Effects of moisture and thermal ageing on structural stability of sandwich panels

Abstract

The structures employed in the space are frequently manufactured with composite materials, which can successfully optimize the exigences of lightness, strength and stiffness, as required by the nature of the missions. On the other hand, the ground and the space environments can modify the properties of these materials in a permanent way, so that an accurate evaluation of these impacts becomes necessary, in order to minimize the risk of degradations or not expected behaviours. These considerations have great importance in the design of large antenna reflectors, for which the operational frequencies are increasing and an high dimensional stability is required. As a consequence, their in-orbit thermal distortions must be minimized, considering the actual value of their Coefficient of Thermal Expansion (C.T.E.). Most of the reflectors are made of sandwich panels with composite skins, and their electrical performances are strongly sensitive to the C.T.E. value. So that, any cause which can even weakly influence this parameter in a irreversible way, like little variations in the structural integrity due to particular environment conditions, can lead to significant effects on the global antenna behaviour. Either moisture sorption and thermal ageing are able to change the physical relationship between the fibers and the matrix in a composite: the former, degrading the properties of the matrix itself, the latter, inducing micro-cracking inside the structure. Both these effects can influence the C.T.E. and there is a strong coupling between them. The moisture content is in fact a function of this thermal ageing and furthermore has an impact on the matrix micro-cracking. In the open literature very little has been published about the influence of this environment on the thermo-mechanical stability of sandwich structure for space applications. In order to understand more clearly the influence of moisture and thermal ageing on the C.T.E. of composites, both these parameters will be investigated separately by testing laminate and sandwich specimens. A mathematical model to predict the moisture absorption/desorption of sandwiches and laminates will be developed and compared with the test results. Comparison with existing data shall be presented.