Shape fixity ratio

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%... 

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 ... 

Shape-memory properties can be quantified in cyclic, stimuli-specific mechanical tests [23,40]. Each cycle consists of the SMPC and the recovery of the original, permanent shape. From the data obtained, the shape fixity ratio (Rf) and the shape recovery ratio (/ r) can be determined (see, e.g., [40-42] and Chapter Characterization Methods for Shape-Memory Polymers in this volume). Rf describes the ability of the switching segment to fix a mechanical deformation, e.g., an elongation to applied during SMCP resulting in the temporary shape. Rr quantifies the ability of the material to memorize its permanent shape. Different test protocols have been developed. They differ in SMCP, which can be performed under constant strain or constant stress conditions (see Chapter Characterization Methods for Shape-Memory Polymers in this volume). The recovery process under stress-free condition enables the determination of the switching temperature Tsw for thermally-induced SMP. 

These fibers exhibited also excellent shape fixity ratios. The fiber then shrank or recovered to its original permanent shape substantially by heating. Cyclic, thermomechanical experiments were performed to characterize the thermally-induced... 

The shape fixity ratio Rf can be determined for quantification of the effect of programnfing. Rf describes the ability to fix the mechanical deformation, which has been appUed during the programming process, i.e., Rf is equal to the amplitude ratio of the fixed deformation to the total deformation (see Fig. 8) [4],... 

Cyclic, photomechanical experiments allow - similar to thermomechanical experiments - the determination of the shape fixity ratio Rt N) of the Vth cycle as well as the strain recovery ratio Rr (N). Polymers from the IPN polymer system (permanent network formed from n-butyl acrylate with 3.0 wt% polyCpropylene glycol)-dimethacrylate as crosslinker) showed R of 20-33% and Rt of more than 88%. When the cyclic, photomechanical experiment was performed at lower jnax (such as 20%), higher values for Rt were obtained. 

Ri, achieved during the programming module was quantified by the shape fixity ratios after creation of shape B (C —> B) or shape A (B A) ... 

In fibre form, SMP is more easily applied in textiles. In comparison with shape memory alloy threads, SMP flbres have several advantages like soft feel, no protrusion, larger elongation and greater weaving ability. The SMP fibres have better compatibility with human bodies as a result of their polymeric nature. They can give a look and feel similar to conventional fabrics. SMP flbres are much cheaper than shape memory alloy threads. At present, SMP flbres produced in the lab can have tailorable switch temperature (5—60°C) and tailorable shape fixity ratios (10—90%). A plant scale of the SMP flbres is underway. 

The cyclic strain-stress curves of fiber obtained by thermo-mechanical cyclic tensile testing are shown in Fig. 9.14. The fiber has a fixity ratio of more than 85.8% and a recovery ratio of more than 95.4%. The detailed shape fixity ratios and recovery ratios are recorded in Table 9.3. At 60°C, in the first cycle, the maximum stress at full elongation is 0.07 cN/dtex. 

Table 9.3 Detailed shape fixity ratios and recovery ratios of fiber...

Table 11.14 shows the shape fixity and shape recovery ratios of the specimens after thermal-hnmidity treatments for 100 and 190 horns, respectively. The shape fixity ratios for specimens that are treated below 50°C are 91.4 to 92.5%, which are close to the 92.4% of the imtreated specimen. After the treatmerrts at 80°C, the shape fixity ratios rednce to 89 to 90% (except for specimen h-190). Frrrthermore, the shape recovery ratios increase from 79 to 81-84% after the thermal-hrrmidity treatments. 

Some holes appear on the surface of fibers that are treated at 80°C (Figs 11.20 to 11.24), and this distortion is one of the reasons for the reduction of shape fixity ratios in specimens after a high temperature treatment. When this is combined with the FT-IR results, it supports the argument that high temperature and humidity conditions will increase the decomposition rate of PB A-based fibers, and reduce shape fixity ratios. 

As indicated by the shape memory test results, when the concentration of solvent increases, the shape fixity ratio increases rapidly from about 70 to 90%, while the fixity ratio remains unchanged until the concentration is increased to 8%. 

Figure 4. Effect of cooling time on shape fixity ratio (upper) and recovery ratio (lower)...

Figure 8. Shape fixity ratios of organoclay and SiC composites...

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