Crack propagation mechanics

Many other methods of strengthening are based on formation of composites by inclusion of fibers or whiskers or by crystallization to form glass-ceramics. Phase separation may also affect strength by altering crack propagation mechanisms. Transformation toughening has also been attained by formation of a small concentration of zirconia crystals in glasses. 

There must be a crack propagation mechanism available. 

In order to understand the crack propagation mechanisms in HAP, and in particular the role of water, relaxation tests were conducted on the same specimen at room temperature in three different media, namely in ambient air, liquid water and silicon oil (Rhodorsil 710, Rhone Poulenc, France). To obtain reproducible results in liquid environments, precracked specimens were cleaned by ultrasonic cleaner in acetone and then in ethyl alcohol for 5 minutes to insure a perfectly clean crack path. They were subsequently put in a desiccator under 10 hPa vacuum for 1 hour and dropped into the test liquid environment while vacuum was maintained for an additional hour. They were further transferred to the testing jig without any contact with ambient atmosphere. This procedure insures perfect wettability of the liquid along the crack path. In order to increase the significance of the results, three successive relaxation curves were conducted on the same specimen, first in air, then in water and oil. This ensures that no variation occurs from one sample to another. 

Figure 17 Schematic diagram of stage I corrosion fatigue crack propagation mechanism [21],...

BWR environments) in which it is assumed [1] that the slip dissolution model is a reasonable working hypothesis for the crack propagation mechanism. This baseline prediction methodology is then extended to treat the effects of irradiation on the cracking of stainless steel. Further extension to other alloys (e.g., nickel-base, low-alloy steels) and environments (PWR) is also outlined. 

It is postulated that specific ions are absorbed and interact with strained bonds at the surface of the crack tip, thus reducing the bond strength, and permitting continued brittle fi acture. This theory has been supported by observations in SCC. By chemisorption of the environmental species on the crack tip, the local fracture stress of the metal lattice is reduced. The theory has been applied to hydrogen embrittlement and liquid metal embrittlement. The adsorption phenomenon may be used to interpret the crack propagation mechanism of alloys which fail by hydrogen embrittlement, such as aluminum alloy 7075. 

Figure 9. Crack propagation mechanisms driven by the pressure and traction from a wheel contact. (a) Shearing action driven primarily by surface friction, (b) Opening action driven by fluid pressure.

---