The energy dissipation during fatigue crack growth

Fatigue failures are a major concern in aeronautics. The growth of fatigue cracks in metallic structures and adhesively bonded joints has been studied for decades. Nowadays, most researchers evaluate and predict the fatigue damage growth rate (da/dN) with empirical and phenomenon based approaches, using Linear Elastic Fracture Mechanics with various fracture mechanics parameters (ΔK and ΔG for example). However, it does not reveal the scientific nature of fatigue crack growth from a physics perspective, which results in the need of additional corrections to fit the phenomenal observation. Therefore, the purpose of this thesis is to provide a better understanding on the nature of fatigue crack growth at a fundamental level rather than providing a da/dN prediction model.

The fatigue crack growth in metallic structures and adhesively bonded joints are considered in this thesis to be the same phenomena, which should be described with a single theory containing similar parameters. An energy equation is proposed in this thesis to describe the phenomenon in both materials, similar to the work on static crack growth by Griffith and Irwin. This equation describes that the external work done over a full cycle is equal to the sum of the surface energy dissipated by the formation of new fatigue crack surface, the plastic dissipation and the elastic strain energy stored throughout one full cycle. It is assumed that the surface energy should be uniquely related to the fatigue crack propagation, so the relation between surface energy and da/dN is assumed to be a one-to-one relationship, independent of external loading conditions. On the contrary, plastic dissipation and elastic strain energy stored are consequences accompanying fatigue crack growth, and their relation with da/dN is assumed to depend on loading conditions like the stress ratio. To verify this assumption, fatigue crack growth tests on 7075-T6 and FM94 adhesive joints were performed to obtain the relationship between the energy components involved and da/dN.