Polymer composites subjected to cyclic loading would exhibit damage precursors, such as crazes and microcracks, during the first few load cycles. However, damage precursors are not readily detectable with existing sensing techniques, and as such current service life prediction methods depend on macroscopic damage measures. For critical airframe structures, information on macroscopic damage does not provide adequate warning time for corrective actions. This article explores the feasibility of embedding particulate magnetostrictive particles for sensing damage precursors during the early stage of fatigue damage. The sensing is based on the notion that magnetostrictive particles undergo irreversible changes in magnetization intensity when subjected to cyclic loading, and that this change can be captured with an induction coil sensor. In the sequel, Terfenol-D particles are embedded between layers of pre-preg AS4/3501-6 material system. The specimen is then subjected to fatigue loading while monitoring the change in the strength of the magnetic flux density using pickup coil. Results show that the embedded system exhibits a change in magnetic state, in tens to hundreds of millivolts of pickup coil, starting from the first few load cycles. Scanning electron microscopy and acoustic emission data were used to validate the observed results.

Abstract:The integrity of composite structures gradually degrades due to the onset of damage such as matrix cracking, fiber/matrix debonding, and delamination. Over the last two decades, great strides have been made in structural health monitoring (SHM) community using various sensing techniques such as acoustic emission, eddy current, strain gages, etc., to diagnose damage in aerospace, mechanical and civil infrastructures. Embedded sensing offers the prospects of providing for real-time, in-service monitoring of damage were weight savings is a major factor in Aerospace Industry. In this present work, magnetostrictive particles such as Terfenol-D were embedded in a composite structure, along with multiple SHM techniques, to capture the damage in an IM7-carbon fiber reinforced polymer composite system undergoing fatigue loading. As the internal stress state increases, the change in the magnetization flux intensity was captured using a non-contact magnetic field sensor. A damage diagnosis system was established along with an acoustic emissions technique to further validate the damage captured by the embedded system. The goal of this project is to identify the change in the mechanical and magnetic property within a composite material during the evolution of damage. Several characterization techniques were used to determine interfacial fiber-matrix interactions which will provide for a more comprehensive understanding of the composite interfaces.

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