Failure behavior of carbon nanotube yarns embedded in polymer matrices

Abstract Purpose: Carbon nanotube yarns (CNTYs), composed of twisted nanoscale carbon nanotubes, are promising candidates for high-performance composite materials. However, the tensile strength of a CNTY embedded in a polymer matrix is reduced compared to that observed under atmospheric conditions. This study provides a direct, side-by-side quantification that links the suppression of the self-tightening effect to the strength loss in embedded CNTY. Methods: CNTY statistical strengths in both conditions were assessed through single-yarn tensile and fragmentation tests. Failure behavior was characterized using photoelastic birefringence to identify first-break events, in situ optical tracking of diameter and twist under load, and fractography and transmission electron microscopy near the fracture plane. Results: Embedding reduced the CNTY characteristic strength by approximately 95%, and the failure strain from 6.72 to 0.60%; by contrast, carbon fiber changed little across conditions. Weibull analysis revealed a substantial decrease in both characteristic strength $$(\sigma_{0} )$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mo>(</mml:mo> <mml:msub> <mml:mi>σ</mml:mi> <mml:mn>0</mml:mn> </mml:msub> <mml:mo>)</mml:mo> </mml:mrow> </mml:math> and shape parameter ( $$m$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>m</mml:mi> </mml:math> ) values for embedded CNTY. Transmission electron microscopy indicated an outer mixed-phase region and a largely dry CNT core, consistent with partial infiltration and suppressed self-tightening as the proximate cause of the reduced scale and shape parameters. Conclusion: Together, the observations and statistical analysis explain the reductions in Weibull scale and shape for embedded CNTY as consequences of the suppressed self-tightening effect and an observed outer mixed-phase region, motivating the need for yarn-specific models beyond continuous fiber assumptions and informing yarn selection and matrix design to mitigate strength loss in CNTY composites. Graphical abstract