Hierarchical (Multiscale) Fiber

1. Hierarchical (Multiscale) Fiber Reinforced Composites (FRC): Dramatic Enhancements in Toughness & Fatigue Resistance Nikhil A. Koratkar Full Professor, Department of Mechanical, Aerospace & Nuclear Engineering Rensselaer Polytechnic Institute 110 8th Street, Troy, NY-12180 Presentation at Nano Bangalore 2008 Conference, December 11, 2008

2. Fiber Reinforced Composites (FRC) • FRC are increasingly the ‘Material of Choice’ in Structural Applications • Matrix • Polymer (Epoxy) • Fiber (Micro scale) • Kevlar fibers • Glass fibers • Graphite fibers

3. FRC: Pros and Cons • Advantages • High stiffness • High strength • Low density • Disadvantages • Low strain-to-failure • Low toughness & ductility • Poor fatigue & resistance to crack growth • Poor impact damage tolerance

4. Our Concept: Hierarchical (Multiscale) FRC • Add Intermediate Length Scale • Carbon Nanotubes (CNT) • Diameter: 1-10 nm • Length: 1-10 microns • CNT Bridges Length Scales • Forms ‘Interconnected’ • Hierarchical Networks Such hierarchical architectures can alleviate the classical failure problems of FRC

5. Fabrication Process

6. Fracture Properties (Static Loading)

7. Tensile Failure Tests Kevlar/Nanotube/Epoxy Composite Kevlar/Epoxy Composite Multiwalled nanotube content: 0.05% by weight

8. Strength Enhancement • Effect of Multiwalled Nanotube (MWNT) weight fraction ~ 20% Increase Failure Stress (MPa) 0.07% Weight CNT No CNT 0.03% Weight CNT 0.05% Weight CNT

9. Toughness Enhancement MWNT SWNT A-MWNT NH2 Diameter:10 nm Length: 1 micron Diameter:1.4 nm Length: 1 micron Diameter:10 nm Length: 1 micron ~ 40% Increase Energy Absorption (J/m3) A-MWNT SWNT MWNT No CNT 0.05% Weight 0.05% Weight 0.05% Weight

10. Mechanism for Toughening:Hierarchical Structure Epoxy Matrix Carbon Nanotubes Kevlar Fibers Hierarchical (Multiscale) Structure Nanotubes link the microfibers to the epoxy matrix Stresses at the macro (material) level are efficiently redistributed to lower levels of Hierarchy Nanotube-matrix interfaces and nanotube-microfiber interfaces must fail prior to failure at macro level; dissipates more energy Bio-inspired Composites Mimics structure of natural composites such as Bone, Silk etc.

11. Evidence of Energy Dissipation at Lower Levels of Hierarchy Interfacial De-bonding and Nanofiber Pull-Out 300 nm

12. Fatigue Properties (Dynamic or Cyclic Loading)

13. Typical Example of Fatigue Failure in FRC: Helicopter Rotor Flexbeams Flexbeam (glass-fiber/epoxy composite) Resin Pockets Simplified Flexbeam Schematic Helicopter Rotor

14. Model Flexbeam Testing • Glass-fiber/Nanotube/Epoxy • Hierarchical Flexbeam Model • A-MWNT weight fraction: 0.05% • Dynamic 4-Point Bend Test • Force: 120 N, Frequency: 2 Hz

15. Model Flexbeam Testing Ultrasound C-SCAN Imaging 12 Plies 6 Plies 12 Plies Ply Drop Regions

16. Fatigue Suppression Baseline Composite ~ 3-fold Reduction in Delamination Length Hierarchical Nano-Composite with 0.05% weight A-MWNT

17. Multifunctionality (Health Monitoring)

18. Detection of Impact Damage 8-Ply Kevlar/Nanotube/Epoxy composite Impact Energy = 2.3 J No visible damage is observed on surface, Internal damage can be monitored by voltage distribution mapping.

19. Solving the “Inverse” Problem- Calculated Conductivity from Measured Grid Potentials (1) (2) ~ 0.5 inch Where: : Electrical Conductivity : Electrical Potential (Voltage) is calculated such at both Equation (1) & (2) are satisfied. Local electrical conductivity changes after impact due to distortion of carbon nanotube network and de-laminations/internal void formation.

20. Ultrasonic C-Scan Verification ~ 0.6 inch • Size of damage determined from ultrasonic scan shows good correlation with the voltage mapping result. • Resolution can be further enhanced by refining the grid spacing.

21. Concluding Remarks • Hierarchical reinforcing using nanofibers (carbon nanotubes) has significant impact. • Over 20% increase in strength and 40% increase in toughness of conventional FRC with only 0.05% weight penalty. • Up to 3-times reduction in crack/delamination growth in fatigue environments. • Multi-functionality: Nanofibers sense damage in real-time mode.

22. Related Journal Publications W. Zhang, R. C. Picu, and N. Koratkar, “Suppression of fatigue crack growth in carbon nanotube composites”, Applied Physics Letters 91, 193109 (2007). W. Zhang, R. Picu, N. Koratkar, “Effect of nanotube dimensions & dispersion on the fatigue behavior of epoxy nanocomposites” Nanotechnology 19, 285709 (2008). W. Zhang, V. Sakalkar, and N. Koratkar, “In situ health monitoring and repair in composites using nanotube additives”, Applied Physics Letters 91, 133102 (2007). J. Suhr, N. Koratkar, P. Keblinski, and P. Ajayan, “Viscoelasticity in carbon nanotube composites,” Nature Materials 4, 134 (2005). J. Suhr, W. Zhang, P. Ajayan, N. Koratkar, “Temperature activated interfacial friction damping in carbon nanotube polymer composites,” Nano Letters 6, 219 (2006).

23. Acknowledgements • My Ph.D. Student (Andrew Proper) • Funding Support - USA National Science Foundation (CAREER Award) - US Army (VLRCOE, DURIP Awards)

24. NANO-DEVICES SENSORS ACTUATORS MEMBRANES NANO-COMPOSITES MECHANICAL PROPERTIES HEALTH MONITORING Koratkar Lab at RPI NANO-COATINGS SUPER-HYDROPHIC/HYDROPHILIC ENHANCED BOILING SURFACES NANO ENERGY CONVERSION CATALYSTS - SOLAR CELL - FUEL CELL ELECTRODES - BATTERIES