Softening cyclic

Almost no effect of the ratio is observed in SiC/SiC, as seen in the above figure, and the entire damage is caused by the applied tensile stress. In Fig. 7.22, cyclic softening may be observed where the strain amplitude, As/2, is plotted against the... 

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... 

When a material is subjected to cyclic loading, its stress-strain response may change with the number of applied cycles. If the maximum stress increases with the number of cycles, the material is said to cyclically harden . If maximum stress decreases over the number of cycles, the material is said to cyclically soften . If the maximum-stress level does not change, the material is said to be cyclically stable . As seen in Fig. 7.33, the nature of these transformation-induced hysteresis loops is cyclically stable when the stress level is considered. However, the strain of these cycles upon unloading and under compression are different, possibly due to the asymmetric stress characteristic of phase transformation (the peak strain at compression point E is less than that at tension point B). 

In the work of Liu and Chen [19], a series of hysteresis phenomena is described for 3YTZP and Mg-PSZ ceramics, as are the mechanisms controlling the evolved loop characteristics and the concept of cyclic softening. For more details on hysteresis please refer to the original work. 

Fig. 7.40 Tensile stress-strain behavior, the cyclic curves are for a stress ratio, R of 0.1 and a frequency of 0.1 Hz at room temperature, a The submicron sized 3Y-TZP ceramics (0.35 pm) showing cyclic hardening, b the nanocrystalline 3Y-TZP ceramics (100 nm) showing cyclic softening [37]. With kind permission of John Wiley and Sons...

Fig. 7.41 TEM micrographs of microstructure a for the submicron material after cyclic hardening up to fracture (N > 500 cycles) showing that transformation occurred in many micro-locahties without microcracking b the nanocrystalline material after cyclic softening up to fracture (N > 500 cycles). The microcracks had coalesced and propagated into a main crack along the tetragonal grain boundary [37]. With kind permission of John Wiley and Sons...

All of the forging heat treat combinations tested in this study cyclically softened. Most of the stress reduction occurred early In the test. For relatively short lives in low-cyde fatigue, the load never completely stabilizes. 

Figure 6.3 (a) Variation of normalized stress during cyclic loading at different temperatures (steel grade 91, variation in the total applied strain AE = 1%, elastic shear modulus dependent on the temperature, T p) [42] (b) comparison of cyclic softening measured on different steels with different generations ( 0.35%, 550°C, strain rate 2 x 10 s ) [43]. 

Significant cyclic softening is demonstrated even for cyclic amplimdes corresponding to significantly lower stresses than the conventional yield stress. No saturation is... 

For components subjected to cyclic thermal transients, for example, due to start ups and shut downs, such as those of sodium-cooled fast reactors, the prevention of creep-fatigue failure is one of the most important points for a 60-year design. Conventional ferritic-martensitic steels show lower thermal expansion coefficients, which is advantageous in terms of thermal stresses. However, unlike conventional austenitic stainless steels, Grade 91 steels are a cyclic softening material, and this point is to be taken into account appropriately in the evaluation of creep-fatigue. 

B. Fournier, M. Sauzay, A. Renault, F. Barcelo, A. Pineau, Microstructural evolutions and cyclic softening of 9%Cr martensitic steels, J. Nucl. Mater. 386—388 (2009) 71—74. 

Fig. 1.20 a Cyclic hardening of a crystal of annealed copper subjected at —75 °C to ten strain controlled cycles [26], b cyclic softening of copper initially hardened by cold-working [26]... 

Fig. 1.23 Cyclic softening found by Laird [29] conducting yield strength measurements on cold-worked nickel at different number of cycles...

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