Dielectric Properties of Composite Materials during Damage Accumulation and Fracture

Fiber reinforced polymer matrix composite materials have many unique properties and their high performance makes them available to use in many advanced technologies i.e. aerospace, microelectronics, and energy storage. There is a correlation that exists between the long term behavior of those materials under combined mechanical, thermal, and electrical fields, and the functional properties and characteristics of the composite materials that requires a fundamental understanding of the material state changes caused by deformation and damage accumulation. This will ultimately lead, for example, to the design and synthesis of optimal multifunctional material systems. Composite materials are heterogeneous and the complex morphology of these material systems has been investigated for decades to achieve multi-functionality and reliable performance in extreme environments. These heterogeneous materials are inherently dielectric. Broadband Dielectric Spectroscopy (BbDS) is a robust tool for dielectric material characterization often used in polymer industries. In composite processing this method is employed to monitor the composite curing process. Dielectric spectra of heterogeneous materials are altered by many factors, e.g., electrical and structural interactions between particles, morphological heterogeneity, and shape and orientation of the constituent phases of the material system. During the service life of composites, damage occurs progressively and accumulates inside the materials. The process of microdefect interaction and accumulation to create a final fracture path is an active research area. The present research is designed to investigate material state change using a new non-invasive interrogation method for establishing not only internal integrity but also the nature and distribution of internal material structure and defect morphology changes by using Broadband Dielectric Spectroscopy (BbDS) to detect and characterize permittivity changes during the history of loading. Interpretations of the results by analysis of discrete local details, and prognosis of performance will be discussed by the introduction of a new technique, called Generalized Compliance, which directly and quantitatively reflects material state changes. A two dimensional computational model was also developed using COMSOLTM. The effects of volume fraction and the distribution of the defects inside the material volume, and influence of the permittivities and ohmic conductivities of the host material and defects on the effective dielectric behavior of the resulting composite as a function of applied frequency spectra are discussed. Single frequency dielectric behavior with increasing defect development inside the composite is used to interpret the in-situ BbDS experimental results for the progressive damage of the material systems investigated.