Abstract:
A plurality of reversible active composite materials are disclosed, including composites based on a shape memory alloy member (SMA) and a shape memory polymer member (SMP), as well as composites based on two different SMP members. Each different member (SMA or SMP) will be trained to remember a specific shape at a specific temperature. Where two different SMP members are employed, the members exhibit different glass transition temperatures. Such composite materials can be implemented in many form factors, including two generally planar members, a single generally planar SMP member with SMA fibers distributed throughout the SMP, and a SMA fiber/wire coated with a SMP coating. In particular, the SMA fiber/wire coated with a SMP layer can be used to form helical coils that can be used in paired hinges to achieve reversible bending of a structure into which such paired hinges are incorporated.

Description:
RELATED APPLICATIONS 
       [0001]    This application is based on two prior copending provisional applications, Ser. No. 60/984,866, filed on Nov. 2, 2007, and Ser. No. 60/985,390, filed on Nov. 5, 2007, the benefits of the filing dates of which are hereby claimed under 35 U.S.C. § 119(e). 
     
    
     GOVERNMENT RIGHTS 
       [0002]    This invention was made with government support under grant number FA9550-05-1-0196 awarded by the Air force Office of Scientific Research. The government has certain rights in the invention. 
     
    
     BACKGROUND 
       [0003]    Actuators are relatively simple mechanical components that are often incorporated into more complex mechanical systems, including those found in automobiles, aircraft, manufacturing facilities, and processing facilities. A conventional solenoid is one example of an actuator that has found broad application across many types of industries and technologies. 
         [0004]    With respect to aerospace structures in particular, morphing aerospace structures can be achieved using several approaches: by using fixed shape components and actuators which will move those fixed shaped components; by using variable shape materials (i.e., morphing materials); and by using combinations thereof. It would be desirable to provide morphing materials capable of accommodating relatively large deformations while also exhibiting relatively high mechanical strengths, for use in aerospace structures and other applications. 
         [0005]    Electroactive polymers, shape memory alloys (SMA), and shape memory polymers (SMP) exhibit such desirable properties. Electroactive polymers generally require a relatively bulky power unit. SMAs and SMPs can change shape in response to temperature changes. Performance of such active materials can be evaluated in terms of specific active strain, which is the maximum strain divided by the weight of the active material and required infrastructure (i.e., any power supply required, or means for controlling temperature). Both SMAs and SMPs can provide relatively high specific active strain. 
         [0006]    Aerospace structures (whether implemented using fixed shape materials or morphing materials) often require a reversible change between two configurations. Unfortunately, SMAs and SMPs generally exhibit a one-way shape memory property, which is not ideal for reversible morphing structures. It would be desirable to enable techniques for using one-way SMAs and SMPs in structures requiring a reversible change between two configurations. Such structures will likely find application in aerospace and medical industries. 
       SUMMARY 
       [0007]    This application specifically incorporates by reference the disclosures and drawings of each patent application and issued patent identified above as a related application. 
         [0008]    The concepts disclosed herein encompass reversible active materials based on SMAs and SMPs. Such a composite reversible active material will include a SMP member and a SMA member. The SMP member will be configured to return (i.e., to remember) to a specific shape at a first temperature. The SMA member will be configured to return (i.e., to remember) to a specific shape at a second temperature. In at least one embodiment, the first temperature is less than the second temperature. 
         [0009]    A related embodiment employed no SMA, but rather a first SMP member and a second SMP member, where the different SMP members exhibit different glass transition temperatures. 
         [0010]    Such composite materials can be implemented in many form factors, including two generally planar members (including configurations where one or more of the generally planar members is trained to achieve a bent configuration), a single generally planar SMP member with SMA fibers distributed throughout the SMP, and a SMA fiber/wire coated with a SMP coating. In particular, the SMA fiber/wire coated with a SMP layer can be used to form helical coils that can be used in paired hinges to achieve reversible bending of a structure into which such paired hinges are incorporated. 
         [0011]    This Summary has been provided to introduce a few concepts in a simplified form that are further described in detail below in the Description. However, this Summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
     
    
     
       DRAWINGS 
         [0012]    Various aspects and attendant advantages of one or more exemplary embodiments and modifications thereto will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
           [0013]      FIG. 1  is a block diagram illustrating an exemplary method for achieving a composite reversible active material including a SMP member and a SMA member; 
           [0014]      FIG. 2  is a block diagram illustrating an exemplary method for using a composite reversible active material including a SMP member and a SMA member; 
           [0015]      FIG. 3  schematically illustrates the methods of  FIGS. 1 and 2 ; 
           [0016]      FIG. 4  schematically illustrates a composite reversible active material including a generally planar SMP member and a generally planar SMA member; 
           [0017]      FIG. 5  schematically illustrates a composite reversible active material including a generally planar SMP member and a plurality of SMA member fibers; 
           [0018]      FIG. 6  schematically illustrates a composite reversible active material including a generally planar SMP member and a SMA member implemented using a fibrous mat; 
           [0019]      FIG. 7  schematically illustrates a composite reversible active material including a SMA fiber coated with a SMP; 
           [0020]      FIG. 8  schematically illustrates an exemplary method for making the composite reversible active material of  FIG. 7 ; 
           [0021]      FIG. 9  schematically illustrates a structure including a hinge implemented using a composite reversible active material including a SMP member and a SMA member; 
           [0022]      FIGS. 10A and 10B  schematically illustrate the operation of the hinge of  FIG. 9 ; and 
           [0023]      FIG. 11  schematically illustrates an exemplary method for achieving a composite reversible active material including a first SMP member and a second SMP member, where different SMPs are employed. 
       
    
    
     DESCRIPTION 
     Figures and Disclosed Embodiments are not Limiting 
       [0024]    Exemplary embodiments are illustrated in referenced Figures of the drawings. It is intended that the embodiments and Figures disclosed herein are to be considered illustrative rather than restrictive. No limitation on the scope of the technology and of the claims that follow is to be imputed to the examples shown in the drawings and discussed herein. 
         [0025]      FIG. 1  is a block diagram illustrating an exemplary method for achieving a composite reversible active material including a SMP member and a SMA member. In a block  10 , the SMP member is trained to remember a specific configuration or shape at a first temperature T 1 . In at least one embodiment the SMP (at T m ) is formed into a straight configuration and allowed to cool to T 1 . In a block  12 , the SMA member is trained to remember a specific configuration or shape at a temperature T 3 , where T 3  generally corresponds to the austenite finishing temperature of the SMA. In at least one embodiment, the SMA is trained to achieve a bent configuration. In a block  14 , the trained SMA and SMP members are combined together to form the composite reversible active material. It should be recognized that the specific configurations the SMA and SMP are trained to remember are intended to be exemplary, rather than limiting. 
         [0026]    In such a method, T 1 &lt;T 2 &lt;T 3 &lt;T m  (melting temperature of the SMP), where T 2 =T g  of the SMP. Those of ordinary skill in the art will recognize that T g  is the glass transition temperature of a non-crystalline material (i.e., the critical temperature at which the material changes its behavior from being hard and brittle (and therefore relatively easy to break) to being elastic and flexible). For example, at room temperature, striking a piece of glass with a hammer will shatter the glass, while striking a piece of rubber will not shatter the rubber, as the rubber would simply absorb the energy by momentarily deforming or stretching. However, if the same piece of rubber is submerged in liquid nitrogen, the cold rubber will shatter just like glass at room temperature, because the cold rubber is below its glass transition temperature. 
         [0027]      FIG. 2  is a block diagram illustrating an exemplary method for using a composite reversible active material including a SMP member and a SMA member. In a block  16  temperature conditions are manipulated such that the composite achieves temperature T 1  and an initial shape. In a block  18  the temperature conditions are manipulated such that a temperature required for the composite to morph into the trained SMP shape (i.e., T SMP ) is achieved. The temperature T SMP  is less than T 3  but greater than or equal to the T g  of the SMP (i.e., T 3 &gt;T SMP &gt;=T 2 ). In a block  20  the temperature conditions are manipulated such that a temperature required for the composite to morph into the trained SMA shape (i.e., T SMA ) is achieved. The temperature T SMA  is less than T m  of the SMP but greater than or equal to the T 3  of the SMA (i.e., T m &gt;T SMA &gt;=T 3 ). 
         [0028]      FIG. 3  schematically illustrates the methods of  FIGS. 1 and 2 , showing the composite material including SMP member  19  and SMA member  21  being formed into a composite structure that reversibly transitions (or morphs) between two configurations. 
         [0029]      FIG. 4  schematically illustrates a composite reversible active material  22  including a generally planar SMP member  24  and a generally planar SMA member  26 . 
         [0030]      FIG. 5  schematically illustrates a composite reversible active material  28  including generally planar SMP member  24   a  and a plurality of SMA member fibers  26   a . The SMA fibers are added to the SMP when the SMP is in a molten state, and the SMP is then molded into the desired shape. 
         [0031]      FIG. 6  schematically illustrates a composite reversible active material  30  including a generally planar SMP member  24  and a SMA member implemented using a fibrous mat  26   b.    
         [0032]      FIG. 7  schematically illustrates a composite reversible active material  32  including a SMA fiber  26   c  coated with a SMP layer  24   b .  FIG. 8  schematically illustrates an exemplary method for making the composite reversible active material of  FIG. 7 . SMA fiber  26   c  is introduced into a container  38  including a SMP solution. The SMA fiber coated with the SMP solution is then introduced into a volume  40  configured to solidify the SMP coating on the SMA fiber. For example, container  38  could contain molten SMP and volume  40  could be cooled to a temperature below T m  of the SMP, so that the SMP coating on the SMA solidifies. Alternatively, container  38  could include SMP dissolved in a solvent, and volume  40  could be heated to a temperature selected to volatilize the solvent, leaving behind the SMP coating on the SMA. A system of rollers  42  can be used to control a motion of the SMA fiber. It should be recognized that such a method is intended to be exemplary, rather than limiting. 
         [0033]      FIG. 9  schematically illustrates a structure  44  (i.e., a plate) including a plurality of hinges implemented using a composite reversible active material including both a SMP member and a SMA member. The hinges are employed in pairs, including an upper hinge  46   a  and a lower hinge  46   b.    
         [0034]    Each hinge is formed using a composite coil substantially corresponding to the composite material of  FIG. 7  (i.e., a SMA fiber or wire coated with a SMP, each of which is shape trained as discussed above in connection with  FIG. 1 ). A generally helical coil is formed using the composite material  FIG. 7  for each upper and lower hinge. 
         [0035]      FIGS. 10A and 10B  schematically illustrate the operation of the hinge of  FIG. 9 , which enables reversible bending of structure  44  to be achieved. In  FIG. 10A , the temperature of upper spring is controlled so that the coil spring remains in a first compact configuration, while the temperature of lower spring is controlled so that the coil spring of the lower hinge morphs to a second expanded configuration. In  FIG. 10B , the temperature of upper spring is controlled so that the coil spring hinge morphs to the second expanded configuration, while the temperature of lower spring is controlled so that the coil spring of the lower hinge remains in a first compact configuration. 
         [0036]    Manipulating the temperature of the upper and lower springs enables reversible bending of the plate structure. While such a plate structure is likely to be usable in many types of actuators, it should be recognized that such paired hinges can be employed in other types of structures to enable reversible shape changes, and the plate structure of  FIGS. 9 ,  10 A and  10 B is intended to be exemplary, and not limiting. 
         [0037]    Temperature control of the hinges can be implemented by passing a current through the SMA portion of the composite coil spring, thereby heating the hinge. When the current is removed, the hinge will be cooled to a lower temperature by the ambient environment. If faster switching times are required, cooling can be implemented using the thermoelectric effect, also referred to as the Peltier-Seebeck effect. 
         [0038]      FIG. 11  schematically illustrates an exemplary method for achieving a composite reversible active material including a first SMP member and a second SMP member, where different SMPs are employed. The SMP each have a different T g . 
         [0039]    In Step  1 , two different SMPs are prepared, generally as discussed above. SMP 1  will have a larger T g  than SMP 2 . SMP 1  is trained to remember a curved or bent configuration, while SMP 2  is trained to remember a flat or straight configuration. 
         [0040]    In Step  2 , a composite is formed using SMP 1  and SMP 2 , at a temperature T i  (where T i &lt;T g  of SMP 1 ). The different SMPs are forced together, such that the SMP 2  is partially bent (i.e., the composite will be partially curved, but to a lesser extent than SMP 1 ). 
         [0041]    In Step  3 , the composite is heated to a temperature T 1 , where T g  of SMP 1 &lt;T 1 &lt;T g  of SMP 2 . This causes the SMP 2  portion of the composite to soften, and the composite becomes bent to a greater degree (i.e., the SMP 1  portion of the composite moves closer to its original bent configuration). 
         [0042]    In Step  4 , the composite is heated to a temperature T 2 , where T 2 &gt;T g  of SMP 1  (which is &gt;T g  of SMP 2 ). This causes the SMP 1  portion of the composite to soften, and the composite becomes bent to a lesser degree (i.e., the SMP 2  portion of the composite moves closer to its original straight configuration). The composite is then cooled to T 1 , and the composite remains in the configuration of Step  2 . 
         [0043]    Heating and cooling of the composite can be achieved generally as discussed above. It should be recognized that the disclosed heating and cooling techniques are intended to be exemplary, rather than limiting, and other heating and cooling techniques known in the art can also be employed. 
         [0044]    Although the concepts disclosed herein have been described in connection with the preferred form of practicing them and modifications thereto, those of ordinary skill in the art will understand that many other modifications can be made thereto within the scope of the claims that follow. Accordingly, it is not intended that the scope of these concepts in any way be limited by the above description, but instead be determined entirely by reference to the claims that follow.