Concept of Smart Materials
Smart Materials: Numerous new, high-quality, and cost-effective materials have been introduced into structural engineering as a result of material science advancements. A strong example is smart material. Since the Naval Ordinance Laboratory first investigated nickel-titanium smart materials, they have quickly found uses in a variety of fields including aerospace, electronic, and biomedical engineering. Nowadays, an increasing number of researchers are concentrating their efforts on smart materials due to their unique characteristics and performance in civil engineering applications.
At different temperatures, smart material will occur in two phases: Austenite, which exists at high temperatures, and Martensite, which exists at low temperatures. As the exterior temperature or stress state increases, one of these two steps will turn into the other. During the transitions between these two stages, smart material shows a variety of unique properties, including shape memory, superelasticity, and two-way memory.
Perhaps the most vivid demonstration of shape memory alloy’s ability has been the tv advert in which an eyeglasses pair is wrapped around someone’s wrist. When the wearer lets go, the eyeglasses snap back into their original form. This phenomenon is referred to as superelasticity. Form Memory Alloys (SMA) are able to do so due to their shape memory property. When a wire is twisted, the added tension allows the wire to undergo a phase transition from Austenite to Martensite. When the wire is released, the stress decreases to zero and the wire reverts to its initial Austenite phase and form.
Smart materials and structures are increasingly evolving as a result of technical advancements in engineering materials, sensors, actuators, and image processing. Self-adaptability, self-sensing, memory, and various functionalities of materials or systems are also terms used to characterize smartness. These characteristics allow these materials and structures to be used in a wide variety of applications in aerospace, construction, civil infrastructure systems, and biomechanics. Self-adaptive properties of smart structures are a significant advantage that take advantage of embedded adaptation in smart materials such as form memory alloys. Smart materials can detect flaws and fractures by altering their properties, making them useful as a diagnostic tool. This property can be used to compensate for the fault by activating the smart material contained in the host material appropriately. This is referred to as the self-repairing effect.
Shape Memory Alloy (SMA) is a type of adaptive material that is capable of directly converting thermal energy to mechanical work. Multiple heat treatments produce this effect in a number of alloys when they are correctly combined. Since the early 1930s, the shape memory effect in various alloys has been reported, resulting in a range of commercial products in the mechanical and aviation applications. Ni-Ti is the most frequently used SMA material.
As SMA is deformed at low temperatures T As (where As is the Austenite start temperature), it undergoes a mechanical twining mechanism in which each neighboring layer of atoms shifts by one lattice parameter. As heated above Af (where Af is the Austenite finish temperature), the sample undergoes a phase transition, recovering the majority of the mechanically induced deformation and returning to the specimen’s initial “memory” shape. This is known as the one-way memory effect. A two-way memory effect can be accomplished by a special heat treatment technique. As heated and cooled, the two-way memory alloys exhibit significant and opposite deformations, making them ideal for use as two-way actuators. That is, heating the SMA produces one memorized shape, while cooling produces another. The SMA will expand or contract in response to this two-way impact, allowing it to add and eliminate stress in a system on an as-needed basis, resulting in a smart structure.
Not only does the phase transition temperature depend on the material structure, but also on the state of stress. As the stress level increases, the phase transition temperatures can change as well. This means that the phase transition can also be triggered without a change in temperature, as shown by the eyeglass illustration above. This property has a number of potential applications in self-repair. As concrete structures begin to crack, the increased stress in the SMA can cause phase transition, resulting in compressive stress in the structures, thus limiting the cracking. Another significant property of SMA is that it can shrink as a result of phase transformation. Additionally, this shrinkage should be used to compensate for thermal expansion. As a result, SMA can be an extremely promising material for resolving problems associated with temperature-induced stress/cracking.
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