The mechanisms of fatigue crack growth in ductile metals and alloys, which were discussed in the preceding chapter, involve cyclic dislocation motion along one or two glide systems at the crack tip. The ensuing subcritical crack growth process in very ductile metals can typically cover a stress intensity factor range which spans a (threshold) value of 1–7 MPa√m to one that corresponds to final failure of well over 100 MPa√m. Another noteworthy feature of fatigue cracking in ductile alloys is that there exist some distinct microscopic markings, such as striations, on the fracture surfaces from which the occurrence of failure unique to cyclic loading can be clearly identified in many situations.

Subcritical crack growth in brittle solids under cyclic loads, however, involves much more complex phenomena. In many brittle solids, there are no known differences between the micromechanisms of static and cyclic crack growth at low temperatures. Despite this apparent similarity of deformation and failure mechanisms, the superimposition of a cyclic load on a static mean stress can lead to noticeable differences in the lifetime and, in some cases, the rate of crack growth may become a strong function of cyclic frequency. Furthermore, some macroscopic crack growth phenomena may occur solely as a consequence of the imposition of cyclic loads as, for example, in the case of crack initiation ahead of stress concentrations under fully compressive cyclic loads (see Chapter 5).

This chapter deals with the growth of cracks in brittle solids subjected to fluctuating loads.