Section 1.3.2. Design Category
Slow Crack Growth structure consists of those design concepts
where flaws or defects are not allowed to attain the critical size required for
unstable rapid crack propagation.
Safety is assured through slow crack growth for specified periods of
usage depending upon the degree of inspectability. The strength of slow crack growth structure with sub-critical
damage present shall not be degraded below a specified limit for the period of
unrepaired service usage.
Fail Safe structure
is designed and fabricated such that unstable rapid propagation will be stopped
within a continuous area of the structure prior to complete failure. Safety is assured through slow crack growth
of the remaining structure and detection of the damage at subsequent
inspections. Strength of the remaining
undamaged structure will not be degraded below a specified level for the period
of unrepaired service usage.
In the development of the guidelines, it was recognized that
multiple load path and crack arrest type structure have inherent potential for
tolerating damage by virtue of geometric design features. On the other hand, it is not always possible
to avoid primary structure with only one major load path, and therefore some
provisions are necessary to ensure that these situations can be designed to be
damage tolerant. It is the intent of the
guidelines to encourage the exploration of the potentials for damage tolerance
in each type of structure. Single load
path or monolithic structures must rely on the slow rate of growth of damage
for safety and thus, the design stress level and material selection become the
controlling factors.
Single load path “monolithic” structures must be qualified as
Slow Crack Growth. However, the
guidelines allow flexibility for qualification of multiple load path
cases. The decision may be made to
qualify multiple load path structure as Slow Crack Growth, instead of Fail
Safe, if sufficient performance and life cycle costs advantages are identified
to offset the burdens of the inspectability levels for Fail Safe
structure. Therefore, the method of
construction may not agree with the design category selected, i.e. all multiple
load path structure is not Fail Safe.
When deciding on the design category option, the most important factor
to consider is that once a design category is chosen, the structure must meet
all the guidelines in the guidelines that cover that category.
The mere fact that a structure has alternate load paths (local
redundancy) in some locations does not necessarily qualify it as Fail
Safe. Examples are helpful in
illustrating this point. Examples 1.3.1 and 1.3.2
illustrate the fact that a structure is often locally redundant (usually good
design practice), but in an overall sense may have some restriction such that
one is not able to take advantage of the localized redundancy in order to
qualify the structure as Fail Safe.
Considerable judgment is required for the selection of
potential initial damage locations for the assessment of damage growth patterns
and the selection of major load paths.
The qualification as Fail Safe is thus a complex procedure entailing
judgment and analysis. Because of this,
the choice is often made to qualify the design as Slow Crack Growth regardless
of the type of construction. As stated
in JSSG-2006 A3.12.2.3 Requirement Lessons Learned "There are currently no
aircraft in the Air Force inventory which have been qualified as fail-safe
crack arrest structure under Air Force criteria".
EXAMPLE 1.3.1 Identifying
Non-Redundant Structure – Lug Example of Slow Crack Growth Structure
The lug fitting
illustrated here has multiple lug ends at the pinned connection. Failure or partial failure of one of the
lugs (A) would allow the load to be redistributed to the remaining sound
structure. Localized redundancy is
often beneficial, and in this case is good design practice. However, the fitting cannot be qualified as
Fail Safe Multiple Load Path structure since the occurrence and growth of
damage at a typical location (B) would render the structure inoperative. The only means of protecting the safety of
this structural element would be to qualify it as Slow Crack Growth.
EXAMPLE 1.3.2 Choice
Options for Redundant Structure – Wing Box Example
As shown here, a wing box
is attached to the fuselage carry through structure by multiple fittings. The upper and lower skin is one piece for
manufacturing and cost reduction. The
substructure consists of multiple spars spaced to attach to the individual
attachment fittings. A case could be
made to qualify this structure as Fail Safe Multiple Load Path. Depending upon the amount of bending carried
by the spars, it would be possible to design the structure such that damage in
the skin would be arrested at a spar prior to becoming critical. The design might also tolerate failure of
one spar cap and a portion of the skin, prior to catastrophic failure. The attachment system could be designed to
satisfy Fail Safe guidelines with one fitting failed.
On the other hand, if the
skin was the major bending member with a design stress of sufficient magnitude
to result in a relatively short critical crack length, then the skin and spar
structure could only be qualified as Slow Crack Growth structure.
