Fatigue load frequency

Figure 6.45 Fatigue life data, S-N curves, for a high-strength steel under different environmental conditions. Stress ratio R — — 7. Loading frequency 7 Hz for tests in 0.6 M NaCIsolution. Horizontal arrows indicate failure condition not attained. OCP = open-circuit potential102...

Cyclic load frequency is the most important factor that influences corrosion fatigue for most material environment and stress intensity conditions. The dominance of frequency is related directly to the time dependence of the mass transport and chemical reaction steps involved for brittle cracking. 

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...

A coincidence between the proportional limit, fatigue limit, o-fi, has also been observed for other composite systems fatigued at loading frequencies of 10 Hz or lower. For example, as shown in Fig. 6.6, under tension-tension fatigue at 1000°C, 0° SCS-6 SiQ/HPSN43 composites exhibit a fatigue limit (5 x 106 cycles) at the monotonic proportional limit stress of 200 MPa. 

Fig. 6.12 Influence of loading frequency on the surface temperature rise measured during the tension-tension fatigue of a woven 0790° CVI Q/SiC composite. The fatigue experiments were conducted at 20°C between fixed stress limits of 10 MPa and 250 MPa. After Holmes and Shuler.51...

On the other hand, very high temperatures and low cyclic loading frequencies (high cycle times) promote near-tip creep-fatigue conditions where... 

The same critical potential is observed for intergranular fracture under fatigue conditions as for monotonic loading (SCC), provided that the loading frequency in fatigue is sufficiently low (/ < 0.1 Hz for R = 0.5, AK = 20 MPam / ) [28]. 

Figure 9.17. Demonstration of the change in fatigue crack path from a transgranular to intergranular mode with reduction in loading frequency. [10]...

AK) relationships as a function of cyclic load frequency for the ferritic carbon steel SA 105 in hydrogen gas (adapted from Walter and Chandler [44]). Fatigue crack growth rate data in helium gas are included for comparison. 

The rate dependence of fatigue strength demands carefiil consideration of the potential for heat buildup in both the fatigue test and in service. Generally, since the buildup is a function of the viscous component of the material, the materials that tend toward viscous behavior will also display sensitivity to cyclic load frequency. Thus, TPs, particularly the crystalline polymers like polyethylene that are above their glass-transition temperatures, are expected to be more sensitive to the cyclic load rate, and highly crosslinked plastics or glass fiber reinforced TS plastics are much less sensitive to the frequency of load. 

Loading frequency and moderate variation of temperature have little effect upon the number of cycles to fracture in dry fatigue. These factors are more important in CF. The effect of frequency is significant because flie corrosion effect is time dependent, and the temperature is known to be an important factor in connection with corrosion. These two factors are mentioned at the same time because they both represent possible ways of accelerating CF tests. Reliable test results can, however, be obtained only if the fatigue acceleration factors are quantitatively known. 

In conjunction with the environmental effect, the loading frequency is an important factor. Since the environmental effect is due to the reaction between the environment and the freshly exposed material t the crack tip, the period of time for reaction is important. The degree of crack extension due to environmental crack tip damage during each cycle has been studied and reported by Wei [ ] and Bradshaw and Wheeler [" j. Also, test frequency can have a marked effect on results, if creep is a contributor to the fatigue processes [" ]. 

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