Cracking mechanism

The general features of the cracking mechanism involve carbonium ion formation by a reaction of the type... 

A crack may be defined as a continuous separation in a metal component. The conditions under which cracking occurs are many and varied. However, one condition is necessary, although not necessarily sufficient, for all cracking mechanisms—stress. Stress may be residual andtor applied, static andhr cyclic, and of a high or low level. 

Most cracking problems in cooling water systems result from one of two distinct cracking mechanisms stress-corrosion cracking (SCC) or corrosion fatigue. 

Gross cracks may be visually observable. Nondestructive testing for the presence of cracks includes using dye penetrant, ultrasonics, and radiography. Determination of the cracking mechanism will require metallographic analysis. 

Cracking mechanisms in which corrosion is implicated include stress corrosion cracking, corrosion fatigue, hydrogen-induced cracking and liquid metal embrittlement. Purely mechanical forms of cracking such as brittle failure are not considered here. 

Zeolite, or more properly, faujasite, is the key ingredient of the FCC catalyst. It provides product selectivity and much of the catalytic activity. The catalyst s performance largely depends on the nature and quality of the zeolite. Understanding the zeolite structure, types, cracking mechanism, and properties is essential in choosing the right catalyst to produce the desired yields. 

There has been some controversy as to whether s.c.c. occurs by active path corrosion or by hydrogen embrittlement. Lack of space does not permit a full treatment of this subject here. References 14 and 15 are recent reviews on the s.c.c. of high strength steels and deal with the mechanism of cracking (see also Section 8.4). It is appropriate to discuss briefly some of the latest work which appears to provide pertinent information on the cracking mechanism. It should be noted, however, that cracking in all alloy systems may not be by the same mechanism, and that evidence from one alloy system need not constitute valid support for the same cracking mechanism in another. 

Fig. 8.3 Schematic representation of the stress corrosion cracking mechanism of the pit (after Pickering and Swann ). (a) Tubular pits initiated at solute-rich slip step. The pits may, but need not necessarily, follow the slip plane once they are initiated, (b) Ductile tearing along a plane containing the tubular pits. The stress is increased across the plane because of the reduced cross section and the stress raising effect...

Despite considerable developments in the study of stress-corrosion cracking mechanisms in recent years, it remains difficult to draw a clear distinction between those situations which involve hydrogen embrittlement, and those... 

At low temperature (375 and 400 °C), the product distribution obtained with the catalysts is very different from the one obtained under thermal cracking. With the catalytic cracking (ZSM-5), the obtained products are mainly n-alkanes, isomerised alkanes and alkenes with a carbon number between 1 to 6 whereas with the thermal cracking the whole range of n-alkanes with 1 to 9 carbon atoms and the 1 -alkenes with 2 to 10 carbon atoms are observed. This difference of product distribution can easily be explained by the cracking mechanisms. In one hand, the active intermediate is a carbocation and in the other hand it is a radical. 

At the lower temperatures (375 and 400 °C), the n-dodecane conversions is higher with a catalyst. Moreover, the products distributions are very different. This is explained by the cracking mechanisms (free radical and carbocation) and maybe by the supercritical conditions. This is no more the case at 425 °C as the catalysts seem to deactivate rapidly by coking. So the formed products come mainly from the thermal cracking. 

Mechanism for protonation of alkenes was previously discussed in Section 13.5.1. In general, protonation of alkenes is an exothermic process. Protonation of alkanes was discussed in Section 13.5.2. There wiU be further discussion on this step in Section 13.8.4 within the context of alkane cracking mechanisms. The formation of a penta-coordinated carbonium ion from alkane protonation is typically an endothermic process, the reverse being true for deprotonation. 

The duality of cracking mechanisms is summarized in Fig. 5, where RH paraffin feed, R -C=C = olefinic product, Kq = equilibrium constant of olefin chemisorption. Free Bronsted acid sites HZ interact directly with the paraffin feed by protonation, producing monomolecular cracking. When the acid sites are covered with adsorbed olefins to form... 

Fig. 6.2.7 (a). Diagram of parameters of chevron cracks associated with spherical indenter action and (b) crack mechanism seen in cross-section of Hertz s cone (C—C crack plane). Crack growth tends to enlarge in (r, 0) in relation to the initiation of the crack peak, which optimizes the conditions for energy liberation. (After Lawn et at., 1975)... 

Once the critical load is exceeded (P > Pc), the crack starts to grow substantially (normally R > 2a, Fig. 6.2.7). For the boundary case of a true cone (i.e., R- 0), crack mechanics becomes independent of the developments in the contact region (i.e., r and Cf°), which is given by the equilibrium equation derived by Roesler (1956)... 

Figure 4-17 Hypothetical thermal cracking mechanism for an amphoteric asphaltene... 

Figure 4-18 Hypothetical thermal cracking mechanism for a neutral polar asphaltene molecule. 

The fractures on a plane surface, created by the collisions of hard spherical particles at low-impact velocities, may form a conical crack according to the Hertzian quasi-static stress theory. In a multiple-impact situation, the conical cracks meet those extending from neighboring impact sites, and then the brittle material becomes detached. Once appreciable damage is done, the cracking mechanism may be altered because the particles no longer strike on a plane surface nevertheless the brittle removal continues by the successive formation and intersection of cracks. 

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