Shape fixity

Other measurable features in SMPs relate to the ability of the polymer to fix the imposed strain when subjected to deformation after cooling and offloading, otherwise known as shape fixity (Tobushi et al., 1998). Another relates to the ability of the polymer to recover from the collated strain during deformation after reheating to its former rubbery state. This is often referred to as shape recovery (Hu, 2007 Tobushi et al., 1998). Further elaborations on these important parameters are beyond the scope of this chapter and readers are referred to the targeted literature (Liu et al., 2007 Kang and Nho, 2001 Adler et al., 1991 Tobushi et al., 1998). 

Tobushi, H., Hashimoto, T., Ito, N., Hayashi, S., Yamada, E., 01/1998. Shape fixity and shape recovery in a film of shape memory polymer of polyurethane series. Journal of Intelligent Material Systems and Structures 9 (2), 127—136. http //dx.doi.org/10.1177/... 

Table 3.1 Shape fixity and recovery ratios for the pure SMP and SMP based syntactic foam under stress-controlled programming and free shape recovery at 79 °C...

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... 

With 10% pre-strain, which is about 3% higher than the yield strain, a tendency similar to 30% pre-strain is observed. Therefore, as long as the pre-strain is over the yield strain, a certain amount of shape fixity can be realized. Of course, as the pre-strain increases, the shape fixity ratio also increases. For example, at the zero stress relaxation time, the shape fixity is about 62.5% for the 10% pre-strain level, which is lower than the corresponding shape fixity of 73% for the 30% pre-strain level. It is also observed that the shape fixity with 10% pre-strain plateaus earlier than that with 30% pre-strain as the stress relaxation time increases, possibly due to less viscoelastic and viscoplastic deformation with the lower pre-strain level. 

In summary, the test results show that cold compression is an effective and efficient method for programming. It is found that the pre-strain level must be larger than the yielding strain of the SMP in order to fix a temporary shape at temperatures below Tg. It is also found that a longer stress relaxation time leads to a larger shape fixity ratio. The upper bound of the shape fixity is determined by the differcnee between the pre-strain and the springback, which is the ratio of the relaxed stress over the relaxed modulus. 

The strain evolution with time during the material programming process can be observed in Figure 3.36. A decent shape fixity ratio (70.5% for 20% pre-strain and 72.6% for 30%... 

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 (%)... 

From Figure 5.28, the shape fixity ratio can be defined as the residual strain over the prestrain ... 

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... 

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