8.1 Introduction

Macroscopic aspects of fatigue crack paths, corresponding to the top two scales in Table 3.3, have been of industrial interest since the earliest fatigue investigations (see Sections 2.2.4 and 5.3.3.3). Microscopic aspects are of interest in the development of fatigue crack propagation resistant materials. Macroscopic aspects are emphasised in this chapter. Much current interest in fatigue crack paths is in the collection of data for the solution of specific engineering problems. The fatigue crack path in a critical structure in aerospace, automotive, offshore and other industries can determine whether a fatigue failure is benign or catastrophic (Grandt 2004). There is relatively little information on how to ensure that a fatigue crack path is benign. Knowledge of potential crack paths is also needed for the selection of appropriate non destructive testing procedures.

In the last four decades there have been substantial advances in the understanding of macroscopic aspects of fatigue crack paths and in their prediction. This is largely through developments in fracture mechanics and computer software (Carpinteri and Pook 2005). Theoretical prediction of fatigue crack paths in three dimensions is an active area of research. Nevertheless, despite recent theoretical advances, fatigue crack paths in structures are still often determined by large scale structural tests. For example, Figure 8.1 shows an experimentally determined fatigue crack path in a fighter aircraft centre section. It was tested using a load history representative of service loads. The times shown are equivalent flying hours. Where two times are shown the first is...