Fatigue maximum stress

Otnax = maximum stress level of the fatigue cycle... 

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... 

Corrosion Fatigue Limit—the maximum stress that a metal can endure without failure. This is determined in a stated number of stress applications under defined conditions of stressing and corrosion. 

Endurance limit To develop S-N curves the fatigue specimen is loaded until, for example, the maximum stress in the sample is 275 MPa (40 ksi) (Fig. 2-43). At this stress level it may fail in only 10 cycles. These data are recorded and the stress level is then reduced to 206 MPa (30 ksi). Tliis specimen may not break until after 1,000 stress cycles at this rather low stress level. 

Figure 4.123. Dynamic fatigue of polyimide two examples of maximum stress (MPa) at 10 cycles versus temperature...

Stress ratio. Rates of corrosion fatigue crack propagation generally are enhanced by increased stress ratio R, which is the ratio of the minimum stress to the maximum stress. 

The growth or extension of a fatigue crack under cyclic loading is principally controlled by the maximum load and stress ratio (minimum/maximum stress). However, as in crack initiation, there are a number of additional factors that may exert a strong influence, especially in the presence of an aggressive environment. 

Figure 6.69 Venn diagram illustrating the interrelationship among stress corrosion, corrosion fatigue, and hydrogen embrittlement R — ratio of minimum stress to maximum stress (Phull)5...

In corrosion fatigue experiments, a notch is cut in the specimen at which maximum stress will occur. Crack initiation and growth thus occur at the tip of this notch and may be measured with the aid of a microscope. 

Fig. 6.5 Cyclic stress-strain behavior observed during the room temperature fatigue of unidirectional SiQ/LAS-II at a maximum stress below the 105 fatigue limit (loading frequency = 10 Hz, crmin/crmax = 0.1). The cyclic stress-strain curves show very limited, if any, hysteresis. Note that the strength and strain capability is retained after cyclic loading below the proportional limit stress. After Prewo.42...

Fig. 6.6 Fatigue life diagram for the tension-tension fatigue of unidirectional SiCf/Si3N4 at 1000°C and a stress ratio (o /cr, ) of 0.1. Fatigue run-out (5 x 106 cycles) was observed when the maximum stress was below apt. After Holmes et al.43...

A comparison of the crack velocities measured under static and cyclic loads is illustrated in Fig. 7.2. For this purpose, the crack velocity under cyclic loads, da/dt = da/dN x vc, plotted against the maximum stress intensity factor of the fatigue cycle, Kmax = AA7(1 — R), from the results shown in Fig. 7.1. The static crack velocity da/dt is also plotted against the stress intensity factor Kj corresponding to the applied load. In the intermediate range of crack growth, the static crack velocity generally follows the power-law relationship... 

Fig. 8.16 Variation of static and cyclic fatigue crack velocity, daldt, with the applied (maximum) stress intensity factor, KIy for fatigue tests on A CVSiCw composites conducted at 1400°C. The inset shows a schematic of the change in crack velocity for a change from static- cyclic-> static loading at fixed Kt.51...

Fig. 14a and b. Fatigue fracture surfaces of HIPS tested at 21 Hz in a tensile mode. The maximum stress was 17.2 MPa and the minimum stress was 3.4 MPa a Low magnification scan b High magnification scan... 

Fig. 44a ami b. Fatigue fracture surface of an ABS sample tested at 21 Hz in an alternating tensile mode at a maximum stress of 27,5 MPa and a minimum stress of zero a Low magnification scan b Higher magnification of region near source... 

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