Constrained shape recovery

Figure 3.9 Setup used for isothermal stress-strain testing and constrained shape recovery testing. The fixture provided a 1-D external confinement and the furnace was used to trigger the shape memory effect by heating the specimen above its Tg. The MTS machine was used to record the resuiting recovery stress...

Programming Using the 3-D Stress Condition and Constrained Shape Recovery... 

Figure 6.9 Schematic of an SMP with a crack to he closed hy constrained shape recovery...

Mechanism for Confined shape recovery by Constrained shape recovery by resisting... 

Figure 7.39 Effect of fiber length on crack closure during constrained shape recovery of short programmed SMPFs embedded in composite beams. Source [14] Reproduced with permission fiom Elsevier...

Figure 3.12 Schematic of classical tension programnting (steps 1 to 3), free shape recovery (step 4), fuUy constrained stress recovery (step 5), and schematic of molecular mechanisms for shape fixing and shape recovery...

Figure 5.31 Schematic of the alignment change of amorphous molecules during free shape recovery and fiiUy constrained stress recovery.

Figure 6.8 Schematic of further crack opening during shape recovery (crack closing) process of tension programmed SMP rod specimen with constrained houndary condition...

The CTH approach is able to control the crack width, which can be closed by controlling the compression pre-strain level. A simple equation has been established by Li et al. [54] to correlate the crack width to be closed and the pre-strain level during compression programming. Cracks with different opening widths can be closed based on the level of compression programming (of course it is limited by the maximum allowed compression pre-strain level). In the unconstrained shape recovery approach such as the SMASH approach, it has not been demonstrated that it can close a wide-opened crack with constrained boundary. Also, it cannot control the crack width that can be closed because it depends on the external load to perform programming and relies on the shape recovery ratio and boundary condition of specimens to perform crack closing. 

In this beam model (length 2L), the two ends of the beam are elamped with a central crack of width IW. Now we want to narrow this crack using SM material. Let us insert an SM rod of length IW with the same cross-section as the beam, and tightly glue it to the fracmred two half beams. Assuming the SM material is pre-stretched so that it has a fully constrained recovery stress of Go (maximum recovery stress) and free shape recovery strain of (maximum... 

Figure 5.12 Thermomechanical behavior of SMPFs by both cold and hot tension programmings, (a) Stress-strain-time diagram for Sample 2. Steps 1 to 5 complete programming and Step 6 completes stress recovery, where step 1 is to stretch the fiber bundle to 100% strain at a rate of200 ram/min at 100 °C step 2 is to hold the strain constant for 1 hour step 3 is to cool the fiber to room temperature slowly while holding the pre-strain constant step 4 is to release the fiber bundle from tbe fixture (unloading) step 5 is to relax the fiber in the stress-free condition until the shape is fixed and step 6 is to recover the fiber at 150 °C in the fully constrained condition (adapted from Reference [20]) (b) Stress-strain-time diagram for Sample 3. Steps 1-4 complete programming and step 5 completes stress recovery, where step 1 is to stretch the fiber bundle to 100% strain at a rate of 200 mm/min at room temperature step 2 is to hold the strain constant for 1 hour step 3 is to release the fiber bundle from fixtures (unloading) step 4 is to relax the fiber in the stress-free condition until the shape is fixed and step 5 is to recover the fiber at 150 °C in the fully constrained condition (adapted from Reference [20]) (c) Stress evolution with time for Sample 2 (d) Stress evolution with time for Sample 3.

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