  Soft actuators have attracted a great deal of interest in the context of
rehabilitative and assistive robots for increasing safety and lowering costs as
compared to rigid-body robotic systems. During actuation, soft actuators
experience high levels of deformation, which can lead to microscale fractures
in their elastomeric structure, which fatigues the system over time and
eventually leads to macroscale damages and eventually failure. This paper
reports finite element modeling (FEM) of pneu-nets at high angles, along with
repetitive experimentation at high deformation rates, in order to study the
effect and behavior of fatigue in soft robotic actuators, which would result in
deviation from the ideal behavior. Comparing the FEM model and experimental
data, we show that FEM can model the performance of the actuator before fatigue
to a bending angle of 167 degrees with ~96% accuracy. We also show that the FEM
model performance will drop to 80% due to fatigue after repetitive high-angle
bending. The results of this paper objectively highlight the emergence of
fatigue over cyclic activation of the system and the resulting deviation from
the computational FEM model. Such behavior can be considered in future
controllers to adapt the system with time-variable and non-autonomous response
dynamics of soft robots.
