Macro-composites containing ferromagnetic microwires for structural health monitoring

E-glass fibre/epoxy composites containing periodic arrays of ferromagnetic microwires have been designed and investigated in terms of dielectric properties in the presence of a permanent magnetic field in the gigahertz frequency range. Negative effective permittivity is demonstrated at zero magnetic field and 25 A/m, while it changes to a positive sign when 50 A/m of external field applied and increases with the applied field from 900 MHz to 3.5 GHz. Thereupon the magnetic field serves as a switch to manipulate the sign and magnitude of effective permittivity. In addition, the field tunability of the composites is found to depend intimately on the intrinsic properties and concentration of the wires, and there exists a remarkable field dependence of scattering spectra. A radical change of effective permittivity is observed when the wire breakage occurs. The extremely small volume fraction of wires allow them retain the mechanical integrity and performance of resultant composites evidenced by mechanical tests. The realization of electromagnetic functionalities in the structural composite makes it a promising candidate for structural health monitoring, sensing and biomedical applications.