Fatigue and fracture mechanics of light-alloy parts

When in service, aeronautical structures are subjected to stresses that fluctuate over time. Examples include fuselage pressurization, pilot maneuvers, atmospheric turbulence... Experience shows that the repetition of stress cycles modifies and degrades material properties, and can eventually lead to component failure. This phenomenon is commonly referred to as "fatigue" or "fatigue damage". It can occur at relatively low stress levels, below the material's elastic limit. In aeronautics, fatigue generally occurs without overall plastic deformation, but with highly localized plastic deformation around accidents of shape (notches, bores, connection fillets, etc.).

Fatigue must be taken into account right from the design stage.

The difficult question facing aircraft manufacturers is how to reconcile economic requirements (longest possible service life, lowest possible structural weight), technical requirements (availability and intrinsic performance of materials, technology, processing, design, etc.) and regulatory requirements (resistance of a structure to extreme loads, maintenance of airworthiness, etc.).

Choosing the right materials is particularly important. For a long time, it was believed that materials with the highest possible resistance to deformation should be sought first and foremost. In many cases, however, the focus has gradually shifted to finding materials with a better compromise between strength and toughness or, more generally, ductility. Oversizing is not a good solution either.

Light alloys are used extensively for aircraft structures.

The first part of this text briefly presents the general characteristics of aluminum and titanium alloys.

Fatigue and fracture mechanics calculation methods adapted to these alloys are described in detail in a second section.