Fatigue behavior and recovery of a van der Waals-layered material, highly oriented pyrolytic graphite, under cyclic interlayer shear loading

van der Waals (vdW)-layered materials possess both high stiffness and flexibility owing to strong in-plane bonding and weak interlayer interactions and are characterized by reversible deformation through interlayer slip. These structural features suggest the possibility of a unique fatigue response that is distinct from that of conventional materials. However, a fundamental question remains unanswered: Does fatigue occur in such systems at all? This study aims to elucidate the fundamental fatigue behavior of vdW-layered materials, with a particular focus on their characteristic interlayer slip. Highly oriented pyrolytic graphite (HOPG), a representative vdW-layered material composed of stacked graphene layers, was employed. Microscale cantilever specimens were subjected to one-sided (pulsating) and two-sided (alternating) cyclic shear loading under in situ observation using transmission and scanning electron microscopes. In the one-sided fatigue tests, cyclic softening was observed with increasing cycle number, as indicated by the decreases in the shear modulus and nominal shear stress range, accompanied by the accumulation of plastic deformation. Furthermore, the reduced deformation resistance was partially restored over time. Cyclic softening was also observed in the two-sided fatigue tests; however, unlike in the one-sided case, no accumulation of plastic deformation occurred, and the reduced deformation resistance was fully restored over time. These findings indicate that, while HOPG exhibits a transient decrease in the deformation resistance characteristic of fatigue, it also possesses a self-recovery mechanism enabled by reversible interlayer slip, suggesting that its mechanical properties can be retained semi-permanently.