Muddasani, Maithri (2008-08). Nonlinear viscoelastic behaviors of multilayered (pultruded) composites at various temperatures and stresses. Master's Thesis.
Thesis
This study presents experimental works and finite element (FE) analyses for understanding nonlinear thermo-viscoelastic behaviors of multilayered (pultruded) composites under tension. Uniaxial isothermal creep tests in tension are conducted on Eglass/ Polyester pultruded composites of 0o, 45o and 90o off-axis fiber orientations subject to combined temperatures and stresses. The temperatures range from 0?F to 125?F, and stress levels range from 20% to 60% of the ultimate tensile strength of the composite specimen. The creep responses seem to accelerate with temperature for higher temperatures (75oF to 125oF) and do not behave in any particular manner for lower temperatures (0oF to 50oF). Isochronous curves of time-dependent material responses show that the nonlinearity increases with time and also temperature for higher temperatures while there is no particular trend seen at lower temperatures. Also, the creep responses of the axial specimens show negligible nonlinearity when compared to that of the transverse and 45o off-axis specimens. The Poisson's effect is studied and orthotropic material symmetry conditions are satisfied. A nonlinear viscoelastic constitutive model, based on convolution integral equation, is presented for orthotropic materials. The nonlinear stress-temperature-dependent material parameters are coupled in the product form and are calibrated using the experimental data. Overall good predictions are shown but for a slight mismatch in the prediction of the responses at temperatures below 50 o F owing to the random behavior of the creep responses at lower temperatures. The numerical integration algorithm for the nonlinear viscoelastic model of orthotropic composite materials developed by Sawant and Muliana (2008) was used to integrate the constitutive material model to FE structural analyses. Sensitivity analysis is conducted to check for error in experiments by numerically simulating the testing procedure. A practical structural analysis is carried out on composite slabs using ABAQUS and our model is used to predict the responses of slabs under combined stress and temperature loading.
