Crack Initiation and Propagation:

As elaborates previously, brittle failure commonly occurs since a flaw or crack propagates by the material. The beginning of a fracture at low stresses is determined through the cracking tendencies at the tip of the crack. The structure is not endangered since the metal mass surrounding the crack will support the stress if a plastic flaw exists at the tip. While brittle fracture occurs (under the conditions for brittle fracture stated above), the crack will initiate and propagate by the material at great speeds (speed of sound).  It should be noted in which smaller grain size, higher temperature, and lower stress tends to mitigate crack initiation. A  Larger grain size, lower temperatures, and higher stress tend to favor crack propagation. There is a stress level below that a crack will not propagate at some temperature. This is known as the lower fracture propagation stress. As the temperature raise, a higher stress is needed for a crack to propagate. The relationship among the temperature and the stress needed for a crack to propagate is known as the crack arrest  curve, that is display on Figure as Curve  D. At temperatures above which denotes on this curve, crack propagation will not happen.