Synthesis, characterization, and mode I fracture toughness of aligned carbon nanotube polymer matrix nanocomposites
Author(s)Lidston, Dale L. (Dale Leigh)
Massachusetts Institute of Technology. Department of Aeronautics and Astronautics.
Brian L. Wardle.
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In an effort to fully understand the contribution of carbon nanotubes (CNTs) to strength and toughness enhancement in hierarchical nanoengineered composites, particularly steady state Mode I fracture toughness, RTM6 and EPON 862/W epoxy based vertically-aligned carbon nanotube (A-CNT) polymer nanocomposites (A-PNCs) are manufactured. These A-PNCs can be tested to isolate structure-property relationships between the polymer matrix and the A-CNTs without the presence of the micro-scale fibers. Additionally, A-CNT volume fraction can be varied via a densification process to realize 1-30% volume fraction (vf.%) A-PNCs. An investigation of the Mode I initiation fracture toughness via single edge notch beam (SENB) testing of A-PNCs with 1-5 vf.% uniaxially densified A-CNT forests finds that RTM6 baseline and A-PNC samples have a KIc,i of ~ 1 MPa-m¹/², with the exception of 1 vf.% having 1.33 ± 0.09 MPa-m¹/², which needs to be further explored due to process-structure questions of specimen quality. No statistically significant change is observed in EPON 862/W A-PNCs at 1-5 vf.% over baseline specimens having a KIc,i of 1.49 ± 0.06 MPa-m¹/² , indicating that A-CNTs do not offer any toughening at initiation in this system. Scanning electron microscopy of the fracture surface for both A-PNC systems reveals that < 10% of the A-CNTs available are engaged during crack bridging, i.e., in a 5 vf.% A-PNC specimen, at most 0.5 vf.% of the A-CNTs are engaged during fracture, and pull-out from the matrix is less than 1 pm. Thus, the pullout and debonding toughening contribution offered by the A-CNTs is expected and measured to be negligible. It is possible that these A-CNTs may offer steady-state toughening, however, results are not achieved due to limitations in specimen size and geometry. While changes in Mode I initiation toughness are not observed, significant changes in properties from both quasi-static and dynamic nanoindentation testing are observed. The A-CNT alignment confers a non-isotropic mechanical response when quasi-statically tested with A-CNTs parallel or perpendicular to the indentation load. An ~ 270% modulus increase over baseline for the 30 vf.% EPON 862/W A-PNC parallel configuration and a ~ 140% increase in the perpendicular configuration, are observed and dynamic nanoindentation supports this finding with, e.g., a storage modulus increase of ~ 200% in the parallel orientation. RTM6 A-PNCs show less of an increase in indentation modulus (~ 33% in 10 vf.% specimens in the parallel direction) over baseline when compared to those of EPON 862/W, likely due to the relatively stiff RTM6 (modulus is ~ 1.5x larger than EPON 862/W) and therefore there is a relatively smaller A-CNT stiffness contribution. Looking forward, the A-CNTs used in this work are noted to be unmodified/as-grown, and the A-CNT fracture results highlight the need for modifying the A-CNTs towards increased A-CNT strength (defect density reduction), and/or reducing the strength of the CNT-polymer interface, to increase A-CNT toughness contribution further.
Thesis: S.M., Massachusetts Institute of Technology, Department of Aeronautics and Astronautics, 2017.Cataloged from PDF version of thesis.Includes bibliographical references (pages 199-215).
DepartmentMassachusetts Institute of Technology. Department of Aeronautics and Astronautics.
Massachusetts Institute of Technology
Aeronautics and Astronautics.