Chapter 3 Fatigue Fracture From the practical point of view,fatigue fracture is the most important dam- age process.Available statistics show that,including corrosive assistance,fa- tigue is a leading cause of material failures registered during a long-term per- formance of engineering components and structures [246.From the microme- chanical point of view,the fatigue process can be understood as a sequence of the following stages:nucleation of cracks,stable propagation of short(small) cracks,stable propagation of long cracks and unstable fracture [247]. This chapter is divided into four sections.In Section 3.1,morphological patterns reflecting all crack growth stages on the fracture surface are briefly described.Moreover,the topological methods widely utilized in fatigue re- search and the quantitative fractography are outlined.These topics are im- portant for all subsequent sections of the chapter. The second section is devoted to the propagation of fatigue cracks under the remote opening mode(mode I).Mechanisms of nucleation and growth of short cracks are briefly reported,although these initial stages of the fatigue process were not a special subject of our research.Inclusion of these topics was,however,inevitable in order to provide a self-contained description of fatigue micromechanisms.The growth of short cracks is governed by shear stress components in favourably oriented crystallographic slip systems that are inclined at about 45 with respect to the maximal principal stress.This means that the short cracks grow in a local mixed-mode I+II and the pic- ture of dislocation emission from the crack tip as well as the related growth micromechanisms are completely different from those related to long cracks. After a certain incipient period that incorporates crystallographic and transient growths of short cracks,the fatigue cracks incline towards a di- rection perpendicular to the maximal principal stress,i.e.,nearly towards mode I loading of the crack tip.This means that the long cracks keep prop- agating so that the crack tip plasticity is produced in the opening loading mode.Thus,the second section provides the reader with a micromechanical interpretation of all important phenomena accompanying this,most frequent, type of fatigue crack growth.Knowledge of these micromechanisms is essen- 125Chapter 3 Fatigue Fracture From the practical point of view, fatigue fracture is the most important dam￾age process. Available statistics show that, including corrosive assistance, fa￾tigue is a leading cause of material failures registered during a long-term per￾formance of engineering components and structures [246]. From the microme￾chanical point of view, the fatigue process can be understood as a sequence of the following stages: nucleation of cracks, stable propagation of short (small) cracks, stable propagation of long cracks and unstable fracture [247]. This chapter is divided into four sections. In Section 3.1, morphological patterns reflecting all crack growth stages on the fracture surface are briefly described. Moreover, the topological methods widely utilized in fatigue re￾search and the quantitative fractography are outlined. These topics are im￾portant for all subsequent sections of the chapter. The second section is devoted to the propagation of fatigue cracks under the remote opening mode (mode I). Mechanisms of nucleation and growth of short cracks are briefly reported, although these initial stages of the fatigue process were not a special subject of our research. Inclusion of these topics was, however, inevitable in order to provide a self-contained description of fatigue micromechanisms. The growth of short cracks is governed by shear stress components in favourably oriented crystallographic slip systems that are inclined at about 45◦ with respect to the maximal principal stress. This means that the short cracks grow in a local mixed-mode I+II and the pic￾ture of dislocation emission from the crack tip as well as the related growth micromechanisms are completely different from those related to long cracks. After a certain incipient period that incorporates crystallographic and transient growths of short cracks, the fatigue cracks incline towards a di￾rection perpendicular to the maximal principal stress, i.e., nearly towards mode I loading of the crack tip. This means that the long cracks keep prop￾agating so that the crack tip plasticity is produced in the opening loading mode. Thus, the second section provides the reader with a micromechanical interpretation of all important phenomena accompanying this, most frequent, type of fatigue crack growth. Knowledge of these micromechanisms is essen- 125