Applied Physics Research; Vol. 13, No. 1; 2021 ISSN 1916-9639 E-ISSN 1916-9647 Published by Canadian Center of Science and Education 33 Fracture Nucleation Phenomena and Thermally Activated Crack Dynamics in Monocrystals Leonardo Golubović 1 & Dorel Moldovan 2 1 Physics and Astronomy Department, West Virginia University, Morgantown, WV 26506-6315, USA 2 Department of Mechanical and Industrial Engineering, Louisiana State University, Baton Rouge, LA 70803 Correspondence: Leonardo Golubović, Physics and Astronomy Department, West Virginia University, Morgantown, WV 26506-6315, USA. E-mail: lgolubov@wvu.edu Received: July 9, 2020 Accepted: September 29, 2020 Online Published: June 30, 2021 doi:10.5539/apr.v13n1p42 URL: http://dx.doi.org/10.5539/apr.v13n1p42 Abstract We explore irreversible thermally activated growth of cracks which are shorter than the Griffith length. Such a growth was anticipated in several studies [Golubović, L. & Feng, S., (1991). Rate of microcrack nucleation, Physical Review A 43, 5223. Golubović, L. & Peredera, A., (1995). Mechanism of time-delayed fractures, Physical Review E 51, 2799]. We explore this thermally activated growth by means of atomistic Monte-Carlo dynamics simulations of stressed monocrystals. This crack growth is stepwise. Each step is marked by nucleation of a microcavity close to the crack tip, and by creation of a passage connecting the microcavity and the crack. If the external tensile stress is weak, many such nucleation events occur before the crack length reaches the Griffith size. In addition to the simulations, we also present an analytic theory of the stepwise thermally activated crack growth. The theory explains surprising observation form our simulations that the thermally activated crack growth remains fairly well directed in spite of the stochastic nature of the crack growth process. Keywords: Crack dynamics, Fracture nucleation phenomena, Atomistic dynamics 1. Introduction Solids under external tensile stresses are examples of metastable states of matter similar to supercooled liquids or magnetic systems [Lifshits & Pitaevski, 1981]. The threshold of fast (“instantaneous”) failure is in fact a metastability limit, i.e., spinodal point. At this point, the external stress  as a function of the strain reaches its maximum,   . However, stressed solids can break under stresses that are significantly weaker than   . It may however take a long time before weakly stressed solids break. That is, the fracture is time-delayed with relatively long life time which is a function of temperature and the applied stress [Brenner, 1965]. Time delayed fracture is physically related to the phenomena of micro-failure nucleation and growth [Golubović & Feng, 1991; Golubović & Peredera, 1995]. These phenomena are similar to nucleation phenomena in metastable states of matter [Lifshits & Pitaevski, 1981]. The classical work of Griffith (1921) contains some elements suggesting a phenomenological theory of microcrack nucleation. The critical, Griffith crack is somewhat similar to critical nucleus in a metastable state: Cracks longer than the Griffith crack rapidly and irreversibly grow [Mott, 1948]. The growth of cracks shorter than the Griffith crack size may seem unlikely due to energy reasons. Yet, such a growth still does occur in realistic systems due to thermally activated processes of failure nucleation [Brenner,