Mechanical and Piezoresistive Self-Sensing Behavior of Polymer Nanocomposites Processed via Selective Laser Sintering

Abstract

This thesis is concerned with the design, fabrication, characterization and mechanical testing of multi-functional polymer-based nanocomposite structures enabled via selective laser sintering (SLS) for applications in self-sensing structural systems. Two thermoplastic powders, namely polyamide (PA12) and ultra-high molecular weight polyethylene (UHMWPE), and two reinforcing nanofillers, multi-walled carbon nanotubes (MWCNTs) and graphene nanoplatelets (GNP), were used in this work. The structural, morphological, thermal, mechanical and piezoresistive properties of the 3D printed composites were evaluated. The developed nanocomposite powders were also used to 3D print hexagonal and re-entrant 2D lattice structures and their mechanical and piezoresistive responses were measured under different loading conditions (i.e, tension, compression, three-point bending, impact). Observations of the MWCNT/PA12 microstructure revealed that the porosity level in the sintered parts significantly increased with increasing MWCNT loading, and this was identified as the main cause for the reduction in mechanical performance. The 3D printed MWCNT/PA12 composite with the lowest MWCNT loading (0.3 wt.%) provided the highest (initial) gauge factors, reporting 1.2 and 31 for uniaxial tension and compression, respectively. The 3D printed hexagonal honeycomb nanocomposite structures exhibited excellent strain sensing capabilities in the elastic regime, as evinced by gauge factors in the range of 25. For the MWCNT/UHMWPE composites, we found enhancements in the mechanical properties as compared to neat UHMWPE. The 0.5 wt.% MWCNT/UHMWPE composite possessed superior mechanical and piezoresistive characteristics among all compositions. Furthermore, MWCNT/UHMWPE 2D hexagonal lattice structures exhibited a nearly linear piezoresistive response with a constant gauge factor of 1. Incorporating GNP (up to 1.5 wt.%) into 3D-printed UHMWPE components was shown to increase mechanical characteristics under tensile loading. In terms of gauge factors, the maximum values for strain sensing and damage sensing under tensile loading were found to be 9.6 and 18, respectively, for GNP/UHMWPE composites with 1.5 wt.% GNP loading. The developed nanocomposites exhibited stable piezoresistive response under different cyclic loading conditions which is desirable for strain sensing applications in smart biomedical implants, soft robots or human−machine interface technologies.

Date of Award	Aug 2023
Original language	American English
Supervisor	Andreas Schiffer (Supervisor)