Cyclic Hardening Softening

SEM observations of the fatigue-fracture zone reveal that plastic deformation has occurred in some micro-areas after cyclic deformation at RT in the nano-crystaUine (100 nm) 3Y-TZP, as seen in Fig. 7.42. This was afiirmed by AFM, as 

It is interesting to observe the curved slip lines on the fractured surface of the nanocrystalhne 3Y-TZP ceramic, where the role played by the dislocations in the plastic deformation is evident. Also note the appearance of the curved slip lines that resemble the microstructures observed in metals (the beach markings see, for example, Polakowski and Ripling). These lines represent hiatuses between working and rest periods during fatigue in metals (see Fig. 7.44). 

The accumulated plastic strains during the cycles were continuously recorded after unloading for the nanocrystalline and the submicron-sized 3Y-TZP ceramics (see Fig. 7.45). This accumulated plastic strain saturated after 100 cycles or so, but was obviously different in nature and extent than that of metal. Such strain accumulation would eventually lead to failure. Under a 50 MPa maximum cyclic-tensile stress, very little strain accumulation was found under fully-cyclic loading 

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). 

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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... 

Cyclic hardening softening Damage L.J MSC — nucleation j formation Macro crack LjJ p pg formation rj, gi re... 

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...

If A r or Ae,i is maintained constant by the test machine, then Ao increases tuid A p decreeises until the hysteresis loop stabilizes at a constant form for a cyclically hardening material, cind vice-versa for a softening alloy [7,37]. The stabilized-loop value of axial Ae is used in the Cof a-Manson Law to correlate Nf. When substantial crack growth occurs, these... 

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-... 

Fig. 1.18 Time distribution in terms of cycles JVy (i = 1, 2, 3, 4) of the four fatigue phases (1) cyclic hardening or softening (2) slips with MSC formation (3) macro crack formation (4) growth of macro crack to failure, at four selected stress amplitude Sj...

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. 1.27 Examples of cyclic hardening or softening in some alloys [32]...

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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