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

ContentContent

Introduction

Test Programme

Constant Amplitude Cyclic Fatigue

Tension-Tension

Compression-Compression

Tension-Compression

Multiple Step T-T Block Loading

Concluding Remarks

IntroductionIntroduction

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 ProgrammeTest 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) TensionOpen-Hole (Notched) TensionTension-Tension FatigueTension-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 GratingsEmbedded Fibre Bragg Gratings

18 36

End-tab 125

= 6

150 50 50

50 50 7.5

x

y

Strain Gauges and FBGs

FBG

Multiple-Plexed FBGsMultiple-Plexed FBGs

215 mm buffer free region centred on middle grating

12 mm grating length 38 mm end-to-end spacing

50 mm centre-to-centre spacing

3 gratings at different centre wavelengths ~1540, 1550 and 1560nm

Length – 660 mm

Core – glass, 9 m diameter

Coating - 125 m diameter (acrylate re-coated)

Cladding – glass, 125 m diameter

This work is part of a small task in a project we are doing, the aim being to monitor strain during fatigue of an open hole tension (OHT) sample made from a QI 913 glass epoxy composite. Comparisons to be made between extensometers, strain gauges & embedded FBGs at 3 strain levels. Due to the non-uniform strain distribution we ideally want to look at the strain at 3 different points on the sample.

We would need a total of 6 fibres. Depending on the success of the FBG work, the outcome of the project will be written up as either an NPL measurement note or as a section of a larger report detailing all the outcomes of the project tasks plus a possible journal/conference paper. Although you are not permitted to sell the fibres, any work published would acknowledge City University’s help with the provision of the fibres.

50mm

50mm

Centre positions of FBG’s

Quasi-Static Strain MeasurementsQuasi-Static Strain Measurements

0 5000 10000 15000 200000

50

100

150

200

250

Strain Gauge Extensometer Fibre Bragg Grating Digital Image Correlation

Ap

plie

d S

tres

s (M

Pa)

Strain ()

Quasi-Static LoadingQuasi-Static LoadingDIC DIC xxxx Strain Strain MapsMaps

LaVision® DIC System

Single megapixel (1280 x 1024 pixel) video camera

Image recording frequency: 1 Hz

LaVision® Strainmaster software

Data capture/analysis

40.3 kN 42.5 kN

Quasi-Static LoadingQuasi-Static Loadingxxxx Strain Strain Across Specimen Mid-lengthAcross Specimen Mid-length

-0.5

0

0.5

1

1.5

2

2.5

3

0 3 6 9 12 15 18 21 24 27 30 33 36

Distance across specimen width (mm)

xx

(%)

0 kN

5.40 kN

11.80 kN

17.67 kN

20.17 kN

26.28 kN

32.03 kN

35.70 kN

Increasing Load

T-T Cyclic FatigueT-T Cyclic Fatigue

Fatigue Damage (55% UTS)Fatigue Damage (55% UTS)

5,000

20,000

30,000

Nf = 27,979 ± 9,142 cycles

0

5,000

10,000

15,000

20,000

25,000

30,000

T-T Cyclic FatigueT-T Cyclic Fatigue

Pulse Thermography (55% UTS)Pulse Thermography (55% UTS)

T-T Cyclic FatigueT-T Cyclic FatigueNormalised Stress-Cycle (S-N) CurvesNormalised Stress-Cycle (S-N) Curves

100 101 102 103 104 105 106 1070.0

0.2

0.4

0.6

0.8

1.0

MAX

/UTS

= 1 - 0.09 logNf

MAX

/UTS

= 1 - 0.10 logNf

No

rmal

ised

Str

ess

(M

AX/ U

TS)

Number of Cycles to Failure (Nf)

R = 0.1 R = 0.5

T-T Cyclic FatigueT-T Cyclic FatigueResidual Stiffness (40 % UTS)Residual Stiffness (40 % UTS)

0 200000 400000 600000 8000000.0

0.2

0.4

0.6

0.8

1.0I

II

III

No

rmal

ised

Res

idu

al S

tiff

nes

s (E

/E0)

Number of Cycles (N)

Extensometer Fibre Bragg Grating

0 1x105 2x105 3x105 4x105 5x105 6x105 7x105 8x1050.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ised

Sti

ffne

ss (

E/E

0)

Number of Cycles (N)

0 200 400 600 800 10000.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ised

Sti

ffne

ss (

E/E

0)

Number of Cycles (N)

T-T Cyclic FatigueT-T Cyclic FatigueResidual StiffnessResidual Stiffness

70% UTS 40% UTS

0 5000 10000 15000 20000 25000 300000

50

100

150

200

250

300

350

Experimental Boltzmann distribution

Res

idua

l Str

engt

h (M

Pa)

Number of Cycles (N)

Monotonic decrease in stiffness is not accompanied by decrease in residual strength during fatigue life

T-T Cyclic FatigueT-T Cyclic FatigueResidual Strength (55% UTS)Residual Strength (55% UTS)

255 ± 6 MPa

T-T Cyclic FatigueT-T Cyclic Fatiguexxxx Strain Distribution vs. Loading Cycles Strain Distribution vs. Loading Cycles

1 0 , 0 0 0 3 0 , 0 0 0 4 0 , 0 0 0 5 0 , 0 0 0 6 0 , 0 0 0 7 0 , 0 0 0 8 0 , 0 0 0

Fatigue: 44% UTS (f = 5 Hz, R = 0.1)

Static load for measurements: 20 kN

T-T Cyclic FatigueT-T Cyclic FatigueStrain Distributions vs. Loading CyclesStrain Distributions vs. Loading Cycles

1 0 , 0 0 0 3 0 , 0 0 0 4 0 , 0 0 0 5 0 , 0 0 0 6 0 , 0 0 0 7 0 , 0 0 0 8 0 , 0 0 0

yy

xy

T-T Cyclic FatigueT-T Cyclic Fatiguexxxx Strain Strain Across Specimen Mid-lengthAcross Specimen Mid-length

-10123456789

10

0 6 12 18 24 30 36

Distance across specimen width (mm)

xx (

%)

10,000 cycles30,000 cycles40,000 cycles50,000 cycles60,000 cycles70,000 cycles80,000 cycles

Increasing Cycles

Fatigue: 44% UTS (f = 5 Hz, R = 0.1)

Static load for measurements: 20 kN

T-T Cyclic FatigueT-T Cyclic FatigueMaximum Maximum xxxx Strain at Hole Perimeter Strain at Hole Perimeter

0.0 2.0x104 4.0x104 6.0x104 8.0x1040

1

2

3

4

5

6

7

8

xx

= 0.97 + 8.35 x 10-5N

Max

imu

m S

trai

n xx

(%

)

Number of Cycles (N)

Experimental Linear fit

T-T Cyclic FatigueGlobal xx Strain Values

0

mean

0

maximean

imax

fmax EE

2

R1maxmean

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

599389862093

2262

OHT QI Laminate (T-T Cyclic Fatigue)Maximum Failure Strain f

max

100 101 102 103 104 105 106 1070.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Experimental Linear fitM

axim

um F

ailu

re S

trai

n (f m

ax (

%))

Number of Cycles to Failure (Nf)

f10fo

f

maxfmax Nlogk

E

T-T Cyclic FatigueT-T Cyclic FatigueHysteretic Heating Effects (40% UTS)Hysteretic Heating Effects (40% UTS)

0 200000 400000 600000 80000020

30

40

50

60

70

80S

urf

ace

Tem

per

atu

re (

o C)

Number of Cycles (N)

T-T Cyclic FatigueT-T Cyclic FatigueMaximum Surface Temperature (ºC)Maximum Surface Temperature (ºC)

Measured at hole perimeter

Frequency is 5 Hz (unless otherwise specified)

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

T-T Cyclic Fatigue (55 %UTS)T-T Cyclic Fatigue (55 %UTS)Normalised Residual Fatigue StiffnessNormalised Residual Fatigue Stiffness

20 40 60 80 100 1200.0

0.2

0.4

0.6

0.8

1.0N

orm

alis

ed R

esid

ual S

tiff

ness

(E

/E0)

Surface Temperature (oC)

Experimental Linear fit

OHT QI Laminate (T-T Cyclic Fatigue)OHT QI Laminate (T-T Cyclic Fatigue)Normalised Residual Fatigue StiffnessNormalised Residual Fatigue Stiffness

20 40 60 80 100 1200.0

0.2

0.4

0.6

0.8

1.0

70% UTS 55% UTS 40% UTS Linear fit

Nor

mal

ised

Res

idua

l Sti

ffne

ss (

E/E

0)

Surface Temperature (oC)

AT1E

E

0

Open-Hole (Notched) CompressionOpen-Hole (Notched) Compression

Compression-CompressionCompression-Compression

Unnotched

SCxx (MPa): 617 ± 19

Open-Hole Compression (OHC)

Strength (MPa): 346 ± 54

C-C Cyclic Fatigue Damage/FailureC-C Cyclic Fatigue Damage/Failure

C-C Cyclic Fatigue C-C Cyclic Fatigue Normalised S-N Curve Normalised S-N Curve

100 101 102 103 104 105 106 1070.0

0.2

0.4

0.6

0.8

1.0

MAX

/UTS

= 0.54 + 0.57/(1 + Nf/74.42)0.31

MAX

/UTS

= 1 - 0.07 logNf

No

rmal

ised

Str

ess

(M

AX/

UT

S)

Number of Cycles to Failure (Nf)

Experimental Linear fit Sigmoidal fit

C-C Cyclic FatigueC-C Cyclic Fatiguexxxx Strain Strain Across Specimen Mid-lengthAcross Specimen Mid-length

-2.50

-2.00

-1.50

-1.00

-0.50

0.00

0 6 12 18 24 30 36

Distance across specimen width (mm)

xx

(%)

10,000 cycles20,000 cycles30,000 cycles40,000 cycles50,000 cycles60,000 cycles

Increasing Cycles

Fatigue: 61% UCS (f = 5 Hz, R = 10)

Static load for measurements: -25 kN

C-C Cyclic FatigueC-C Cyclic FatigueMaximum Maximum xxxx Strain at Hole Perimeter Strain at Hole Perimeter

1x104 2x104 3x104 4x104 5x104 6x1040.0

0.5

1.0

1.5

2.0

xx

= 0.97 + 1.46 x 10-5N

Experimental Linear fit

Max

imu

m C

om

pre

ssiv

e S

trai

n xx

(%

)

Number of Cycles (N)

C-C Cyclic FatigueC-C Cyclic FatigueHysteretic Heating Effects (5 Hz)Hysteretic Heating Effects (5 Hz)

* Unnotched

Applied Stress

MAX/UTS

Surface Temperature

(°C)

60% 41

65% 54

70% 59

70%* 45

Open-Hole (Notched) CompressionOpen-Hole (Notched) Compression

Tension-CompressionTension-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 FatigueT-C Cyclic FatigueNormalised S-N CurveNormalised S-N Curve

100 101 102 103 104 105 106 1070.0

0.2

0.4

0.6

0.8

1.0

run outMAX

/UTS

= 1 - 0.12 logNf

No

rmal

ised

Str

ess

(M

AX/

UT

S)

Number of Cycles to Failure (Nf)

Experimental Linear fit

T-C Cyclic FatigueT-C Cyclic Fatiguexxxx Strain Strain Across Specimen Mid-lengthAcross Specimen Mid-length

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

5.00

0 6 12 18 24 30 36

Distance across specimen width (mm)

xx (

%)

10,000 cycles

20,000 cycles

30,000 cycles

40,000 cycles

50,000 cycles

60,000 cyclesIncreasing Cycles

Fatigue: 61% UTS/UCS (f = 5 Hz, R = -1)

Static load for measurements: 15 kN

T-C Cyclic FatigueT-C Cyclic FatigueMaximum Maximum xxxx Strain at Hole Perimeter Strain at Hole Perimeter

1x104 2x104 3x104 4x104 5x104 6x1040

1

2

3

4

xx

= 0.41 + 5.37 x 10-5N

Experimental Linear fitM

axim

um

Ten

sile

Str

ain

xx (

%)

Number of Cycles (N)

T-C Cyclic FatigueT-C Cyclic FatigueFully Reversed Loading S-N ResponseFully Reversed Loading S-N Response

100 101 102 103 104 105 106 1070.0

0.2

0.4

0.6

0.8

1.0N

orm

alis

ed S

tres

s (

max

/U

LT)

Number of Cycles to Failure (Nf)

Experimental Predicted

Multiple-Step T-T Block LoadingMultiple-Step T-T Block Loading

QI E-glass/913 laminate

OHT: Tension-tension

Ni = 1,000 cycles

40% 25%, 55% 25%, 55% 40% UTS

50% 40% 25% UTS (repeated)

Ap

plie

d S

tres

s

T i m e

T-T Block LoadingT-T Block LoadingGlobal Global xxxx Strain Values (R = 0.1) Strain Values (R = 0.1)

Stress

(% UTS)

Stress (MPa) Initial Strain (%) Final Strain (%)Nf

(cycles)mean max mean max mean max

40-2 5

25

40

47.6

76.2

86.6

138.6

0.230

0.368

0.418

0.669

0.353

0. 581

0.642

1.065

1980585

990000

990584

55-2 5

25

40

47.6

104.8

86.6

190.6

0.244

0.538

0.444

0.978

0.515

0.940

0.684

1.548

74796

37000

37796

55-40

25

40

76.2

104.8

138.6

190.6

0.385

0.530

0.700

0.963

0.591

0.812

1.047

1.478

82569

41000

41569

55-40-25

25

40

55

47.6

76.2

104.8

86.6

138.6

190.6

0.238

0.381

0.524

0.433

0.693

0.953

0.303

0.485

0.666

0.681

0.994

1.574

51564

17000

17000

17564

T-T Cyclic FatigueT-T Cyclic FatigueGlobal Strain ValuesGlobal Strain Values

0

mean

0

maximean

imax

fmax EE

f10fo

f

maxfmax Nlogk

E

2

R1maxmean

T-T Block Loading T-T Block Loading (55%(55%40%40%25% UTS)25% UTS)Surface TemperatureSurface Temperature

0 10000 20000 30000 40000 50000 600000

20

40

60

80

100

40% UTS

25% UTS

55% UTS

Surf

ace

Tem

pera

ture

(o C

)

Number of Cycles (N)

Concluding RemarksConcluding RemarksAlignment and rigidity of loading chain is critical for compression-compression 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

AcknowledgementsAcknowledgements

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