Shape memory programming

Deformation can be performed either above or below [15,16]. When the sample is deformed at T T, only the inelastic component of the strain is simultaneously retained. On the other hand, when the sample is deformed at T , the deformed shape is frozen upon subsequent cooling below 

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Fig. 8.3 shows the shape memory programming to obtain stress variation in an SMP under constraint. Shape A indicates the original shape of the SMP. After stretching the SMP at a higher temperature T > T ), the force (internal stress) will be developed. If the SMP is allowed to relax under constraint at a higher temperature (T > T ), the stress will reduce due to stress relaxation, and finally all the elastic stress remains at a higher temperature in state B. Further cooling (T < T ) leads to decrease in the internal force, and aU the stress will be frozen in the final state B. State B is temporarily fixed with zero or no stress at a lower temperature. If state B is heated under constraint... 

Fig. 22 (a-f) Time series photographs showing recovery of shape-memory tube from start to finish of the process total time 10 s, at 50 °C. The tube was made of PCL-dimethacrylate polymer network that had been programmed to form a flat helix. Reprinted from [323] with permission... 

Programmed temperature vaporization (PVT), gas chromatography, 4 612-613 6 421-422, 423 Programmer consoles, 20 669 Programming, of shape-memory polymer, 22 355-356... 

Macroscopically, the shape memory effect can be described and evaluated using a cyclic tensile deformation experiment under a temperature programming, represented as a three-dimensional (3-D) plot of strain versus temperature and force as depicted in Figure 4.2. The sample is deformed to a certain level of strain at T > and then can be fixed during cooling,... 

Kolesov and Radusch prepared peroxide cross-linked binary and ternary blend SMPs from high density polyethylene and two ethylene-l-octene copolymers with medium and high degrees of branching [47]. The blends were prepared by a melt mixing and subsequently are cross-linked with 2 wt% of liquid peroxide 2,5-dimethyl-2,5-di-(tertbutylperoxy)-hexane at 190°C. The blends showed multiple shape memory behavior that appeared only at consequent stepwise application of convenient programming strains and temperatures. Obviously, that is caused by multiple melting behavior of these blends with many poorly separated peaks. 

In order to perform programming and recovery effectively, the glass transition temperature (T ) must be determined first as the shape memory effect of thermosetting shape memory polymers revolves around a temperature range centered at the Tg. Below this range the material is rigid or glassy and above it is in a rubbery-elastic state. Recovery must be conducted within or above... 

Figure 3.32 SEM observation of the original microstmcture of the foam (a), the effect of programming on changing the microstmcture ((b), (d), and (e)), and the effect of shape memory on recovering the microstmcture (c). Source [42] Reproduced with permission from Elsevier...

Behavior of Thermoset Shape Memory Polymer Based Syntactic Foam Trained by Hybrid Two-Stage Programming... 

Figure 3.49 Schematic showing the shape memory mechanism involved in the two-stage biaxial programming and recovery. Note the contribution of the compression programming to the further segmental alignment along the tension direction. Source [59] Reproduced with permission from the American Society of Civil Engineers...

Xu, W. and Li, G. (2011) Thermoviscoplastic modeling and testing of shape memory polymer based self-healing syntactic foam programmed at glassy temperature. ASME Journal ( Applied Mechanics, 78, paper 061017 (14 pages). 

Li, G. and Xu, W. (2011) Thermomechanical behavior of thermoset shape memory polymer programmed by cold-compression testing and constitutive modeUng. Journal of the Mechanics and Physics of Solids, 59, 1231-1250. 

Xu, T. and Li, G. (201 l)CycUc stress-strain behavior of shape memory polymer based syntactic foam programmed by 2-D stress condition. Polymer, 52, 4571 580. 

Li, G., King, A., Xu, T., and Huang, X. (2013) Behavior ofthermoset shape memory polymer based syntactic foam sealant trained by hybrid two-stage programming. ASCE Journal of Materials in Civil Engineering, 25, 393-402. 

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. 

Based on this understanding, a mechanism based constitutive model incorporating the nonlinear structural relaxation model into the continuum finite-deformation thermoviscoelastic framework was developed as follows. The aim of this effort was to estabUsh a quantitative understanding of the shape memory behavior of the thermally responsive thermoset SMP programmed at temperamres below Tg. To simplify the formulation, several basic assumptions were made in this study ... 

As discussed in Chapter 3, pseudo-plastic deformation is the key for cold-programmed thermosetting SMP to display shape memory functionality. Therefore, the deformation includes both plastic/viscoplastic and elastic/viscoelastic deformation. The thermomechanical cycle also includes thermal deformation. Based on Figure 4.5, the deformation gradient F can be multiplicatively decomposed into thermal Fj and mechanical Fm, which are further decomposed into plastic F and elastic F, as follows ... 

Thermoviscoplastic Modeling of Cold-Compression Programmed Thermosetting Shape Memory Polymer Syntactic Foam... 

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