Recovered strain

These are commonly presented graphically as fractional recovered strain against log (reduced time) curves as typified in Figure 9.11. In the case of the acetal... 

However, since the stress was removed after 100 hours the recovered strain after a further 100 hours will be the same as sdT), i.e. 0.747%. Thus the... 

Requires IVF to recover strain Requires appropriate oocyte donor strain Some strains and mutations adversely affect IVF success. Offspring will be heterozygotes... 

Shape memory polymers are defined by their ability to store and recover strains when subjected to a particular thermo-mechanical cycle. Shape-memory polymers can recover their original shape by being heated above their transition temperature, which are defined by different phases in the materials. In particular, the shape-... 

When the stress is suddenly removed at D, the pattern consists of an elastic recovery (D-E) followed by a retarded elastic recovery (E-F). Because bonds between structural units are broken in region C-D, a part of the structure is not recovered. For values of time after the applied stress is removed, the recovered strain approaches a maximum value, 5r, called recoverable shear (Dealy, 1982) ... 

When plastics are unloaded, the creep strain is recoverable. This contrasts with metals, where creep strains are permanent. The Voigt linear viscoelastic model predicts that creep strains are 100% recoverable. The fractional recovered strain is defined as 1 — e/cmax, where e is the strain during recovery and Cmax is the strain at the end of the creep period. It exceeds 0.8 when the recovery time is equal to the creep time. Figure 7.9 shows that recovery is quicker for low Cmax and short creep times, i.e. when the creep approaches linear viscoelastic behaviour. 

Fractional recovered strain vs. reduced time (= recovery time/creep time) for acetal copolymer at 20 °C and 65% humidity. The data from different creep times and stresses do not superimpose (from Thermoplastics and Mechanical Engineering Design, ICI Plastics Division, booklet GI 17). 

Pressure-cast bodies of ceramics also have problems, when they are cast from either a dispersed or a flocculated slurry. As the last portion of the sluny is consolidated, the pressure gradient across the cast becomes zero, so that the total applied pressure is transferred to the cast. Therefore, upon the applied pressure is removed, the cast expands, i.e., undergoes strain recovery, due to the stored elastic energy. However, the nature of the strain recovery is different from that in the die compaction of dry powders, in a way that the strain recovery for the compacts of the pressure casting is time dependent. This time-dependent strain recovery is attributed to the fact that the fluid, either liquid or gas, must flow into the compact to allow the particle network to expand and relieve the stored strain. The magnitude of the recovered strain increases with increasing consolidation pressure nonUnearly, which can be described by the following Hertzian elastic stress-strain relation ... 

The shape recovery ratio is defined as the recovered strain over the fixed strain ... 

Fig. 4.38 Variations of input strain eneigy at 300% elongation E c, and first cycle recovered strain en-eigy with concentration U of urethane groups (expressed as equivalent ure-thanic groups/l(X)g PU), in the case of a PU system based on DBD1-PTHF2ooO"UEG (polymers PU1-PU4) [61]...

Default strain in a cyclic, thermomechanical experiment Recovered strain in a cyclic, thermomechanical experiment Fixed strain after unloading in a cyclic, thermomechanical experiment Stress... 

Figure 15.7. Empirical models of A. density and B. modulus from compression graphed vs. total Si concentration and fraction of APTES-derived Si, and C. recovered strain graphed vs. total Si concentration and BTMSH fraction. Reprinted from [11], Copyright 2009 American Chemical Society.

Here7/j,ff(t, oo) is the total recovered strain after cessation ofcreep for timer under stress viscous deformation in a time interval f and for the linear case would be [Pg.519]

Figure 2.8 Fractional recovered strain vs. reduced time (PBT at 20°C)...

When a load is removed from a metal part, the recovery from strain, if it occurs, is instantaneous. With plastics, much of the strain is recovered reasonably quickly but a proportion is recovered only after a period of time. Thus, recovery, like creep, is time-dependent. Recovery data are provided as fractional recovered strain vs. reduced time (Figure 2.8), where ... 

In this region e(t) is decreasing and this process is known as recovery. If the recovered strain e (t - 6) is defined as the difference between the strain that would have occurred if the initial stress had been maintained and the actual strain then... 

An experiment closely related to creep is creep recovery or constrained recoil. After a creep experiment has proceeded for a time fg, the shear stress is suddenly removed, and the recoil or recovered strain is measured as a function of time. It is often found that more precise measurements can be made in this test than in a creep test. The recovered strain, y, is equal to the strain at time tg minus the strain at times greater than tg, as shown below. 

Note that the strain t( t) at time t is less than that at a smaller time fg, because the recoil process removes some of the previously imposed strain from the sample. We see that the recovered strain depends on both the preceding creep time tg and the duration of time (f- fg) since the stress was removed. Figure 4.8 is a sketch showing the strain as a function of time for a creep and recovery experiment. 

If, at time, tg, the creep experiment has reached a steady-state (constant shear rate) then the recovered strain no longer depends on tg, and it is convenient to set tg equal to zero, thus resetting the clock . This leads to ... 

Figure 10 shows the curves that represent the viscoelastic response at an applied stress of 5 Pa for the three hydrogels obtained at 37 °C, in a creep test followed by recovery. The creep curves comprise three parts the instantaneous strain, the retardation strain, and the viscous strain. When the applied stress is removed, the recovery process starts, and first the instantaneous strain is recovered, then the retardation one, and finally remains the viscous part. The high elasticity of the hydrogels can be observed, where the reached strain after the stress of 5 Pa was applied for 60 s is very high, and the recovered strain represents 52 % from the maximum value reached by the strain in the creep test. 

The DMA was used to conduct the free shrink test. During this test, the length (strain) of the film was measured as a fimction of temperature. A small force of 0.01 N (minimal based upon instrument capability) was apphed to the sample to get rid of wrinkle. After these two tests, the shrink force-temperature profile from the fixed length test and sample recovered strain (length change/initial length)-temperature profile from the free shrink test were point-wise mapped relative to the common temperature for each coordinate pair and displayed on a fictive stress-strain curve. 

---