in Table 1.0.1.
Relevant sample problems are presented in
each Section. Additional sample
problems are included in the Sample Problem section of the Handbook. For the convenience of the user, links to
the appropriate USAF structural specifications are contained as an Appendix to
this handbook. Any conflict or
discrepancy in information contained in this handbook and/or the Joint Service
Specification Guide is unintentional and in all cases, the governing document
is the current version of the Guide.
This new version of the Handbook is presented as a web-based
document, allowing easy access for all users from any location. The web page will allow timely updates as
new methodologies emerge and technologies advance. Finding information will be easier with the search capabilities
available in electronic documents.
Hyperlinks are provided for sub-sections, figures, tables, and
references within the handbook, as well as to other related web sites. Links are provided to websites where
referenced papers can be found, software can be downloaded, and additional
in-depth information is provided.
Advantages of this are to give the user the most accurate, up-to-date
information without reprinting the Handbook.
In addition to the web pages, each Section of the Handbook, as
well as the Sample Problems, is available as a
file in .pdf format, that can be downloaded and printed.
Table
1.0.1. Summary of Sections of
Damage Tolerance Design Handbook
|
Section
|
Title
|
Description
|
|
2
|
Fundamentals of Damage
Tolerance
|
Basic elements of the
methodology for damage tolerant analysis.
|
|
3
|
Damage Size
Considerations
|
Appropriate NDI practice,
state-of-the-art procedures, demonstration programs to qualify NDI, in
service NDI practice and specific examples illustrating how damage is assumed
to exist in structures.
|
|
4
|
Residual Strength
|
Theory, methods,
assumptions, material data, test verification, and gives examples for
estimating the final fracture strength or crack arrest potential of cracked
structures.
|
|
5
|
Analysis of Damage Growth
|
Current practice for
estimating the rate of crack growth as a function of time, cyclic and
sustained load occurrence; gives examples indicating limitations of methods,
use of material data and suggested testing to support predictions and
establish confidence.
|
|
6
|
Example Damage Tolerance
Analyses
|
Detailed analysis of
typical structural examples illustrating methodology and assumptions
required.
|
|
7
|
Damage Tolerance Testing
|
Methods and recommended
tests to verify methods, full-scale testing to verify residual strength and
slow crack growth rates.
|
|
8
|
Force Management/
Sustainment Engineering
|
Force structural
maintenance (FSM) planning and methods available to account for usage
variations for individual aircraft based on a crack growth model.
|
|
9
|
Structural Repairs
|
Factors that should be
considered when designing a repair, in order to ensure that the basic damage
tolerance present in the original structure is not degraded by the repair.
|
|
10
|
Guidelines for Damage
Tolerant Design and Fracture Control Planning
|
Methods and procedures
for development and implementation of a damage tolerance control plan as
required in MIL-HDBK-1530.
|
|
11
|
Summary of Stress
Intensity Factor Information
|
Stress intensity factors
|