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Determination of Crack Growth
Rate Curves in Mode II Tests by
Means of the Compliance Real
Time Monitoring
J. Renart, L. Carreras, J. Costa, A.Turon
COMPTEST 2015, 9th of April 2015
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COMPTEST, 2015
Contents
1: Introduction / Motivation
2: Methodology
3: Results
4: Conclusions
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Introduction: interlaminar fatigue tests
Interlaminar fatigue tests are used to analyze the composite structures under
service conditions
Typical coupons: DCB, ENF, MMB
Onset curves
Crack growth rate curves
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COMPTEST, 2015
Introduction: interlaminar fatigue tests
Interlaminar fatigue tests are used to analyze the composite structures under
service conditions
Typical coupons: DCB, ENF, MMB
Onset curves
Crack growth rate curves
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Introduction: fatigue test methodology
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Introduction: fatigue test methodology
The damage in the specimen is measured by an increase in the compliance (C)
The crack length is estimated
with compliance calibration
tests
Real time monitoring of the
compliance
–
The compliance and the maximum
load are monitored continuously at
every cycle
Continuous da/dN curve
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Introduction: fatigue test methodology
DCB fatigue test:
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Introduction: fatigue test methodology
DCB fatigue test:
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Introduction: fatigue test methodology
ENF fatigue test:
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Introduction: fatigue test methodology
ENF fatigue test:
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Introduction: fatigue test methodology
ENF fatigue test:

In order to obtain a whole
curve you need to perform a
series of tests at different
levels of %ERR

As the data gets closer to the
threshold the crack growth
segment gets smaller
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Introduction: fatigue test methodology
ENF fatigue test:

In order to obtain a whole
curve you need to perform a
series of tests at different
levels of %ERR

As the data gets closer to the
threshold the crack growth
segment gets smaller
Objective
Cover the full range of the crack growth with a continuous curve
Determination of the threshold energy release rate
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Methodology: test procedure
Impose a range of G to measure
before the crack arrives to the
loading arm.
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Methodology: test procedure
The displacement control is performed by an internal variable and the calculated channels
option of MTS Test Star software
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Results: experimental test set-up
A series of displacement controlled ENF fatigue tests were performed with UD CRFP
specimens. Test frequency: 5 Hz
Specimen dimensions (length, width, thickness): 200 x 25 x 3 mm
The half span length was set to L = 60 mm
% ERR
(a0 = 45 mm)
80%
60%
35%
17%**
(a0 = 35 mm)
80 to 10%*
50 to 10%
* The 10% could not be reached, the crack propagation
was greater than expected
** No crack propagation occurred during the test
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Results: crack growth rate curves
Curves obtained from the fatigue tests at constant displacement
G17% < Gtho
1.E-03
80%
da/dN (mm/cycle)
60%
35%
1.E-04
1.E-05
1.E-06
1.E-07
0%
20%
40%
60%
GII max/GIIc VIS
80%
100%
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Results: crack growth rate curves
1.E-03
80% → 10%
da/dN (mm/cycle)
50% → 10%
1.E-04
1.E-05
1.E-06
1.E-07
0%
20%
40%
60%
GII max/GIIc VIS
80%
100%
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Results: crack growth rate curves
1.E-03
80% → 10%
da/dN (mm/cycle)
50% → 10%
1.E-04
1.E-05
Difference in the slopes of both curves
Threshold located around a 20% of %ERR
1.E-06
1.E-07
0%
20%
40%
60%
GII max/GIIc VIS
80%
100%
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Results: onset curves
The onset points are considered as a an increment of 5% in the compliance
1.00E-02
80%
9.80E-03
70%
60%
9.40E-03
9.20E-03
9.00E-03
8.80E-03
C5% = 8.75·10-3 mm/N
N5% = 82821 cycles
8.60E-03
8.40E-03
8.20E-03
5.0E+04
1.0E+05 1.5E+05
N (cycles)
50%
40%
30%
20%
10%
C0 = 8.33·10-3 mm/N
8.00E-03
1.0E+02
GIImax/GIIc (VIS)
C (mm/N)
9.60E-03
2.0E+05
Onset fatigue test at 35% of %ERR
0%
1E+3
1E+4
1E+5
N (cycles)
1E+6
Run-out number of cycles: 2.5·106
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Results: crack growth rate curves
G17% < Gtho
1.E-03
80% → 10%
da/dN (mm/cycle)
50% → 10%
1.E-04
1.E-05
1.E-06
1.E-07
0%
20%
40%
60%
GII max/GIIc VIS
80%
100%
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Results: crack growth rate curves
G17% < Gtho
1.E-03
80% → 10%
da/dN (mm/cycle)
50% → 10%
1.E-04
1.E-05
1.E-06
1.E-07
0%
Gth
20%
40%
60%
GII max/GIIc VIS
80%
100%
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Conclusions
A methodology to obtain full crack growth rate curves has been developed
for the mode II ENF test. A variable displacement is applied during the
fatigue test.
An experimental testing campaign has been performed in which the crack
growth rate curve is covered by only two tests.
Differences have been obtained in the slopes of these curves, more tests
are required in order to determine if it is caused by the uncertainty of the
material or by the crack propagation speed.
The method allows the determination of the threshold value that is in
agreement with the results of the onset curves.
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1.E-03
1.E-03
1.E-04
1.E-04
da/dN(mm/cycle)
da/dN(mm/cycle)
Results: location of the threshold
1.E-05
1.E-06
1.E-07
30
35
40
a(mm)
45
50
1.E-05
1.E-06
1.E-07
0%
10%
20%
30%
GIImax/GIIc VIS
40%
50%
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Results: location of the threshold
1.E-03
1.E-03
1.E-04
1.E-04
da/dN(mm/cycle)
da/dN(mm/cycle)
Threshold
1.E-05
1.E-06
1.E-07
30
35
40
a(mm)
45
50
1.E-05
1.E-06
1.E-07
0%
10%
20%
30%
GIImax/GIIc VIS
40%
50%