Fatigue life time

Fatigues tests have been run on a series of ABS samples under reversed tension-compression at a stress amplitude of 27.6 MPa, at four different frequencies, viz. 0.02, 0.2, 2, and 21 Hz. The average fatigue lifetime as a function of frequency is shown in Fig. 46 and, for comparison purposes, the data for HIPS obtained at 17.2 MPa is also shown. The average fatigue life-time of ABS increases in a linear manner with frequency on this log-log plot. The rate of increase, however, is reduced compared to that of HIPS, or of PS (Fig. 15). The reduced frequency sensitivity is perhaps a result of a reduced magnitude of the p-transition in the SAN copolymer compared to that in PS. A reduced frequency sensitivity of FCP rate in ABS vs. a HIPS-modified PPO has also been noted... 

The Nonzero Minimum-Life Case. In many situations, no failures are observed during an initial period of time. For example, when testing engine bearings for fatigue life no failures are expected for a long initial period. Some corrosion processes also have this characteristic. In the foUowing it is assumed that the failure pattern can be reasonably weU approximated by an exponential distribution. 

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... 

For cables such as in elevators or cranes, the service life might be based on 95% of the fatigue life of the material. In such circumstances, a composite cable would clearly last far longer than a metal cable. Moreover, the composite cable length, if measured, would be found to be longer than originally as time passes (an elevator would have to be periodically adjusted to stop precisely at each floor). Such measurements when recorded would be an evidential record of the cable per-... 

Lognormal distribution Similar to a normal distribution. However, the logarithms of the values of the random variables are normally distributed. Typical applications are metal fatigue, electrical insulation life, time-to-repair data, continuous process (i.e., chemical processes) failure and repair data. 

A careful study of the above tables and figure 12.1 will indicate why a rubber chemist spends a lot of time in designing compounds with various cross link densities for oil field service as well as for other critical applications. It can also be observed that tear strength, fatigue life and toughness, all important requirements for oil field, rubber seals pass through an optimum at low cross link density and fall off with increase in cross link, whereas the most important sealing properties such as hysteresis and compression set improve with increased cross link. 

The thin-wall bellows element should be designed for membrane stresses to conform to code-allowable stresses. The sum of membrane and secondary bending stresses should not exceed 1.5 times the yield stress in order to prevent the collapse of the corrugations caused by pressure. Bellows subjected to external pressure can be analyzed in a manner similar to a cylinder, utilizing an equivalent moment of inertia. The fatigue life can be estimated based on the sum of deflections and pressure stresses as compared to S/N curves based on bellows test data or using the curves in B31.3 Appendix X, Metal Bellows Expansion Joints. Formulas for the stress analysis of bellows are available in the Expansion Joints Manufacturing Association (EJMA) Standards (37). 

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 number of permissible load cycles, which results from the fatigue stress evaluation, does not necessarily determine the life time expectancy of the pressure vessel, but sets the intervals for fissure tests. 

Mechanical Characterization of Sulfur-Asphalt. The serviceable life of a pavement comes to an end when the distress it suffers from traffic and climatic stresses reduces significantly either the structural capacity or riding quality of the pavement below an acceptable minimum. Consequently, the material properties of most interest to pavement designers are those which permit the prediction of the various forms of distress—resilient modulus, fatigue, creep, time-temperature shift, rutting parameters, and thermal coefficient of expansion. These material properties are determined from resilient modulus tests, flexure fatigue tests, creep tests, permanent deformation tests, and thermal expansion tests. 

The search for quantitative structure-property relationships for the control and prediction of the mechanical behaviour of polymers has occupied a central role in the development of polymer science and engineering. Mechanical performance factors such as creep resistance, fatigue life, toughness and the stability of properties with time, stress and temperature have become subjects of major activity. Within this context microhardness emerges as a property which is sensitive to structural changes. 

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