Stress cycle resistance

Failure Rate and Stress Cycle Resistance. The interrelationship between the failure rate as a reliability characteristic and the conventional stress cycle resistance 5 for components under pressure—defined here as the ratio of mean endurable number of stress cycles No and design stress cycle frequency N—can be obtained from the above in the following manner. 

It has already been established that the results of interior cyclic pressure tests [5-14]-[5-20] demonstrate a spread pattern around the curves of mean endurable dynamic load which are similar to a Gaussian density distribution (Figure 5.17). Furthermore, it has been established that the dispersion of the logarithms of the stress cycles is practically independent of the magnitudes of the amplitudes, i.e., independent of KG. Under these circumstances. Equation (5-27) permits the computation of the failure probability for any combination of stress cycles Nq and N. The result of these computations, together with the stress cycle resistance, has been demonstrated in Figure 5.20,... 

Figure 5.20 Interrelationship between stress cycle resistance and failure probability in a pressure-stressed pipe.

Applies to essentially noncorroded piping. Corrosion can sharply decrease cyclic life therefore, corrosion resistant materials should be considered where a large number of major stress cycles is anticipated. 

FIGURE 6.3.10 The results of the endurance test performed by measuring stress cycles, where each cycle comprised an extension of the film by 20% followed by the release of the stress. The resistance between both edges of the test structures is monitored as a function of the number of tension cycles. The up arrow represents the breaking of either one side or the other of the gold wire. The test structure is shown in the inset. 

Rotational injection molding n. A modified injection-molding process applicable to hollow, axisymmetrical articles such as cups and beakers, in which the male half of the mold is rotated during the molding cycle until the material has hardened to a pre-chosen degree. The rotation produces orientation and increased crystallinity of some polymers, resulting in improved toughness and stress-craze resistance. 

Figure 14.7 (a) Thin film resistivity of PEDOT PSS dispersions (PEDOT PSS ratio = 1 6) vs number of shear stress cycles being applied, (b) Particle size distribution of dispersions XO, X2 and X5 determined by analytical ultracentrifugation. For method, see Ref [57], Part (b) from 5. Kirchmeyer and K. Reuter, Scientific importance, properties and growing applications of poly(3,4-ethylenedioxythiophenes), J. Mater. Chem. 15, 2077-2088 (2005). Reproduced by permission of The Royal Society of Chemistry... 

One of the most frequently used tests for fatigue-resistance evaluation is the well-known plotting of stress versus the number of cycles, usually referred to as the S-N (curve) relation . Various wave forms of cyclic stresses may be applied to a specimen to test its suitability to withstand prolonged strain. Machine elements are assessed to determine their practical endurance of industrial applications to which they may be exposed. Such tests focus on the nominal stress required to cause fatigue failure at some number of cycles. A logarithmic scale is almost always used for N, the number of cycles to failure. A schematic S-N plot is shown in Fig. 7.1. Note the horizontal line in plot (a), known as a knee , which represents the endurance limit . As implied by its name, at this level of stress, the specimen is characterized by its ability to endure a large number of stress-cycles at the stress level of the horizontal line and below it without failure. In plot (b), no such horizontal line is observed and the curve continues to decrease, indicating that the stress must be reduced for the test specimen to be able to withstand a certain number of cycles. 

X 10 stress cycles. Test results showed load resistance and rolling life to be in the following order (best to worst) silicon nitride, zirconia, silicon carbide, and then alumina. 

The components can be tested for SCC to evaluate the effects of applied and residual manufacturing tensile stresses leakage resistance after salt spray or temperature-humidity cycling tests and contact resistance after a salt spray test, temperature-humidity cycling, or a hydrogen sulfide exposure test. 

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