[Paper Review] Interlaminar toughening in structural carbon fiber/epoxy composites interleaved with carbon nanotube veils

Yunfu Ou, Carlos González+1 more|arXiv (Cornell University)|May 22, 2019

Carbon Nanotubes in Composites63 references4 citations

TL;DR

This study demonstrates a 60% increase in Mode-I fracture toughness of carbon fiber/epoxy composites by integrating thin, continuous carbon nanotube (CNT) veils directly onto carbon fiber fabrics via a semi-industrial gas-phase process. Crack propagation alternates above and below the CNT-toughened interlayer, enhancing interlaminar toughness when CNT veils are deposited as low-density layers, whereas densified veils act as defects.

ABSTRACT

The susceptibility to delamination is one of the main concerns in fiber reinforced polymer composites (FRPs). This work demonstrates improvements of 60% in Mode-I fracture toughness after integration of thin (~30 micron), continuous veils of carbon nanotubes (CNTs) directly deposited onto carbon fiber fabric as the CNT are drawn from the gas-phase using a semi-industrial process. A combination of optical imaging, scanning electron microscopy and a Raman spectroscopy provide a new rapid tool to unambiguously determine the crack propagation path by simple visual inspection of fracture surface. The results show that interlaminar crossing between CNT veil/CF interfaces is of paramount importance. The crack front alternatingly propagates above and below the CNT-toughened interlayer, significantly improving the fracture toughness of resultant laminates. This mechanism is strongly influenced by the method used to integrate the veils onto the CF. CNT veils directly deposited onto the fabrics as a low-density layer lead to large improvements in interlaminar properties, whereas compact CNT veils densified by solvent exposure prior to their integration in the lay-up act as defects.

Motivation & Objective

• To address the critical issue of delamination in fiber-reinforced polymer composites, which limits structural reliability.
• To develop a scalable, low-cost interlaminar toughening strategy using carbon nanotube veils for industrial application.
• To investigate the influence of CNT veil integration method (direct deposition vs. solvent densification) on interlaminar fracture properties.
• To establish a rapid, visual method for tracking crack propagation paths using optical imaging, SEM, and Raman spectroscopy.
• To identify the dominant toughening mechanism in CNT-veil-reinforced laminates through microstructural analysis.

Proposed method

• Fabricated continuous, ~30 µm thick CNT veils using a semi-industrial chemical vapor deposition (CVD) process.
• Directly deposited CNT veils onto carbon fiber fabric as a low-density layer without solvent treatment.
• Prepared control specimens with CNT veils that were densified by solvent exposure before lay-up integration.
• Performed notched end-notched flexural tests to measure Mode-I fracture toughness (GIC).
• Used optical microscopy, scanning electron microscopy (SEM), and Raman spectroscopy to analyze crack propagation paths and interfacial interactions.
• Correlated microstructural observations with mechanical test results to identify toughening mechanisms.

Experimental results

Research questions

• RQ1 How does direct deposition of CNT veils onto carbon fiber fabric affect interlaminar fracture toughness in carbon fiber/epoxy composites?
• RQ2 What is the role of CNT veil morphology and integration method (e.g., solvent densification) in influencing delamination resistance?
• RQ3 Can optical imaging and Raman spectroscopy provide a rapid, reliable method for identifying crack propagation paths in fracture surfaces?
• RQ4 What microstructural mechanisms underlie the observed toughening effect in CNT-veil-reinforced laminates?
• RQ5 Why do densified CNT veils act as defects rather than toughening agents in the composite structure?

Key findings

• A 60% improvement in Mode-I fracture toughness (GIC) was achieved in composites with directly deposited, low-density CNT veils.
• Crack propagation alternately crossed above and below the CNT-toughened interlayer, indicating effective energy dissipation through crack deflection and bridging.
• CNT veils that were densified by solvent exposure prior to integration acted as defects, reducing interlaminar toughness compared to the control.
• The combination of optical imaging, SEM, and Raman spectroscopy enabled unambiguous identification of crack paths through visual inspection of fracture surfaces.
• Interlaminar crossing at the CNT veil/carbon fiber interface was identified as the dominant toughening mechanism.
• The method of CNT veil integration—specifically, maintaining low density and direct deposition—was critical to achieving enhanced fracture resistance.