Strain fixity

Strained silicon wafers, in scaling to deep submicron dimensions, 22 256 Strainers, in refrigeration systems, 27 539 Strain fixity rate (R ), in testing shape-memory polymers, 22 361... 

Cross-linked PUs with shape memory properties were prepared by Galia, Meier et al. using linear polyols synthesized by ADMET [140]. In this work, ADMET of a 10-undecenoic acid-derived a,co-diene containing a hydroxyl group was performed in the presence of 0.1 mol% of C4. 10-Undecenol was used as chain stopper, and the mixture of oligomers and diols (from 10-undecenol SM) obtained was cross-linked with MDI. The PUs obtained displayed outstanding values of strain fixity and recovery. 

PTBA also exhibits excellent strain fixity (ability to retain its actuated shape upon cooling) and strain recovery. In its softened state PTBA also possesses excellent actuation properties with a breakdown field strength in excess of 250 MV/m, a maximum strain of 335% in area, a maximum actuation stress of 3.2 MPa and an energy density of 1.2 J cm , values that rival even the best of the conventional dielectric elastomer materials. The BSEP is the first active material that possesses bistable actuation with high strain and specific power density. 

The strain fixity rate R describes the ability of the switching segment to fix the mechanical deformation which has been applied during the programming process. It describes how exactly the sample can be fixed in the stretched shape after a deformation to m- The resulting temporary shape u(.N) always differs from the shape achieved by deformation. 

The strain fixity rate Rf is given by the ratio of the strain in the stress-free state after the programming step in the Nth cycle, and the maximum strain m... 

Strain fixity representing the capability of material to keep its temporary shape when it has been deformed and cooled and the stress released. 

Lendlein and co-workers (2005a) synthesized two different configurations with cinnamic add in the matrix, grafting these molecules and producing an interpenetrating polymer network. By this method they obtained excellent values for strain recovery, around 98%. However, the strain fixity had small values, 52% in the best case. 

Li et al. (2007) also described a type of supramolecular shape memory elastomer that contains reversibly associating side groups, namely ureidopyridinone (UPy). In addition to its excellent shape memory effects (SMEs) - about 90% strain fixity and 100% strain recovery - a unique feature of this new SMP is the dynamies of its shape-memory response. More recently, Zhu et al. (2009) synthesized a novel supramolecular polyurethane for utiUzation as a SMP, by grafting the UPy nnit to the elastic polyurethane. Supramolecular SMPs based on o-cyclodextrin (a-CD) network through the inclusion of either PEG or polycaprolactone (PCL) with a-CD. In this system, the a-CD-PEG inclusion crystalhtes and a-CD-PCL crystallites serve as a fixing phase, while the naked PEG crystallites and PCL crystallites act as a reversible phase (Luo et al., 2008 Zhang et al., 2008). Fan... 

In the model, in order to describe the frozen stress and its activation in SMP, a linear viscoelastic theory was used as the first trial. Since the linear theory is limited to small deformations, subsequently a nonlinear viscoelastic theory was adopted for large deformations. The linear viscoelastic model was found to predict the characteristics of SMPs, especially the strain fixity and recovery properties for small deformations with some discrepancy between the experimental and calculated values. The main source for the error was found to be the reduced rigidity of SMPs due to the thermal treatment. This should be avoided for better shape memory performance of SMPs. 

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). 

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 ... 

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