Interrogation system for passive solid-state sensors embedded in composites

Development of passive solid-state ferromagnetic sensors suitable for assessing damage accumulation in composite materials requires an accurate, reproducible detection technique. The solid state sensors gradually transform from a paramagnetic phase to a ferromagnetic phase as a function of applied strain. The irreversible and strain dependent formation of ferromagnetic material passively provides a magnetic signature useful in nondestructively assessing peak strain exposure. This peak strain signature can be used to identify and quantitatively estimate the degree or extent of accumulated composite material damage. Embedded sensors were continuous 254-μm diameter wires positioned beneath the first or second surface ply along the longitudinal axes of tensile specimens. Single embedded wire sensor configurations were evaluated. This paper discusses the development and results obtained for a giant magneto-resistive (GMR) sensor detection system for measuring the ferromagnetic response of the sensor wires after strain-controlled tensile testing. Tensile testing was done at levels ranging from 0-4% total strain