Interlaminar to intralaminar mode I and II crack bifurcation due to aligned carbon nanotube reinforcement of aerospace-grade advanced composites

Abstract

Aerospace-grade unidirectional carbon microfiber reinforced epoxy prepreg composite laminates were reinforced in the relatively weak interlaminar regions with high densities (>10 billion nanofibers per cm2) of uniformly-distributed vertically aligned carbon nanotubes (A-CNTs), creating a hierarchical architecture termed “nanostitch”. Such nanostitched laminates have been shown to increase laminate in-plane and interlaminar shear strengths. Here, the Mode I and Mode II fracture behavior and associated toughening mechanisms are investigated experimentally by performing double cantilever beam and end-notched flexure tests, respectively, of unidirectional 0° laminates following the ASTM Standards. Investigation of the crack surfaces via microscopy and micro-computed tomography (μCT) show that in both Mode I and II, the interlaminar crack bifurcates into the intralaminar region from the interlaminar precrack, and then propagates within the intralaminar region parallel to the nanostitched interlaminar region as an “intralaminar delamination” in steady state. This before unobserved phenomenon is attributed to the A-CNTs adding interlaminar toughness to a level that causes the interlaminar crack to bifurcate into the less tough intralaminar region. Microscopy and μCT analyses reveal that the A-CNTs do not increase the interlaminar thickness, and drive the crack into the intralaminar region within 1–2 mm of crack initiation in both Mode I and II, with the distance of the “intralaminar delamination” crack from the interlaminar region (at the laminate centerline) being greater in Mode II than in Mode I (~30 μm vs. ~15 μm, respectively). Finite element simulation of the crack bifurcation in Mode I predicts a minimum of 10% increase in interlaminar toughness due to the A-CNTs to propagate the crack in the intralaminar region in steady state, as observed experimentally. This unique crack behavior in advanced composites provides new insights into the magnitude and effects of reinforcement induced by A-CNTs that influence the macroscopic fracture and failure behavior of laminates, and suggests new opportunities for toughening laminates.