Characterization and modeling of viscoelastic composite laminates with nonisothermal physical agin

Advanced fiber-reinforced composite materials are often used at temperatures that lead to time-dependent material behavior; such behavior must be understood and accounted for to ensure adequate design. This dissertation considers the time-dependence caused by physical aging, which is the evolution towards the equilibrium state in glassy solids, and its effect upon the mechanical response of a viscoelastic composite laminate. A predictive methodology is presented to determine the laminate stress-strain response to a general loading function during an arbitrary time-temperature history. This characterization assumes that the material is thermorheologically simple, that it remains linear viscoelastic, and that effective time theory can be used to incorporate the effects of physical aging.;The first portion of the dissertation studies physical aging. A new method for recovering isothermal aging parameters that utilizes both load and unload test data is demonstrated; the results compare favorably to the traditional approach. The Kohlrausch compliance function, commonly used in physical aging studies, is shown to be an invalid material function at long times; a Prony series is a preferable representation. This method is then extended to characterize nonisothermal physical aging. It is demonstrated that a new parameter, called "effective aging time," adequately describes the nonisothermal aging state. A model to predict this parameter given the thermal history is presented and shown to adequately describe experimental results.;Once the effective aging time is known, classical lamination theory (CLT) can be used with linear viscoelasticity to predict mechanical response. An approach is presented to calculate modulus behavior (convenient for CLT) from compliance behavior (typical result of testing). A prediction method is developed to incorporate the resulting modulus functions into CLT while maintaining the distinct aging behavior in the shear and transverse directions for each lamina. Several prediction results using this approach are shown.;These methods provide the tools required to accurately analyze physical aging test data, determine the aging response for a temperature history, and predict the mechanical response of a general laminate. Such information is crucial if advanced fiber-reinforced composite materials are to be employed at the full limits of their operating range.