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In the present study, the transition of a polymeric material from the linear to the nonlinear viscoelastic behavior and the determination of the nonlinearity stress threshold variation with time and temperature are investigated. For this purpose a systematic experimental program consisted of thermal and mechanical characterization of certain polymeric material followed by isothermal creep tests at different stress levels and temperatures was conducted. Through isochronal curves that occurred from creep tests, the nonlinearity threshold dependence on both time and temperature is presented. The reported results provide information that is critical for the design and development of polymer structures and components.

The aim of the present investigation is to study both the influence of particle weight fraction (0-50%) and the effect of the notch length on the static mechanical properties of aluminum particle epoxy composites. Experimental results in both cases were compared with three different theoretical models, previously developed by the first author and presented in a series of publications. First, for the evaluation of the maximum strength the particle sectioning model (PSM) was applied. Next, for the evaluation of the elastic modulus as a function of aluminum powder weight fraction, the interphase model (IM) was applied. Finally, in the case of notches' length influence, the residual property model, (RPM), was applied. This model can be applied for the description of the residual behavior of materials after any type of damage. In all cases, predicted values showed a satisfactory agreement with experimental findings. PREDICTIVE MODELS Predictive models for the elastic modulusThe mechanical properties of particulate-filled composites are affected by a great number of geometrical, topological, mechanical, etc. parameters.

In this study the effect of carbon nanotubes content as well as of the tensile stress level applied upon the linear viscoelastic creep response of carbon nanotube polymer nanocomposites was investigated. Experimental findings were modeled by means of the newly developed hybrid viscoelastic interphase model, which constitutes an extension of the previously developed hybrid interphase model. According to this model, the viscoelastic interphase thickness has not of constant value but is dependent upon the property considered at the time as well as on the creep time. In addition, the parameter of imperfect bonding is introduced through the degree of adhesion. Experimental findings combined with analytical results gave a better understanding of the viscoelastic response of epoxy resin carbon nanotubes nanocomposites.

All structural components when under working conditions are subjected to damages resulting to respective deterioration of their mechanical response. The type and the extent of damage depend not only on the materials' structure, but also on the working and general environmental conditions. The main causes, for damage development in polymeric materials, are: water absorption (or moisture) from the environment, random simple impact, repeated impact, mechanical, thermal or hydrothermal fatigue, creep and any combination of the above mentioned damage sources. As a result, damage deteriorates polymeric materials' response and occasionally their fracture behaviour. In the present study, the effect of water uptake on the creep behaviour of glass-epoxy composites was investigated and experimental results were compared with respective theoretical predictions as derived from the application of a viscoelastic model.