Ageing of organic composite materials Modelling tools

The use of organic matrix composites (OMCs) for aeronautical structures offers weight savings of between 25% and 30% compared with more conventional solutions (aluminum alloys); however, in civil aviation today, the weight of composite structures represents only around 20% of the total weight. These materials have very good specific properties, strength and rigidity, and exceptional fatigue resistance under 'standard' environmental conditions, but their performance deteriorates in the presence of aggressive environments, due to the onset of ageing phenomena. For these reasons, the integration of CMOs into structures facing aggressive environments remains difficult, as the problem of their durability arises. The aging of CMOs is the result of the combined action of temperature, humidity and the pressure of the gaseous environment, but also of the conditions under which they are used (type of process, firing cycle inducing residual stresses) and the structure of the material. It corresponds to an often irreversible evolution of the material's properties, and its study requires complex approaches.

For example, the structural parts of a future supersonic aircraft (planned for the 2020-2030 timeframe) will be subjected to fairly significant cyclic hygro-thermomechanical stresses: moisture desorption phenomena at high temperatures and successive passages at low temperatures can promote the initiation and propagation of damage. In the first stages of turbojet engines ('lukewarm' structures), the temperatures involved are of the order of 120°C-190°C, and the presence of oxygen in the surrounding environment induces thermo-oxidation processes in the organic matrices, leading to severe degradation phenomena and a significant reduction in service life.

Controlling the degradation phenomena associated with these conditions of use calls for multiphysical and multiscale approaches, both experimental and theoretical, and raises scientific as well as technological questions of great scope and complexity.

This article focuses on the durability and ageing of composite materials for aeronautical applications, with the aim of presenting simple and relevant characterization and modeling methodologies that can be used immediately to understand physical phenomena, and to identify, simulate and optimize real-life cases. It begins with a background and bibliographical overview of aging phenomena affecting organic matrix composites, and a review of the main modeling tools available for their prediction and interpretation. This is followed by a description of the mechanical/thermodynamic framework within which composite material ageing modelling has been developed, to enable us to interpret the observations schematically described and to approach their simulation....