Inelastic analysis of fiber reinforced polymeric composites

Abstract: Fiber reinforced polymer composites are seeing increased use in the outomotive industry and other applications. This is because of the potential for rational and economical production in large series of complex shaped parts. Properties such as high strength to weight ratio and superior impact properties are of particular interest. The presented thesis contains results of experimental and theoretical studies of the time dependent processes in fiber composites: creep, strain recovery and stress relaxation. The first part of the thesis is dedicated to unidirectional fiber reinforced composites. Viscoplastic strains of unidirectional continues fiber composite (HEXCEL GF/EP prepreg system) are studied experimentally and theoretically. The Schapery's nonlinear viscoelastic viscoplastic constitutive equations are used and generalized to describe inelastic behavior of unidirectional composite under isothermal creep and strain recovery conditions. Necessary experimental data are collected in creep and strain recovery tests. The methodology to quantify the viscoplastic strains with respect to applied stress is proposed. Source of the laminate viscoplastic strains is the viscoplastic shear strain in material symmetry axis. Assumptions has been used and validated that the function describing the stress and time dependence of viscoplastic strain can be presented as a product of two time and stress dependent master curves. Since the viscoelastic response is found to be linear, the nonlinearity is included only in viscoplastic terms. The micromechanical Hashin's Concentric Cylinder Assembly model in viscoelastic formulation is used to predict the viscoplastic behavior of the composite using matrix viscoelasticity. The time and stress dependence of viscoplastic strains is obtained from off-axis creep tests as difference between experimental measured creep strains and predicted linear viscoelastic strains. The developed material model is validated in creep tests on [0n 90m]s and [+/-45n]s specimens. Irreversible strains developed in UD composites in creep tests with large shear stress are attributed to micro damage evolution. Scanning Electron Microscopy is used to study damage in glass fiber epoxy composite laminates subjected to creep and strain recovery conditions. It is found that damage in fibers, in predominant shear loading, occurs more often as compared with debonding and damage in matrix. It is concluded that fiber damage is the main mechanism that initiates other damage mechanisms and is responsible for plastic strain developing during the creep loading. The effect of the interface between fibers and matrix is studied experimentally and theoretically. The carbon fiber vinyl ester long fiber composites are used for this study, where the fibers have two kinds of sizings giving two types of interface properties. The experimental and theoretical tools developed in papers 1-3 are utilized and generalized for case of nonlinear viscoelasticity. SEM is used to identify the damage mechanisms. The last part of thesis deals with glass mat reinforced thermoplasts (GMT) with a certain fiber distribution. Creep tests and relaxation tests are performed on polypropylene and GMT specimens. It is proved that in the range of applied loads materials may be considered as linear viscoelastic and the complete description of viscoelastic properties of both materials are obtained. The micromechanical model developed in the first part of thesis is generalized for the case of non-unidirectional fiber orientation distribution. For this reason the GMT material is considered as consisting of an infinite number of thin unidirectional plates with a given fiber orientation distribution and laminate theory approach is applied. The model is used to predict the time dependent behavior of GMT material. Predictions are in good agreement with experimental results. Hence, the developed models for characterization of viscoelastic properties of composites are validated. They present a powerful tool to analyze the effect of different geometrical and viscoelastic characteristics of constituents on material performance.