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.

L’étude se situe dans le cadre de la recherche de gains de performance de structures d’isolations passives pour l’offshore profond. Le travail proposé a pour support des analyses expérimentales et numériques de tubes revêtus par des matériaux isolants utilisés en eau profonde pour transporter du fluide chaud. Le raboutage des tubes en acier, préalablement revêtus en atelier, nécessite un dégagement du revêtement aux extrémités pour réaliser l'opération d'assemblage (généralement par soudure). La partie dégagée est ensuite recouverte par un nouveau matériau pouvant être appliqué sur site. Ainsi l’isolation de cette partie du tube (Field Joint), qui est soumise à des chargements thermomécaniques en service, doit être optimisée pour assurer une durée de vie compatible avec les contraintes de l’exploitation offshore en eau profonde. Le travail comporte principalement quatre parties : - la modélisation du comportement thermique pour analyser l’évolution en temps et en espace de la température du matériau au cours de la fabrication, de la pose et en service sachant que pour les matériaux d’isolation le comportement mécanique est fortement dépendant de la température,- une partie expérimentale pour l’analyse du comportement des matériaux isolants en fonction de la température et en fonction de la pression hydrostatique qui est le principal chargement mécanique de ces structures en service,- la modélisation du comportement mécanique des isolants,- et une partie modélisation et simulation du comportement en service d'assemblages multi-matériaux de type industriel, avec prise en compte du comportement non-linéaire des constituants. / Ultra deep offshore oil exploitation presents new challenges to offshore engineering and operating companies. Such applications require the use of pipelines with an efficient thermal protection. Passive insulation materials are commonly used to guarantee the thermal performance of the pipes, and syntactic foams are now the preferred material for this application. The mechanical behaviour of such insulation materials is quite complex, associating time-dependent behaviour of polymers with damage behaviour of glass microspheres. In order to allow an optimisation of such systems, while ensuring in-service durability, accurate numerical models of insulation materials are thus required. During the service life in deep water, hydrostatic pressure is the most important mechanical loading of the pipeline, so this study aims to describe the mechanical behaviour of the material under such loading. Using a hyperbaric chamber, the analysis of the evolution of the volumetric strain with time, with respect to the temperature, under different time-evolutions of the applied hydrostatic pressure is presented in this paper. Such experimental results associated with the mechanical response of the material under uniaxial tensile creep tests, allow the development of a thermo-mechanical model, so that representative loadings can be analysed.