1. Namas Chandra and Sirish Namilae
Mechanics of Atomic Scale Interfaces in Carbon Nanotube Reinforced Composites
Namas Chandra and Sirish Namilae
Department of Mechanical Engineering
Florida State University
I like to thank all of you …This work was funded by ARO and FSU RF
Thesis deals with deformation of nanoscale or atomic scale materials
Nanoscale gained prominence in recent times …appl range from nanosensors to nano composites.
Defects play a big role as the def/totvol ratio is much higher
The focus is to look at the role of defects in mechanical behavior

2. Carbon Nanotubes (CNT)
Carbon Nanotubes: Graphite sheet rolled into a tube
Single wall and Multiwall nanotubes
Zigzag, armchair and chiral nanotubes
Length ~ 100 nm to few m Diameter~ 1 nm
Applications
E ~ 1 TPa Strength ~150 GPa
Conductivity depends on chirality
High strength
composites
Energy
storage
Nano sensors
Medical applications
Nano
electronics
Functional
composites
Do these properties extend to
CNT reinforced composites ?

3. Answer: Currently NO!!! Parallel model Upper Bound Series model
Lower Bound

4. Critical Issues Critical issues in nanotube composites
Alignment
Dispersion
Load Transfer
Load transfer and to some extant Dispersion affected by interfaces
Interface  Bounding surface with physical / chemical / mechanical discontinuity
CNT-matrix interfaces
Vanderwall’s forces
Mechanical interlocking
Chemical bonding

5. Functionalized Nanotubes
Change in hybridization (SP2 to SP3)
Experimental reports of different chemical attachments
Application in composites, medicine, sensors
Functionalized CNT are possibly fibers in composites
Sensors check how ? Pyrene what check?
How do fiber properties differ with chemical modification of surface?

6. Functionalized nanotubes
Increase in stiffness observed by functionalizing
Vinyl and Butyl
Hydrocarbons
T=77K and 3000K
Lutsko stress
Stiffness increase is more for higher number of chemical attachments
Stiffness increase higher for longer chemical attachments

7. Radius variation
Increased radius of curvature at the attachment because of change in hybridization
Radius of curvature lowered in adjoining area
Sp3 Hybridization here

8. Evolution of defects in functionalized CNT
Defects Evolve at much lower strain of 6.5 % in CNT with chemical attachments
Onset of plastic deformation at lower strain. Reduced fracture strain

9. Different Fracture Mechanisms
Fracture Behavior Different
Fracture happens by formation of defects, coalescence of defects and final separation of damaged region in defect free CNT
In Functionalized CNT it happens in a brittle manner by breaking of bonds
Chemical Physics Letters (2004)

10. Multiscale approach to composite problem
Molecular Dynamics
Computationally expensive
Multiscale
MD+continuum
Molecular Dynamics
Computationally expensive
Multiscale
MD+continuum
Need for multiscale approach ?
Why hierarchical modeling ?
Molecular dynamics of pull out tests
Atomically informed cohesive zone model for interfaces
Finite element model for composites with CZM interfaces
Step I: MD
Step II: FEM
Step I:
Atomic e.g. MD
Step II:
Continuum e.g. FEM
Pass qualitative
& quantitative
information
Hierarchical Modeling
Time scale problem
MD simulations
of CNT Pullout
CZM based FEM
for composite
Atomically
Informed CZM
For interfaces
Hierarchical model for CNT composite

11. Atomic simulation of CNT pullout test
Simulation conditions
Corner atoms of hydrocarbon attachments fixed
Displacement applied as shown 0.02A/1500 steps
T=300K

12. Debonding and Rebonding of Interfaces
Failure

13. Debonding and Rebonding
Matrix
Matrix
Energy for debonding of chemical attachment 3eV
Strain energy in force-displacement plot 20 ± 4 eV
Energy increase due to debonding-rebonding

14. Variation in interface behavior

15. Force distribution along the interface

16. Force Distribution - continued
Variation of reaction force with time as shown
Black interface region fully loaded, white region is unloaded or fractured

17. Cohesive zone model for interfaces
Assumptions
Nanotubes deform in linear elastic manner
Interface character completely determined by traction-displacement plot

18. Cohesive zone Models for nanoscale interfaces

19. Finite element simulation: Composite stiffness

20. Parametric studies
Variation of fiber stiffness for different interface
strengths

21. Summary and conclusions
Functionalized CNT as fibers
Marginal increase in stiffness
Inelasticity sets in at lower strains
Mechanism of fracture is different
Nanoscale interfaces in CNT based composites
Very high interface strength can be obtained by engineering chemically bonded interfaces
Interface strength depends on various factors, primarily extant of bonding
Unique features such as debonding-rebonding observed in nanoscale interfaces
Multiscale model with interface described by atomically informed CZM developed
Effect of interface strength on stiffness of composite studied using this multiscale model