Advanced composite architectures have employed carbon nanotubes (CNTs) to reinforce the inter- and intralaminar regions using various morphologies. Understanding the toughening mechanism seen in these hierarchical composites can be furthered using a polymer nanocomposite (PNC) representative volume element. Aligned CNT (A-CNT) forests ～2mm in height densified to volume fractions between 1 and 10% are infiltrated with aerospace grade epoxy to form aligned PNCs (A-PNCs) that are manufactured into single edge notch beam (SENB) specimens to assess fracture toughness including crack initiation and growth. Preliminary SENB tests have been performed on the baseline (pure epoxy) and densified A-PNC specimens. While initiation toughness K_k remains unchanged, the energy absorbed during crack propagation is 50-70% higher in the A-PNC specimens when compared to the baseline, due to crack arrest behavior. In the 1% V_f A-PNC, A-CNT pullout is observed suggesting crack tip bridging as a toughening mechanism. Voids present in the higher volume fraction specimens make it difficult to elucidate the toughening mechanisms at work as CNT volume fraction is increased.
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