Formation of microcracks

On the other hand the appearance of submicrocracks in loaded polymers has been known for many years since the Leningrad school [17,18, 27, 28] employed X-ray scattering techniques to search for it. Such submicrocracks were found in PE, PP, PVC, PVB, PMMA, and PA 6. The authors noted two important regularities of the submicrocrack formation [28]. First of all, the submicroscopic cracks appear at finite sizes with their transverse dimensions practically independent of load duration, stress value, and temperature (Table 8.3). Secondly the transverse size of the submicrocracks is determined by the polymer structure. For oriented crystalline polymers the transverse size coincides with the microfibril diameter for non-oriented, amorphous polymers having a globular structure it coincides with the diameter of the globules [28]. 

Two explanations have been given for the formation of these submicrocracks Zakrewskii [20] proposes a radical chain reaction and Peterlin [58] suggests that the ends of microfibrils are the crack nuclei. 

The Zakrewskii-mechanism was proposed by the Russian school following their studies of the formation of free radicals and of end-groups. They had noted that the concentration of free radicals was either by a factor of 4000 (PE-HD) smaller than the concentration of newly formed end-groups or even unmeasurably small (PP). This led them to search for a reaction by which chains might break without increasing the concentration of free radicals. 

The authors suggest [17, 18] that the 3300 new end groups formed per submicrocrack in PE-HD (5000 in PP) are created by such a mechanism. Their data indicate also that there is less than a single pair of free radicals available per submicrocrack (0.4 in HOPE, 0.3 in PA 6 and an immeasurable small fraction in PP). These discrepancies, the unfavorable conditions for thermal equilibrium in the splitting reaction, and the absence of a well defined termination of the reaction chain raise considerable concern. They make it difficult to believe that such a reaction is the major reason for the formation of submicrocracks which remain fairly stable during extended and repeated loading periods [220]. 

Peterlin [58] has based his proposal on the origin of submicrocracks on morphological considerations and analysis of the previously mentioned X-ray data 

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Polyamides, like other macromolecules, degrade as a result of mechanical stress either in the melt phase, in solution, or in the soHd state (124). Degradation in the fluid state is usually detected via a change in viscosity or molecular weight distribution (125). However, in the soHd state it is possible to observe the free radicals formed as a result of polymer chains breaking under the appHed stress. If the polymer is protected from oxygen, then alkyl radicals can be observed (126). However, if the sample is exposed to air then the radicals react with oxygen in a manner similar to thermo- and photooxidation. These reactions lead to the formation of microcracks, embrittlement, and fracture, which can eventually result in failure of the fiber, film, or plastic article. 

At surface temperatures exceeding about 1800 K (see Fig. 13.4) the silicon evaporation begins to overcome the sputtered Si-flux leading to an increase of the Si concentration in the plasma and of the total radiation losses. Surface analysis revealed the formation of microcracks and holes. A depletion of silicon was observed in areas of the highest power load with values of 0.03% in and 0.02% between the fibres. Part of the released silicon was found on the limiter surface in the vicinity of the tangency point. 

Zhurkov et al. used light scattering and low angle X-ray scattering (LAXS) to study the time-dependent formation of microcracks or cavities in stressed polymers. The concentrations of microcavities were found to be as high as 10 - 10 cm , uniformly distributed throughout the specimen, thougji more numerous close to the specimen surface. 

E. values in some of them. The authors attributed the increase to two stmcture-forming roles of the elastomers (i) reduction in the size of the sphemlites, and (ii) prevention of the formation of microcracks by being located preferentially in the intersphemlitic regions. 

To some extent, the formation of microcracks was probably related to the quantity and the category of the precipitated crystals. As known, the TEC and internal stress strongly depended on the category of the crystals. 

In oriented samples, oxidation is much more concentrated in the surface layer, thus decreasing the formation of microcracks within the bulk of the sample which increases its ability to resist failure under UV radiation. 

This type of approach has also been used to understand the formation of microcracks at residually stressed inclusions. Figure 8.38 illustrates three possi-... 

This reaction is linked with a decrease of volume however, as the crystalhzation of gypsum is taken into account, the overall volume is increasing and can be estimated as 18 cm /mole of AFm. But the crystallization of gypsum is less probable in the concentrated solution of chlorides and rather the formation of anhydrite will be more probable, with the decrease of total volume. These examples are showing that the effect of chloroaluminate hydrate on the formation of microcracks and deterioration of concrete is negligible. 

It has been suggested, but is not widely accepted (Kutti, 1992), that—in addition to this C-S-H phase— another amorphous phase that is siUca-rich and which exhibits properties similar to those of silica gel is formed in the initial stage of hydration It contains significant amounts of loosely bound water, which escapes upon drying and is responsible for the shrinkage of the hardened cement paste and the formation of microcracks. 

Figure 17.6 The formation of microcracks and crack pinning in metal oxide nanoparticle reinforced...

Fij . 17. Weibull plots of optical glass fibers subjected to pulse-irradiation and continuous wave treatments. The use ol pulsed exeitner radiation lowered the liber t racture strength by as rniich as a factor of 4 compared to the liber that was subjected to a CW argon-ion laser. The lower fracture strength ol pulse-irradiated liber is a result of the formation of microcracks in the material (after Varelas el al.. 1997). 

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