Crucially, the team has a complete theoretical model of the substances’ piezo resistivity – its resistance change as it deforms.
“Imparting piezoresistive behaviour to 3D-printed cellular materials gives them the ability to monitor their own performance without any additional hardware,” said engineering professor Shanmugam Kumar. “While researchers have known about these properties for some time now, what we’ve not been able to do is provide a way to know in advance how effective new attempts at creating self-sensing materials will be. Instead, we have often relied on trial and error.”
The engineering polymer PEI (polyetherimide) was mixed with carbon nanotubes as a source material for the experiments, which was printed into various strong (105MPa) conductive lattices which are stiff (3,368MPa) and showed good sensitivity (13 gauge factor).
Accurate modelling required multi-scale finite element analysis: on microscopic changes in the bulk material, on the unit-cell of the lattice, and on the total lattice ant its surroundings.
In one example (right) resistance drops steadily as the material is squashed, allowing strain to be measured, then drops suddenly once parts of the matrix touch each other parts, allowing impending failure to be detected.
Other lattices did not have this built-in reversible switching action, but progressively crumpled in failure, again causing sudden resistance drops.
Predicted current flow was compared with heat-sensitive photographs taken of real current flowing through, and therefore heating, real printed objects.
“They found that their models could accurately predict how the materials would respond to various combinations of stress and strain, and how their electrical resistance would be affected,” according to the University. “The results could help underpin future developments in additive manufacturing by providing insights into how proposed materials will perform before the first real-world prototype is printed.”
“While we focused on PEI materials with embedded carbon nanotubes in this paper, the multiscale finite element modelling could be easily applied to other materials which can be created through additive manufacturing too,” said Kumar.
Glasgow University collaborated with Istanbul Technical University in this project, which is published as ‘Autonomous sensing architected materials‘ in Advanced Functional Materials – full paper available without payment.
