An unifying concept for fatigue: The reference crack growth rate

The conventional definition of fatigue crack growth rate (CGR) is shown to be fractographicly confusing in cases of different variable cycle loadings. A new concept of reference crack growth rate (RCGR) is proposed to cover all fatigue cracks regardless of the type of loading. RCGR may be estimated directly from images of fracture surfaces by means of the methods of textural fractography. Physical interpretation is suggested on the basis of cycle-by-cycle crack growth description. Both approaches and the conventional treatment are compared in application—quantitative fractography of three specimens from aluminium alloy loaded by constant cycle, constant cycle with periodic overloading, and a random block, respectively. Common morphologic features of all fracture surfaces are found that are closely related to RCGR.