The Cracking Mechanism

When we first think of adhesion experiments, we imagine that we can carry out the sort of tests shown schematically in physical chemistry textbooks, as shown in Fig. 7.8(a), in order to measure the surface forces as the bodies approach and begin to adhere. This was the experiment of Tabor and Winterton, and of Israelachvili. hi fact, they could only obtain gaps down to around 20 nm by this means. Remembering that 99% of adhesion energy is below 1 nm, we know that full molecular adhesion was not being measured in those tests. 

In practice, this idealized experiment is impossible because, when the surfaces get close together, the adhesion foree increases very rapidly and an instability occurs such that the surfaces jump into contact. Essentially, the adhesion force is so strong that it overcomes the elastic resistance of the materials. So the situation shown in Fig. 7.8(b) cannot exist instead, the system goes to the position shown in Fig. 7.8(c), however much we try to control the positions of the surfaces. This is the crack geometry. Molecular contact is made over part of the surface, and there is no contact over the rest of the bodies. 

This elastic cracking mechanism is very common in adhesion studies, except when adhesion is much weakened by contamination. It allows us to make two excellent advances the first is the experimental one of seeing the adhesion experimentally by optical interference the second is the theoretical approximation that adhesion is perfect on one side of the crack line and zero on the other. This is the fundamental premise of fracture mechanics explained in the next section, allowing the use of a one parameter model. 

The reason for the visibility of cracks is seen in Fig. 7.9. A light source shining on an adhesive joint is not much reflected at the adhesive interface, because the difference in refractive index at the interface is not large. Therefore, in reflection the adhered interface looks dark because almost no light is reflected 

The first truly reversible adhesion cracking experiments were carried out by Obreimoff in 1930 on mica and by Johnson et al in 1971 using smooth elastic rubber spheres. The diameter of the black contact zone was measured in reflected light, and plotted against the applied force to compare with the thermodynamic cracking theory. The results were reasonably reversible and fitted the thermodynamic work of adhesion theory. These experiments are described more fully in Chapters 4 and 9. 

---

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

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. 

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. 

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. 

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. 

Solid alkalis might catalyse the cracking reactions of polymers as is the case with acidic catalysts. According to experimental work solid alkalis catalyse the degradation of polystyrene more efficiently than acidic catalysts [53]. This phenomenon could be explained by differences in the cracking mechanism of polymers. The main components in the oils obtained by solid acids were styrene monomer and dimer. Since cracking of hydrocarbons on solid acids has been explained in terms of P-scission of C-C bonds [19, 20], these were probably produced by P-scission of C-C bonds in the PS main chains as follows ... 

The cracking mechanism of polymers follows first-order kinetics. 

A general mechanism of the oxidative coupling of methane over reducible oxide catalysts has been proposed by Lee and Oyama. Their reaction sequence is based on the cracking mechanism suggested by Kolts and Delzer which was adapted to the methane dimerization process. The similarities between these two processes as indicated by Lee and Oyama were as follows (1) the same materials (Mn/MgO, Fe/MgO, LajOj, Ce02) are active in both reactions,... 

The model of the cracking mechanism of olefins proposed by Voevodsky should lead to a strong dependence of olefin cracking products on temperature and pressure. This was not observed. The author proposes a chain olefin cracking mechanism when chains are being formed via a series of displacement and addition reactions of the radicals on w-bonds. Some influence of the paraffinic and cychparaffinic products on the kinetics and mechanism is considered. Decomposition of olefins (primary cracking products) defines the kinetic of summary reaction. 

A further theory on the cracking mechanism is that it is a purely electrochemical process in the base of the crack (active path corrosion), the rapid crack propagation rate observed in most cases due to greatly increased dissolution of the material as a result of plastic deformation at the crack tip with simultaneous passivation of the crack walls. Greatly increased dissolution during the plastic deformation of a metal has been demonstrated (Gerischer 1955). 

The reason for this strengthening of the joint is the cracking mechanism. Although there does not seem to be a crack at the edge of the wire, there is a virtual crack because the rigid material can be replaced by an elastic half-space as shown in Fig. 13.4(b). The stress in the elastic material rises to infinity at the edge according to the Boussinesq analysis, because of the (1 — pressure... 

The tests involve the removal of a structural member to subject it to different loads in order to monitor their shear and/or flexural strength. These tests are particularly effective for detecting problems of localization due to cracks. It should also be verified that the cracking mechanisms would occur on the actual structure as well. 

The test method uses an ammonia atmosphere to simulate service conditions under which stress corrosion cracking may occur. This test method is suitable only for products fabricated from copper alloys that are known to be susceptible to stress corrosion cracking in ammonia vapor atmospheres. It is intended to create an enviromnental condition of reproducible severity, but it is well known that the critical step in the cracking mechanism is the development of an environment in the condensate film that occurs on the test specimen, which is rich in complex copper ions. 

There is usually a crack bridging zone just behind the crack tip, with a small (sub-millimetre) microcrack region at the crack tip, and the cracking process zone extends over several mm. This is the cracking mechanism in some largegrained ceramics, and in whisker reinforced ceramics. See crack bridging toughness. 

Some mechanism has to be introduced with the energy balance the cracking mechanism tends to be more important than Galileo s stress mechanism. 

Loading rate, or dK- /dt, affects SCC initiation and growth in precracked specimens [113] (Fig. 37) and a good appreciation of the cracking mechanism is... 

---