Nanoengineered hierarchical advanced composites with nanofiber interlaminar reinforcement for enhanced laminate-level mechanical performance

Abstract

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2020Cataloged from PDF of thesis.Includes bibliographical references (pages 157-177).At present, there is a need for novel, scalable, and high-performance structural materials that offer unprecedented combinations of stiffness, strength, and toughness at a low density, which can serve in a variety of applications in the aerospace, transportation, defense, and energy industries. To date, composite materials, specifically advanced carbon fiber reinforced plastics (CFRPs), which are comprised of high specific stiffness and strength continuous carbon microfibers and lightweight, relatively compliant polymers, have been among the most attractive materials and are used extensively in the aerospace sector. However, most CFRPs are fabricated by stacking plies in a layer-by-layer fashion, resulting in a weak polymer-rich region, known as the interlaminar region, at each ply interface that leads to poor properties through the laminate thickness.Although the mechanically superior microfibers are designed to be the primary load carriers, the much weaker polymer matrix causes the laminates to be prone to premature failure with interlaminar delamination, which negatively affects both in-plane and out-of-plane performance. This key shortcoming is known as the Achilles' heel of CFRPs, which hinders their design and wider adoption in critical structural applications. In this dissertation, a novel nanoengineering approach to address the longstanding problem of weak ply interfaces of CFRPs is developed and demonstrated. High densities (>10 billion nanofibers per cm²) of uniformly-distributed vertically aligned carbon nanotubes (A-CNTs) are placed between neighboring plies to bridge the weak polymer-rich interlaminar region in existing prepreg-based laminated composites, creating a hierarchical architecture termed "nanostitch".The effectiveness of nanostitching is evaluated via various mechanical tests including short-beam shear (SBS), Mode I and II fracture, and double edge-notched tension (DENT), in all of which the nanostitched composites have demonstrated enhanced mechanical performance. Furthermore, the multiscale reinforcement mechanisms resulting from the CNTs are elucidated via a variety of ex situ and in situ damage inspection techniques, including optical microscopy, scanning electron microscopy, lab-based micro-computed tomography, and in situ synchrotron radiation computed tomography (SRCT). Specifically, in SBS, despite no increase in static strength, a 115% average increase in fatigue life across all load levels (60 to 90% of static strength), with a larger increase of 249% in high-cycle (at 60% of static strength) fatigue, is observed.In Mode I and Mode II fracture, it is revealed that 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 unique crack bifurcation phenomenon has never been previously observed and 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. In DENT, an 8% increase in ultimate tensile strength (UTS) is observed and is attributed to the A-CNTs suppressing critical interlaminar delaminations very close to final failure (greater than 90% UTS) via in situ SRCT.In addition to the positive reinforcement results observed for the nanostitched composites, a next-generation higher volume fraction nanostitched composite with additional levels of beneficial hierarchy termed "buckled nanostitch" or "nanostitch 2.0" is created by exploiting the unique buckling behavior displayed by patterned A-CNT forests under compression. This multilevel hierarchical architecture further enhances the composite mechanical performance: SBS strength by 7% and DENT strength by 28%, compared to the baseline composites. The dissertation not only presents a controllable, scalable manufacturing method to produce engineered structural materials that are hierarchically designed down to the nanoscale with enhanced mechanical performance, but it also establishes key new understanding of the complex and coupled strengthening and toughening mechanisms acting at different scales, as well as their effects on macroscopic laminate-level mechanical properties.A particular focus has been the seminal use of in situ SRCT to study the effects of the hierarchical nanoscale reinforcements, and thus the methods established provide an experimental path forward for future work in this area. Together, these advances open up new opportunities for creating next-generation engineered materials with a suite of programmable properties by controlling their structures and constituents across multiple length scales.by Xinchen Ni.Ph. D.Ph.D. Massachusetts Institute of Technology, Department of Mechanical Engineerin