Fatigue Crack Growth Analysis in Aircraft Components Using a Physically Based Universal Function for Fatigue

The physical connection between the crack length and the fatigue cycles elapsed during fatigue crack growth, in aircraft components undergoing fatigue, is discussed. It has been found that there exists a universal physical relationship between the normalized remaining (uncracked) section size and the normalized remaining fatigue life during fatigue crack growth. This relationship is also found to exist in data from various specimen geometries including purely tension, purely bending and combined bending and tension specimens such as compact tension (CT) and single-edge-notched-tension (SENT) specimens. It enables a physical understanding of the crack growth process in the macroscopic sense. It is shown that the role of crack growth in fatigue is to reduce the size of the uncracked ligament to the extent that it can be fractured monotonically in the last fatigue cycle. This is new and is different from the traditional representation of data from a fatigue crack growth experiment where the data is normally presented as crack length (a) as a function of elapsed fatigue cycles (N). The present approach helps to view the fatigue cack growth process from the viewpoint of what happens to the uncracked ligament as opposed to the crack, even though both are changing with fatigue cycling. On the basis of experimental data generated from tension-dominated, bending-dominated and combined tension+bending specimens, the proposed functional is shown to be valid for a several materials. The functional agrees extremely well with the experimental data in all the cases, confirming the universality of this relationship. A surrogate form of the functional has been shown to provide an excellent description of crack length versus cycles, thus giving a much simpler relationship to express the crack length data in fatigue. Practical application to aircraft fatigue analysis is discussed.