The unified approach to subcritical crack growth and fracture

The initiation and growth of any crack require very high stresses, since the required stress, generally, is inversely proportional to the square root of its length, starting from elastic Griffith's crack to elastic-plastic Orawan crack. Hence, in principle, incipient short cracks require infinite stresses. However, in reality, crack initiation and growth occur at reasonably applied stresses that can be experimentally determined. To accomplish such nucleation and growth of incipient cracks, local internal stresses must be created by plastic deformation involving dislocations and their pile-ups, or deformation bands. If preexisting notches are present, depending on the stress concentration factor, k(t), root radius, rho, and applied stress, sigma(apl), a crack may get initiated but may get arrested if the stress gradients are too sharp and cannot sustain its growth. Thus, crack initiation and growth in materials require pre-existing or in-situ generated stress concentrations with their characteristic stress gradients that provide the needed internal stresses to augment the remote applied stresses. This is true for most of the crack growth processes starting from subcritical crack growth to overload fracture that occurs under fatigue, stress corrosion sustained load crack growth, creep crack growth and their combinations. In this paper, we present our Unified Approach to analyze these cracks using the modified Kitagawa diagram that connects the behavior of smooth, notched and fracture mechanics specimens. The role of internal stresses in relation to applied stresses in accentuating the crack growth process is discussed.