Composite systems with a polymeric phase exhibit time-dependent hygrothermomechanical properties. This time dependent behaviour is global and local. Global for the properties of fibre reinforced lamina and local by the stress transfers between matrix and fibres in a lamina and between lamina in a laminate. The design of a polymer based composite structural component at time t_o for an imposed life time, under an estimated complex mechanical loading history in interaction with environmental variations, needs not only a materials data basis for characteristics measured at time t_o but a test program over a period #DELTA#t in order to obtain the necessary elements to predict the time dependent variations of stiffness and strength in interaction with some damage development. A model is presented including onolinear viscoelastic behaviour, viscoplastic behaviour and damage development in order to predict ultimate delayed failure as the result of the combined stiffness degradation and damage development. Application of this model to the analysis of a graphite-epoxy system will be discussed in comparison in comparison with available methods for durability analysis.
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