Grain creep fracture

Microcracks are also known to occur in the elevated temperature creep of alumina with little or no preexisting glass phase. For example, the development of microcracks during creep fracture of two hot-pressed aluminas, which were free of grain boundary glass films, was studied by Wilkinson et al.52 who employed tensile and four-point bend specimens. They found that the concentrations and morphology of the cavities and microcracks were strongly... 

Natural ice has large grains typically 10 mm or more (it is like a casting which has solidified very slowly). Then this equation gives a tensile fracture strength of 1 MPa -precisely the observed strength. So when ice is loaded in tension, it creeps when the stresses are less than 1 MPa the strength is limited to a maximum of 1 MPa by fast fracture. 

The final stage of creep is known as tertiary creep at which time the creep rate increases rapidly culminating in failure. This acceleration in creep is due mainly to the formation of voids and microcracks in the material which form along the grain boundaries causing the fracture path to be predominantly intercrystalline. 

The essential difference between treatments of chemical processes in the solid state and those in the fluid state is (aside from periodicity and anisotropy) the influence of the unique mechanical properties of a solid (such as elasticity, plasticity, creep, and fracture) on the process kinetics. The key to the understanding of most of these properties is the concept of the dislocation which is defined and extensively discussed in Chapter 3. In addition, other important structural defects such as grain boundaries, which are of still higher dimension, exist and are unknown in the fluid state. 

Crack growth models in monolithic solids have been well document-ed. 1-3,36-45 These have been derived from the crack tip fields by the application of suitable fracture criteria within a creep process zone in advance of the crack tip. Generally, it is assumed that secondary failure in the crack tip process zone is initiated by a creep plastic deformation mechanism and that advance of the primary crack is controlled by such secondary fracture initiation inside the creep plastic zone. An example of such a fracture mechanism is the well-known creep-induced grain boundary void initiation, growth and coalescence inside the creep zone observed both in metals1-3 and ceramics.4-10 Such creep plastic-zone-induced failure can be described by a criterion involving both a critical plastic strain as well as a critical microstructure-dependent distance. The criterion states that advance of the primary creep crack can occur when a critical strain, ec, is exceeded over a critical distance, lc in front of the crack tip. In other words... 

Figure 12.4 (a) Intergranular fracture of an alumina sample showing creep cavitation due to compressive creep at 1600°C. Note closely spaced cavities along the two-grain facets. (b) Schematic of cavity formation in viscous grain boundary films as a result of applied tensile stress. 

The tertiary stage, or tertiary creep, is characterised by a rapid increase in the strain, leading to failure or creep rupture (Figure 10.28). At this stage, voids form in the region of the fracture and there is considerable grain boundary movement. As would be anticipated, this part of the curve is difficidt to analyse theoretically. 

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