Intercrystalline 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. 

Also the character of fracture under the DF in corrosive medium is changed samples of steel Fe76.iCriiNiioMo2Tio.9 after HE with e < 5 have brittle intergranular fracture pattern with traces of plastic deformation at some grain boundaries. The initial (undeformed) state is typical of brittle fracture (trans- and intercrystalline one) with no indication of a plastic deformation. After HE with e >... 

Samples used in this study, their formation, petrographic, petrophysical and mineralogical characteristics. C crystal carbonate M-W mudstone, wackestone P-G packstone-grainstone Vac vugs iX intercrystalline pores iM intramatrix pores iG intragranular pores IG intergranular pores K karsts Fr fractures F formation factor m cementation factor n saturation exponent. 

Porosity. The fraction of total (bulk) volume occupied by the voids is defined as the porosity of the porous medium. A porous medium can be classified according to the type of porosity involved. In sandstone and unconsolidated sand, the voids are between sand grains, and this type of porosity is known as intergranular. Carbonate rocks are more complex and may contain more than one type of porosity. The small voids between the crystals of calcite or dolomite constitute intercrystalline porosity (47). Often carbonate rocks are naturally fractured. The void volume formed by fractures constitutes the fracture porosity. Carbonate rocks sometimes contain vugs, and these carbonate rocks constitute the vugular porosity. Still some carbonate formations may contain very large channels and cavities, which constitute the cavernous porosity. 

Figure 8 from (Jochum 2013) shows the behavior in cutting of brittle hard materials, namely Circonium oxide and Yttrium oxide as it is used for dental implants. The picture shows ductile material behavior due to high compressive stresses and thus ploughing in the upper scratch. In the lower scratch an intercrystalline fracture is shown, which is due to the interaction between grain and material possibly as shear fracture, mechanism similar to the one pointed out by Kragelski in Fig. 6. 

Microstmctural studies on the above samples determined that the addition ofYaOa or MgO caused a transition from a transcrystalline to intercrystalline fracture. This transition was due to softening of the glassy phase, which resulted in slow crack growth at low stress levels. 

In most cases, shear fracture is transcrystalline (through the grains), but, depending on the material state, intercrystalline fracture (fracture along the grain boundaries) may also occur. 

When grain boundaries are embrittled (for example, by precipitates, see section 6.4.4), cleavage fracture may be intercrystalline. In this case, the grain structure can be clearly seen in a scanning electron microscope picture (see figure 1.10(b)). 

Hot spot formation mechanisms have been studied in some detail by Field and his co-workers at Cambridge University (15-19), following on from the pioneering work of Bowden and Yoffe (13). Among the mechanisms proposed have been (i) fracture of explosive crystals (20,21), (ii) plastic flow with dislocation pile-up (22) and adiabatic shear flow (23-26), (iii) friction, (27,28), (iv) adiabatic compression of gas pockets (27) and (v) shock-void interactions (29). Chaudhri has very recently examined these possibilities (30) and concludes that intercrystalline friction and the interaction of strong shocks with voids producing fast jets are dominant mechanisms. Plastic flow can also be effective when the strain is high. 

As a consequence of the above mentioned effects, but in contrast to many other metallic materials, the fracture toughness of Mo and W is strongly reduced with increasing degree of recrystallization. With increasing plastic deformation, the fracture toughness increases (see Sect. 3.1.9.4), combined with a transition from intercrystalline to transcrystalline cleavage and to a transcrystalline ductile fracture [1.147,158,159]. 

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