A viscoelastic-damage constitutive model for a class of advanced composite material systems

Advanced composite material systems are most generally defined as those being comprised of continuous fibers, having high strength and stiffness, embedded in a polymeric binder or matrix. After manufacturing, the resulting composite material system is, at the micro-level, a representative three-phase system consisting of: a fiber phase, a matrix phase, and an interphase (which is simply a mesophase with transitional properties different than both the fiber and matrix phases). When subjected to working loads, these material systems experience strength and stiffness loss as a function of the interaction of several internal damage mechanisms. This interaction process, which will eventually lead to failure of the structural system or component, starts with the initial imperfections normally occurring during the manufacturing process. Under a constant applied load, for example, internal voids and microcracks in the matrix material increase in size due to the viscoelastic deformation of the matrix material. Correspondingly, areas of weak fiber-matrix bonds begin to break. This increases the viscoelastic effect and causes additional disbonding and increased crack and void growth. Eventually, the fibers that have been isolated from the matrix by the disbonding phenomenon will break, which further exacerbates the fiber-matrix disbonding and crack growth. All these damage mechanisms interact at an increasing rate until failure occurs. This complex phenomenon represents the natural process of progressive failure of this class of advanced composite material systems. It is commonly known as creep-rupture.;In the approach taken herein, a viscoelastic analysis model is developed for this type of material system through the application of a constitutive damage model wherein the effects of internal damage are characterized through the use of stress-dependent coefficients. The developed model includes the effects of the matrix material nonlinearity, arguably attributed to microcracking, at the constituent level, and the effects of the fiber-matrix disbonding, at the composite material system level, as the primary causes of the nonlinear behavior of the composite material system. The complex phenomenon is quantitatively described in four coefficients, which at the macro-level are used to model the structural response of the degraded composite material system to the applied load. A Time-Stepping Algorithm is used to perform the time-dependent viscoelastic analysis. The method of cells (MoC) micromechanics model is used to include and combine the effects of matrix nonlinearity and fiber-matrix disbonding explicitly. Limited creep tests were carried out for the purpose of developing nonlinear response curves of a specific composite material system.;Thus, the primary objective of this research is the development of a nonlinear viscoelastic material model which can be employed in the analysis of polymer-matrix based advanced composite material systems. The model can be used to perform analyses wherein the effects of matrix material nonlinearity and fiber-matrix disbonding are included. The effects of progressive damage in the composite material system can be studied through the use of a time-dependent function for the matrix material average equivalent stress.