Experimental and numerical study of the Poynting effect in additively manufactured TPU for soft robotic applications
Hynes, Daniel (2026)
Kandidaatintyö
2026
School of Energy Systems, Konetekniikka
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Julkaisun pysyvä osoite on
https://urn.fi/URN:NBN:fi-fe2026060260757
https://urn.fi/URN:NBN:fi-fe2026060260757
Tiivistelmä
This thesis investigates a non-linear material elongation due to torsion, the Poynting effect, in 3D-Printed TPU. A test bench was designed and constructed using additive manufacturing techniques, and a physical experiment was carried out to measure the Poynting effect in two soft TPU materials, Filaflex 70A and Select Flex 80A. A numerical simulation was carried out using finite element method to evaluate predictive accuracy against the physical experiments.
The results show a distinct and measurable Poynting effect in both tested materials. Experimental data strongly correlates with the numerical simulation. At low-to-mid twist angles, up to 360 degrees, after which physical limitations of the test bench, such as slipping of the specimen within its constraints, caused the experimental elongation to plateau, diverging the theoretical model. The softer TPU70A exhibited higher initial responsiveness, while the slightly stiffer TPU80A maintained structural integrity longer, resulting in a higher identified maximum elongation.
Based on the findings, torsion induced elongation should be taken into account in soft robotic applications. Not accounting for the Poynting effect in soft robotic systems may lead to unintended linear motion, overshooting kinematic models, joint jamming and possible mechanical failure. This applies especially in small scale systems.
The results show a distinct and measurable Poynting effect in both tested materials. Experimental data strongly correlates with the numerical simulation. At low-to-mid twist angles, up to 360 degrees, after which physical limitations of the test bench, such as slipping of the specimen within its constraints, caused the experimental elongation to plateau, diverging the theoretical model. The softer TPU70A exhibited higher initial responsiveness, while the slightly stiffer TPU80A maintained structural integrity longer, resulting in a higher identified maximum elongation.
Based on the findings, torsion induced elongation should be taken into account in soft robotic applications. Not accounting for the Poynting effect in soft robotic systems may lead to unintended linear motion, overshooting kinematic models, joint jamming and possible mechanical failure. This applies especially in small scale systems.