Fatigue time dependent life

Extend the safe useful operation life of major HT/HP power plant items, subject to time-dependent creep and thermal fatigue damage, with benefits in terms of delayed costs for component replacement. 

Fatigue is a dynamic and time-dependent phenomenon. When a component is subject to alternating stresses repeatedly, it fails at a much lower stress than the material yield strength due to fatigue. Most mechanical failures are related to dynamic loading therefore, the safety assessment in fatigue life plays a critical role in reverse engineering. 

The result of most fatigue testing is the generation of S-N curves. These curves plot stress S against the number of cycles to failure N. The data are obtained by testing many specimens to failure at different stress levels. As with any type of time-dependent strength property, the life to failure increases as the stress is decreased. Also, an endurance limit is reached, i.e., a maximum stress above which the material will not withstand cyclic stress indefinitely. For experimental convenience and time required, fatigue tests are usually discontinued if failure has not occurred by 10 million cycles. 

Applicability of Fatigue Life Prediction Method to Polymer Composites. Mechanics of Time-Depend-Materials 3, (1999), p. 141-157... 

On the other hand, relatively low stresses (e.g., below the yield stress) that are applied at reduced loading rates cause the deformation to shift into the solder material. These conditions characterize the service life environment of most solder interconnections as well as accelerated aging tests. As a consequence, the solder deformation will include time-dependent or creep deformation. In the case of cyclic loading environments, the solder deforms by a combination of creep and fatigue responses. Temperature vari-... 

Y. Miyano, M. Nakada, R. Muki (1997) Prediction of fatigue life of a conical shaped joint system for fiber reinforced plastics under arbitrary frequency, load ratio and temperatwe. Mechanics of Time-Dependent Mater. 1,143. 

Service lifetime prediction of polymers and/or polymer based materials may be undertaken from different types of tests, such as creep behavior tests (linear and non-linear creep, physical aging, time-dependent plasticity), fatigue behavior tests (stress transfer and normalized life prediction models, empirical fatigue theories, fracture mechanics theory and strength degradation) and standard accelerated aging tests (chemical resistance, thermal stability, liquid absorption) [32]. 

The second method was called the dominant damage approach and assumes that fatigue and creep are independent mechanisms and that crack growth is determined by whichever is dominant. In this case, crack growth should be predominantly either cycle or time dependent. For example, if creep is the dominant damage mechanism, then plots of dn/df against a suitable TDFM should be independent of frequency and can be used to predict the fatigue life in terms of time under load rather than number of cycles. 

Perhaps the most important stress factor affecting corrosion fatigue is the frequency of the cyclic stress. Since corrosion is an essential component of the failure mechanism and since corrosion processes typically require time for the interaction between the metal and its environment, the corrosion-fatigue life of a metal depends on the frequency of the cyclic stress. Relatively low-stress frequencies permit adequate time for corrosion to occur high-stress frequencies may not allow sufficient time for the corrosion processes necessary for corrosion... 

A distinction must be made regarding the length of service of the pressure reducing systems. Fatigue failure of any mechanical system depends on time, i.e., the number of cycles to failure. Therefore, the treatment required for a continuous service may not be justified for a short term service. A System in short term service is defined as one which operates a total of 12 hours or less during the life of the plant. Pressure relief valves typically meet this limit. Systems in short term service exceeding the screening criteria indicated above should be evaluated. 

Regenerator cyclones have a typical life of 15-30 years depending upon erosion and mechanical fatigue. The base metal of the cyclones will deteriorate with time leading to graphitization. Once this happens, the metal cannot be welded upon and hence cannot be repaired during a normal unit TAR. This phenomenon is dependent upon time and temperature. COP is used to minimize the temperature to extend the cyclone life. 

Also in this case there is a kind of temp era ture-time equivalence the fatigue life of a polymer is generally reduced by an increase of temperature. The temperature-dependence can usually be expressed by ... 

The relationship between fatigue life, oxidation extent and UV exposure is illustrated in Fig. 3 where both logNp and log [carbonyl] are plotted vs. UV exposure (for m = 10 cycles.) It can be seen that as the time of UV exposure increases, i.e., as [carbonyl] increases, Np decreases sharply. Fig. 4 shows that logNp depends in a linear fashion on log[carbonyl]. Such a de-... 

Figure 2. Dependence of fatigue life (Nj, the number of cycles to failure) on UV exposure time for LDPE. (Ae = 8%, u = 10 cycles/min).

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