Free growth approximation

It is interesting to note that the deviations from the Avrami expression occur at a level that is given by the limit of applicability of the free growth approximation in Fig. 3.55. Similar data for a single molar mass, but at different temperatures, are shown in Fig. 3.101. All crystallizations seem to approach a common limit, but deviate at different temperatures from the Avrami equation. 

Another key problem is establishing a theoretical isotherm that explains the development of overall crystallinity over the complete extent of the transformation. Since the free growth approximation explains the experimental results as well as the Avrami type theory, the impingement cessation mechanism is not adequate. It... 

The qualitative approach of Harkins was put on a quantitative basis bv Smith and Ewart. Because 10 radicals are produced per second and can enter between 10 " and 10 particles. Smith felt that a free radical can enter a particle once every 10 to 100 seconds. It can cause the polymerization to occur for 10 to 100 seconds before another free radical would enter and terminate chain growth. A period of inactivity would follow that would last 10 to 100 seconds and then the process would repeat itself. Such a stop and go mechanism implies that a particle contains a free radical approximately half of the time. It can also be said that the average number of radicals per particle is 0.5. This is predicted on condition that (a) the rate of chain transfer out of the particle is negligible, and (b) the rate of termination is very rapid compared with the rate of radical entry into the particle. 

To compare experimental results with theory we start with the G61er-Sachs free-growth expressions, approximated by Eqs. (9.12) to (9.14). Accordingly, a plot... 

It is often difficult to conduct laboratory tests in which both the environmental and stressing conditions approximate to those encountered in service. This applies particularly to the corrosive conditions, since it is necessary to find a means of applying cyclic stresses that will also permit maintenance around the stressed areas of a corrosive environment in which the factors that influence the initiation and growth of corrosion fatigue cracks may be controlled. Among these factors are electrolyte species and concentration, temperature, pressure, pH, flow rate, dissolved oxygen content and potential (free corrosion potential or applied). 

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