If the industry generally considers composite materials as potentially interesting, the lack of durability makes their often limited employment. Indeed, the design of composite structures often requires taking account of the long-term behaviour under various types of complex solicitations. This study is based on the development of a methodology for modeling the long term behavior under constant load for multilayer composite structures subjected to complex solicitations. This study is on the evaluation of the long-term behaviour of tri-layers glass/polyesterwood composite material. The objective is the evaluation of the long term strength of this material by the Goldenblat-Kopnov criterion. The studied model is an orthotropic tri layer plate of sandwich type with upper and lower flanges (skins) are in composite material glass/polyester KACT-B and the intermediate layer (soul) is pine wood. The character of loading of the plate is reflected in the model by the functions σ ik (ξ), σ mn (ξ) which can be of different natures. For example, to load in the long term by constant constraints in time (the weight own element for example), we will have: σ ik = const, σ mn = const. The results of the calculations show that, for this type of material, the conditions of strength to the dangerous points of the plate are met.

A wide range of applications is based nowadays on analytical developments which allow a precise and effective approach and short time of computations compared with the time required for numerical methods; in this way these developments are suitable for calculations in real time. This work proposes an approach for solving a two-dimensional harmonic problem of a rectangular plate under local surface loading using Vlasov's symbolic method of initial functions and a general solution of the harmonic equation for a rectangle. Substituting the harmonic functions in symbolic form for the corresponding solutions allows us to give the exact solution of the problem in trigonometric form.

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One of the major problems encountered in prediction of hereditary viscoelastic behavior of polymeric composites is the determination of heredity kernels. This issue comes down to identification of the model characterizing the viscoelastic properties of these materials. The purpose of this work is to propose a model for prediction of viscoelastic nonlinear behavior of laminate composite with polyester matrix, through the study and analysis of heredity kernels and their influence on the life time of this material. Identification of this model required experimental determination at room temperature, of viscoelastic parameters of heredity kernels by macroscopic approach. These data provide predictive tools for establishment of the life time and long term stress limit under static complex loading for this type of material.

One of the major problems encountered in the prediction of the hereditary viscoelastic behavior of polymeric composites, is the determination of the heredity kernels. This issue comes down to the identification of the model characterizing the viscoelastic properties of these materials. The purpose of this work is to propose a model for prediction of viscoelastic nonlinear behavior of a composite laminate matrix polyester KACT-B by the study and analysis of the heredity kernels and their influence on the life time of this material. The identification of this model required the experimental determination at room temperature, of the viscoelastic parameters of the heredity kernels by a macroscopic approach. These data provide predictive tools for the establishment of the life time under static complex solicitation and yield strength in the long term for this type of material. Sufficiently interesting in terms of concept and methodology, experimental tests in this study, performed on polymeric fiberglass composite, are still few but develop at a pace that we can look in the near future access suitable for purposes of design data.