Critical stress concentration

Physical Factors. Unsatuiated elastomers must be stretched for ozone cracking to occur. Elongations of 3—5% are generally sufficient. Crack growth studies (10—18) have shown that some minimum force, called the critical stress, rather than a minimum elongation is required for cracking to occur. Critical stress values are neady the same for most unsaturated mbbers. However, polychloroprene has a higher critical stress value than other diene mbbers, consistent with its better ozone resistance. It has been found that temperature, plasticization, and ozone concentration have httie effect on critical stress values. 

The critical stress is 64% greater than the longitudinal stress. However, the change in section from a cylinder to a sphere produces something akin to a stress concentration when this is taken into account the failure is accurately predicted. 

All the above modes of fracture are affected by the environment around the crack tip. This behaviour is typified by the phenomenon of stress-corrosion cracking where a crack, which is subjected to a subcritical stress concentration, will grow in a corrosive environment when /f, the critical stress concentration for stress-corrosion cracking). Therefore, to predict accurately the occurrence of cracking and crack growth rate, not only the materials properties are required but also information on the immediate environmental conditions. 

Klc = critical stress intensity factor in mode I, MPam1/2 Km = stress concentration factor Me = average molar mass between crosslinks, kg mol 1 Mn = number-average molar mass, kg mol 1 p = hydrostatic pressure, Pa Tg = glass transition temperature, K... 

Figure 4.40 shows the shear-thinning behavior of an aqueous solution of ethyl hydroxyethylcellulose as a function of the concentration. The pseudoplastic behavior is observed at lower polymer concentrations as the molecular weight of the polymer increases. An aqueous solution of ethyl hydroxyethylcellulose becomes pseudoplastic at concentrations of less than 1%. Above the critical value of the shear stress the flow behavior is non-Newtonian, and viscosity decreases with the increasing shear stress. The critical stress is in the range of 0.1 N/m2 for the solution. 

As mentioned before, the conventional drawing process leads to the well-known fibrillar structure, which still contains a majority of folded chains (Fig. 19.14). In drawing above the "natural" draw ratio further unfolding takes place. Such a process at very high draw ratios must be conducted with the utmost care since critical concentrations of stress on the folded chain surface of the crystal blocks must be avoided they lead to fracture. The super-drawing can be carried out in one and two stages. 

A critical minimum stress exists, below which delayed cracking will not take place. The critical stress decreases with increase in hydrogen concentration. These effects are shown in Figure 6.61 for SAE 4340 steel (0.4% C) charged with hydrogen by cathodic polarization in sulfuric acid, then cadmium plated to help retain hydrogen, and finally... 

Using the critical stress concentrations found in Figure 13 for the drawn material and Equation 8 for the undrawn material, the relative tensile strength ratio is obtained from Equation 25 for axial samples. The results are shown in Figure 14. 

Fig. 4. Critical stress cr for craze initiation vs diluent concentration c for various esters of the phthalic acid...

Chapter 3. In-plant measurement of flow behavior of fluid Foods. Using a vane-in-a-cup as a concentric cylinder system. The vane yield stress test can be used to obtain data at small- and large-deformations. Critical stress/strain from the non-linear range of a dynamic test. Relationships among rheological parameters. First normal stress difference and its prediction. 

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