Material-Level Degradation of Magnetorheological Fluids Under Long-Term Cyclic Shear

Abstract

The long-term functional stability of magnetorheological fluids (MRF) remains a key limitation for their reliable use in adaptive systems and continuously operated magnetorheological devices. In this study, the intrinsic evolution of rheological properties in a commercial MRF (MRC-C1L) is systematically examined under prolonged cyclic loading, with the aim of isolating material-level degradation mechanisms independent of device-related effects. The fluid was subjected to 120,000 low-strain oscillatory shear cycles under a constant magnetic field, simulating extended service conditions while avoiding macroscopic structural failure. Rheological measurements revealed a progressive decrease of approximately 13 percent in magnetically induced shear stress, indicating partial irreversibility in field-induced particle chain formation and microstructural densification within the suspension. Microstructural characterization using scanning electron microscopy confirmed the morphological integrity of the carbonyl iron particles, with no evidence of fragmentation or severe wear. However, energy-dispersive X-ray spectroscopy detected a measurable increase in surface oxygen content from 1.4 percent to 1.5 percent, suggesting the onset of mild oxidative processes at the particle-fluid interface. These findings indicate that early-stage oxidative interfacial modification, rather than mechanical damage, is the dominant degradation pathway under low-strain cyclic loading, promoting irreversible particle clustering and a gradual loss of magnetorheological performance. The results provide new insight into the physicochemical aging behavior of MRF and offer practical guidance for the design of oxidation-resistant formulations with enhanced durability for demanding automotive and aerospace applications.