Cyclic Hardening and Softening

Studying the fatigue phenomenology, how damage initiates, progresses and accumulates it is of great importance to identify the fundamental steps that 

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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). 

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...

Thus, as indicated above, in the submicron-sized 3Y-TZP ceramic, the stress-induced cyclic hardening, due to transformation taking place, was higher than under static deformation. NanocrystaUine 3Y-TZP softened cyclically, due to the formation of a large number of microcracks. In the submicron structures, this observation basically reflects the effects of dislocations and dislocation-dislocation interactions. In the nanocrystalline 3Y-TZP ceramic, this greater ability to accommodate plastic strain is probably due to grain-boundary sliding, since in nanocrystalline structures dislocations cannot move, because shp distances are on an atomic scale (hke the dimensions of dislocations themselves). 

Cyclic hardening/softening was deduced and cyclic stress-strain curves over wide ranges of plastic strain amplitudes were published in [1.178, 179] for Mo, in [1.172,180,181] for Ta, and in [1.182] for Nb and... 

Fatigue crack nuclei that precede macro crack formation originate in persistent slip bands well before the final failure. It has been said in 1.4.1 that they start to appear on the surface of the material as soon as the hardening/softening process saturate and, in fact, it has been also said that if the cyclic hardening were periodically removed by annealing the specimen subjected to a fatigue test, the life of the specimen would increase enormously to almost become infinite. Saturation... 

Injection molding is the most important molding method for thermoplastics [7—9]. It is based on the ability of thermoplastic materials to be softened by heat and to harden when cooled. The process thus consists essentially of softening the material in a heated cylinder and injecting it under pressure into the mold cavity, where it hardens by cooling. Each step is carried out in a separate zone of the same apparatus in the cyclic operation. 

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