Interphase properties and degradation in advanced composites: Analytical and experimental dynamic mechanical analysis

The goal of this research was to investigate non-uniformity and degradation of the interphase in advanced composites. An analytical study using a micromechanical model was directed towards understanding the effects of interphase characteristics on the composite's overall dynamic behavior. An experimental study was directed towards characterizing interphase degradation and damage caused by either hygrothermal environment exposure or mechanical cyclic loading. The micromechanical model was also correlated with experimental results to determine interphase dynamic properties.; A micromechanical model incorporating a viscoelastic non-uniform interphase and the interphase sub-layer method were developed to obtain closed-form solutions. Imperfect interfacial conditions were modeled by an irregular interphase simulation, in which the quality of interfacial bonding and degree of interphase degradation were modeled by adjusting interphase dynamic properties. The analytical results showed that spatial variations of interphase properties largely determined the loss tangent of the composite, while the influence of the averaged values of non-uniform interphase properties was minimal.; An experimental approach was developed to investigate bond degradation and accumulated damage due to environmental conditioning or mechanical cyclic loading in carbon fiber/epoxy interphase. Experimental results showed that torsional damping response, especially the loss tangent of the composite, was sensitive to changes in the interphase caused by bond degradation and damage accumulation. Bond degradation and damage were found to initiate in the interphase and were characterized by increased damping as well as reduced stiffness of the composite. Interphase degradation affected the composite's dynamic behavior by reducing the stiffness, creating or enhancing damping mechanisms, and introducing stress concentrations in the interphase.; The micromechanical model was used to analyze experimental data. In contrast to models with no interphase or with a uniform interphase, the current model with a non-uniform interphase correctly predicted the dynamic properties of composites with varying fiber volume fractions. The micromechanical model accounted for interphase degradation and localized interfacial debonds. Analytical results showed that the lower interphase storage modulus predominantly resulted in a lower composite storage modulus. An increase in the global damping, however, was caused by the combined effects of increased damping, reduced stiffness and high stress concentrations of the interphase. Interfacial degradation showed more significant influence on damping than on the stiffness of the composite. However, for multiple interfacial debonds, the method of simulating interfacial damage by adjusting interphase viscoelastic properties may have overestimated the composite's damping response.