Fatigue of continuous fiber reinforced composite materials

The penetration of composite materials in many industrial sectors, such as transportation, calls for increased confidence in these materials, supported by reliable dimensioning methods.

The choice of materials is the result of a multi-criteria optimization. Depending on the application, these criteria may include, but are not limited to, mechanical properties, certain functional properties (optical, electromagnetic...), structural lightness to meet environmental requirements, cost constraints... For example, high mechanical properties are required in the aeronautical sector, shock absorption in the automotive sector, and good long-term mechanical resistance in the naval sector.

For several years now, all these criteria have been joined by a crucial requirement for fatigue design. The reason for this is that composite materials are no longer confined to so-called secondary parts, i.e. those that take up very little force, but to primary structures that are subject to high loads, and for which greater safety is required.

The immediate consequence is an increase in laminate thickness, which raises new questions. Laminated structures develop interlaminar stresses in their thickness, which can lead to delaminations between plies, highly detrimental to mechanical strength. Where certain fatigue phenomena could be avoided, they can now become unavoidable.

Can we still speak of exceptional fatigue resistance, a high ratio between endurance limit and tensile strength, and little sensitivity to notch effect compared to aluminum alloys under cyclic loading?

To answer these questions, we first need to understand the mechanisms of fatigue degradation. Designing load-bearing structures in composite materials requires reliable dimensioning and service life prediction tools. The main models of behavior and failure under various cyclic loads are reviewed, and the latest trends in fatigue design are examined.

Note that the case of chopped fiber-reinforced polymers is not covered here.