- Email: [email protected]
epoxy smart composites
Materials Science and Engineering A 390 (2005) 139–143
Basic design guidelines for SMA/epoxy smart composites Y.J. Zhenga,b,∗ , L.S. Cuia , J. Schrootenb b
a Department of Materials Science and Engineering, University of Petroleum, Beijing 102249, China Department of Metallurgy and Materials Engineering, KULeuven, Kasteelpark Arenberg 44, B-3001 Leuven, Belgium
Received 5 April 2004; received in revised form 26 July 2004
Abstract The actuating ability and reliability of shape memory alloy (SMA) hybrid composites were studied in this paper. Results showed that by selecting small hysteresis SMAs such as TiNiCu alloy, SMA hybrid composites have a linear stress–temperature behavior, which is relatively easy to control. The curing process of the epoxy matrix does not affect significantly the actuating ability of the embedded TiNiCu alloy wires. A moderate prestrain of the TiNiCu wires is preferred when giving attention to both the mechanical properties and the reliability of the TiNiCu hybrid composites. Additional reinforcing fibers with a negative thermal expansion coefficient such as Kevlar fibers are helpful to strengthen the reliability of the interface and enhance the actuating ability of the SMA hybrid composites. Based on the experimental results, some basic guidelines for designing shape memory hybrid composites were proposed. © 2004 Elsevier B.V. All rights reserved. Keywords: Shape memory alloy; Composite; Smart material
1. Introduction Shape memory alloy (SMA) hybrid composites are regarded as promising candidates of the so-called “smart materials”. Investigations in the past decade have demonstrated that the shape memory hybrid composites have many novel or improved properties over the un-reinforced matrix, such as enhanced high-temperature mechanical properties, shape control, fatigue resistance, vibration damping, acoustic radiation and transmission control, and impact resistance abilities [1]. Up to now, however, there is still no report of industrialized shape memory hybrid composites, mostly because of the complex behavior that is still not well understood, and the lack of clear guidelines to optimize the design of shape memory hybrid composites. Investigations concerning the design of shape memory hybrid polymer matrix composites have shown the influence of volume fraction [2], surface treatment [3], prestrain of the shape memory components [2,4], and matrix curing process [5] on the properties of SMA hybrid composites. These contributions enabled the production of some prototypes of smart ∗
Corresponding author. Tel.: +86-10-89733200; fax: +86-10-89733203. E-mail address: [email protected] (Y.J. Zheng).
0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2004.07.063
materials, but are not enough to produce a reliable composite. At this moment the biggest known prototype is a scale model of a tail wing at EADS [6]. In fact, the research done up till now did not look at all design problems at once. For example, a high prestrain is preferred if one needs the composites to have an enhanced ability to close crack tips [7] or a higher yield strength [8]. However, according to our research, a high prestrain does not improve significantly the thermomechanical behavior of shape memory hybrid composites [9], but reduces significantly the quality of the interface and the actuating ability of the composites [4,10]. Probably, the design principles will only be available if the complex behavior of SMA hybrid composites is well understood. Nevertheless, one can still draw some basic design guidelines by taking a comprehensive view on the experimental results. The aim of this paper is to provide some basic guidelines based on experimental results, for designing shape memory hybrid composites.
2. Experimental procedure A ternary TiNi 12 wt.% Cu alloy wire of 0.15 mm diameter was obtained from SMA Inc., USA. Strafil G-EPI-140/142
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Y.J. Zheng et al. / Materials Science and Engineering A 390 (2005) 139–143
glass fiber epoxy prepreg was supplied by Hexcel Composites, and Kevlar 29 fibers reinforced LTM217 epoxy prepreg was supplied by Advanced Composites Group in UK. The glass fiber composites were cured in an autoclave at 413 K for 20 min. The Kevlar fiber composites were cured at 343 K for 12 h, and then post cured at 413 K for 4 h. A frame, designed by EPFL, Switzerland, was used to align the SMA wires with an adjustable, constant spacing, prestrain and to maintain the wires at a constant strain during the curing process. The size of the samples used in this research are 0.3 mm × 10 mm × 150 mm with a volume fraction of TiNiCu wires of 11.8%. The transformation behavior of the SMA wires was determined using a TA 2920 (TA Instruments) differential scanning calorimeter (DSC) with a helium gas DSC cell purge. The DSC composite samples were cut into approximately 5 mm × 5 mm using a low speed diamond saw. The heating/cooling rate during the DSC experiments was 5 K/min. The dimensional change of the composites was measured using a DuPont 943 thermomechanical analyzer (TMA). The stress–temperature behavior of the TiNiCu wire and the TiNiCu composites were investigated by using a fully computerized apparatus that enabled the user to program, control and measure the strain, force and temperature of the sample.
3. Results and discussion 3.1. Selection of SMA components TiNi based alloys are the best SMA family for smart material actuator applications mainly because of their excellent mechanical properties. R phase TiNi binary and TiNiCu ternary alloys are of particular interest because of their relatively small hysteresis. Considering that the R phase TiNi alloy is limited in terms of maximum recovery strain and stress, the TiNiCu ternary alloy is more promising. Fig. 1 shows the recovery stress cycles of a 3% prestrained TiNiCu bare wire and a Kevlar/epoxy composite with em-
Fig. 1. Recovery stress of 3% prestrained TiNiCu bare wire and Kevlar/ epoxy composite with embedded 3% prestrained TiNiCu wire.
bedded 3% prestrained TiNiCu wires. One can see that the TiNiCu bare wire shows a relatively small hysteresis. As a result, the hysteresis of the composite is also quite small. It is widely accepted that the hysteresis mainly comes from the energy dissipation associated with the migration of the B19 B2 interface. Our previous investigations have shown that the reverse transformation of the so-called preferentially oriented martensite (POM) is slow and spread over a large temperature window in a constrained heating process [11,12]. The recovery stress, which originates from the reverse transformation of POM, is generated by consuming only a small fraction of POM. It means that the energy dissipation accompanied with the interface migration during a constrained transformation is also quite small. As a result, the small hysteresis of a TiNiCu wire shows an even smaller hysteresis behavior in a constrained transformation process, as can be seen in Fig. 1. Up to now, there is still no reliable mathematical modeling that can simulate the complex thermomechanical behavior of shape memory materials. However, as can be seen from Fig. 1, one can still use a linear function to simulate the behavior of shape memory hybrid composite if a small hysteresis SMA such as TiNiCu wire is used. 3.2. Impact of the matrix curing process Generally, the manufacturing process of SMA hybrid epoxy composites involves prestraining and maintaining the SMA wires from the very beginning to the end of the autoclave run. To study the effect of the autoclave running temperature on the actuating ability of the SMA component, TiNiCu wires were prestrained and maintained at a constant strain, and then the wires were subject to heating processes, as shown in Fig. 2. The wires were first cycled once between room temperature and 413 K, which is the curing temperature of the prepreg used in this research, and then further cycled two times between room temperature and 383 K. The solid circle on each curve denotes the starting point of the heating
Fig. 2. Effects of thermal cycling on the stress–temperature behaviors of prestrained TiNiCu wire. The wires were prestrained and maintained at a constant strain, cycled up to 413 K once, then further cycled two times up to 383 K.
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process. One can see that the recovery stress curves of the first heating up to 413 K are different to the curves in the following thermal cycles. Seemingly, an autoclave run up to 413 K changes the actuating behavior of the embedded SMA wires. However, according to our previous research, it is an intrinsic characteristic of shape memory alloys that the first constrained thermal cycle is always different from the following cycles, because of the separation of the detwinned martensite into the self-accommodating martensite and the fully detwinned martensite [12]. Thus, the result in Fig. 2 does not prove that an autoclave run up to 413 K changes the actuation behavior of the TiNiCu wire. On the contrary, as can be seen in Fig. 2, the behavior of the wires becomes very stable in the following thermal cycles. This means that the curing process of the thermoset matrix will not destroy the actuating ability of the embedded TiNiCu wires. 3.3. Prestrain level of the SMA wires The effects of the prestrain level on the recovery stress can be seen in Fig. 2. Firstly, the recovery stress rate, dσ/dT, decreases with increasing prestrain level. The recovery stress rate dσ/dT is an important parameter for a SMA actuator. However, the nature of the effect of prestrain level on dσ/dT is not fully understood yet. According to previous research, the recovery stress rate may be related to the chemical composition of the alloy, because in TiNi binary alloys, the recovery stress rate tends to increase with increasing prestrain [13], but in TiNiCu ternary alloys the recovery stress rate tends to decrease with increasing prestrain [4]. It is interesting to note here that, according to our previous findings, prestrain levels do not have much effect on the recovery stress rate of SMA hybrid composite [9]. Thus, if the recovery stress rate of a SMA composite needs to be tailored, the best way is to modify the volume fraction, but not to change the prestrain level of the SMA components. Secondly, one can see from Fig. 2 that, for small prestrains such as 1%, the recovery stress reaches zero after one thermal cycle; but for large prestrains such as 6%, the recovery stress level remains at a relatively high level even after several thermal cycles. This means that a large residual compressive stress remains in the epoxy matrix if the embedded SMA wire has a large prestrain level. This residual compressive stress, on one hand, is helpful for improving the tensile strength of the composite. On the other hand, it will exert a large shear stress at the interface between the SMA wires and the matrix, thus making the interface more susceptible to debonding. Fig. 3 shows the strain–temperature curves of glass fiber composites with 3% prestrained TiNiCu wires. The composite expands when the temperature is below As . Above the As temperature, the embedded TiNiCu wire starts building up recovery stresses, resulting in a negative thermal expansion of the composite. There is a point where the composite starts to expand again, indicating the debonding of the interface [10]. The dashed straight lines in Fig. 3 denote the thermal expansion lines of composites without prestrain. The strain
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Fig. 3. Strain–temperature curves of glass fiber composites with 3% prestrained TiNiCu wire. The inserted figure shows the maximum output strain as a function of the prestrain level of TiNiCu wires.
difference between the dashed line and the debonding point is denoted by εmax . Obviously, the physical meaning of εmax is the maximum contracting strain that a composite can generate during heating before the failure of the interface. The inserted figure shows the relationship between εmax and the prestrain level. One can see that εmax decreases almost linearly with increasing prestrain. Fig. 3 shows that a small prestrain level is necessary to produce a reliable SMA composite. Indubitably, the prestrain level of the embedded SMA wires significantly affects the behavior of a SMA composite. It is widely accepted that a higher prestrain can provide a higher ultimate recovery stress. However, as shown above, a high prestrain reduces the reliability of the interface significantly. This implies that a compromise is necessary between mechanical properties and reliability of a SMA composite when selecting the prestrain level of the embedded SMA wires. 3.4. Effects of additional reinforcing fibers Normally SMA composites are expected to be structural materials where adaptive functionality is required. From an engineering point of view, widely used composites such as glass fiber/epoxy composites will be more suitable as a matrix for SMA composites than just pure epoxy. Fig. 4 shows the thermal expansion curves of respective reinforced glass fiber/epoxy and Kevlar/epoxy composites with 3% prestrained TiNiCu wires. The Kevlar fiber composite shows a negative thermal expansion below the As temperature because Kevlar fibers contract on heating. Above the As temperature, the embedded TiNiCu wires start building up recovery stresses, resulting in a more negative thermal expansion of the composite. One can see from Fig. 4 that the Kevlar fiber composite has a higher capacity for outputting recovery strain than the glass fiber composite in two aspects: the maximum contracting strain and the debonding temperature of the interface.
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Fig. 4. Thermal expansion curves of glass fiber/epoxy and Kevlar fiber/epoxy composites with 3% prestrained TiNiCu wires.
In fact, because the embedded SMA wires have a tendency to contract several percent in strain upon heating, SMA composites are more susceptible to interfacial debonding than any other composite. Fig. 4 shows that the additional reinforcing fibers have a significant effect on the interface quality between the SMA wire and the epoxy matrix. This should be ascribed to the thermal dilation properties of the additional reinforcing fibers. Kevlar has a negative expansion coefficient, and can partly relieve the mismatch stress between the TiNiCu wires and the matrix. Therefore, the TiNiCu wire–epoxy matrix interface of the Kevlar fiber composite can survive up to a higher temperature than the glass fiber composite. Fig. 4 shows that a Kevlar fiber/epoxy composite (probably, also carbon fiber/epoxy composite, because carbon fiber also contracts upon heating) is more suitable as a matrix for SMA composites than a glass fiber/epoxy composite. 3.5. The maximum working temperature The working temperature of a thermoset resin composite should be well below the glass transition temperature Tg of the resin matrix to avoid softening of the resin. For a SMA composite, the interface between the SMA wires and the matrix may fail at a temperature far below Tg because of the building up of recovery stresses. Thus, evaluation of the interface degradation upon heating is necessary to define the maximum working temperature of a SMA composite. Due to the complex thermomechanical behavior of SMA materials, commonly used methods for evaluating the thermal stresses in normal composites are not suitable for SMA composites. In our previous research, a quantitative method was developed to calculate the interface stress of a SMA composite [9], which is helpful to evaluate the interface condition. The basic idea of the method is shown in Fig. 5. It has already been shown that a large amount of martensite will be stabilized in a constrained heating process [11,12]. For a SMA composite with a well-bonded interface, the embedded SMA components will show only a small endothermic peak
Fig. 5. DSC curves of Kevlar fiber composites with 3% prestrained TiNiCu wires in two consecutive heating process. The inserted curve shows the relationship between the former heating temperature and the transformation enthalpy of the latter heating process.
in a heating DSC curve, and the area of the peak decreases with increasing prestrain [11,12]. However, if the temperature is too high, the generated recovery stress may exceed the interfacial bond strength between the SMA wires and the matrix. Consequently, the interface debonds little by little, and the stabilized martensite transforms into parent phase little by little, as shown by the little peaks on the gray curve (indicated by arrows) in Fig. 5. Because the partly debonded interface cannot completely constrain the embedded SMA wires, more martensite will participate at the reverse transformation in the second heating. This will result in a larger endothermic peak on the DSC curve, as shown by the dark curve in Fig. 5. The inserted figure in Fig. 5 shows the relationship between the maximum temperature of the first heating and the transformation enthalpy of the second heating. As can be seen, an S-shaped curve is obtained. Obviously, this Sshaped curve shows the interface degradation process of the TiNiCu composite with temperature. For example, point A in the inserted figure of Fig. 5 defines the temperature below which the interface of the composite is intact; point B defines the temperature point above which the interface of the composite debonds completely. By examining the transformation enthalpy of a SMA composite, and comparing it to its own S-shaped curve, one can find out to what extent the interface has degraded.
4. Conclusions Some basic aspects in fabricating reliable SMA hybrid epoxy composites have been discussed in this paper, and the following conclusions can be drawn. 1. By selecting a small hysteresis TiNiCu alloy, SMA composites show a very small hysteresis during thermal cycling, which can be approximated by linear functions.
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2. The curing process of a thermoset epoxy matrix up to 413 K does not degrade the actuating potential of the TiNiCu wires. 3. TiNiCu wires with a high prestrain induce a high internal stress, resulting in (i) a weak interface between the TiNiCu wires and the matrix, and in (ii) a low actuation ability of the composites. A compromise between the mechanical properties and the reliability is necessary when selecting the prestrain level of the embedded SMA wires. 4. Adding fibers with a negative thermal expansion coefficient, such as Kevlar fibers, can release the mismatch stress at the interface between the SMA wires and the matrix, and shift the debonding of the interface to a higher temperature. 5. The maximum working temperature of SMA composites must be below both the Tg of the matrix and the debond temperature of the interface. The debonding extent of the interface can be evaluated by simply measuring the transformation enthalpy of the composite. Acknowledgements This work has been done in the framework of the ADAPTproject that is funded by the European Commission, in the Industrial and Materials Technologies research and technology programme. Y.J. Zheng and L.S. Cui acknowledge the financial support of the Scientific Research Foundation for the
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Returned Overseas Chinese Scholars, State Education Ministry, and the National Natural Science Foundation of China with grant number 50071037.
References [1] Z.G. Wei, R. Sandstr¨om, S. Miyazaki, J. Mater. Sci. 33 (1998) 3763. ˇ [2] J. Schrooten, K.A. Tsoi, R. Stalmans, Y.J. Zheng, P. Sittner, in: A.R. Wilson, H. Asanuma (Eds.), Proceedings of SPIE; Smart Materials, vol. 4234, 2001, pp. 114–124. [3] K. Jonnalagadda, G.E. Kline, N.R. Sottos, Exp. Mech. 37 (1997) 78. [4] Y.J. Zheng, J. Schrooten, K. Tsoi, R. Stalmans, Mater. Sci. Eng. A 335 (2002) 157. [5] D. Vokoun, P. Sittner, R. Stalmans, Scripta Mater. 48 (2003) 623. [6] J. Simpson, C. Boller, in: A.-M.R. McGowan (Ed.), Proceedings of SPIE; Smart Structure and Materials 2002; Industrial and Commercial Applications of Smart Structures Technologies, vol. 4698, 2002, pp. 416–426. [7] A. Shimamoto, H. Ohkawara, F. Nogata, Eng. Fract. Mech. 71 (2004) 737. [8] Y. Yamada, M. Taya, R. Watanabe, Mater. Trans., JIM 34 (1993) 254. [9] Y.J. Zheng, J. Schrooten, L.S. Cui, Intermetallics, in press. [10] Y.J. Zheng, J. Schrooten, K. Tsoi, P. Sittner, Exp. Mech. 43 (2003) 194. [11] Y.J. Zheng, L.S. Cui, D. Zhu, D.Z. Yang, Mater. Lett. 43 (2000) 91. [12] Y.J. Zheng, J. Schrooten, L.S. Cui, J. van Humbeeck, Acta Mater. 51 (2003) 5469. [13] D. Vokoun, R. Stalman, in: V.V. Varadan (Ed.), Smart Structures and Materials SPIE, 1999, p. 825.
Basic design guidelines for SMA/epoxy smart composites Y.J. Zhenga,b,∗ , L.S. Cuia , J. Schrootenb b
a Department of Materials Science and Engineering, University of Petroleum, Beijing 102249, China Department of Metallurgy and Materials Engineering, KULeuven, Kasteelpark Arenberg 44, B-3001 Leuven, Belgium
Received 5 April 2004; received in revised form 26 July 2004
Abstract The actuating ability and reliability of shape memory alloy (SMA) hybrid composites were studied in this paper. Results showed that by selecting small hysteresis SMAs such as TiNiCu alloy, SMA hybrid composites have a linear stress–temperature behavior, which is relatively easy to control. The curing process of the epoxy matrix does not affect significantly the actuating ability of the embedded TiNiCu alloy wires. A moderate prestrain of the TiNiCu wires is preferred when giving attention to both the mechanical properties and the reliability of the TiNiCu hybrid composites. Additional reinforcing fibers with a negative thermal expansion coefficient such as Kevlar fibers are helpful to strengthen the reliability of the interface and enhance the actuating ability of the SMA hybrid composites. Based on the experimental results, some basic guidelines for designing shape memory hybrid composites were proposed. © 2004 Elsevier B.V. All rights reserved. Keywords: Shape memory alloy; Composite; Smart material
1. Introduction Shape memory alloy (SMA) hybrid composites are regarded as promising candidates of the so-called “smart materials”. Investigations in the past decade have demonstrated that the shape memory hybrid composites have many novel or improved properties over the un-reinforced matrix, such as enhanced high-temperature mechanical properties, shape control, fatigue resistance, vibration damping, acoustic radiation and transmission control, and impact resistance abilities [1]. Up to now, however, there is still no report of industrialized shape memory hybrid composites, mostly because of the complex behavior that is still not well understood, and the lack of clear guidelines to optimize the design of shape memory hybrid composites. Investigations concerning the design of shape memory hybrid polymer matrix composites have shown the influence of volume fraction [2], surface treatment [3], prestrain of the shape memory components [2,4], and matrix curing process [5] on the properties of SMA hybrid composites. These contributions enabled the production of some prototypes of smart ∗
Corresponding author. Tel.: +86-10-89733200; fax: +86-10-89733203. E-mail address: [email protected] (Y.J. Zheng).
0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2004.07.063
materials, but are not enough to produce a reliable composite. At this moment the biggest known prototype is a scale model of a tail wing at EADS [6]. In fact, the research done up till now did not look at all design problems at once. For example, a high prestrain is preferred if one needs the composites to have an enhanced ability to close crack tips [7] or a higher yield strength [8]. However, according to our research, a high prestrain does not improve significantly the thermomechanical behavior of shape memory hybrid composites [9], but reduces significantly the quality of the interface and the actuating ability of the composites [4,10]. Probably, the design principles will only be available if the complex behavior of SMA hybrid composites is well understood. Nevertheless, one can still draw some basic design guidelines by taking a comprehensive view on the experimental results. The aim of this paper is to provide some basic guidelines based on experimental results, for designing shape memory hybrid composites.
2. Experimental procedure A ternary TiNi 12 wt.% Cu alloy wire of 0.15 mm diameter was obtained from SMA Inc., USA. Strafil G-EPI-140/142
140
Y.J. Zheng et al. / Materials Science and Engineering A 390 (2005) 139–143
glass fiber epoxy prepreg was supplied by Hexcel Composites, and Kevlar 29 fibers reinforced LTM217 epoxy prepreg was supplied by Advanced Composites Group in UK. The glass fiber composites were cured in an autoclave at 413 K for 20 min. The Kevlar fiber composites were cured at 343 K for 12 h, and then post cured at 413 K for 4 h. A frame, designed by EPFL, Switzerland, was used to align the SMA wires with an adjustable, constant spacing, prestrain and to maintain the wires at a constant strain during the curing process. The size of the samples used in this research are 0.3 mm × 10 mm × 150 mm with a volume fraction of TiNiCu wires of 11.8%. The transformation behavior of the SMA wires was determined using a TA 2920 (TA Instruments) differential scanning calorimeter (DSC) with a helium gas DSC cell purge. The DSC composite samples were cut into approximately 5 mm × 5 mm using a low speed diamond saw. The heating/cooling rate during the DSC experiments was 5 K/min. The dimensional change of the composites was measured using a DuPont 943 thermomechanical analyzer (TMA). The stress–temperature behavior of the TiNiCu wire and the TiNiCu composites were investigated by using a fully computerized apparatus that enabled the user to program, control and measure the strain, force and temperature of the sample.
3. Results and discussion 3.1. Selection of SMA components TiNi based alloys are the best SMA family for smart material actuator applications mainly because of their excellent mechanical properties. R phase TiNi binary and TiNiCu ternary alloys are of particular interest because of their relatively small hysteresis. Considering that the R phase TiNi alloy is limited in terms of maximum recovery strain and stress, the TiNiCu ternary alloy is more promising. Fig. 1 shows the recovery stress cycles of a 3% prestrained TiNiCu bare wire and a Kevlar/epoxy composite with em-
Fig. 1. Recovery stress of 3% prestrained TiNiCu bare wire and Kevlar/ epoxy composite with embedded 3% prestrained TiNiCu wire.
bedded 3% prestrained TiNiCu wires. One can see that the TiNiCu bare wire shows a relatively small hysteresis. As a result, the hysteresis of the composite is also quite small. It is widely accepted that the hysteresis mainly comes from the energy dissipation associated with the migration of the B19 B2 interface. Our previous investigations have shown that the reverse transformation of the so-called preferentially oriented martensite (POM) is slow and spread over a large temperature window in a constrained heating process [11,12]. The recovery stress, which originates from the reverse transformation of POM, is generated by consuming only a small fraction of POM. It means that the energy dissipation accompanied with the interface migration during a constrained transformation is also quite small. As a result, the small hysteresis of a TiNiCu wire shows an even smaller hysteresis behavior in a constrained transformation process, as can be seen in Fig. 1. Up to now, there is still no reliable mathematical modeling that can simulate the complex thermomechanical behavior of shape memory materials. However, as can be seen from Fig. 1, one can still use a linear function to simulate the behavior of shape memory hybrid composite if a small hysteresis SMA such as TiNiCu wire is used. 3.2. Impact of the matrix curing process Generally, the manufacturing process of SMA hybrid epoxy composites involves prestraining and maintaining the SMA wires from the very beginning to the end of the autoclave run. To study the effect of the autoclave running temperature on the actuating ability of the SMA component, TiNiCu wires were prestrained and maintained at a constant strain, and then the wires were subject to heating processes, as shown in Fig. 2. The wires were first cycled once between room temperature and 413 K, which is the curing temperature of the prepreg used in this research, and then further cycled two times between room temperature and 383 K. The solid circle on each curve denotes the starting point of the heating
Fig. 2. Effects of thermal cycling on the stress–temperature behaviors of prestrained TiNiCu wire. The wires were prestrained and maintained at a constant strain, cycled up to 413 K once, then further cycled two times up to 383 K.
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process. One can see that the recovery stress curves of the first heating up to 413 K are different to the curves in the following thermal cycles. Seemingly, an autoclave run up to 413 K changes the actuating behavior of the embedded SMA wires. However, according to our previous research, it is an intrinsic characteristic of shape memory alloys that the first constrained thermal cycle is always different from the following cycles, because of the separation of the detwinned martensite into the self-accommodating martensite and the fully detwinned martensite [12]. Thus, the result in Fig. 2 does not prove that an autoclave run up to 413 K changes the actuation behavior of the TiNiCu wire. On the contrary, as can be seen in Fig. 2, the behavior of the wires becomes very stable in the following thermal cycles. This means that the curing process of the thermoset matrix will not destroy the actuating ability of the embedded TiNiCu wires. 3.3. Prestrain level of the SMA wires The effects of the prestrain level on the recovery stress can be seen in Fig. 2. Firstly, the recovery stress rate, dσ/dT, decreases with increasing prestrain level. The recovery stress rate dσ/dT is an important parameter for a SMA actuator. However, the nature of the effect of prestrain level on dσ/dT is not fully understood yet. According to previous research, the recovery stress rate may be related to the chemical composition of the alloy, because in TiNi binary alloys, the recovery stress rate tends to increase with increasing prestrain [13], but in TiNiCu ternary alloys the recovery stress rate tends to decrease with increasing prestrain [4]. It is interesting to note here that, according to our previous findings, prestrain levels do not have much effect on the recovery stress rate of SMA hybrid composite [9]. Thus, if the recovery stress rate of a SMA composite needs to be tailored, the best way is to modify the volume fraction, but not to change the prestrain level of the SMA components. Secondly, one can see from Fig. 2 that, for small prestrains such as 1%, the recovery stress reaches zero after one thermal cycle; but for large prestrains such as 6%, the recovery stress level remains at a relatively high level even after several thermal cycles. This means that a large residual compressive stress remains in the epoxy matrix if the embedded SMA wire has a large prestrain level. This residual compressive stress, on one hand, is helpful for improving the tensile strength of the composite. On the other hand, it will exert a large shear stress at the interface between the SMA wires and the matrix, thus making the interface more susceptible to debonding. Fig. 3 shows the strain–temperature curves of glass fiber composites with 3% prestrained TiNiCu wires. The composite expands when the temperature is below As . Above the As temperature, the embedded TiNiCu wire starts building up recovery stresses, resulting in a negative thermal expansion of the composite. There is a point where the composite starts to expand again, indicating the debonding of the interface [10]. The dashed straight lines in Fig. 3 denote the thermal expansion lines of composites without prestrain. The strain
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Fig. 3. Strain–temperature curves of glass fiber composites with 3% prestrained TiNiCu wire. The inserted figure shows the maximum output strain as a function of the prestrain level of TiNiCu wires.
difference between the dashed line and the debonding point is denoted by εmax . Obviously, the physical meaning of εmax is the maximum contracting strain that a composite can generate during heating before the failure of the interface. The inserted figure shows the relationship between εmax and the prestrain level. One can see that εmax decreases almost linearly with increasing prestrain. Fig. 3 shows that a small prestrain level is necessary to produce a reliable SMA composite. Indubitably, the prestrain level of the embedded SMA wires significantly affects the behavior of a SMA composite. It is widely accepted that a higher prestrain can provide a higher ultimate recovery stress. However, as shown above, a high prestrain reduces the reliability of the interface significantly. This implies that a compromise is necessary between mechanical properties and reliability of a SMA composite when selecting the prestrain level of the embedded SMA wires. 3.4. Effects of additional reinforcing fibers Normally SMA composites are expected to be structural materials where adaptive functionality is required. From an engineering point of view, widely used composites such as glass fiber/epoxy composites will be more suitable as a matrix for SMA composites than just pure epoxy. Fig. 4 shows the thermal expansion curves of respective reinforced glass fiber/epoxy and Kevlar/epoxy composites with 3% prestrained TiNiCu wires. The Kevlar fiber composite shows a negative thermal expansion below the As temperature because Kevlar fibers contract on heating. Above the As temperature, the embedded TiNiCu wires start building up recovery stresses, resulting in a more negative thermal expansion of the composite. One can see from Fig. 4 that the Kevlar fiber composite has a higher capacity for outputting recovery strain than the glass fiber composite in two aspects: the maximum contracting strain and the debonding temperature of the interface.
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Fig. 4. Thermal expansion curves of glass fiber/epoxy and Kevlar fiber/epoxy composites with 3% prestrained TiNiCu wires.
In fact, because the embedded SMA wires have a tendency to contract several percent in strain upon heating, SMA composites are more susceptible to interfacial debonding than any other composite. Fig. 4 shows that the additional reinforcing fibers have a significant effect on the interface quality between the SMA wire and the epoxy matrix. This should be ascribed to the thermal dilation properties of the additional reinforcing fibers. Kevlar has a negative expansion coefficient, and can partly relieve the mismatch stress between the TiNiCu wires and the matrix. Therefore, the TiNiCu wire–epoxy matrix interface of the Kevlar fiber composite can survive up to a higher temperature than the glass fiber composite. Fig. 4 shows that a Kevlar fiber/epoxy composite (probably, also carbon fiber/epoxy composite, because carbon fiber also contracts upon heating) is more suitable as a matrix for SMA composites than a glass fiber/epoxy composite. 3.5. The maximum working temperature The working temperature of a thermoset resin composite should be well below the glass transition temperature Tg of the resin matrix to avoid softening of the resin. For a SMA composite, the interface between the SMA wires and the matrix may fail at a temperature far below Tg because of the building up of recovery stresses. Thus, evaluation of the interface degradation upon heating is necessary to define the maximum working temperature of a SMA composite. Due to the complex thermomechanical behavior of SMA materials, commonly used methods for evaluating the thermal stresses in normal composites are not suitable for SMA composites. In our previous research, a quantitative method was developed to calculate the interface stress of a SMA composite [9], which is helpful to evaluate the interface condition. The basic idea of the method is shown in Fig. 5. It has already been shown that a large amount of martensite will be stabilized in a constrained heating process [11,12]. For a SMA composite with a well-bonded interface, the embedded SMA components will show only a small endothermic peak
Fig. 5. DSC curves of Kevlar fiber composites with 3% prestrained TiNiCu wires in two consecutive heating process. The inserted curve shows the relationship between the former heating temperature and the transformation enthalpy of the latter heating process.
in a heating DSC curve, and the area of the peak decreases with increasing prestrain [11,12]. However, if the temperature is too high, the generated recovery stress may exceed the interfacial bond strength between the SMA wires and the matrix. Consequently, the interface debonds little by little, and the stabilized martensite transforms into parent phase little by little, as shown by the little peaks on the gray curve (indicated by arrows) in Fig. 5. Because the partly debonded interface cannot completely constrain the embedded SMA wires, more martensite will participate at the reverse transformation in the second heating. This will result in a larger endothermic peak on the DSC curve, as shown by the dark curve in Fig. 5. The inserted figure in Fig. 5 shows the relationship between the maximum temperature of the first heating and the transformation enthalpy of the second heating. As can be seen, an S-shaped curve is obtained. Obviously, this Sshaped curve shows the interface degradation process of the TiNiCu composite with temperature. For example, point A in the inserted figure of Fig. 5 defines the temperature below which the interface of the composite is intact; point B defines the temperature point above which the interface of the composite debonds completely. By examining the transformation enthalpy of a SMA composite, and comparing it to its own S-shaped curve, one can find out to what extent the interface has degraded.
4. Conclusions Some basic aspects in fabricating reliable SMA hybrid epoxy composites have been discussed in this paper, and the following conclusions can be drawn. 1. By selecting a small hysteresis TiNiCu alloy, SMA composites show a very small hysteresis during thermal cycling, which can be approximated by linear functions.
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2. The curing process of a thermoset epoxy matrix up to 413 K does not degrade the actuating potential of the TiNiCu wires. 3. TiNiCu wires with a high prestrain induce a high internal stress, resulting in (i) a weak interface between the TiNiCu wires and the matrix, and in (ii) a low actuation ability of the composites. A compromise between the mechanical properties and the reliability is necessary when selecting the prestrain level of the embedded SMA wires. 4. Adding fibers with a negative thermal expansion coefficient, such as Kevlar fibers, can release the mismatch stress at the interface between the SMA wires and the matrix, and shift the debonding of the interface to a higher temperature. 5. The maximum working temperature of SMA composites must be below both the Tg of the matrix and the debond temperature of the interface. The debonding extent of the interface can be evaluated by simply measuring the transformation enthalpy of the composite. Acknowledgements This work has been done in the framework of the ADAPTproject that is funded by the European Commission, in the Industrial and Materials Technologies research and technology programme. Y.J. Zheng and L.S. Cui acknowledge the financial support of the Scientific Research Foundation for the
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Returned Overseas Chinese Scholars, State Education Ministry, and the National Natural Science Foundation of China with grant number 50071037.
References [1] Z.G. Wei, R. Sandstr¨om, S. Miyazaki, J. Mater. Sci. 33 (1998) 3763. ˇ [2] J. Schrooten, K.A. Tsoi, R. Stalmans, Y.J. Zheng, P. Sittner, in: A.R. Wilson, H. Asanuma (Eds.), Proceedings of SPIE; Smart Materials, vol. 4234, 2001, pp. 114–124. [3] K. Jonnalagadda, G.E. Kline, N.R. Sottos, Exp. Mech. 37 (1997) 78. [4] Y.J. Zheng, J. Schrooten, K. Tsoi, R. Stalmans, Mater. Sci. Eng. A 335 (2002) 157. [5] D. Vokoun, P. Sittner, R. Stalmans, Scripta Mater. 48 (2003) 623. [6] J. Simpson, C. Boller, in: A.-M.R. McGowan (Ed.), Proceedings of SPIE; Smart Structure and Materials 2002; Industrial and Commercial Applications of Smart Structures Technologies, vol. 4698, 2002, pp. 416–426. [7] A. Shimamoto, H. Ohkawara, F. Nogata, Eng. Fract. Mech. 71 (2004) 737. [8] Y. Yamada, M. Taya, R. Watanabe, Mater. Trans., JIM 34 (1993) 254. [9] Y.J. Zheng, J. Schrooten, L.S. Cui, Intermetallics, in press. [10] Y.J. Zheng, J. Schrooten, K. Tsoi, P. Sittner, Exp. Mech. 43 (2003) 194. [11] Y.J. Zheng, L.S. Cui, D. Zhu, D.Z. Yang, Mater. Lett. 43 (2000) 91. [12] Y.J. Zheng, J. Schrooten, L.S. Cui, J. van Humbeeck, Acta Mater. 51 (2003) 5469. [13] D. Vokoun, R. Stalman, in: V.V. Varadan (Ed.), Smart Structures and Materials SPIE, 1999, p. 825.