Stress recovery

In the computation of the volume-recovery curves in Figures 4.12 and 4.13, only thermal agitation, or Brownian motion, was assumed for the driving force for recovery. Stress field effects were not taken into account. However, the release of pressure from the densified glass is equivalent to an application of a negative pressure (i.e., an... 

Although SMPs show promising shape-memory effects, there are several limitations affecting their performance and desirability as follows. 1) Slow recovery speed. The rate of shape recovery process is mostly determined by the thermal conductivity of a thermo-responsive SMP. Typical SMPs have very poor thermal conductivity ( 0.3 W m 2) Low recovery stress. The... 

The low recovery stress (<10 MPa) of thermo-responsive SMPs originates from their intrinsically low modulus, in the order of 0.1-1 GPa below the thermal transition temperature. CNTs have proven to enhance the mechanical properties, particularly modulus, of various polymers. ... 

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. ... 

Figure 3.9 Setup used for isothermal stress-strain testing and constrained shape recovery testing. The fixture provided a 1-D external confinement and the furnace was used to trigger the shape memory effect by heating the specimen above its Tg. The MTS machine was used to record the resuiting recovery stress...

Since the programming is the same for the free recovery specimens and fully constrained specimens, the focus will be on step 4 of the thermomechanical cycles. The stress-temperature behavior under a fully constrained recovery condition is shown in Figure 3.19 for the two programming stresses (47 kPa and 263 kPa). The recovery stress-time behavior of the foam programmed at 47kPa pre-stress is also highlighted by the inset in Figure 3.19. The recovery stress comes from two parts thermal expansion stress and entropically stored stress or back stress. Since this is a 1-D fully constrained recovery, the thermal stress can be calculated as... 

Figure 3.50 Programming (step 1 loading—> step 2 cooling—> step 3 unloading) for the foam and recovery stress-temperature response (step 4) under fully constrained conditions for the programmed foams exposed to UV alone and simultaneously immersed in saltwater and rainwater. Source [84] Reproduced with permission from Elsevier...

It is noted that while the majority of constitutive modeling focuses on thermally induced dual-shape memory behavior, triple-shape and multishape SMPs have been developed recently and they call for constimtive modeling [1]. In addition, the effect of programming temperature and strain rate on the constimtive behavior also needs modeling [2]. Furthermore, some recent smdies have found that while the shape recovery ratio can be 100%, other mechanical properties such as recovery stress or modulus become smaller and smaller as the thermomechanical cycles increase, which has been explained by the shape memory effect in the microscopic scale [24]. Obviously, these new findings also call for constitutive modeling. 

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