Confinement of MWCNTs in PA6 3D-printed fibreglass-reinforced composites to enhance piezoresistive properties

A continuous fibreglass (CFG)-reinforced polyamide 6 (PA6) sandwich structure with self-sensing capabilities was developed by confining multi-walled carbon nanotubes (MWCNTs) within the material volume through a step-by-step process involving a) 3D printing of specimens with a designed porous structure, b) embedding MWCNTs onto the surface of polyamide pores swollen with acid-formic solutions containing various filler contents, and c) hot-pressing the resulting specimens to close the porosity. Sandwiched specimens, designed with top-bottom skins at control layup (no reinforcement CFG, namely “noGF”), the quasi-isotropic (with CFG oriented 0/45/90/-45°s, namely “qiGF”), and the longitudinal layup (with CFG oriented at 0°, namely “longGF”) were subjected to steady and cyclic three-point bending tests and mechanical and piezoresistive characterized. The results show a correlation between applied strain and measured electrical resistance, with a gauge factor (GF) of 23 at a strain of 0.83% for the sample containing 0.05 wt% MWCNTs. The fibre reinforcement, together with the porous sandwich design, proved effective in reducing electrical hysteresis and improving measurement repeatability. The sample containing 0.05 wt% of MWCNTs and longGF shows a significant improvement in sensing performance. These findings confirm that confining MWCNTs within 3D-printed PA6 sandwich structures is an effective strategy for enhancing the piezoresistivity.