Stress-number of cycles

Rotating Beam Fatigue Test for Steel Cords. The purpose of this test method is to evaluate steel cord for pure bending fatigue (121). The test sample consists of a 3-mm diameter mbber embedded with steel cord. Different bending stress levels are appHed and the time to failure is recorded. The test stops at 1.44 million cycles. The fatigue limit is calculated from S—N (stress—number of cycles) curve. 

PVDC polyvinylidene chloride S-N stress-number of cycles... 

R Ratio of minimum stress to maximum stress during fatigue loading S/N Stress/number of cycles (fatigue lifetime plot)... 

Stress-number of cycles (S-N) curve of 5 mm (0.24 in.) as-welded butt joints of 2024-T3 compared to the S-N curves of thinner as-welded joints, skimmed joints, and base-metal curves. FSW, friction stir welded. Source Ref 15... 

Fig. 5.8 Stress-number of cycles (S-N) curves (R = 0.1) of parent plate and friction stir welded joints in the...

Xxx) instead of the maximum cyclic load values, Gomatam (2002) utilized the maximum and minimum principal and von Mises stresses obtained from nonlinear elastoplastic finite element analyses to calculate the ratios of peak stress values obtained at corresponding maximum load levels. These stresses were then compared to the intercept ratios obtained from the P-N curves at given environmental conditions. This comparison procedure revealed the maximum principal stress as the appropriate parameter to be used in the form of stress-number of cycles (S-N) curves, that is. 

Fig. 16. Maximum shear stress at bore vs number of cycles to failure for specimens of EN25 T steel at various k values. +, / = 1.2 Q = 1-4 ...

Machine components ate commonly subjected to loads, and hence stresses, which vary over time. The response of materials to such loading is usually examined by a fatigue test. The cylinder, loaded elastically to a level below that for plastic deformation, is rotated. Thus the axial stress at all locations on the surface alternates between a maximum tensile value and a maximum compressive value. The cylinder is rotated until fracture occurs, or until a large number of cycles is attained, eg, lO. The test is then repeated at a different maximum stress level. The results ate presented as a plot of maximum stress, C, versus number of cycles to fracture. For many steels, there is a maximum stress level below which fracture does not occur called the... 

Fatigue is another property of considerable interest to the design engineer. CycHc deflections of a predeterrnined ampHtude, short of giving immediate failure, are appHed to the specimen, and the number of cycles to failure is recorded. In addition to mechanically induced periodic stresses, fatigue failure can be studied when developing cycHc stresses by fluctuating the temperature. 

Here is the number of cycles to fracture under the stress cycle in region i, and Nj/Nf is the fraction of the lifetime used up after N, cycles in that region. Failure occurs when the sum of the fractions is unity (eqn. (15.4)). This rule, too, is an empirical one. It is widely used in design against fatigue failure but if the component is a critical one. Miner s Rule should be checked by tests simulating service conditions. 

This represents the locus of all the combinations of Ca and Om which cause fatigue failure in a particular number of cycles, N. For plastics the picture is slightly different from that observed in metals. Over the region WX the behaviour is similar in that as the mean stress increases, the stress amplitude must be decreased to cause failure in the same number of cycles. Over the region YZ, however, the mean stress is so large that creep rupture failures are dominant. Point Z may be obtained from creep rupture data at a time equal to that necessary to give (V cycles at the test frequency. It should be realised that, depending on the level of mean stress, different phenomena may be the cause of failure. 

In a small polypropylene component a tensile stress of 5.6 MN/m is applied for 1000 seconds and removed for 500 seconds. Estimate how many of these stress cycles could be permitted before the component reached a limiting strain of 1%. What is the equivalent modulus of the material at his number of cycles The creep curves in Fig. 2.5 may be used. 

Typical S-N (stress versus number of cycles) curves for various metals and composite materials are shown in Figure 6-4 [6-3]. The boron-epoxy composite material curve is much flatter than the aluminum curve as well as being flatter than the curves for any of the metals shown. The susceptibility of composite materials to effects of stress concentrations such as those caused by notches, holes, etc., is much less than for metals. Thus, the initial advantage of higher strength of boron-epoxy... 

As pointed out earlier, even if the metal is under stress for an infinite number of cycles, it will not fail as long as the stress level is below its fatigue limit. However, if the metal is damaged or notched in any way, the fatigue resistance decreases, as shown in Figure 4-443. Note that the nonferrous metals have no... 

Stainless steels are subject to fatigue failure under dry conditions as are all metallic materials, having distinct fatigue limits where level is dependent on steel type and heat treatment. The limits can be depressed by the simultaneous action of a corrodent, the degree depending upon the nature of the corrodent. Under severe conditions the limit can be displaced to very low values and it is customary to describe resistance by an endurance limit, that is the cyclic stress to give rupture at a specific number of cycles when in contact with a specific corrodent. Some comparative data are in Table 3.25. 

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