A method for monitoring of damage progression due to combined mechanical andhygroscopic loading in polymer composite materials is presented. Polymer-basedmaterials have a tendency to absorb moisture from their operating environment.Dielectric properties of these materials are significantly affected by the total amount ofabsorbed moisture and the degree of its interaction with the host polymer. Boundwater molecules which are restricted in their ability to rotate with an appliedelectromagnetic field contribute less to the bulk relative permittivity. 'Free' watermolecules rotate without impediment and are therefore associated with a higherrelative permittivity. The bulk relative permittivity as a function of total water contentof a contaminated composite is a unique function of the internal physical and chemicalcharacteristics of the specimen. Holding chemical contributions constant, physicalcharacteristics dominate. Thus, relative permittivity provides insight into the physicalstate of composite, including amount of free space from processing-induced voids or,critically, the presence of physical damage such as cracks and voids across multiplelength scales. Here, we demonstrate a method for leveraging this phenomenon toprovide insight into the initiation and accumulation of physical damage in moisturecontaminatedcomposites. This is accomplished using a split-post dielectric resonanttechnique operating in the low GHz frequency range, where dipolar contributions torelative permittivity dominate. Further, continuous and non-contact monitoring ofrelative permittivity is achieved by integrating a resonant cavity with a fatigue loadingframe. Preliminary experimental assessment of this test method is supportive of itspotential in damage tracking. Water-contaminated 12-ply bismaleimide (BMI) / quartzlaminate specimens were tested in impact and flexural fatigue, while a 4-plyglass/epoxy laminate was tested in tensile fatigue while changes in relativepermittivity were recorded. The results show a distinct rise in relative permittivityconsistent with the expected magnitude and progression of damage in all cases.
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