The cyclic deformation and fatigue crack initiation characteristics of semicrystalline and noncrystalline solids are the subjects of discussion in this chapter. Attention is devoted to a consideration of the response of polymers and organic composites. The principal objective of this chapter will be to document the prominent mechanisms of fatigue in amorphous materials and, in particular, to present a detailed description of the similarities and differences between the fatigue characteristics of crystalline and noncrystalline materials. As in the case of brittle ceramics and composites, the discussion of cyclic deformation and crack initiation is presented here in a single chapter because in many instances, microscopic cracking processes (such as crazing) constitute prominent mechanisms which influence fatigue response. Also addressed in this chapter is the topic of fatigue crack initiation at stress concentrations in polymers. Stress-based and strain-based approaches to fatigue life of polymers are considered in Chapters 7 and 8, respectively. Fatigue crack growth in semi-/noncrystalline solids is taken up in Chapter 12.

Deformation features of semi-/noncrystalline solids

Basic deformation characteristics

A noncrystalline material can be a metallic glass (e.g., Pd–20 at.% Si alloy), an inorganic glass (e.g., silicate glass), or an organic glass (i.e. a polymer). Amorphous materials exhibit a variety of permanent deformation modes depending on their basic structural unit. This fundamental unit is a single atom for a metallic glass whereas, for an inorganic glass, the building block is a SiO4 tetrahedron.