Unveiling unexpected mechanical softening/stiffening in carbon nanotube composites under cyclic deformation: experiments and predictive modeling

Abstract

Observation and prediction of the electrical and mechanical properties of nanocomposites under dynamic deformation conditions are critical for wearable devices and soft electronics. Despite extensive research, a comprehensive understanding of the mechanical characteristics of composites subjected to various repetitive deformations remains limited. The intrinsic mechanical properties of a composite undergo significant changes after cyclic deformation, and these changes are strongly influenced by the magnitude of deformation, type and content of fillers, and other variables. This study identified softening and unexpected stiffening effects in carbon nanotube-based composites after repeated tensile deformation. The Mullins effect was evident during cyclic stretching within the pre-strain region; however, a stiffening effect occurred beyond this region. To understand this behavior, we quantitatively evaluated three key factors—filler aspect ratio, pre-strain level, and number of cycles—to determine the mechanical properties of the composite under cyclic deformation. This was achieved using systematic experiments and molecular dynamics simulations. Existing theoretical models that predict the mechanical properties of composites fail to account for the property changes under dynamic deformation. To address this limitation, we developed a formula using symbolic regression to predict the tensile strength of the composites after cyclic deformation, demonstrating its robustness and broad applicability.