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 E xx (GPa): 21.9 ± 0.4, xx : 0.31 ± 0.01 Strength (MPa): 484 ± 18 Open-Hole Tension (OHT) E xx (GPa): 20.6 ± 0.3 Strength (MPa): 347 ± 5

Embedded Fibre Bragg Gratings Strain Gauges and FBGs FBG

Multiple-Plexed FBGs Length – 660 mm Core – glass, 9  m diameter Coating  m diameter (acrylate re-coated) Cladding – glass, 125  m diameter

Quasi-Static Strain Measurements

Quasi-Static Loading DIC  xx Strain Maps 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 Loading  xx Strain Across Specimen Mid-length Increasing Load

T-T Cyclic Fatigue Fatigue Damage (55% UTS) 5,000 20,000 30,000 N f = 27,979 ± 9,142 cycles

0 5,000 10,000 15,000 20,000 25,000 30,000 T-T Cyclic Fatigue Pulse Thermography (55% UTS)

T-T Cyclic Fatigue Normalised Stress-Cycle (S-N) Curves

T-T Cyclic Fatigue Residual Stiffness (40 % UTS)

T-T Cyclic Fatigue Residual Stiffness 70% UTS 40% UTS

Monotonic decrease in stiffness is not accompanied by decrease in residual strength during fatigue life T-T Cyclic Fatigue Residual Strength (55% UTS) 255 ± 6 MPa

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

T-T Cyclic Fatigue  xx Strain Across Specimen Mid-length Increasing Cycles Fatigue: 44% UTS (f = 5 Hz, R = 0.1) Static load for measurements: 20 kN

T-T Cyclic Fatigue Maximum  xx Strain at Hole Perimeter

T-T Cyclic Fatigue Global  xx Strain Values Stress (% UTS) Stress (MPa)Initial Strain (%)Final Strain (%) N f (cycles)  mean  max  mean  max  mean  max R = R =

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

T-T Cyclic Fatigue Hysteretic Heating Effects (40% UTS)

T-T Cyclic Fatigue Maximum Surface Temperature (ºC) Measured at hole perimeter Frequency is 5 Hz (unless otherwise specified) Test Condition (% UTS) InitialFinalUltimate failure R = (1 Hz) R =

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

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

Open-Hole (Notched) Compression Compression-Compression Unnotched S C xx (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

C-C Cyclic Fatigue  xx Strain Across Specimen Mid-length Increasing Cycles Fatigue: 61% UCS (f = 5 Hz, R = 10) Static load for measurements: -25 kN

C-C Cyclic Fatigue Maximum  xx Strain at Hole Perimeter

C-C Cyclic Fatigue Hysteretic Heating Effects (5 Hz) * Unnotched Applied Stress  MAX /  UTS Surface Temperature (°C) 60%41 65%54 70%59 70%*45

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

T-C Cyclic Fatigue  xx Strain Across Specimen Mid-length Increasing Cycles Fatigue: 61% UTS/UCS (f = 5 Hz, R = -1) Static load for measurements: 15 kN

T-C Cyclic Fatigue Maximum  xx Strain at Hole Perimeter

T-C Cyclic Fatigue Fully Reversed Loading S-N Response

Multiple-Step T-T Block Loading QI E-glass/913 laminate OHT: Tension-tension N i = 1,000 cycles 40%  25%, 55%  25%, 55%  40% UTS 50%  40%  25% UTS (repeated)

T-T Block Loading Global  xx Strain Values (R = 0.1) Stress (% UTS) Stress (MPa)Initial Strain (%)Final Strain (%) N f (cycles)  mean  max  mean  max  mean  max

T-T Cyclic Fatigue Global Strain Values

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

Concluding Remarks Alignment 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

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