Cold-drawing programming

Strengthening of SMPFs Through Strain Hardening by Cold-Drawing Programming... 

Figure 5.6 Engineering stress-engineering strain behavior of cyclic cold-drawing programming of SMPF. Source [15] Reproduced with permission from John Wiley Sons, Ltd...

Figure 5.8 First cycle of DSC test results of the SMPF after cold-drawing programming to 100%...

Because the stress applied during cold-drawing programming is usually very large as compared to the classical programming above the transition temperature, it is interesting to find... 

Direct evidence of the increase in stiffness after cold-drawing programming comes from the dynamic mechanical analyzer (DMA) test result. The DMA 2980 tester from TA instruments following the ASTM D 4092 standard was conducted. A dynamic load at 1 Hz was applied to SMPF bundles. Each fiber bundle contained 10 filaments. The diameter of the fiber was 0.04 mm. The temperature was ramped from room temperatures of 25 to 160 °C at a rate of 3 °C/ min. The length of the fiber was set to be 15 mm. The as-spun SMPF and SMPF after three cycles of cold drawing to 250% strain were conducted (see Figure 5.11). It is clear that the... 

From Figure 5.17, it is clear that cold-drawing programmed SMPF (Sample 3) has the highest glass transition temperature and the highest damping ratio, followed by the control... 

Figure 5.28 Schematic of stress-strain behavior of SMPFs in the entire thermomechanical cycle by cold-drawing programming (step 1, loading step 2, holding the strain constant for a while (stress relaxation) step 3, unloading step 4, structural relaxation step 5, fiiUy constrained stress recovery)...

Obviously, the stress recovery ratio is much lower than the strain recovery ratio. The difference is even bigger for cold-drawing programming. In other words, the SMPF has a good memory of strain or shape, but a poor memory of stress or load. Actually, this is trae not only for SMPFs but also for almost all shape memory polymers. On the one hand, this shows the limitations of SMPs in certain applications, such as those that need a large recovery force. On the other hand, this also provides a unique opportunity for researchers to develop SMPs with a higher recovery stress and a higher stress recovery ratio. 

Figure 5.29 Stress-strain behavior of SMPFs during the entire thermomechanical cycle, (a) Sample 2 (hot-drawing programmed sample) step 1, stretch the fiber bundle to 100% strain at a rate of 200 mm/min at 100 °C step 2, hold the strain constant for 1 hour step 3, cool the fiber to room temperature slowly while holding the pre-strain constant step 4, release the fiber bundle from the fixture (unloading) step 5, relax the fiber in the stress free condition until the shape is fixed step 6, recover the fiber at 150 °C in a fully constrained condition (b) Sample 3 (cold-drawing programmed sample) step 1, stretch the fiber bundle to 100% strain at a rate of 200 mm/min at room temperature step 2, hold the strain constant for 1 hour step 3, release the fiber bundle from fixtures (unloading) step 4, relax the fiber in a stress free condition until the shape is fixed step 5, recover the fiber at 150 °C in a fully constrained condition. (Because the curves are black and white, please refer to Figure 5.12 to better differentiate each step).

Figure 7.17 DMA test results of the storage modulus of an as-spun SMPF and SMPF after three-cycle cold drawing programmed to 250% with a diameter of 40 pm. Source [20] Reproduced with permission from the Royal Society...

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