EXPERIMENTAL DETERMINATION OF KEY PARAMETERS
FOR MODELLING THE TENSILE AND COMPRESSIVE
FATIGUE BEHAVIOUR OF NOTCHED GRP LAMINATES
Bill Broughton, Mike Gower, Maria Lodeiro,
Gordon Pilkington and Richard M. Shaw
5th International Conference on Composites Testing and
Model Simulation, EPFL, Lausanne, 2011
Content
Introduction
Test Programme
Constant Amplitude Cyclic Fatigue
Tension-Tension
Compression-Compression
Tension-Compression
Multiple Step T-T Block Loading
Concluding Remarks
Introduction
Aims and Rationale: Ensuring the long-term
structural integrity and safety of composite
structures throughout in-service lifetime
Develop and validate fatigue test methods for
composites
Identify and evaluate key parameters for
modelling tensile and compressive fatigue
behaviour of FRPs
Test Programme
E-glass/913 (Hexcel Composites)
Quasi-isotropic (QI) lay-up [45°/0°/-45°/90°]4S
Open-hole tension (OHT)
Open-hole compression (OHC)
Quasi-static loading
Constant amplitude cyclic loading (f = 5 Hz)
Tension-tension (OHT): R = 0.1 and 0.5
Compression-compression (OHC): R = 10
Tension-compression (OHC): R = -1
Stress: 80, 70, 55, 40 and 25% UTS/UCS
Strain measurement
DIC, FBGs, strain gauges, extensometry
Open-Hole (Notched) Tension
Tension-Tension Fatigue
Unnotched
Exx (GPa): 21.9 ± 0.4, xx: 0.31 ±
0.01
Strength (MPa): 484 ± 18
Open-Hole Tension (OHT)
Exx (GPa): 20.6 ± 0.3
Strength (MPa): 347 ± 5
Embedded Fibre Bragg Gratings
FBG
50
150
50
=6
x
18
7.5
End-tab
50
36
50
y
125
Strain Gauges and FBGs
Multiple-Plexed FBGs
Centre positions of
FBG’s
50mm
50mm
3 gratings at different centre wavelengths
~1540, 1550 and 1560nm
50 mm centre-to-centre spacing
We would need a total of 6 fibres. Depending on the success of the FBG work, t
of the project will be written up as either an NPL measurement note or as a sectio
report detailing all the outcomes of the project tasks plus a possible journal/confe
Although you are not permitted to sell the fibres, any work published would a
City University’s help with the provision of the fibres.
38 mm end-to-end spacing
12 mm grating length
215 mm buffer free region centred on middle grating
Length – 660 mm
Core – glass, 9 m diameter
Coating - 125 m diameter (acrylate re-coated)
Cladding – glass, 125 m diameter
Quasi-Static Strain Measurements
250
Applied Stress (MPa)
200
150
100
Strain Gauge
Extensometer
Fibre Bragg Grating
Digital Image Correlation
50
0
0
5000
10000
Strain ()
15000
20000
Quasi-Static Loading
DIC xx Strain Maps
40.3 kN
LaVision® DIC System
Single megapixel (1280 x 1024 pixel)
video camera
Image recording frequency: 1 Hz
LaVision® Strainmaster software
Data capture/analysis
42.5 kN
Quasi-Static Loading
xx Strain Across Specimen Mid-length
0 kN
3
5.40 kN
11.80 kN
Increasing
Load
2.5
17.67 kN
20.17 kN
2
26.28 kN
 xx (%)
32.03 kN
35.70 kN
1.5
1
0.5
0
0
3
6
9
12
15
18
21
24
27
-0.5
Distance across specimen width (mm)
30
33
36
T-T Cyclic Fatigue
Fatigue Damage (55% UTS)
5,000
20,000
30,000
Nf = 27,979 ± 9,142 cycles
T-T Cyclic Fatigue
Pulse Thermography (55% UTS)
0
5,000
10,000
15,000
20,000
25,000
30,000
T-T Cyclic Fatigue
Normalised Stress-Cycle (S-N) Curves
Normalised Stress (MAX/UTS)
1.0
0.8
MAX/UTS = 1 - 0.09 logNf
0.6
0.4
MAX/UTS = 1 - 0.10 logNf
0.2
R = 0.1
R = 0.5
0.0
0
10
1
10
2
10
3
10
4
10
5
10
Number of Cycles to Failure (Nf)
6
10
7
10
Normalised Residual Stiffness (E/E0)
T-T Cyclic Fatigue
Residual Stiffness (40 % UTS)
I
1.0
II
0.8
III
0.6
0.4
0.2
Extensometer
Fibre Bragg Grating
0.0
0
200000
400000
600000
Number of Cycles (N)
800000
1.0
1.0
0.8
0.8
Normalised Stiffness (E/E0)
Normalised Stiffness (E/E 0)
T-T Cyclic Fatigue
Residual Stiffness
0.6
70% UTS
0.4
0.2
0.6
40% UTS
0.4
0.2
0.0
0.0
0
200
400
600
Number of Cycles (N)
800
1000
0
1x10
5
2x10
5
3x10
5
4x10
5
5x10
5
Number of Cycles (N)
6x10
5
7x10
5
8x10
5
T-T Cyclic Fatigue
Residual Strength (55% UTS)
350
255 ± 6 MPa
Residual Strength (MPa)
300
250
200
150
100
50
Experimental
Boltzmann distribution
0
0
5000
10000
15000
20000
25000
30000
Number of Cycles (N)
Monotonic decrease in stiffness is not accompanied
by decrease in residual strength during fatigue life
T-T Cyclic Fatigue
xx Strain Distribution vs. Loading Cycles
Fatigue: 44% UTS (f = 5 Hz, R = 0.1)
Static load for measurements: 20 kN
T-T Cyclic Fatigue
Strain Distributions vs. Loading Cycles
yy
xy
 xx (%)
T-T Cyclic Fatigue
xx Strain Across Specimen Mid-length
10
9
8
7
6
5
4
3
2
1
0
-1
10,000 cycles
30,000 cycles
40,000 cycles
50,000 cycles
60,000 cycles
70,000 cycles
80,000 cycles
Increasing
Cycles
0
6
12
18
24
30
Distance across specimen width (mm)
Fatigue: 44% UTS (f = 5 Hz, R = 0.1)
Static load for measurements: 20 kN
36
T-T Cyclic Fatigue
Maximum xx Strain at Hole Perimeter
8
Maximum Strain xx (%)
7
-5
xx = 0.97 + 8.35 x 10 N
6
5
4
3
2
Experimental
Linear fit
1
0
0.0
4
2.0x10
4
4.0x10
4
6.0x10
Number of Cycles (N)
4
8.0x10
T-T Cyclic Fatigue
Global xx Strain Values
Stress
(% UTS)
Stress (MPa)
Initial Strain (%)
Final Strain (%)
Nf
(cycles)
mean
max
mean
max
mean
max
R = 0.1
40
55
70
76.2
104.8
133.4
138.6
190.6
242.5
0.363
0.534
0.698
0.658
0.949
1.375
0.663
0.835
1.081
1.100
1.421
2.117
822804
26976
706
R = 0.5
40
55
70
103.8
142.7
181.7
138.6
190.6
242.5
0.412
0.570
0.763
0.554
0.765
1.017
0.753
1.063
1.331
1.004
1.373
1.775
5993898
62093
2262

f
max

i
max

i
mean
 mean   max
max mean


E0
E0
1  R 
2
OHT QI Laminate (T-T Cyclic Fatigue)
Maximum Failure Strain fmax

3.0
f
max
2.5
 max

  of  k log10 Nf
Ef
f
Maximum Failure Strain ( max (%))
3.5
2.0
1.5
1.0
0.5
0.0
0
10
Experimental
Linear fit
10
1
10
2
10
3
10
4
10
5
Number of Cycles to Failure (N f)
10
6
10
7
T-T Cyclic Fatigue
Hysteretic Heating Effects (40% UTS)
80
o
Surface Temperature ( C)
70
60
50
40
30
20
0
200000
400000
600000
Number of Cycles (N)
800000
T-T Cyclic Fatigue
Maximum Surface Temperature (ºC)
Test Condition
(% UTS)
Initial
Final
Ultimate failure
R = 0.1
40
55
55 (1 Hz)
70
33
46
30
23
46
78
34
65
87
104
41
68
R = 0.5
40
25
33
Measured at hole perimeter
Frequency is 5 Hz (unless otherwise specified)
T-T Cyclic Fatigue (55 %UTS)
Normalised Residual Fatigue Stiffness
Normalised Residual Stiffness (E/E0)
1.0
0.8
0.6
0.4
0.2
Experimental
Linear fit
0.0
20
40
60
80
100
o
Surface Temperature ( C)
120
OHT QI Laminate (T-T Cyclic Fatigue)
Normalised Residual Fatigue Stiffness
Normalised Residual Stiffness (E/E0)
1.0
E
 1  AT
E0
0.8
0.6
0.4
70% UTS
55% UTS
40% UTS
Linear fit
0.2
0.0
20
40
60
80
100
o
Surface Temperature ( C)
120
Open-Hole (Notched) Compression
Compression-Compression
Unnotched
SCxx (MPa): 617 ± 19
Open-Hole Compression (OHC)
Strength (MPa): 346 ± 54
C-C Cyclic Fatigue Damage/Failure
C-C Cyclic Fatigue
Normalised S-N Curve
Normalised Stress (MAX/UTS)
1.0
MAX/UTS = 1 - 0.07 logNf
0.8
0.6
MAX/UTS = 0.54 + 0.57/(1 + Nf/74.42)
0.31
0.4
Experimental
Linear fit
Sigmoidal fit
0.2
0.0
0
10
1
10
2
10
3
10
4
10
5
10
Number of Cycles to Failure (Nf)
6
10
7
10
C-C Cyclic Fatigue
xx Strain Across Specimen Mid-length
0
6
12
18
24
30
36
0.00
 xx (%)
-0.50
-1.00
-1.50
Increasing
Cycles
10,000 cycles
20,000 cycles
30,000 cycles
40,000 cycles
50,000 cycles
60,000 cycles
-2.00
-2.50
Distance across specimen width (mm)
Fatigue: 61% UCS (f = 5 Hz, R = 10)
Static load for measurements: -25 kN
C-C Cyclic Fatigue
Maximum xx Strain at Hole Perimeter
Maximum Compressive Strain xx (%)
2.0
1.5
1.0
-5
xx = 0.97 + 1.46 x 10 N
0.5
Experimental
Linear fit
0.0
4
1x10
4
2x10
4
3x10
4
4x10
Number of Cycles (N)
4
5x10
4
6x10
C-C Cyclic Fatigue
Hysteretic Heating Effects (5 Hz)
Applied Stress
MAX/UTS
Surface Temperature
(°C)
60%
41
65%
54
70%
59
70%*
45
* Unnotched
Open-Hole (Notched) Compression
Tension-Compression
Open-Hole Compression (OHC)
Strength (MPa): 346 ± 54
Requirements
Rigid test frame and well aligned grips
Max. Bending Strains: < 8% (C) and < 3% (T)
T-C Cyclic Fatigue
Normalised S-N Curve
Normalised Stress (MAX/UTS)
1.0
0.8
0.6
0.4
run out
MAX/UTS = 1 - 0.12 logNf
0.2
Experimental
Linear fit
0.0
0
10
1
10
2
10
3
10
4
10
5
10
Number of Cycles to Failure (Nf)
6
10
7
10
T-C Cyclic Fatigue
xx Strain Across Specimen Mid-length
5.00
10,000 cycles
 xx (%)
4.50
20,000 cycles
4.00
30,000 cycles
3.50
40,000 cycles
50,000 cycles
3.00
60,000 cycles
Increasing
Cycles
2.50
2.00
1.50
1.00
0.50
0.00
0
6
12
18
24
Distance across specimen width (mm)
Fatigue: 61% UTS/UCS (f = 5 Hz, R = -1)
Static load for measurements: 15 kN
30
36
T-C Cyclic Fatigue
Maximum xx Strain at Hole Perimeter
Maximum Tensile Strain xx (%)
4
3
-5
xx = 0.41 + 5.37 x 10 N
2
1
Experimental
Linear fit
0
4
1x10
4
2x10
4
3x10
4
4x10
Number of Cycles (N)
4
5x10
4
6x10
T-C Cyclic Fatigue
Fully Reversed Loading S-N Response
Normalised Stress ( max/ULT)
1.0
0.8
0.6
0.4
0.2
0.0
0
10
Experimental
Predicted
10
1
10
2
10
3
10
4
10
5
Number of Cycles to Failure (N f)
10
6
10
7
Multiple-Step T-T Block Loading
Applied Stress
QI E-glass/913 laminate
OHT: Tension-tension
Ni = 1,000 cycles
40%  25%, 55%  25%, 55%  40% UTS
50%  40%  25% UTS (repeated)
Time
T-T Block Loading
Global xx Strain Values (R = 0.1)
Stress
(% UTS)
40-2 5
25
40
55-2 5
25
40
55-40
25
40
55-40-25
25
40
55
Stress (MPa)
mean
47.6
76.2
47.6
104.8
76.2
104.8
47.6
76.2
104.8
max
86.6
138.6
86.6
190.6
138.6
190.6
86.6
138.6
190.6
Initial Strain (%)
Final Strain (%)
mean
mean
0.230
0.368
0.244
0.538
0.385
0.530
0.238
0.381
0.524
max
0.418
0.669
0.444
0.978
0.700
0.963
0.433
0.693
0.953
0.353
0. 581
0.515
0.940
0.591
0.812
0.303
0.485
0.666
max
Nf
(cycles)
0.642
1.065
1980585
990000
990584
0.684
1.548
74796
37000
37796
1.047
1.478
82569
41000
41569
0.681
0.994
1.574
51564
17000
17000
17564
T-T Cyclic Fatigue
Global Strain Values

f
max


f
max
i
max

i
mean
max mean


E0
E0
 max

  of  k log10 Nf
Ef
 mean   max
1  R 
2
T-T Block Loading (55%40%25% UTS)
Surface Temperature
80
55% UTS
o
Surface Temperature ( C)
100
40% UTS
60
25% UTS
40
20
0
0
10000
20000
30000
40000
Number of Cycles (N)
50000
60000
Concluding Remarks
Alignment and rigidity of loading chain is critical for compressioncompression and tension-compression tests
DIC suitable for monitoring local and global strains
Providing critical information on changes in strain distribution around
the hole of notched laminates due to damage formation/growth incurred
through either increasing load or number of loading cycles
Optical fibres (FBGs) suitable for monitoring fatigue performance – superior
fatigue performance compared with strain gauges
Longitudinal strain and stiffness along with surface temperature – indication
of level of remnant life of notched components
Possible to estimate fatigue life for fully reversible and block loading
conditions from T-T and C-C cyclic data
Acknowledgements
The work was supported by United Kingdom Department
for Business, Innovation and Skills (National Measurement
Office), as part of the Materials 2007 Programme.
The authors would also like to thank:
Hexcel Composites Limited
Dr F Surre and Dr T Venugopalan - City University London