THE CLEAVAGE ENERGIES OF A DINAMICALLY PROPAGATING CRACK IN BRITTLE CRYSTALS

Crack initiation in brittle single crystals is usually dictated by the energy requires to create two new surfaces, known as Griffith barrier and equals twice the free surface energy, 2γ_s [1]. We investigated, experimentally, the fundamentals of crack initiation and propagation, with emphasis on the cleavage energy at initiation and propagation. The experiments were performed using our Coefficient of Thermal Expansion Mismatch method (CTEM) [2]. The method possesses the advantage of controlling and varying the quasi static energy release rate (ERR) gradient to the crack front, dG₀/da, which we denote as Θ. The cleavage energies of dynamic cracks on the two low-energy cleavage systems (LECSs) of brittle silicon crystal, (110)[1-10] and (111)[11-2], were evaluated by comparing our dynamic fracture experiments with the theoretical energy-speed relationship formulated by Freund equation of motion [3]. The energy release rate, G₀, was evaluated using finite elements analysis (FEA), and the crack speed was measured using Potential Drop Technique (PDT).

The experiments revealed that the energy required to initiate and propagate the crack for Θ >0.5 J/m²/mm is higher than 2γ_s and linearly increase with increasing Θ for both cleavage planes of silicon. Surprisingly, the Θ dependent cleavage energy for initiation and propagation remains constant during the event of fracture, meaning, it is not crack speed dependent.

References:

[1] Griffith, A. A. (1921). "The phenomena of rupture and flow in solids". Philosophical Transactions of the Royal Society of London: 163-198.

[2] Gleizer, A. and D. Sherman (2014b). "The cleavage energy at initiation of (110) silicon". International journal of fracture, 187: 1-14.

[3] Freund, L. B. (1990). Dynamic fracture mechanics, Cambridge university press.