Fatigue stress controlled

SA mixes exhibit significantly higher fatigue lives than comparable conventional mixes under stress-controlled testing. 

Fi. 37. Fatigue curves for rolled Cu foils (99.99% purity) that were measured in the stress-controlled tension-tension loading mode parallel and perpendicular to the rolling direction. Thickness 25 pm. 

The mathematical objective of the optimization process is to find a minimum feasible weight. All relevant wing box sizing criteria comprising of limit, ultimate and fatigue stresses, buckling criteria, manufacturing requirements, control surface... 

The reason for the occurrence of strain softening seems to be associated with the initiation and propagation of cracks in the SiC matrix. The effect of frequency in stress-controlled fatigue in SiC/SiC, at a ratio of R = 0.1 is indicated in Fig. 7.23. 

Fig. 7.22 Strain amplitude versus number of cycles for stress-controlled low cycle fatigue [39]. With kind permission of Elsevier...

A typical fracture surface of fibers with a mirror and hackle feature in SiC/SiC ceramics is shown in Fig. 7.24. The cyclic softening observed in SiC/SiC occurs either under stress control or strain control. Since this observation is crack initiation- and propagation-related, it is possible that similar phenomena may be more frequent after softening during fatigue deformation. The Manson-Coffin... 

Figure 3.298. Variation of optical birefringence of the fatigued and control samples with the applied stress and solicitation period [877].

Thermal fatigue is observed mainly in stress-controlled loading because the strain amplitude increases in this case due to the reduction of stiffness with increasing temperature. The heat generated per cycle thus increases with time. If the loading is strain-controlled, thermal fatigue is usually not problematic because the stress decreases in this case. 

If a strain-controlled fatigue experiment is performed at a non-zero mean strain, cyclic relaxation may occur in addition to cyclic hardening or softening, with the mean stress decreasing over time (figure 10.31(a)). If, on the other hand, the experiment is stress-controlled at a non-zero mean stress, the hys-... 

The fatigue resistance of nano- and microcomposites was also compared by Mallick and co-workers [46,56]. In particular, th performed stress-controlled tension-tension fatigue tests on PP nanocomposites reinforced with 5 wt% of organoclay and on 40 wt% talc-filled PP. As reported in Figure 9.15, the S-N diagram of nanocomposites was slightly better than that of neat PP and much higher than that of talc-filled composites. 

Figure 9.15. Fatigue S-N curves of (a) neat PP, (b) PP - 5 wt% organoclay, and (c) PP -40 wt% talc. Tests have been performed at room temperature under stress-control with R = 0.1 and a frequency of 1 Hz [56]...

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