|M.Sc Student||Belenky Alexander|
|Subject||Static and Dynamic Fracture of Transparent Nanograined|
|Department||Department of Mechanical Engineering||Supervisors||Professor Daniel Rittel|
|Dr. Avraham Dorogoy|
|Full Thesis text|
The resistance to dynamic crack extension is an important mechanical property called the dynamic fracture (crack initiation) toughness. Many quasi-brittle materials are known to possess a dynamic fracture toughness that is significantly higher than their quasi-static counterpart, but the underlying reasons are not yet understood. Transparent polycrystalline nanograined alumina (PCA), investigated in the present work, has a great technological potential for highly demanding applications which take advantage of its superior mechanical properties like hardness, wear resistance and strength, in addition to its optical performance in the infrared and visible domain. As this material is still under development, accurate fracture properties (toughness) are rather scarce in the quasi-static regime and almost non-existent in the dynamic regime. In this work the quasi-static and the dynamic fracture toughness were determined experimentally.
The specimens in the present study are all precracked with a sharp crack introduced through the extension of hardness indentations in controlled bending. From an experimental point of view, the selected approach overcomes problems related to the crack-tip sharpness and the usual approximations made during indentation toughness testing. Using a standardized technique, single edge precracked beam (SEPB), the measured static fracture toughness of the nanograined PCA is 3.12±0.08 [MPa*m0.5].
The dynamic fracture toughness was measured using one-point impacted precracked beams. The dynamic initiation fracture toughness of nanograined PCA is 36.1±21.70 [MPa*m0.5]. The present study reveals that polycrystalline nanograined transparent alumina has dynamic fracture toughness values which are quite high with respect to its quasi-static values. The measured high dynamic fracture toughness is correlated with the operation of a specific mixed trans-intergranular fracture mode at initiation that is not observed otherwise. This mechanism is modeled numerically as a geometrical crack-front perturbation, which translates local variations in fracture toughness into equivalent geometrical perturbations of the crack front.
The numerical simulations of various crack front geometries, including straight cracks, show that two factors are responsible for the observed dynamic toughening of the material, namely the geometrical perturbation itself and the kinetic energy imparted by the impact. A comparison of different dynamic test geometries, for which identical toughness values are measured, suggests that the dynamic initiation toughness can be considered a material property whose high values are ascribed to the above mentioned toughening mechanisms.