The attainment of trouble-free and damage tolerant design for
safety-of-flight structure on aircraft is a continuing multiple element process
which begins in the preliminary design phase and extends through manufacturing
into the operational planning and use of the aircraft. The purpose of this process is to develop a
planned approach to the control of fracture damage in the aircraft. Figure 10.0.1
illustrates the main elements of the process.
Figure 10.0.1. Elements of Damage Tolerant Design
The design criteria include the service loads history, the
functional requirements, and the desired life of the structure. The initial damage assumptions to be used
with each type of design concept and degree of inspectability are also
specified in the criteria. These are
discussed in detail in Section 1.3.
Material selection is a critical element of the process. Trade-off studies are conducted between
competing materials and use a comparative property data is necessary in the
selection process. Ultimate strength,
yield strength, fracture toughness and stress corrosion resistance must be
considered together with the expected aircraft environment. The crack growth rate as a function of
stress-intensity factor is required.
The structural configuration development must consider the
effects of design details on fracture control.
The inspection level is defined and a list of critical parts is
begun. Consideration of the inspection
procedures to be used at each critical location is important. The analysis methods used and the
stress-intensity factor computations are a function of the structural
configuration and design details, and are set at this time. Testing methods for each critical part and
assembly are also developed and incorporated into the damage tolerant design
Manufacturing processes must be selected for the critical parts
such that they do not reduce the damage tolerance level required by the
design. Control of processes and
selection of inspection procedures to maintain process quality are the prime
consideration of this element.
Procedures for inspecting the aircraft during operational
maintenance and the development of the force structural management plan
constitute the last element of the total damage tolerant design process.
This introduction of the elements illustrates the strong
connections between design, testing, manufacturing, inspection and use in order
to obtain and maintain the desired damage tolerant structure and to reduce the
incidence of fracture related failures and loss.
The documents that outline the requirements for this activity
and describe the various functions are military standards and
establishes the requirements for aircraft structural integration program (ASIP)
and JSSG-2006 describes the airplane damage tolerance design requirements.
Wood  provides discussion of the ASIP technology. Tiffany  presents extensive discussions
of the durability and damage tolerance problem of USAF aircraft
structures. In particular, the review
by Mr. C.F. Tiffany, “Durability and Damage Tolerance Assessments of United
States Air Force Aircraft,” provides a thorough review of the background of
This section of the handbook describes these tolerant design
elements in some detail, and when the described functions are followed, the
resultant aircraft structure should have the required level of damage