Strain recovery various temperatures

Further detailed aspects of the viscoelastic behavior of amorphous copolymers that have been reported includes linear stress-relaxation behavior (49) and creep properties in the glass-transition region (50). Chen and co-workers (51) have also reported the large strain-stress relaxation and strain recovery of copolymers at temperatures above Tg, and found that the observed behavior could be rationalized in terms of various network models. 

Strain recovery curves of PUPyBD053 and PUPyBDOSO at various elongation (150 and 100%) and deformation temperatures (120 and 125°C). 

The role of the stress in embrittlement and stress-corrosion processes has been examined in some detail by employing the slow strain-rate technique . Specimens of alloy 7179-T651 tested in air or in vacuum after pre-exposure to water at 70° C or in water at various potentials at ambient temperature exhibited a reversible embrittlement in excess of that arising from testing in moist air . The embrittlement was attributed to hydrogen absorption, and recovery was thought to be due to loss of hydrogen (particularly under vacuum) or to diffusion to traps. Potentiostatic tests revealed... 

Figure 11.15 (a) Recoverable strain yr versus time for a 52.8% liquid-crystalline hydroxypropy-Icellulose solution in water at room temperature, 22°C. The curves from left to right were obtained by imposing stresses of 50, 125, 250, 500, 1000, and 2000 dyn/cm before recovery. These imposed shear stresses correspond to various values of the prior shear rate yq. (b) The same plotted against yot all curves fall within the indicated region between the curves for 50 and 2000 dyn/cm. (From Larson and Mead 1989, with permission from the-Joumal of Rheology.)... 

For example, Koerner et al. ° have found that 8.5 wt% MWNTs could increase the room temperature rubbery modulus of TPU by a factor of 5. In fact, incorporation of 5 wt% of MWNTs into the TPU matrix results in an increase of the recovery stress by 130%. Similarly, Ni et al. have reported that adding 3.3 wt% MWNTs into the TPU matrix leads to an increase in the recovery stress by 100%. The recent study on mechanical properties of CNT-TPU at various nanotube loadings suggests that the reinforcement mechanism is largely due to the immobilization of TPU soft segments by adsorption on to the nanotube surface, which suppresses the glass transition and increases the stress at low strains. ... 

Compressive creep tests allow measurement of strain as a function of time when a constant stress is applied. These can be conducted at several stress levels for aerogel of various densities. Loads are removed at the end of the creep test, and strains as a function of time are monitored to determine the recovery behavior. Compressive relaxation tests can be conducted at different strain levels. The relaxation functions determined at the same strain level at different temperatures can be shifted horizontally to determine whether a master curve can be formed for use to determine the long-term behavior. Recovery behavior after relaxation can also be characterized by monitoring the stress as a function of time after removing partially the step strain. For aerogels that contain polymers such as X-aerogels... 

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