ESTIMATION OF THE VISCOELASTIC PARAMETERS OF LAMINATED COMPOSITES. PART I. ANALYTICAL CONSIDERATIONS

Abstract

In an effort to study the nonlinear viscoelastic behavior of neat and carbon fiber-reinforced Polyether-etherketone (PEEK) and epoxy resin, creep tests were perfomed on [904]s and [+454]s laminates as well as on neat resin specimens. The laminates [904]s and [+454]s are used to study the time dependent transverse and shear properties, respectively. Series of 10-hour isothermal tensile creep tests were conducted on each laminate at four temperatures (up to 140°C for the epoxy system and up to 120°C for the PEEK system) and different stress levels. An analytical procedure based on the Mori-Tanaka's mean field theory but extended into viscoelastic domain is used to account for the time dependent overall response of the composite body. Directed toward this, Schapery's nonlinear constitutive equation is input into the analysis which makes the proposed method capable of accounting for time dependent behavior of the polymeric matrix. The nonlinearity factors in the Schapery's formulation axe found based on the creep tests performed at different stress levels and test temperatures. The stress and temperature dependence of the nonlinearity factors was evaluated.using a numerical procedure based on least squares techniques. The results for the pure resin and 90 degree specimens show good agreement between experiment and the predicted data. For the 45 degree specimen, the above correlation is less impressive, nevertheless the general trend in the curves of the predicted data closely match those generated by experiment. The accuracy of the results will be more improved if the gradual change of the fiber orientation specially at high values of temperature and stress is accounted for in the analysis.