4. What is the cast of failure ?
Physical harm to people or the
environment
Loss or destruction of property or
equipment
Loss of productivity or use of the failed
“system” or device
Damaged reputation
6. Infinite Life Design
Unlimited safety is the oldest criterion.
For parts subjected to many millions of
cycles, like engine valve springs, this is still
a good design criterion.
This criterion may not be economical or
practical in many design situations.
endurance limit of the material is
important in this design criterion.
8. Safe-life Design
Safe-life refers to the philosophy that the
component or system is designed to not
fail within a certain, defined period.
The benefit of safe-life designs includes
reducing the likelihood of unplanned
maintenance and reducing the likelihood
of any failure
10. Safe life process
evaluating the highest operational stress
on the component
safety factors are often applied to ensure
that catastrophic failures
Comparing S-N curve
It has infinite life or limited life
13. In order to overcome this shortcoming of
the safe-life approach, the methods were
must developed that assume the structure
contains initial cracks.
Damage tolerance analysis
14. Damage tolerance analysis
we have category :
Slow crack growth
structures are
designed such that
initial damage will
grow at a stable,
slow rate under
service
environment
Fail-safe
structures are
designed such that
propagating
damage is safely
contained after
failing a major load
path by load shift to
adjacent intact
elements
15. Slow crack growth
damage tolerance (and thus safety) is
assured only by the maintenance of a
slow rate of growth of damage, a residual
strength capacity
sub-critical damage will either be
detected at the depot or will not reach
unstable dimensions within several design
life times.
16. Fail-safe
damage tolerance is assured by the
allowance of partial structural failure
the ability to detect this failure prior to
total loss of the structure
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
18. Lug Example of Slow Crack Growth Structure
The lug fitting illustrated here
has multiple lug ends at the
pinned connection
occurrence and growth of
damage at a typical location
(B) would render the structure
inoperative.
19. Wing Box Example
a wing box is attached
to the fuselage carry
through structure by
multiple fittings.
A case could be made
to qualify this structure
as Fail Safe Multiple
Load Path.
if the skin was the major bending member with a design stress of
sufficient magnitude to result in a relatively short critical crack
length.
20. Damage concept
the majority of the life is spent growing the
resultant cracks to failure.
analyses of in-service fractures, cracking
instances, etc. have indicated that a
major source of cracks is the occurrence
of initial manufacturing defects such as
sharp corners, tool marks
21. typical growth behavior for a crack
structural element as it moves from an initial damage size
to a damage size that causes structural failure
• Crack increment
(Δa)
• number of loading
events (ΔN)
• critical value (acr)
23. Damage growth effect
Quality
Note that the shape of the crack growth curve (for a
given configuration and loading) remains essentially
constant for any given crack growth increment.
The effect of initial crack size is significant.
24. Damage growth effect
load history
The stress history experienced at each location on the
aircraft will also differ due to changes in bending
moment, twisting moment, shear loading.
The loading spectra for a lower surface location is
typically more severe than a corresponding upper
surface location.
25. Damage growth effect
material properties
The crack growth rate (Δa/ΔN) can be derived
experimentally for each material
the alloy having the slower growth rate characteristics
(i.e. 2024-T3) will have a longer life
26. Damage growth effect
Structural Properties
The most complex of the parameters affecting crack
growth behaviorare the structural properties.
27. Life Prediction Methodology
(Initial Flaw Distribution)
For predictions of safety limits, the initial cracks
larger than detectability limit are of principal concern.
29. Life Prediction Methodology
(Usage)
The sum of the load
levels that a structure
is expected to
experience is
determined by a
projection of the
amount of usage
expected over the
life in the various
possible missions
31. Life Prediction Methodology
(material properties)
Crack growth data are
generated in the laboratory
under constant cyclic
loading on simple
specimens with accepted
characterizing stress intensity
factors.
32. Life Prediction Methodology
(Crack Tip Stress Intensity Factor)
The crack tip stress intensity factor(K)
interrelates the crack geometry, the
structural geometry, and the load on the
structure
It defines as :
β- geometric termfor structural
configuration
σ- stress applied to the structure
a- crack length
33. Damage Size Characterizations
Reference Documat is JSSG-2006 .
This approach assumes that cracks are
present in all critical locations.
periods between inspections are greatly
influenced by the crack lengths assumed
at the beginning of a usage period.
36. Residual Strength
The strength of a structure can be
significantly affected by the presence of
a crack
The basic concept in damage tolerance
design is to ensure the safety of the
structure throughout the expected service
life.
38. Residual Strength
Slow crack growth Fail-Safe
detection of this
failure prior to total
loss of the structure
safely within the
partial failure prior
to inspection
40. Built-Up Structure Residual Strength Diagrams
In built-up structures, due to the complex
geometrical configuration, one or more failure
criterion may have to be considered
42. Damage Tolerance Analysis Procedure
Step 1. Determine the stress-intensity
factor (K) as a function of crack size for
each member
Step 2. derive the stress history for the
location under consideration.
Step 3. Obtain baseline crack-growth
data (da/dN as a function of ΔK and R)
for all the materials
43. Damage Tolerance Analysis Procedure
(continue)
Step 4. Using
the results of
Steps, 1, 2, and
3, calculate the
crack-growth
curve for each
element
Start with a
0.02 inch flaw
44. Damage Tolerance Analysis Procedure
(continue)
Step 5. By using the results of the residual
strength analysis plot the critical crack sizes,
aDMC and aLTC
Step 6. For slow crack growth structure
I. whether BD is equal to or greater than 2 design lifetimes.
II. whether CE (or C’E) is equal to or greater than ½ design
lifetime.
Step 7. For safe fail
I. whether AF is equal to or greater than 1 design lifetime.
II. whether CG (or C’G) is equal to or greater than ¼
design lifetime.
