First cycle hysteresis

We indicated all relative quantities by asterisks. Thus we defined the relative first cycle residual strain, and relative first cycle hysteresis energy dissipation by... 

To understand the relative first cycle hysteresis we need to consider the processes of both first loading and unloading. In terms of the description above, its value is expected to be sensitive to the relative importance of the two damage processes identified (I) and (H). Thus its reduction with replacement of DBDI by MDI, or with increasing phase separation (Fig. 4.29 and 4.30) could be caused by increase in contribution from process II, relative to process I, in these cases, presumably because of weaker binding of segments to the hard phase. 

A typical example, from the extensive study by Kamakin on an alumina-silica gel, is shown in Fig. 3.32. When the mercury pressure was reduced to 1 atm at the end of the first cycle, 27 per cent of the intruded mercury was retained by the sample a second intrusion run followed a different path from the first, whereas the second extrusion curve agreed closely with the first. Change in f re structure of the kind described above could perhaps account for the difference between the two intrusion curves, but could not explain the reproducibility of the remainder of the loop. There is no doubt that hysteresis can exist in the absence of structural change. 

Fig. 47 Typical stress-strain curves of PTFE-based chloroprene composites. The first six hysteresis cycles are shown with solid lines. The seventh curve (dotted line) was investigated using an optical method...

The relaxation phenomenon described herein is also called first-cycle effect, secondary break-in, waittime effect, memory effect, and hysteresis phenomenon which is in connection with the characteristic changes of the cyclic voltammograms obtained for -> polymer-modified electrodes [iii-vi]. 

Figure 26.16 Effect of plasma treatment on dynamic hysteresis, which is the difference in the d5mamic first cycle advancing line and the receding line, (AF/L)d (mN/m) = (F/L)D,a,i — (F/T)D,r,i IS a direct result of the changing shape of the meniscus during a wetting cycle.

Figure 26.17 Effect of plasma treatment on the intrinsic hysteresis, which is the difference in second cycle immersion line and the first cycle immersion line, (AC/L) = (C/Llu 2 — Fj T)D,a,i intrinsic hysteresis is directly proportional to the extent of surface configuration change of the surface state.

F. 2.18 Swelling and desweUing of thin films of a planar aligned smectic network (a) and a homeotropic smectic network (b) (scale bar corresponds to 500 pm). Swelling depends on the concentration of covalent crosslinker and on the direction with respect to the molecular orientation (c) and shows hysteresis upon cycling during the activation step and the first cycle (d) and repeated cycle (e) Reproduced from Ref. [77] by permission of The Royal Society of Chemistry... 

During cyclic loading, the stress-strain behavior of clays and silts is nonlinear and hyster-etic. For San Francisco Bay silty marine clay, consecutive hysteresis loops obtained for the first cycle of dynamic loading obtained at different controlled strain levels illustrates the stiffness reduction as the enlargement of the loops as they deform and tilt with increasing strain amplitude (Figure 9.20) (Idriss et al., 1978). 

The maximum mechanical hysteresis and residual strain, which were strongly dependent on the hard block content, occurred during the first cycle and the residual elongations and hysteresis decreased with temperature. 

Notable features were the pronounced hysteresis, unrecovered strain and Mullins effect (whereby re-loading follows a stress-strain path closer to the unloading path than the original loading path). From curves such as these we calculated several quantifiers of the inelasticity. Consider the first cycle for material PU1, shown above. It defines three zones A, B and C in the stress-strain diagram. 

First cycle work input, unrecovered strain and hysteresis... 

Fig. 4.29 First cycle relative residual stram on load-ing/unloading to e = 3, plotted versus first cycle relative hysteresis, showing correlation and trends with respect to choice of DI and MD. Symbols as in Fig. 4.24 [135]...

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