|Ph.D Student||Shaheen Mualim Merna|
|Subject||The Fundamentals of Cracks Dynamics in Brittle|
Crystals at High Driving Force
|Department||Department of Materials Science and Engineering||Supervisor||Dr. Dov Sherman|
|Full Thesis text|
Crack initiation in single crystals is usually dictated by the energy required to create two new surfaces, 2γs. Past experiments performed in silicon single crystal at low energy and speed crack propagation, indicated that the cleavage energy for crack propagation is comparable with 2γs, which means that the energy supplied to the crack tip was mainly dissipated by bond rupture and kinetic energy.
In this investigation, we experimentally examined the fundamentals of crack propagation at high-energy and speed, with emphasis on the cleavage energy at initiation and propagation, and the effect of reflected stress wave on crack speed. The effect of reflected waves was investigated owing to the commonly accepted statement that when a dynamic crack propagates at high speed, the reflected stress wave is responsible for significant crack speed reduction. This assumption, however, has not yet been fully verified experimentally in general, and in brittle single crystals in particular.
Our experiments were performed using the Coefficient of Thermal Expansion Mismatch method (CTEM), developed in our laboratory in recent years, which possesses the advantage of controlling and varying the energy release rate gradient to the crack tip, Q=dG0/da. The cleavage energies of high Q dynamic cracks on the two low-energy cleavage systems (LECS) of brittle silicon crystal, (110) and (111), were evaluated by comparing experimental and theoretical energy-speed relationship. The propagating crack energy release rate was evaluated using Finite Elements Method (FEM), where the crack speed was measured using Potential Drop Technique (PDT).
The experimental results showed that for crack with high Q, the energy required to break the bonds and propagate the crack is higher than 2gs and increase with increasing Q. We suggest that this complex behavior can be explained by the crack advance mechanisms in the form of atomistic steps or kinks along the propagating front, which is governed by various energies. Moreover, we show that the dynamic cleavage energy, G, at the high energy and speed regime depends on Q and the crystallographic arrangement.
Finally, our quasi-statically loaded experiments show small variations in the crack speed due to the interaction between the propagating crack tip and the reflected stress wave with a speed reduction of less than 10%.