Model embrittlement

Antioxidants may be assessed in a variety of ways. For screening and for fundamental studies the induction period and rate of oxidation of petroleum fractions with and without antioxidants present provide useful model systems. Since the effect of oxidation differs from polymer to polymer it is important to evaluate the efficacy of the antioxidant with respect to some property seriously affected by oxidation. Thus for polyethylene it is common to study changes in flow properties and in power factor in polypropylene, flow properties and tendency to embrittlement in natural rubber vulcanisates, changes in tensile strength and tear strength. 

Figure 6.8. Compositional map made with an early model of the scanning electron microprobc. The pictures show the surface segregation of Ni. Cu and Sn dissolved in steel as minor constituents the two latter constituents enriched at the surface cause hot shortness (embrittlement at high temperatures), and this study was the first to demonstrate clearly the cause (Melford I960).

The implication of the foregoing equations, that stress-corrosion cracking will occur if a mechanism exists for concentrating the electrochemical energy release rate at the crack tip or if the environment in some way serves to embrittle the metal, is a convenient introduction to a consideration of the mechanistic models of stress corrosion. In so far as the occurrence of stress corrosion in a susceptible material requires the conjoint action of a tensile stress and a dissolution process, it follows that the boundary conditions within which stress corrosion occurs will be those defined by failure... 

As approaches the crack can also propagate by hydrogen embrittlement processes during the higher load parts of the stress cycle. This forms the basis of various models which have been developed to describe corrosion fatigue, probably the best-known of which are the superposition models due to Wei"". In its most recent version this model takes the form ... 

Figure 6.49 Schematic presentation of (a) the slip dissolution and (b) the hydrogen embrittlement models 02...

Adsorption-induced brittle fracture. This model is based on the hypothesis that adsorption of environmental species lowers the interatomic bond strength and the stress required for cleavage. This model of chemical adsorption can explain the fact that a certain alloy is susceptible to specific ions. An important factor in support of this mechanism is the existence of a critical potential below which the SCC does not occur in some systems, and this model underlines the relation between the potential value and the capacity of adsorption of the aggressive ion. It also explains the preventive action of SCC for some systems by cathodic protection. This model may interpret the rupture of plastic materials or glass. It is referred to as the stress-sorption model, and similar mechanisms have been proposed for HE and LME. In this model, the crack should propagate in a continuous way at a rate determined by the arrival of the embrittling species at the crack tip. The model does not explain how the crack maintains a sharp tip in a normally ductile material.156... 

E. E. Glickman and V. I. Igoshev, Micromechanism of Solid Metal Induced Embrittlement Fractography and Kinetic Model, Surface Physics, Chemistry, Mechanics, No. 3, 104—112, (1989) (in Russian). 

Relating Kinetic Models to Embrittlement in Polymer Oxidative Aging... 

A previously described kinetic model for polyethylene radical chain oxidation is tentatively extended to include the conditions relevant to embrittlement behavior in the case of thermal oxidation at 90°C. The important roles of chemi-crystallization and morphology as a follow-up to initial chemical changes are discussed. The philosophy of how chemical reactions will ultimately lead to physical polymer changes apparent in Mw and lamellar properties, and how these processes could be discussed in terms of advanced modeling strategies is briefly reviewed. 

It is well known that thermal oxidation in polyethylene (PE) in the presence of oxygen leads to sudden and deep embrittlement, in which oxidative chain scission plays a key, but perhaps indirect, role. We have developed a kinetic model, derived from a branched radical chain mechanistic scheme, able to predict accurately molecular structural changes (7). 

To extend this model to the prediction of embrittlement time, one needs first to identify the causal chain leading to embrittlement, second to express mathematical relationships corresponding to the elementary links of the chain, and third to define pertinent end life criteria. According to previous studies (2,3,4), two causal chains are possible in the case under study (Figure 1). 

This model is considered to be useful to improve the knowledge of the role played by the factor of hydrogen accumulation in prospective rupture sites by stress-assisted diffusion, one of the key items in hydrogen embrittlement, a very dangerous phenomenon that frequently accompanies structural metals and alloys in service. 

Proposed computational model seems to be a promising tool as an aid to develop the life-prediction analyses for metallic components and structures subjected to any king hydrogen embrittlement in service. 

The modeling was first developed for crack growth in gaseous environments in which hydrogen is the embrittling species. It is assumed that ... 

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