**4. Summary**

A detailed analysis of crack initiation and growth in crystalline materials is provided both from the perspective of continuum mechanics and dislocation dynamics. Basic principles underlying crack initiation and growth are highlighted. The role of pre-existing or in situ generated stress concentrations in the initiation of the incipient cracks in crystalline materials is outlined. It is shown that stress concentrations are essential for the nucleation of cracks. They provide the local internal stresses and gradients needed for crack initiation and growth. The application of the modified Kitagawa–Takahashi diagram in accounting for the role of internal stresses for the initiation and growth of short cracks, for both subcritical crack growth and fracture, is discussed. Discrete dislocation models are presented, and the effect of hydrogen on crack growth kinetics is analyzed.

**Author Contributions:** All authors have read and agreed to the published version of the manuscript. Conceptualization; K.S., I.A., K.N.S. Methodology K.S., I.A., K.N.S., A.K.V.; Software I.A., K.N.S., Validation, I.A., K.N.S.; Formal Analysis, K.S., I.A., K.N.S.; Investigation, I.A., K.N.S.; resources K.S., I.A., K.N.S., A.K.V.; data curation, K.S., I.A., K.N.S.; Writing-original draft preparation, K.S., I.A.; writing review and editing, K.S., I.A.; Visualization, K.S.; Supervision, K.S., K.N.S.; project administration, Nagaraja Iyer, Technical Data Analysis, funding acquisition.

**Funding:** This research is supported by funds received from the Office of Naval Research, USA, under Technical Data Analysis Contract # N68335-16-C-0135 with Anisur Rahman as the Project Monitor.

**Conflicts of Interest:** The authors declare no conflict of interest.
