EQUIVALENT INTIAL FLAW SIZE

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• EQUIVALENT INTIAL FLAW SIZE
• IN AL 2024-T3 RIVETED SPECIMENS

Pedro M G P Moreira, Paulo F P de Matos, P M S T
de Castro
IDMEC e Departamento de Engenharia Mecânica e
Gestão Industrial, Faculdade de Engenharia da
Universidade do Porto, Rua Dr. Roberto Frias,
4200-465 Porto, Portugal
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OBJECTIVES
One of the problems of aeronautical structures is
structural damage due to fatigue, for example in
the connection of components
Lap splice with three rivets rows and one rivet
column single-rivet lap joint
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SUMMARY
Fatigue test data of stress versus number of
cycles Measurements of fatigue striation
spacing along two perpendicular
directions Measurements of fatigue striation
spacing along two perpendicular directions The
EIFS was used for predicting fatigue lives
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SPECIMENS
specimen type lap splice with 3 rivet rows
and 1 rivet column material 2024-T3 Alclad
sheets sheet thickness 1.2 mm specimen
length 260 mm, 60 mm overlap specimen width
20 mm rivets 2117-T4, ? 3.2 x 6.4 mm
riveting force according to typical production
standards
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MAIN FATIGUE TESTS
MTS 312.31
maximum load 3840 N minimum load 192 N R ratio
0.05 test frequency 10 Hz
160 MPa
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MAIN FATIGUE TESTS
IDMEC fatigue tested 45
m 77688 cycles s 18320 cycles
As observed in testing, cracks occurred in the
first rivet plane
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MAIN FATIGUE TESTS
Statistical analysis Weibull 2P
Weibull 2P, Weibull 3P, Lognormal and Normal
distributions were tested The Weibull
distribution showed good versatility, due its
good adjustment to different shapes
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SEM ANALYSIS
specimens 93, 95, 224 and 229
specimen 224
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SEM ANALYSIS
fatigue striation - specimen 93
16 striations
transversal
longitudinal
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SEM ANALYSIS
The transversal crack growth rates are almost
constant The longitudinal crack growth rates
are similar to the transversal crack growth rates
for small cracks The longitudinal crack growth
rate increases exponentially with increasing
crack length
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SEM ANALYSIS
Assembled fatigue striations spacing
The fatigue striation spacing was converted in
crack growth rate da/dN
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EIFS BY BACK EXTRAPOLATION
EIFS is the size of an initial flaw that under
stated cyclic loading would lead to a prescribed
endurance The back extrapolation technique and
fatigue crack growth rule for long crack, Paris
rule, were used
The value of EIFS (2c) for symmetric crack was
calculated
m 3,3736 C 5,0227E-11 MPa.m0,5
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EIFS BY BACK EXTRAPOLATION
Results of Pisa Univ and Naples Univ were
used (90, 120 e 160 MPa)
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EIFS BY BACK EXTRAPOLATION
Predicted fatigue life linear EIFS
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EIFS BY BACK EXTRAPOLATION
Using the SIF calibration, a simple Paris law and
back extrapolation, the EIFS was calculated for
each fatigue testing stress level.
Using a value of EIFS dependant upon stress level
increases the accuracy of predicted fatigue lives
The EIFS is a fictitious crack length used as the
starting crack length for a crack propagation
calculation that makes it possible to access the
fatigue life
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