INTRODUCTION

What are CNT Bundles?
• CNT bundle consists of several twisted or untwisted CNTs.
• Several Production Techniques - Spinning from CNT arrays, spinning from CNT aerogel, twisting/rolling of CNT films.

Bundle properties / Applications
• Could have higher specific modulus and specific strength.
• Can be used in multi-functional composites.

Objectives
• Determine CNT bundle properties under mechanical loading using MD Simulations

MODEL DEVELOPMENT

Bundle Model
• Four models consisting of 3, 4, 7, and 19 (5,5) CNTs are considered.
• In 4-tube bundle, CNTs are packed in square geometry. In other cases, CNTs are packed in hexagonal geometry.
• Nearest CNT surface-to-surface distance is considered as 0.34 nm.

MOLECULAR SIMULATION CONDITIONS

MD Simulation Parameters & Loading Conditions
• Temp = 300 K.
• Time Step = 0.5 fs.
• Initial structure relaxed for 50 ps.
• Axial displacement rate = 10 m/s.
• Torsional displacement rate = 10 Grad/s.
• Axial force and torque are calculated by summing up the atomic force and torque of the moving end atoms.

RESULTS - TENSION

• Bundle tensile forces are slightly less than single CNT at failure,
• This indicates inter-CNT vdW interaction in bundle does not affect the CNT tensile strength significantly.
• Failure starts at the end of the bundle with fixed displacements. End tearing also observed in previous studies with standalone CNTs.

RESULTS - COMPRESSION

• Peak load per CNT in bundle is significantly less (12 % - 21 %) than that of the standalone CNT.
• Due to vdW interaction buckling loads and failure modes change.
• CNT surface distortion occurs in bundle due to vdW interaction, and this distortion lowers compressive load
• Three cases studies to check the vdW interaction effects.
-vdW interaction is switched off, inter-tube spacing = 0.34 nm.
-vdW interaction is on, inter-tube spacing = 1.0 nm.
-vdW interaction is on, spacing = 0.34 nm, only the center CNT loading keeping the surrounding CNT rigid.

FORCE / TUBE vs. Displacement
• When vdW is not active, bundle load per CNT is the same as the standalone CNT load.
• When vdW is not active, buckling directions of the CNTs are random.
• Central CNT takes higher compressive loads when the surrounding CNTs are rigid and it buckles in shell mode.

RESULTS - TORSION

• Average per CNT torque significantly increases with increase in CNTs number in the bundle.
• While the standalone CNT buckle, end tearing of the outer CNTs occurs in the bundle under torsional loading.

CONCLUSIONS

• Bundle stiffness and load carrying capacity increase with increase in the bundle diameter (i.e., increase in CNT number in the bundle).
• vdW interaction affects the bundle compressive properties by introducing CNT surface imperfections.