Shape recovery ratio

Li et al. reported that immiscible high-density polyethylene (HDPE)/ poly(ethylene terephthalate) (PET) blends, prepared by means of melt extrusion with ethylene-butyl acrylate-glycidyl methacrylate (EBAGMA) terpoly-mer as a reactive compatibilizer, can exhibit shape memory effects [32]. They observed that the compatibilized blends showed improved shape memory effects along with better mechanical properties as compared to the simple binary blends. In the blend, HDPE acts as a reversible phase, and the response temperature in the shape recovery process is determined by of HDPE. The shape-recovery ratio of the 90/10/5 HDPE/PET/EBAGMA blend reached nearly 100%. Similar behavior was observed for immiscible HDPE/ nylon 6 blends [33]. The addition of maleated polyethylene-octene copolymer (POE-g-MAH) increases compatibility and phase-interfacial adhesion between HDPE and nylon 6, and shape memory property was improved. The shape recovery rate of HDPE/nylon 6/POE-g-MAH (80/20/10) blend is 96.5% when the stretch ratio is 75%. 

Additionally, as shown in Figure 3.24 (c), the horizontal and vertical strains after 10 thermomechanical cycles are close to the strain evolution of the first thermomechanical cycle. The shape fixity ratio is 98.5% and the shape recovery ratio is 88.3%. It is noted that both the shape fixity ratio and shape recovery ratio are slightly lower than those in the first thermomechanical cycle (99.2% and 91.6%, respectively) under the same pre-stress level (300.7 kPa). This is because more unrecoverable damages have aeeumulated during eaeh... 

The effect of the pre-strain level on the thermomechanical behavior can be evaluated by the shape fixity and shape recovery ratios. In this study, the shape fixity and shape recovery ratios are determined using the following equations ... 

Table 3.4 Shape fixity and shape recovery ratios in each direction ...

Shape fixity ratio (%) Shape recovery ratio (%) Shape fixity ratio (%) Shape recovery ratio (%) Shape recovery ratio (%)... 

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. 

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

It is noted that this definition for the shape recovery ratio is different from the definition by Behl and Lendlein [111]. In their definition, the shape recovery ratio for the first thermomechanical cycle is... 

In a manner similar to the shape fixity and the shape recovery ratio, we can define the stress fixity and the stress recovery ratio. From Figure 5.28, the temporarily fixed stress can be obtained by subtracting the springback stress from the total stress (peak stress). Once the... 

The CTH approach is able to control the crack width, which can be closed by controlling the compression pre-strain level. A simple equation has been established by Li et al. [54] to correlate the crack width to be closed and the pre-strain level during compression programming. Cracks with different opening widths can be closed based on the level of compression programming (of course it is limited by the maximum allowed compression pre-strain level). In the unconstrained shape recovery approach such as the SMASH approach, it has not been demonstrated that it can close a wide-opened crack with constrained boundary. Also, it cannot control the crack width that can be closed because it depends on the external load to perform programming and relies on the shape recovery ratio and boundary condition of specimens to perform crack closing. 

It is interesting to note that, while the CP content has a significant effect on the stress recovery, its effect on shape recovery is comparatively small. The reason is that shape recovery is basically a global or macroscopic behavior, while stress recovery is dependent more on the microstmeture and internal parameters. For instance, the recovery stress depends on the stiffness at the recovery temperature, while the recovery strain does not. Therefore, the shape recovery ratio and the stress recovery ratio usually do not have the same value and the stress recovery ratio is usually lower than the strain recovery ratio. In this sense, the stress recovery ratio is a more rigorous indicator of shape memory functionality. 

Again, here we notice the considerable difference between the stress recovery ratio and the strain (shape) recovery ratio. The reason is that shape recovery is a macroscopic measurement... 

Shape memory (SM) properties are typically quantified by the shape fixity (ffj) and shape recovery ratios (/ ,). means the extent of fixing of the externally applied deformation in the temporary shape. Its value is 100% when the applied deformation, introduced above is fully kept below in the temporary... 

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