A Dielectric Resonant Cavity Method for Monitoring of Damage Progression in Moisture-Contaminated Composites

Abstract

A method for monitoring of damage progression due to combined mechanical and hygroscopic loading in polymer composite materials is presented. Polymer-based materials have a tendency to absorb moisture from their operating environment. Dielectric properties of these materials are significantly affected by the total amount of absorbed moisture and the degree of its interaction with the host polymer. Bound water molecules which are restricted in their ability to rotate with an applied electromagnetic field contribute less to the bulk relative permittivity. 'Free' water molecules rotate without impediment and are therefore associated with a higher relative permittivity. The bulk relative permittivity as a function of total water content of a contaminated composite is a unique function of the internal physical and chemical characteristics of the specimen. Holding chemical contributions constant, physical characteristics dominate. Thus, relative permittivity provides insight into the physical state of composite, including amount of free space from processing-induced voids or, critically, the presence of physical damage such as cracks and voids across multiple length scales. Here, we demonstrate a method for leveraging this phenomenon to provide insight into the initiation and accumulation of physical damage in moisturecontaminated composites. This is accomplished using a split-post dielectric resonant technique operating in the low GHz frequency range, where dipolar contributions to relative permittivity dominate. Further, continuous and non-contact monitoring of relative permittivity is achieved by integrating a resonant cavity with a fatigue loading frame. Preliminary experimental assessment of this test method is supportive of its potential in damage tracking. Water-contaminated 12-ply bismaleimide (BMI) / quartz laminate specimens were tested in impact and flexural fatigue, while a 4-ply glass/epoxy laminate was tested in tensile fatigue while changes in relative permittivity were recorded. The results show a distinct rise in relative permittivity consistent with the expected magnitude and progression of damage in all cases.