High temperature properties of HFPE-II-52 polyimide resin and composites

Polyimides and polyimide matrix composites have been used for various high temperature applications and have recently been proposed for use as secondary structures in high temperature aerospace applications. Motivated by the potential use of the polyimide/polyimide matrix composites, this work focuses on characterizing high temperature behavior of the polyimide and polyimide composites. In particular, this thesis covers a diverse set of experiments performed to characterize HFPE-II-52, a polyimide developed at NASA Glenn Research Center, and to characterize composite laminates of the polyimide and T650/35 carbon fibers.; Steam induced delamination, blistering, swelling, plasticization and residual stresses are some of the common problems associated with moisture absorption in polyimides and polyimide matrix composites. Motivated by this, experiments were performed to determine the moisture diffusion properties of the polyimide. The experiments include determining the moisture diffusivity, equilibrium moisture absorption and swelling strains for the polyimide. Moisture diffusivity was determined over the temperature range of 25--200°C using weight loss and weight gain experiments. Sample size restrictions as proposed by ASTM standard were relaxed and diffusivity was determined by fitting a 3-D solution to the experimental data. Equilibrium weight gain was determined as a function of relative humidity and the validity of several models such as GAB equation, Henry's law and Flory-Huggins equation were evaluated. Swelling behavior of the polyimide in presence of water was determined and an estimate of free volume was obtained.; A difficult to determine, but important property is the shear strength of composites. Shear strength of unidirectional polyimide composite was determined using an Iosipescu specimen. The design of the Iosipescu specimen was obtained using finite element analysis. A simple and easily fabricated fixture was devised to perform the shear tests. The shear strength of the polyimide matrix composite was determined as a function of temperature and moisture conditioning. A full finite element analysis including the test fixtures was performed to determine the relation between load-point displacement and average shear strain in the gauge section and an estimate of modulus was obtained.; To study the limiting behavior of polyimide at high temperatures, a study was performed to characterize the nonlinear viscoelastic and viscoplastic properties of polyimide as a function of temperature. The nonlinear viscoelastic model is based on Schapery's theory of nonlinear viscoelasticity and the viscoplastic model is based on the state variable model proposed by Boyce et al. Three different types of experiments were performed at four different temperatures. The experiments included (i) constant displacement rate tests, (ii) creep recovery tests and (iii) multistep relaxation tests. Nonlinear viscoelastic and viscoplastic model parameters were fit to the creep recovery tests with emphasis on capturing the overall material behavior in all three type of tests. A comparison of linear viscoelastic viscoplastic and nonlinear viscoelastic viscoplastic model was also performed.