Fatigue Cycling of Shape Memory Polymer Resin Part 1

Abstract

Shape memory polymers have attracted great interest in recent years for application in reconfigurable structures (for instance morphing aircraft, micro air vehicles, and deployable space structures). However, before such applications can be attempted, the mechanical behavior of the shape memory polymers must be thoroughly understood. The present study represents an assessment of viscoelastic and viscoplastic effects during multiple shape memory cycles of Veriflex-E, an epoxy-based, thermally-triggered shape memory polymer resin. The experimental program is designed to explore the influence of multiple thermomechanical cycles on the shape memory performance of Veriflex-E. The effects of the deformation rate and hold time at elevated temperature on the shape memory behavior are also investigated.

Introduction

Shape memory polymers (SMPs) can change their shape in a predefined way from a locked-in (deformed) shape to their original shape when exposed to an appropriate stimulus, as illustrated with heat as the stimulus in Figure 1. The material begins at state A1 with a relatively high "glassy" modulus. Heat is applied to the sample which causes the modulus to drop by several orders of magnitude to its "rubbery" modulus. While in this high temperature state (B in the figure), the sample is deformed into its new shape (in the present study it is deformed in axial tension as signified in C). The deformed shape is held in place while the sample is cooled back to its "glassy" modulus. Once it is cooled, the sample is in the locked-in state D. When heat is reapplied to the locked-in sample, the material reaches its "rubbery" modulus again and the unconstrained sample returns to its original "memorized" shape at state E. Finally, the sample is cooled to state A2 which is close to state A1. The better the "memory" properties of the SMP, the closer state A2 is to state A1. In the current work, the path-dependent behavior of Veriflex-E is evaluated by means of a shape memory cycle similar to the schematic in Figure 1.


For an ideal shape memory material, the original state is exactly achieved. However, in reality, shape memory materials achieve a state close to their original shape. The shape recovery parameter is a measure of how closely the material returns to that original shape in the shape memory cycle (A2 comparison to A1 in Figure 1). Various researchers have established performance parameters to compare the response of SMP materials. Tobushi et al. [1] defined a set of performance parameters termed strain fixity and strain recovery. The strain recovery term was defined specifically to evaluate material performance throughout multiple shape memory cycles by comparing the material strain following recovery (state A2) in the current cycle to the strain following recovery of the previous cycle. Many researchers have employed Tobushi’s definition. The drawback of this definition is that it does not give a full picture of the material capability. For example, in [1] the permanent strain following the shape memory cycle increases from cycle 1 to 10 (see Fig 5b in [1]), Tobushi’s shape recovery parameter indicates a recovery performance which begins low and increases toward 100% as the cycling is continued. This increase leads the reader to believe that the material recovery is improving while in fact it is simply stabilizing to a constant state of performance which is worse than that in the first shape memory cycle.

Schematic of shape memory cycle with free recovery for heat activated SMPs

Fig. 1 Schematic of shape memory cycle with free recovery for heat activated SMPs

Schmidt et al. [2] examined functional fatigue of Veriflex (a styrene based thermally triggered SMP). They used their own definition of shape recovery which compares the recovered strain to the maximum programmed strain (and therefore the programmed length in addition to the sample original length) and found that the styrene based resin exhibited recovery values between 65 and 85% with a large decay from the first cycle to the 18th. The main experimental setup difference between the current work and Schmidt et al. is that they had no way to directly measure the strain in the sample. So they consider the cross-head displacement to be equivalent to the displacement in the gage section of the sample. While the cross-head displacement can give an approximation of the sample gage displacement, they are not equivalent as variations in the sample shape near the clamped section are evident in the photograph that Schmidt et al. present of a deformed sample (Fig 2b "after programming" [2]). They do not adjust for the grip thermal expansion and contraction while the specimen is cycled through the heating and cooling stages of the shape memory cycle (the grips are heated/cooled along with the specimen). In addition, they did not study the influence of the deformation rate on the shape memory response of the SMP.

Liu et al.[3] also defined a set of performance parameters including a shape fixing parameter and a shape recovery parameter from the displacement at various points in the shape memory cycle. Ratna and Karger-Docsis [4] defined two similar parameters, also measured from similar shape memory cycles, which measure the performance of the material when subjected to multiple cycles. These various pre-defined parameters are useful in comparing the performance of candidate SMP materials.

The present study focuses on the durability performance utilizing the shape fixity and linear shape recovery parameters defined by Tandon et al.[5]. The shape fixity, Rf, is defined as

tmp10-183_thumb

Where sp is the prescribed axial strain (at point C in Figure 1) and e„ is the measured strain after unloading to zero force (measured at the end of D in Figure 1) before reheating the sample. The shape recovery, Rr, is defined by Tandon et al. [5] as

tmp10-184_thumb

where Lf is the final gage length after free recovery (measured at state A2 in Figure 1) and Lt is the initial gage length before the shape memory cycle (measured at state Al in Figure 1). For multiple cycles Lt is still the initial lengthbefore the first shape memorv cvcle, such that the length after each cvcle is compared back to the original gage length. Equation (2) can be rearranged in terms of the final strain after free recover

tmp10-185_thumb

as follows:

tmp10-186_thumb

Material and experimental arrangements

The material studied was Veriflex-E, a two-part epoxy-based shape memory polymer (SMP) resin system manufactured by Cornerstone Research Group, Inc. (CRG). The material was purchased from CRG designed with a glass transition temperature (Tg) of 105 °C. The panel fabrication, cure, and post-cure were conducted following the details discussed by the authors in [6],

The uniaxial tensile tests were conducted on an MTS machine surrounded by an MTS 651 environmental chamber and an optical window for making observations inside the chamber. An 890 N water-cooled load cell and MTS Advantage 200 N pneumatic grips were used. An MTS FlexTest 40 digital controller was employed for input signal generation and data acquisition. Strain control was accomplished using an MTS laser extensometer with a 25.4-mm gauge length to the retro-reflective tape applied to each sample.

For elevated temperature testing, type K thermocouples were attached to test specimens using Kapton tape to verify the furnace calibration on a periodic basis. The temperature controller (using a non-contacting thermocouple exposed to the ambient environment near the test specimen) was adjusted to determine the power setting needed to achieve the desired temperature of the test specimen. The determined power setting was then used in the actual tests. Thermocouples were not attached to the test specimens after the environmental chamber was calibrated. A standard temperature ramp rate of 2.5 °C/min was used for heating/cooling the chamber to the desired temperature with a 60 min soak time to ensure thermal equilibrium.

Experimental observations

Shape memory cycle—baseline shape memory response

The experimental program is founded on the shape memory cycle illustrated in Figure 1 conducted in the MTS tensile load frame. The shape memory cycle begins with heating the sample from 25 °C to 130 °C at 2.5 °C/min followed by a 60 min soak time at 130 °C. The sample is then deformed in actuator displacement control at rates of 0.5, 5, or 50 mm/min until the sample is strained to 60% (measured by the laser extensometer). At this point, the control mode is instantaneously switched to laser displacement control, and the sample is held at a constant 60% strain for hold times of 5, 30, or 60 min. Next, the sample is cooled at 2.5 °C/min to 25 °C while the strain is still held constant at 60%\ after which the sample is held (at constant temperature and strain) for 60 min to ensure thermal equilibrium in the sample. After this hold, the bottom grip is released from the sample, and free recovery is conducted on the sample while still held in the top grip to enable continued strain measurements with the laser extensometer. The free recovery consists of heating the sample (again at 2.5 °C/min) to 130 °C, holding the temperature for 60 min, cooling the sample to 25 °C (at 2.5 °C/min), and finally holding the sample at 25 °C for 60 min to ensure thermal equilibrium. This procedure is also laid out step-by-step in Table 1.

Table 1 Steps in shape memory cycle experiment

State in Figure 1

State in Figures 2 and 3

Temperature

Force

Cross-head displacement

Strain

AltoB

Heat at 2.5 °C/min from 25 °C to 130 °C

Hold at zero

B

Hold at 130 °C for 60 min

Hold at zero

BtoC

1 to 2

Hold at 130 °C

Increase to 60% strain at 0.5, 5, or 50 mm/min

C

2 to 3

Hold at 130 °C

Hold at 60% strain for 5, 30, or 60 min

CtoD

3 to 4

Cool at 2.5 °C/min from 130 °C to 25 °C

Hold at 60% strain1

D

4 to 5

Hold at 25 °C for 60 min

Hold at 60% strain1

5 to 6

Hold at 25 °C

Decrease at 50 mm/min until force in sample is zero

Unclamp bottom grip from sample

DtoE

6 to 7

Heat at 2.5 °C/min from 25 °C to 130 °C

No force applied, only top of sample is gripped

E

7 to 8

Hold at 130 °C for 60 min

No force applied, only top of sample is gripped

Eto F

Cool at 2.5 °C/min from 130 °C to 25 °C

No force applied, only top of sample is gripped

F

Hold at 25 °C for 60 min

No force applied, only top of sample is gripped

The baseline shape memory cycle was conducted at a displacement rate of 50 mm/min with a hold time of 5 min. The temperature, strain, and stress are plotted versus time in Figure 2. The results from point 1 to point 8 are also shown in the temperature-stress-strain space in Figure 3. Three samples were tested with each showing similar results. Using the shape fixity and shape recovery definitions in equations (1) and (2) the average values during the first cycle were calculated as R/ = 98.8% (standard deviation 0.37%) and Rr = 98.3% (standard deviation 0.45%), respectively. In addition, the maximum stress, CTmax, during the cycle (achieved while locking in the 60% strain from step 4 to 5) was measured and the average value was 8.01 MPa (standard deviation 0.58 MPa).

Fatigue cycling shape memory response—influence of repeated cycles

To evaluate the fatigue effects on the shape memory performance of Veriflex-E, the cycle described in Figures 1 to 3 (hold times of 5 min and deformation rates of 50 mm/min) was performed multiple times per sample on three samples. The total strain sp is kept constant for each cycle at 60% with respect to the original gage length. One sample failed during cycle 7, the other two samples were subjected to 10 cycles and remained unbroken. The shape fixity (Rj), shape recovery {Rr), and maximum stress (amax) are shown for each cycle in Figure 4. Note that the averages as well as the individual values are displayed in the figure. The shape fixity of the material remains quite excellent with values of Rf near 100% throughout the ten cycles. The shape recovery ability however degrades with repeated cycling. The average Rr begins at 98.3% drops to 94.7% by cycle three. For cycles three to ten the shape recovery continues to drop but in slower increments, falling to 91.5% by the tenth cycle. (This continued drop is similar to the response that Schmidt et al. [2] reported for the styrene based Veriflex.) In addition, the maximum stress builds with each cycle beginning at an average CTmax=8.0 MPa and climbing to 11.7 MPa by cycle ten.

Baseline shape memory cycle with free recovery. The circled numbers correspond to the second column in Table 1.

Fig. 2 Baseline shape memory cycle with free recovery. The circled numbers correspond to the second column in Table 1.

Baseline shape memory cycle in temperature-stress-strain space. The numbers correspond to the second column in Table 1.

Fig. 3 Baseline shape memory cycle in temperature-stress-strain space. The numbers correspond to the second column in Table 1.

 Shape fixity, shape recovery, and maximum stress for ten shape memory cycles (deformation rate 50 mm/min followed by 5 min hold time)

Fig. 4 Shape fixity, shape recovery, and maximum stress for ten shape memory cycles (deformation rate 50 mm/min followed by 5 min hold time)

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