Microcracking threshold stress

The above results are all for a perfect solid under stress, with a single microcrack inside. For randomly disordered solids, the appropriate modification of the above Mott formula has not been developed yet. However, some quantitative features of the fracture propagation process in extremely disordered solids, like the percolating solid near its percolation threshold, are quite obvious and interesting. Although the (equilibrium) strength erf of the solid vanishes near the percolation threshold Pc erf (Ap) ), the... 

The structural capsules start to be formed in films subjected to deformation in liquids until some tension threshold. Microcracks and microvoids appear and are filled with the inhibiting liquid under tensile stresses exceeding the polymer flow limit. Capillary channels connecting these voids with the process liquid and with each other start to merge or open in the course of structural transformations but do not disappear fully. The liquid may move over the network of the formed channels beyond the polymer matrix limits or concentrate in some voids able under certain conditions to enlarge the manifold. Thermal treatment of the deformed film intensifies the relaxation processes in the polymer matrix, the film shrinks in the tension direction and the capillaries between voids link up densely, thus insulating liquid particles from each other. If the film is treated in the extended state, a more complex mechanism of microcapsule formation is realized [4]. Cl liberation from microcapsules is related to their ability to break spontaneously under residual... 

Table 7.3 lists the relevant parameters determined for some ceramic materials. In the table AKxh is the fatigue threshold, defined as the maximum value of AK at which growth rates did not exceed 10 ° m/cycle (ASTM E 647 procedure). Fig. 7.28 shows micrographs where a comparison is made between monotonic (monotonicaUy increasing loads) and fatigue fractures. The lengths of selected microcracks, obtained by monitoring the top surface of the cantilever-beam specimens (Fig. 7.27b) at various maximum applied stress levels and plotted as a function of the number of stress cycles, are shown in Fig. 7.28a, b at load ratios R of 0.05 and —1, respectively (Fig. 7.29). 

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