A unified description of micro and macroscopic fatigue crack behaviour

In this paper, the importance of crack front length as a factor controlling growth rate is emphasized. It is shown that fatigue cracks in aluminium alloys do not advance in a coherent manner, but the front is divided into sectors, each of which relates to an individual cracking element. These elements act with some degree of mechanical isolation from their neighbours, and such an arrangement leads to crack front fragmentation and to an increase in the real crack front length.Even on this microscopic scale the crack segments extend as though in a continuum, and, certainly at low stress intensity factors, crack paths are dictated by the local stress direction.