Experimental Investigation of Interlaminar Fracture Micro-mechanisms of Aligned Carbon Nanotube-reinforced Aerospace Laminated Composites

摘要

The relatively weak interlaminar regions of aerospace-grade unidirectional carbon microfiber reinforced epoxy prepreg composite laminates were reinforced with high densities of vertically aligned carbon nanotubes (A-CNTs), creating a hierarchical architecture termed "nanostitching". Such nano-engineered interfaces have been shown to increase laminate short beam strength and substructural in-plane strengths. Here, we investigate the Mode Ⅰ and Mode Ⅱ interlaminar fracture behavior of nanostitched laminates by performing double cantilever beam (DCB) and end-notched flexure (ENF) tests, respectively. Micro-computed tomography (micro-CT) and scanning electron microscopy (SEM) imaging reveal that the A-CNTs increase the toughness of the interlaminar region to an extent that forces the crack to bifurcate into the less tough intralaminar region and propagate there parallel to the interface as an "intralaminar delamination" in both Mode Ⅰ and Ⅱ. As a result, the measured toughness in nanostitched laminates more closely represent the intralaminar toughness, as opposed to the interlaminar toughness. The Mode Ⅰ "intralaminar toughness" is shown to be 6% less than the baseline interlaminar toughness, whereas in Mode Ⅱ the "intralaminar" and baseline interlaminar toughness is the same. This before-unobserved crack behavior due to interlaminar toughening provides new insights into the reinforcement mechanisms at work at the micro and nanoscale induced by A-CNTs that influence the macroscopic fracture behavior of laminates. Future work to evaluate the nanostitch interlaminar toughness required to drive the crack to, and propagate it in, the intralaminar region in Mode Ⅰ and Ⅱ, as well as fatigue behavior of the same geometries is planned.