Concrete is considerably stronger in compression than it is in tension. When cracks form in concrete members, the flexural stiffness of the member decreases and the deflection increases which increases the overall size of the member. Prestressing concrete remedies this problem by inducing a compressive stress in the concrete thereby reducing the net tension in the member and increasing the load required to crack the member. Traditional prestressing is generally limited to large, straight members. During the last decade, shape memory alloys (SMA) have become more prevalent in engineering and civil engineering applications. The shape memory effect refers to the contraction of the SMA when it is heated to its austenite phase. When a prestrain is induced in the SMA, it can be recovered when it goes through the phase change. Nitinol, a NiTi shape memory alloy was used in this research. Thin, steel cables were also tested to provide a comparison. Two different Nitinol alloys were studied in this research. The alloy M wires were elongated to 8% stain while the alloy X wires were prestrained by the manufacturer. The wires were cast into thin concrete beams and once cured, the beams were heated and a phase change from martensite to austenite occurred in the Nitinol. As a result, the Nitinol contracted and compressed the concrete. The SMA fibers are randomly oriented and allow prestressing to occur along all three axis. This is ideal for thin, curved specimens. Third-point bending tests showed that the SMA fibers prestressed the concrete and upon reheating the cracked specimens, the shape memory effect provides partial crack closure.

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Shape memory alloys (SMAs) possess the unique ability to change their shape by undergoing a solid-state phase transformation at a particular temperature. The shape change is associated with a large strain recovery as the material returns to its "remembered" shape. Their ability to act as both sensor and actuator has made them an attractive subject of study for numerous applications. SMAs have many characteristics which are advantageous in space-related applications, including generation of large forces associated with the strain recovery, smooth and controlled movements, large movement to weight ratio, high reliability, and spark-free operation. The objective of this work is the further development and testing of a cryogenic thermal conduction switch as part of NASA funded projects. The switch was developed to provide a variable conductive pathway between liquid methane and liquid oxygen dewars in order to passively regulate the methane temperature. Development of the switch concept has been continued in this work by utilizing Ni-Ti-Fe as the active SMA element. Ni-Ti-Fe exhibits the shape memory effect at cryogenic temperatures, which makes it well suited for low temperature applications. This alloy is also distinguished by an intermediate phase change known as the rhombohedral or R-phase, which is characterized by a small hysteresis (typically 1-2 deg C) and offers the advantage of precise control over a set temperature range. For the Ni-Ti-Fe alloy used, its thermomechanical processing, subsequent characterization using dilatometry and differential scanning calorimetry and implementation in the conduction switch configuration are addressed. This work was funded by grants from NASA KSC (NAG10-323) and NASA GRC (NAG3-2751).

In the range of 25°C - 80°C (ultra-low grade heat), a large quantity of waste heat from various processes is available unused. Special alloys made of nickel and titanium, so-called Shape Memory Alloys (SMA), could be an alternative technology to Organic Rankine Cycles to make this energy usable in the low power range. The 'THEAsmart 2' research project is therefore investigating the service life and energy lifecycle of this material to test the benefits of shape memory alloys in energy recovery and the efficiency levels that can be achieved. To this end, a demonstration prototype is being built that converts thermal energy into rotary motion. The next step is to link the demonstration prototype with a conventional fluid pump to create an SMA fluid pump that is driven by the thermal energy of the fluid to be pumped. The advantage of such a pump would be that it would be energy-independent, i.e. it would be operated solely by the thermal energy of the fluid without an electrical connection. Furthermore, such a pump could contribute to energy savings if it is used in cooling circuits in which the thermal energy of the fluid is the waste product from another process. In this case, it replaces an electric pump and utilizes the 'waste product' heat. The aim of the project is to investigate how and whether coil springs made of shape memory alloy are suitable for energy recovery. This is considered via the energy lifecycle: if more energy is required to manufacture a spring than this spring can convert kinetic energy from thermal energy in its lifecycle, then its use for energy recovery does not make sense in principle. As a secondary result of this research, statements about the efficiency of shape memory alloy coil springs and statements about their service life are expected.

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The Whole Skin Locomotion robot propels itself using a motion similar to the cytoplasmic streaming exhibited by an amoeba. In the robot there are embedded ring actuators which evert the material of the robot to produce forward motion. The robot benefits from a highly flexible exterior allowing it to squeeze into constricted passageways or collapsed structures. The development of actuators for such a motion is performed by a shape memory alloy composite actuator. Unlike a typical composite model which utilizes a homogenization of fiber and matrix properties our model is developed for line loads produced in individual shape memory alloy wires onto the rod structure. The load vectors are determined in the deformed configuration of the actuator to account for the highly deformed actuator profiles that would be seen in operation. Also the load requirements for such actuators are developed in terms of the constriction forces and functional design limits are established. In addition, a helical spring backbone design is considered and stiffness properties for general helical springs are determined. The contact of spring coils is included in the analysis and a coupled constitutive model is developed for the spring when coils are in contact. The static design of helical springs for use in the actuators is performed and deformation and load restrictions are determined for subsequent design efforts. / Ph. D.