Magnetorheological elastomers (MREs) are a kind of active control smart material, and their critical problem is that their ferromagnetic particles are too large, which causes holes to develop and results in MREs with poor mechanical performance and fatigue resistance. In this work, liquid butadiene acrylonitrile rubber (NBR)-synthesized phenolic resin microcapsules were synthesized and applied to MREs as a self-healing agent, effectively reducing the number of holes caused by ferromagnetic particles. The structure of the self-healing agent was determined by Fourier transform infrared (FT-IR) spectroscopy, thermal gravimetric analysis (TGA), and scanning electron microscopy (SEM). The results indicated that a self-healing agent was successfully synthesized, the core of the capsule was spherical liquid nitrile rubber, and the wall of the capsule was composed of phenolic resin microspheres. Furthermore, the SEM images of the MREs showed that the number of cavities caused by ferromagnetic particles was greatly reduced after the addition of the self-healing agent, and the X-ray photoelectron spectroscopy (XPS) results of the MREs indicated the formation of a chemical bond between Fe and O. In addition, the mechanical properties and fatigue resistance of the MRE materials with the self-healing agent were improved. Under 100% strain and with the same number of cycles, the crack growth rate of MREs without self-healing agent is faster by about 329%, and the crack length is longer by about 220% than those of MREs with self-healing agent.

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