Shear strain recovery

The third curve in the figure, denoted 7e, gives the amount of shear strain recovery subsequent to a removal of the external torque. There is an interesting observation The linear increase of % with 7 at low shear rates is exactly... 

Fig. 15. Measured shear strain during creep under a constant shear stress and viscoelastic recovery after cessation of shear for PDMS near the gel point [71] plotted against the time. The solid lines are predicted by the gel equation for finite strain...

The rheological properties of gum and carbon black compounds of an ethylene-propylene terpolymer elastomer have been investigated at very low shear stresses and shear rates, using a sandwich rheometer [50]. Emphasis was given to measurements of creep and strain recovery at low stresses, at carbon black flller contents ranging between 20 and 50% by volume. The EPDM-carbon black compounds did not exhibit a zero shear rate viscosity, which tended towards in-Anity at zero shear stress or at a finite shear stress (Fig. 13). This was explained... 

Experimentally, as indicated in Fig. 12.13, we find that D/Dq depends on the shear stress at the wall xw (a flow variable) and the molecular weight distribution (MWD) (a structural variable) (22). The length-to-diameter ratio of the capillary (a geometric variable) also influences D/Dq. The swelling ratio at constant xw decreases exponentially with increasing L/Dq and becomes constant for L/Dq > 30. The reason for this decrease can be explained qualitatively as follows. Extrudate swelling is related to the ability of polymer melts and solutions to undergo delayed elastic strain recovery, as discussed in... 

Consider the application of a constant shear stress ao to a viscoelastie solid at t = 0 in simple shear (or alternatively in tension as pure shear). The time-dependent response is ideally an instantaneous elastic flexure followed by time-dependent creep as depicted schematically in Fig. 5.1(a), which at a monotonically decreasing rate asymptotieally approaches a constant shear strain proportional to the applied shear stress. Removal of the shear stress at any time t results in an instantaneous elastic recovery followed by a reverse creep response that asymptotically returns the solid to its initial state, as also depicted in Fig. 5.1(a). 

With constant stress, G t) = Gy, where creep strain y t) is constant [y(t) = Gq/G] for a Hookean solid. It would be directly proportional to time for a Newtonian liquid [(y(0 = Go/r])t]. Here t is the initial time at which recovery of the viscoelastic material begins. For a viscoelastic fluid, when stress is applied, there is a period of creep that is followed by a period of recovery. For such liquids, strains return back toward zero and finally reach an equilibrium at some smaller total strain. For the viscoelastic liquid in the creep phase, the strain starts at some small value, but builds up rapidly at a decreasing rate until a steady state is reached. After that the strain simply increases linearly with time. During this linear range, the ratio of shear strain to shear stress is a function of time alone. This is shear creep compliance, J t) The equation of shear creep compliance can be written as follows ... 

By 10% shear strain or before, the flow stress has reached a steady-state value in other words, the specimen is creeping at the imposed strain rate under the resulting flow stress - that is, the work-hardening and recovery rates are equal. 

Oscillatory shear rheology of EPDM plasticized with resol was used to determine an equilibrium shear modulus Ge), relaxation in compression and strain recovery. Ge was analysed with consideration of crosslink density and permanent entanglements, including evaluation of plasticizer and soluble polymer fraction. Relaxation data were modelled with the empirical Chasset-Thirion equation and it was proposed that longer relaxation times were associated with chains pendant from the network. Relaxation times increased with crosslink density. When the crosslink density was low and pendant chains were longer and more numerous, relaxation times were increased and elastic recovery diminished. ... 

An alternative expression relates die swell to the recoverable shear strain at the die wall (y ) which may be measured via creep experiments and the creep recovery function [4] ... 

Figure 7 Dynamic shear moduli recovery for a 19.8 VB wt.% composite with even fabric distribution heated to 250°C at heating/cooling rates of 3.6 C/min (upper plot) and 11.1 °C/min (lower plot). Shown are data for G (— —) and G (— —) during heating and G (— —) and G (—O—) during cooling. Frequency = 10 rad/s. Strain =1%.

The creep test was performed to determine the viscoelastic behavior under static conditions. A constant stress (0.3 Pa) was applied for 1 min and the resulting strain was measured (creep), then the stress was released and the strain was measured for 1 min (recovery). The shear creep compliance, J (reciprocal pascals), was calculated by dividing the measured shear strain, y, by the applied stress, r (pascals). 

Theoretical analyses of titis phenomenon, for flow in roimd capU-laries, are available [41-45] in which the most basic [44] of them is built upon the free recovery calculations set down by Lodge [13] using the theory of Berstein, Kearsley and Zapas [46]. The developed expression for die swell S in which the elastic strain recovery Sr is balanced by the shear stresses arising in the die, is given by,... 

In an experiment which combines stress relaxation and recovery, sudden shear strain 70 is applied to a viscoelastic liquid at time t = ti, and stress relaxation is observed until a time r = 0 at which there is still some residual stress. Then the stress is removed and the decreasing strain is observed as a function of time. It is given by 5... 

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