Experimental Durability Assessment and Multivariable Lifetime Modeling of UHMWPE Twisted String Actuators

Predicting cyclic durability while interpreting geometric and loading effects remains a challenge for ultra-high-molecular-weight polyethylene (UHMWPE) twisted string actuators (TSAs). This study analyzed 390 recorded lifetimes across 39 tested combinations of string diameter (1.0–2.0 mm), string number (two to seven), and applied mass (15–25 kg). A ten-coefficient second-order log-polynomial model was fitted to the logarithms of condition-wise arithmetic mean lifetimes and compared with a refitted power-law model. Lifetime increased with diameter and string number and decreased with applied mass. The second-order model achieved an in-sample mean absolute percentage error (MAPE) of 2.1212%. Leave-one-condition-out cross-validation yielded a MAPE of 2.8274%, compared with 3.4433% for the reduced model, a 17.89% relative reduction, although the maximum held-out error increased slightly. In the fitted surface, a positive diameter–mass interaction indicated greater proportional load sensitivity for thinner strings. Stress-equivalent local sensitivities were compatible with a nominal-stress interpretation at the reference condition, without establishing a universal stress–life law. Surface fuzzing, filament exposure, partial breakage, and pull-out were consistent with damage associated with repeated contact and relative sliding. The model provides a conditional engineering description within the tested domain; independent validation and confirmation of data assignments and record independence remain necessary.