The basic concept in damage tolerance design is to ensure the
safety of the structure throughout the expected service life. To provide the required safety, a structure
must be designed to withstand service loads even when cracks are present or
when part of the structure has already failed; i.e., the structure has to be
damage tolerant. The overriding
philosophy is to maintain a minimum required residual strength so that
catastrophic failure of the structure is prevented.
identifies the major sequence of events that ultimately define the residual
strength requirements. As can be noted
from the figure, once a safety-of-flight-critical element is identified, either
its structural configuration or its degree of inspectability will establish the
allowable structural design concept and the inspection level categories. Every safety-of-flight-critical element must
be qualified in at least one design concept category and in one inspection
category. Each allowable combination of
design concept and inspection category is coupled in JSSG-2006 to a residual
strength requirement, a service life requirement, and a requirement to assume a
level of initial damage.
4.1.1. The Structural Configuration
or Degree of Inspectability Controls the Subsequent Choices of Design Concept
and Inspection Level
Figure 4.1.2 illustrates the residual strength and the
service life interval requirements as well as a residual strength capability
curve. The residual strength capability
curve defines the level of load that the structure can withstand without
failing in the presence of a growing crack.
To account for the change in residual strength capacity as a function of
time, it is necessary to determine the crack size as a function of time. The crack-growth-life curve and its various
properties are shown schematically in Figure 4.1.3.
Shown are the various technology and specification requirements needed to
define the crack growth curve which, in turn, establishes the life limit.
Figure 4.1.2. Relationship Between the Life Expended and Residual Strength
Showing a Monotonic Decrease in Load Carrying
Capacity Due to Damage
Figure 4.1.3. Relationship Between Crack Length and Life
a Monotonic Increase in Crack Length Up Until
As can be seen from Figure 4.1.2,
when the residual strength of the structure falls below the maximum stress in
the service load history, failure can be expected. To avoid such a failure, a thorough understanding of the problem
is essential. Significant advances have
been made in recent years in procedures for analyzing damaged structures. Assessments now consider residual strength,
damage growth, interactive multiple damage sites and quantitative structural
maintenance and in-service evaluations.
The application of existing fracture mechanics solution
techniques has yielded effective methods for analyzing the residual strength of
the cracked structure. The necessary
theories and methods for determining the residual strength capability of
cracked structures are presented in this section. Prior to presenting this information in the following sections, a
few remarks are made about the residual strength requirements for the two
damage tolerant design categories: slow crack growth structure and fail-safe