Reversed stress cycle

Figure 5.40 Variation of stress with time that accounts for fatigue failure by (a) a reversed stress cycle and (b) a repeated stress cycle. Reprinted, by permission, from W. Callister, Materials Science and Engineering An Introduction, 5th ed., p. 210. Copyright 2000 by John WUey Sons, Inc.

Tmint then the stress cycle is reversed. In such cases, the mean stress... 

A large number of experiments on fatigue were performed with completely reversed cycles (indicated in Fig. 7.2a), where the mean stress is zero (o max = o min)- Often in components exposed to fatigue, a pattern (as shown in Fig. 7.2b) is observed, resulting from the superposition of a static preload during the reversed cycle (see 7.2a) also note that the names of the cycle-patterns shown in Fig. 7.2 may have different nomenclature in the literature. This is also often stated differently, namely that the mean stress is ffm(= Cmean). represents a steady-state stress and the alternating stress is a variable stress. This stress cycle is asymmetrical, since the sum of Cmax and Cniin 0. Clearly, machine parts in service exposed to cyclic stresses may experience particular conditions in which min 0 or (7max 5 0-... 

For this purpose we use Equation (5-26) to compute the mean endurable number of stress reversals Nq by using KRq =. 1 and KG= 187.5. The result is a number of stress reversals Afo = 23,400. If the numbers of stress cycles (A = 23,400 and A/=2500) are inserted into Equation (5-27), the result constitutes the degree of probability of the pipe section failing after the operation period during which the number of stress reversals will reach... 

The curves are computed under the assumption that the logarithms of the endurable number of stress reversals demonstrate a spread pattern according to a Gaussian density distribution. The failure rate determined from this shows a tendency to increase along with an increasing number of stress cycles. From a physio-technical point of view, this is understandable, since increasing stress will mean increased wear. 

While fatigue data collected in the laboratory are generated using a fully reversed stress cycle, actual loading applications usually involve a nonzero mean stress. The mean stress can be compressive or zero and it affects the strain-life curve as shown schematically in Eig. 1.33. Mean stress has its largest effects in the high-cycle regime. Compressive means extend life and tensile means reduce it. 

The maximum stress that presumably leads to fatigue fracture in a specified number of stress cycles. If the stress is not completely reversed, the value of the mean stress, the minimum stress, or the stress ratio should also be stated. Compare with endurance limit. 

The two-way memory can be brought about by either, or a combination, of two procedures. First, the specimen is subjected to deformation in the martensitic condition, and then heated to above Af. It is then cooled below Mf and deformed again in exactly the same manner to the same shape, and heated above Af again. This process is repeated a few times and then the specimen is fully programmed. Alternatively, a specimen can be superelastically stress-cycled above M. A stress-induced martensite then forms and in reverse generates the microstress sources. 

Figure 8.17 Variation of stress with time that accounts for fatigue failures, (a) Reversed stress cycle, in which the stress alternates from a maximum tensile stress (+) to a maximum compressive stress (-) of equal magnitude, (b) Repeated stress cycle, in which maximum and minimum stresses are asymmetrical relative to the zero-stress level mean stress cr, range of stress cr and stress amplitude cr are indicated, (c) Random stress cycle.

By convention, tensile stresses are positive and eompressive stresses are negative. For example, for the reversed stress cycle, the value of 7 is -1. 

A cylindrical bar of 1045 steel having the S-N behavior shown in Figure 8.20 is subjected to rotating-bending tests with reversed-stress cycles (per Figure 8.18). If the bar diameter is 15.0 mm, determine the maximum cyclic load that may be applied to ensure that fatigue failure will not occur. Assume a factor of safety of 2.0 and that the distance between loadbearing points is 60.0 mm (0.0600 m). 

A cylindrical 4340 steel bar is subjected to O reversed rotating-bending stress cycling, which... 

Thermal stress cycling is caused by creep or plastic relaxation of thermal stresses. When the power is reduced, so is the thermal gradient and thermal stress, and if significant relaxation has occurred a reverse stress is set up which in turn will relax. As the power is cycled strain is accumulated. Simplifications can be made (58) but there are many other interfering phenomena (59) and it is better to treat the problem more fully with a transient computer code such as CRASH (43). If clad thermal creep is the deformation mechanism, the nonlinear stress dependence of the creep rate decreases the relaxation times in the presence of other stresses (35). 

When R = -lor A = °o, the stress cycle is completely reversed and the mean stress equals zero. One simple example is the sinusoidal tension-compression stress cycle as shown in Figure 4.1a. The correlation between the R and A ratios is described by Equation 4.5 ... 

To determine in the laboratory if a component survives in use, a test bogey is frequentiy estabUshed based on past experience. The test bogey is with the particular test used to dupUcate (or simulate) field conditions. The bogey can be stated in cycles, hours, revolutions, stress reversals, etc. of components are placed on test and each component either survives or faUs. The reUabiUty for this situation is estimated. 

Fig. 2. The shape-memory process, where Tis temperature, (a) The cycle where the parent phase undergoes a self-accommodating martensite transformation on cooling to the 24 variants of martensite. No macroscopic shape change occurs. The variants coalesce under stress to a single martensite variant, resulting in deformation. Then, upon heating, they revert back to the original austenite crystallographic orientation, and reverse transformation, undergoing complete recovery to complete the cycle, (b) Shape deformation. Strain recovery is typically ca 7%.

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