Xinchen Ni1 Reed Kopp1 Nathan Fritz1 Estelle Kalfon-Cohen1 Carolina Furtado2 Albertino Arteiro2 Gregor Borstnar3 Mark Mavrogordato3 Lukas Helfen4 5 Ian Sinclair3 Mark Spearing3 Pedro Camanho2 Brian Wardle1

1, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
2, University of Porto, Porto, , Portugal
3, University of Southampton, Southampton, , United Kingdom
4, European Synchrotron Radiation Facility, Grenoble, , France
5, Karlsruhe Institute of Technology, Karlsruhe, , Germany

Aerospace-grade unidirectional carbon fiber reinforced plastic (CFRP) composite laminates were reinforced in the resin-rich weak interfaces using high densities of aligned nanoscale fibers, in particular, aligned carbon nanotubes (A-CNTs), in a hybrid architecture termed “nanostitching.” Here, we investigate the effect of A-CNT interlaminar reinforcement on laminate strength and toughness via ex situ and in situ mechanical testing, leveraging both synchrotron radiation computed tomography (SRCT) and lab-based high resolution micro-computed tomography (μCT). Mode I and II interlaminar fracture toughness were assessed by conducting ex situ double cantilever beam (DCB) and end-notched flexure (ENF) tests, respectively. We find a ~6% lower steady-state mode I fracture toughness and no improvement of precracked mode II fracture toughness over baseline laminates. However, unique crack propagating behaviors in A-CNT reinforced laminates were revealed by scanning electron microscopy and μCT of fractured specimens. A-CNTs were found to force the crack into the intralaminar region in both DCB and ENF loadings, suggesting a tougher interlaminar region with the addition of A-CNTs. The measured toughness values of the A-CNT reinforced laminates were found to be associated with the toughness inside the ply. In situ SRCT tensile testing of double edge-notched tension (DENT) specimens shows a ~9% increase in ultimate tensile strength (UTS) over the baseline specimens. No significant differences in progressive damage features (up to 90% of UTS) near the notch were observed regardless of the presence of A-CNTs. 3D visualization and damage segmentation software identified matrix cracking and fiber/matrix interfacial debonding as dominant damage mechanisms for loads up to 90%. However, large interlaminar delaminations, which are known to be a primary damage mechanism suppressed by A-CNT interlaminar reinforcement, were revealed by post-mortem CT of DENT specimens. These findings reveal for the first time the multiscale strengthening and toughening mechanisms induced by A-CNTs, which influence the macroscopic behavior of composite laminates. Future work will focus on acquiring 3D data beyond 90% UTS of DENT configurations and performing in-situ mode I testing to non-destructively elucidate the crack propagation behavior in real-time.