1. 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

2. Content Introduction Test Programme Constant Amplitude Cyclic Fatigue Tension-Tension Compression-Compression Tension-Compression Multiple Step T-T Block Loading Concluding Remarks

3. 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

4. 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

5. 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

6. Embedded Fibre Bragg Gratings Strain Gauges and FBGs FBG

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

8. Quasi-Static Strain Measurements

9. 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

10. Quasi-Static Loading  xx Strain Across Specimen Mid-length Increasing Load

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

12. 0 5,000 10,000 15,000 20,000 25,000 30,000 T-T Cyclic Fatigue Pulse Thermography (55% UTS)

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

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

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

16. 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

17. 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

18. T-T Cyclic Fatigue Strain Distributions vs. Loading Cycles  yy  xy

19. 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

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

21. 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 = 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

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

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

24. 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 = 0.1 40 55 55 (1 Hz) 70 33 46 30 23 46 78 34 65 87 104 41 68 R = 0.5 402533

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

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

27. Open-Hole (Notched) Compression Compression-Compression Unnotched S C xx (MPa): 617 ± 19 Open-Hole Compression (OHC) Strength (MPa): 346 ± 54

28. C-C Cyclic Fatigue Damage/Failure

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

30. 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

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

32. 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

33. 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)

34. T-C Cyclic Fatigue Normalised S-N Curve

35. 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

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

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

38. 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)

39. 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 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 17564

40. T-T Cyclic Fatigue Global Strain Values

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

42. 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

43. 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