After a brief introduction, where the question of durability related to com
posite materials is raised concerning the type of applications and the cons
tituents of the composite material, some considerations are given about the
interaction between physical and chemical ageing and its effect on the lon
g-term behaviour of those materials. A comparison of different methodologie
s to define models for the constitutive laws of the material is made, toget
her with a reference to methodologies used for real structures in order to
predict long-term behaviour. An algorithm to predict long-term laminate pro
perties of fibre reinforced composites is presented. In the procedure, the
classical laminate plate theory was extended to include time-related respon
se of composite materials for membrane and bending load. An efficient semi-
direct time integration scheme, providing a stable integration process, was
derived to be included in the numerical procedure. The present formulation
enables solving problems related with creep, stress relaxation and rate de
pendent stress-strain behaviour for in-plane and bending loads. The results
concerning studies related to the influence of water and temperature on th
e long-term behaviour of fibre reinforced composite, hence on the constitut
ive models, are presented. The experimental results shows that a combined t
ime-temperature-moisture superposition principle (TTMSP) is not valid. This
principle simply states that the effects of temperature and moisture on th
e viscoelastic properties can be uncoupled. (C) 2000 Elsevier Science Ltd.
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