# An Ample Review on Compatibility and Competence of Shape Memory Alloys for Enhancing Composites

Authors: T. Gopalakrishnan; M. Chandrasekaran; R. Saravanan; P. Murugan
Journal: Advances in Materials Science and Engineering
 (2022)

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## Abstract

The name shape memory alloy (SMA) reveals its behavior of being an accurate heat-sensitive material in changing its shape based on the temperature. This ample review concentrates on the current scenario of including SMA in polymer matrix composites to achieve desired objectives. Polymer-based shape memory alloys are termed shape memory polymers (SMP), and they consist of deformable materials that are able to switch between their original shapes and temporary shapes, which can be generously designed. SMPs could be classified as smart materials by considering their low density, good biocompatibility, excessive deformation etc. On the other hand, many engineering applications of SMP uses have limitations and disadvantages. In this regard, the importance of SMPs has been analyzed based on the following aspects: synthesis method, fiber reinforcement, parameters that affected the polymer-based SMA, and implementation of multifunctionality materials. Fiber-reinforced polymer composites have more responsibilities for expanding interest in current innovative research and expected mechanical applications because of their significant space compared with conservative materials. A polymer composite presents effectively adaptable product properties, expected high strength-to-weight ratio, high flexibility in the manufacturing process, high corrosion resistance, and easy fabrication at a lower cost.


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## Body

## 1. Introduction

SMPs that are now frequently used in aerospace, renewable power generation, and automotive applications were produced in response to an increasing necessity for reducing weight technology problems. The combinations of adding more materials made polymer matrix composite materials, which establish in nature. The improved production seeking materials with high effective rigidity was one of the important early reasons for the advancement of composites. Composite materials have subsequently been developed that take advantage of their underlying heterogeneity and anisotropy to integrate conventional massive amount capabilities with unexpected functionality in the form of inserted components. The new addition of SMA in polymer composite extends its flexibility or “alert” property of composite materials, and the same could organize thrusters and sensors.The thin SMA wires reinforced polymer composite materials and increased the applications of polymer composites. In the last five decades, the role of SMAs as larger particles is found in a variety of applications, such as controllers and dampening, as well as in the microtooling sector. SMAs have advantages over other actuating technologies, which are as follows: high changeable strains and structure recovery, rising absorption capacity, phase transformation changes of physical and mechanical properties aspects, and the capacity to generate higher absorption burdens once they are stopped from attempting to restore their structure [1]. The thin SMA wires (about 0.2 mm in diameter) are widely utilized to integrate and make modifications in functional synthetic structures while preserving the original microstructure and physical features and preserving the weight reduction with the economic advantages of blends. Consequently, hybrid composites, including SMA wires, might display features that include structural changes, managed thermal contraction, a switch in intrinsic resonant frequencies after initiation and risk in demaging avoidance and restoration. However, since the SMA transformation is generally triggered by changes in temperature or tension, the response time governed by heat transfer dynamics is severely constrained. Furthermore, SMA wire incorporation should be suitable with the polymeric matrix, which necessitates changes to the manufacturing method and the product design to preserve adequate structural qualities while limiting excess weight.The primary objective of this research is to explore SMA cables’ applications in polymer structures, lightweight structures reinforced by natural or artificial fibers from the point of view of composite fabrication, and the properties that result, which include vibration shock absorbers, structure transformation, fracture completion, and low-velocity impact reduction. The focus of this study is on SMA-reinforced polymer composites. The performance of these composites can be influenced by a variety of factors, such as the placement of reinforcing components, curing, and other environmental factors. Biopolymers are also utilized for altering the mechanical and physical properties. Constructing hybrid composites could be reinforced with traditional modern fibers. Rogers et al. [2] were the first to suggest the concept of SMA-based hybrid composites (SMAHCs). SMAHCs have been originally made by strengthening SMA wires and strips in a polymeric matrix. SMAs were adsorbed onto the substrate of the structure, whether straightforwardly incorporated into composite [3]. Wang et al. [4] investigated the Ni-Ti shape memory alloys by polypropylene, which was used as the reinforcement material in geopolymer concretes for improving the mechanical properties of shape memory polymers [5, 6].Prestrained SMAs have been used in many circumstances, so that the natural shape of polymer composite could be obtained by heating to that predefined temperature. There is a corresponding increase in stress involved with slips and displacement action, which already happened [7]. Figure 1 depicts the technique for prestraining SMA as per standard ASTM E3098.Figure 1
Shape memory alloys prestraining.Materials and structures have a fundamental change through technology, mainly in the area of structural engineering, robotics, and aerospace, resulting in an ever-increasing necessity for perfectly capable, lightweight, dynamic systems. The smart material group nature behaviors of SMAs, such as recovery stress, tremendous actuation strain, high-specific actuation energy, etc., lead to extended applications in synthesizing the components of adaptive structures. The superelasticity and shape memory effect (SME) are the recommended properties. SMAs offer these viable individuals for composite materials and appropriate good structures [8]. SME takes the form of shape deformation at the martensite phase at low temperatures, followed by the austenite phase at high temperatures for shape recovery. Unless the recovery is restricted, considerable stress and strain are created in SMA. Superelasticity is defined as a significant recovery of strain after displacement with temperatures above the austenite completion temperature. The accumulated actuation stress level from the SME can range from 100 to 700 MPa, which is considerably high as compared with low-power source actuators that range between 20 MPa and 70 MPa and between 1 MPa and 9 MPa for piezoelectric actuators. Now a days, different types of smart materials have arrived, such as Fe-, NiTi-, and Cu-based SMAs. Specifically, Nitinol alloys are most widely investigated because of superelasticity, maximized tensile strength, high corrosion resistance, force-to-weight ratio, and biocompatibility. Phase transformation by Ni-Ti SMA is used as a sensor with the change of property of resistance for measuring the strain. Furthermore, ferromagnetic shape SMAs are used based on the magnetic field by utilizing ion-based shape memory alloys. FSMAs could be employed as both a magnetic sensitivity and heat-sensitive detector [9]. SMAs are employed in a variety of technical fields to handle a variety of problems. In particular, these alloys can be employed like built components or as reinforcement material. SMAs are available in a variety of topologies, including cables, poles, bands, actuators, aluminum foils, and sometimes even foams.The hybridization of thermoset-based composite materials improves the matrix’s heterogeneous nature and low deformation characteristics. Hybridization with more durable fibers is also a conceptual model for novel hybrid composites that would improve strength properties. The load-bearing performance is enhanced while using the robust graphene fibers. The impact strength of a hybrid composite comprised of S-glass, Kevlar fibers, or graphite/epoxy increased moderately. Created by different graphite/epoxy composites containing SMA fibers, on the other side, can significantly improve their mechanical characteristics. Figure2 shows the process flow cycles of shaped memory compounds embedded with the mechanical process with the desired shape outputs. The strain rate in typical engineering materials, including Kevlar fiber, graphite, and glass fiber, ranges from 13 to 131 MJ/m3, whereas that of conventional materials, such as aluminum and iron, range from 0.689 to 4.13 MJ/m3. Advanced materials matrix-based composites, such as glass/epoxy and graphite/epoxy, get the ability to strengthen functionality by including SMA and certain other fibers. Creeping control, impact stress, acoustic characteristics, vibration control, and shape and position control in the structure have been accomplished using shape memory polymer composites [10, 11]. The programmed pattern of SMPCs is depicted in Figure 2.Figure 2
Shape memory polymer composites programmed pattern.This article mainly revealed the effect of fiber reinforcement and types on shape memory polymers by the effect of a different method of fabrication and their principles. The past research analyses of applications in different scenarios have been illustrated. Furthermore, the process techniques of property enhancement and quantity have been addressed in a separate section. Subsequently, the essential factors such as connector-associated requirements, SMA acceleration and deceleration, thermal effects, best wire positioning, best actuation, and weight impact are to be considered while designing a matrix with encapsulated SMA components. Furthermore, SMP, through existing and future uses, as well as the remaining challenges that need to be overcome before all resulting in the ability, may be widely used in commercial processes.


### 1.1. Overview of Shape Memory Polymer Composites

The complex and hazardous gold-cadmium alloy was the first of this group of metal materials to also be identified in the twentieth century. Nitinol alloys are nontoxic and a little more expensive, and they became the dominant class of SMAs in the 1960s and figured prominently in a diverse range of products [12, 13]. The prepared shape memory alloys are characterized based on the following three main properties:(1)
Reshaped from the state of martensitic deformation while heating a shape memory alloy(2)
Damping and vibrational characteristics in the martensitic state(3)
Austenitic state super elastic nature in SMAsShape memory polymers composite (SMPC) of mono-way (one-way) and dual-way (two-way) transformations are utilized in their crystal structure from Austenite to martensite (Refer to Figure3). The change in the crystal structure is linked to the intersection of the atomic layers above the distance in the atomic layer. As a response, thermodynamically stable martensitic variations occur within the morphology since SMA is chilled in the phase of cubic austenitic to its transformation temperature under no stress. There is no macroscopic structural change because the variations are broken in all directions. Self-accommodated martensite (SAM) is the term applied to the resulting structure.Figure 3
Mono- and dual-way shape memory effects.At the time of the material experiencing external stress, the maximum favorably orientated variations rise without a regard for the smallest favorably directed, generating detwinned martensite (Figure4). The phase transition could be examined using thermal mechanics since such a phase transition is initiated by a bit of Gibbs free energy. The martensitic phases become established when the primary phase GA possesses higher Gibbs free energy than the martensitic GM. The Gibbs free energy diagram clearly shows the equilibrium state for GA and GM in martensite transformation below T0. The cooling process initiated the nucleation and was accomplished by martensite at a lower temperature. The reverse transition occurs during heating, commencing at austenite start and terminating at Af (austenite finish).Figure 4
Stress-strain diagram of shape memory alloy (SMA).In particular, composite materials are significant because various components supplement each other, promote collaboration, or enhance the performance of polymer composite. Composites for SMAs are developed with a couple of objectives in consciousness, which are as follows: strengthening by reinforcement and the establishment of novel and efficient stimulation techniques. Versatile and adaptive shape memory polymer materials, which have self-healing [14–16], drag reduction [17], and refractive features [18–22], are other intriguing research fields. The shaped memory can be used as a substrate for polymer matrix compounds, a matrix, binding agent, or compounds, which are discussed in detail. The simulation model plays a vital role in predicting the reinforcement effects in particularly self-healing functions and advanced subfunctions, which help to examine the SMA composite structures in multiple views, such as different dimensional views and sectional views, and isolate the views for examining the layers of ceramics, films, and metals. To make a versatile composite material, SMP can be employed as the form of fibers. Two-way recovery, functional filler or sensitive materials, and more helpful functionalities can be achieved with a good composite material. The investigation of SMPC is motivated by such appreciable demand to determine the class and quantity of filler in an appropriate manner. The intergraded from shape memory alloy for the shape memory polymer was illustrated in Figure 5. Figure 6 comprehensively demonstrates the various possibilities of SMPC. Excessive filler reduces the thermal qualities of the material, resulting in a decreased transformation temperature. Some of the fillers, instead of enhancing the shape memory effects on polymer composites, degrade the final shape memory effects (refer to Figure 5).Figure 5
Shape memory polymer integrated from a shape memory alloy.Figure 6
Shape memory polymer composites.


### 1.2. Stimulation Methods

Stimulation methods are frequently employed in the recovery of shape memory effects and some programming operations, apart from enhancing mechanical and thermal properties. There seems to be a necessity for nonthermal responses in a diverse variety of polymeric composite materials, including biomedical devices, actuators, and aerospace structures. Figure6 shows different new features, such as the reduction in the image memory effect and the effect of the shape memory polymers because of the addition of smaller fillers. Electromagnetic, ferromagnetic, and optical responses and more precise and quasi-stimulation approaches are crucial stages in the implementation of polymer-based shape memory composites. The alternative option is to use a filler to provide secondary stimulation that is simple and efficient by doping with adsorbing materials and magnetically conductive SMPC, and it can also be stimulated with the use of ultrasonic, solvents, and other methods.


### 1.3. Electric Stimulation

Electric stimulation (ES) is one of the very exact and convenient methods of stimulation. The utilization of the current-driven form has resulted in a massive growth in shape memory composite materials. Implicitly conductive polymers still have not been discovered, leading to limited electrical stability and low permeability. When constructing an ES of EMPC, several difficulties have been taken into consideration [23, 24]. The modified filament is first constructed into something of a three-dimensional active network structure. For example, under a magnetic field, Nickle powder forms a chain [25, 26], and connecting short carbon fibers with carbon black can improve conductivity [27]. Secondly, surfactant [28] and chemical modification are commonly utilized for making the loaded conductor more consistently and permanently absorb into the polymer matrix. The sample of electrically resistive shape memory polymer composite of Ag/CCF/H-EP with 5.4 wt% Ag/CCF activated by electricity under 60 V [29] was presented.


### 1.4. Light Stimulation

Light transmission is also a noncontact stimulation, and it is one of the most common and practical methods. It is ideal for medical instrumentation and biological applications in which regular contact stimulation is problematic. There are two types of approaches to these two modes of transportation: direct steering and indirect steering. Spontaneous redox linkages are common in light-reactive shape memory polymers, such as the cinnamic acid’s cycle-addition reaction [28, 30]. Nevertheless, as the polymer has no intrinsic increase in temperature, the electromagnetic thermal transfer can only be accomplished inadvertently. Magnetic drives are analogous to electrical machines in that they address the doping and matrix connection issues. Other magnetic driving mechanisms have not been documented so far.


### 1.5. Magnetic Stimulation

The magnetic stimulation principle is opposite to the light stimulation method as it directly gets in touch with SMA. By adding soft and hard magnetic materials to shape memory polymers, we observed indirect heating principles. This kind of heating is ideal for medical devices like implanted shape memory polymer scaffolds. Also, this technology is strengthened by a heating technique that combines electromagnetic fields and direct heating [31]. The requirement for ambient heating is variable since magnetic heating is changeable. The material’s perceived switching temperature is thereby altered in this manner. They also created magnetic memory effects to form memory polymer nanoscale composites by adding magnetic elements to SMPCs with temperature memory effects [32]. Smoukov et al. synthesized a thin membrane with Fe3O4 and Nafion material for magnetic driving with the adjustable realization of up to four predefined shapes [13]. Magnetic stimulation is like magnet-induced deformation and shape recovery measurement [33]. The surface temperature is maintained around body temperature (38–400°C) even when the area ambient approaches 800°C, which suggested tremendous promise in clinical uses.


### 1.6. Wetting Stimulation

Solvents can also be used to speed up the shape recovery process. An SMP is with either a hydrogen relationship or a glass state transformation. SMPs are among the modified solvents discovered to govern shape memory polymers. Plasticization is the essence of solution actuation for an SMP with a glass new system implementation. Because of the swelling and polarization effects, the polymer network portions progressively soften as moisture levels rise, and the heat of the glass phase transitions drops. The rate at which solvents permeate utmost plastics, such as phenyl ethylene, epoxy resin, etc., is extremely slow. Leng et al. showed in 2014 that an epoxy resin SMP loaded with sodium dodecyl sulfate may be driven by moisture [34].The effect of form memory is a kind that could be generated by scratching with the help of composites. A nanotechnology polymeric composite material of Poval and oxidized graphite took on transitory forms when heated to a specific temperature and returned to their initial form (shape) when exposed to moisture [35]. The transition period of this SMP has been dependent on the bonds of hydrogen. Adsorbing and heating on the surface might dissolve the chemical bonds, allowing form recovery by unfreezing macromolecular chain sections frozen at the oxidized graphene surface. The elastic thermoplastic polyurethane with microsized cellulose whiskers is included in the polymer structure to enhance shape memory effects [36]. The SMP with manually created cracks obtains tunable hydrophobicity. A new SMP composite (SMPC) was synthesized by [37] that clings to a coating of collagen nanocrystal reinforced carbon black in the polyurethane microsized skeleton, thereby imposing a shape freeze. It can be accomplished by the use of heat, wetness, and a variety of other stimuli. Polyurethane is an entropic elasticity system in the three cases above. However, it did not exhibit shape memory effects. That system remained frozen after the inclusion of nanosized structures capable of forming the bonds of hydrogen, and it exhibited the effects of shape memory at that time, which indicates the development of shape functional decline to a certain measure.The fundamental concepts of SMP are discussed, although no details were given. As a result, most of this article is devoted to a common topic that could be observed in every SMP analysis. The simulation methodology, a crucial asset of the SMP, is not discussed in this section. The shape memory polymers, which are extensively employed in the industry, are based on phase transition. The driving mechanism is limited if the composite material is created. Through the process of switching and SME classification, SMP can be classified into molecular/supramolecular switching and physical bonding. SMPs can be classified as mono-way or dual-way based on the directionality of the SME, such as dual-way shape memory effect, which has only been documented in shape memory alloys, or as dual shape memory effect or multishape memory effect, based on the number of constant configurations (Figure4). There seem to be numerous subtle variations to be made. Their emphasis is on the multiple shape memory polymer-based physical bonding. It is efficient and well-established in industrial applications. Based on the thermal mechanics, the SME actuation method is shown in Figure 7.Figure 7
Shape memory effect on polymer composites.


### 1.7. Principles of SME

The SME in an SMP-based glass transition is predominantly compared with the following configuration of the material: the continuous stage to sustain the natural structure of the structural properties and the bidirectional stage of hardening and softening, according to molecular dynamics. The shape of the bidirectional stage could be maintained whenever the temperature is below the Tg region. Alternatively, thermodynamic elasticity causes the polymeric matrix to return toward its initialization stage. The solid phase assures that the circuit displacement is unidirectional and that external pressures do not cause the flow to be viscous. Physical bonding and chemical bonding in general correlate to thermoplastic and thermosetting shape memory plastics, correspondingly. The retention of such natural morphology could be assured whenever the Tg band of the solid stage is high and softness and loosening need not happen in the ambient temperature with the use of the element. Softening and hardening can happen as the temperature varies because of the low range of bidirectional component Tg, and at higher temperatures, it has a high deformation capacity.


### 1.8. The Course of Action on SMPC

As a result, the shape memory effect encompasses both product and manufacturing impacts. The phase transformation principles of SMPs permit them to appropriately illustrate with the help of a standardized linear solid model (SLS model) for the cycle of SMPs. The presented stress-strain-temperature curves for each state position are most relevant. Researchers demonstrate a dual-SME with rapid programming, short recovering, and no-load [31]. The most noticeable mechanical features of shape memory polymers are identified. The polymer exhibits viscous optimum activity temperatures and general elastic behavior for relatively low temperatures, demonstrating that viscosity has strong temperature dependence. Viscosity fluctuates among the maximal and a smaller quantifiable value throughout the stage transformation period. Initially, the dashpot’s viscosity is decreased to almost undetectable levels after heating, and creep occurs under an impact field to get the desired shape.The viscosity of the fluid increases dramatically, while the shape is preserved by an applied stimulus, and the temperature is reduced underneath the transition ambient temperature. Once the external force is withdrawn, the programmed shape remains the same, except for a minor general elastic response, and the shape construction operation was already complete. The viscosity of the dashpot diminishes when reheated far beyond the crossover range of temperature, and the solution returns to its natural characteristics. This definition describes the shape memory effect’s important mechanical mechanisms, and it is challenging to get an accurate description of the program’s details, thus rendering improving the viscoelastic modeling difficult.As the shape memory polymer’s process of recovery might transmit an external force, the SMA seems to have a propelling performance. However, energies were stored in this process by mechanical deformation during the programed stage, and external stimulation is needed to open the switch. As a result, when we discuss the stimulus afterwards, we avoid using the term “driving technique,” which is susceptible to contradiction. Above that, the shape memory cycles of hot programming, followed by hot recovery, dual-SME without load, and hot programming, have been employed to determine if the SME is positive or negative, which is generally accomplished by defining various indicative shape memory cycle metrics, such as the fastening and retrieval rates. The reduction of the fastening rate is made of two parts: generalized elasticity structure restoration owing here to the elimination of stress factors after that procedure and cold restoration during extended storage. Cold recovery is usually affected by the nature of the material and its loading circumstances, such as chemical environment external force and humidity. The faster cold restoration occurs the larger the external pressures and greater the energy that moves to the previous shape [24, 38, 39]. Recovery can also be triggered by the surrounding environmental conditions, such as moisture. The rate of recuperation is affected by the temperature. Furthermore, the attachment and recovery rates show the material’s effectiveness during a certain shape memory cycle. They are process-dependent quantities that are influenced by strain, temperature, period, and often even the strain rate.


### 1.9. Modeling

The stress-temperature phase diagram for the Brinson model is depicted, [40] in which the transition temperature at zero stress is represented by the intersection of boundaries in the temperature direction. Here on a graph, there are regions that reflect the pure phase and regions that demonstrate the coexistence of many phases [D]. The transition temperature is considered to vary continuously with tension. The stress contribution factors of CA and CM are representing the slopes of the austenite and martensite segment lines, appropriately, as well as the consequences of stress and overall temperature changes. The finish and start of critical stresses are, respectively, denoted as σfcr and σscr. The SMA function may vary with respect to the volume fraction. The temperature-induced twinned martensite and stress-induced detwinned martensite classifications are used for slope behavior (refer to Figure 8).Figure 8
Brinson model.The final volume of martensite is calculated by the following:(1)ξ=ξs+ξT.Stress influenced volume fraction during the detwinned martensite, the terminologiesξs, σ0,ε0,ξ0sand T0 represents the initial state of martensite.(2)σ−σ0=Eε−ε0+Ωξs−ξ0s+θT−T0.θ− represents the E-Young’s modulus and thermal expansion estimated value from the rule of mixture.(3)ξEM−EA+EA=Eξ.Transformation coefficients are as follows:(4)Ω=−εLEεL−Maximumresidualstrain.The condition for detwinned martensite conversion is as follows:Ms < T [41].(5)σscr+T−MsCM<σ<σfcr+T–MsCM,ξs=1−ξ0s2×cosπσscr−σfcrσ−σfcr−CmT−Ms+1+ξ0s2,ξT=ξ0T−ξ0T1−ξ0sξs−ξ0s,ξs=1−ξ0s2×cosπσscr−σfcrσ−σfcr+1+ξ0s2,ξT=∆Te−∆Te1−ξ0sξs−ξ0s,Mf<T<MsandT<T0,∆Te=1−ξ0s−ξ0T2cosam(T−Mf+1−ξ0s+ξ0T2,am=πms−mf,∆Te=ξ0T.The condition for austenite conversion is as follows:As<T(6)T−AfCA<σ<T–AsCA,ξ=ξ02cosaA(T−As−σCA+1,aA=πAs−Af,ξs=ξ0sξ0ξ−ξ0−ξ0s,ξT=ξ0Tξ0ξ−ξ0−ξ0T,where ξ0s and ξ0T are the volume fractions of martensite stress-induced.


### 1.10. Differential Form for Constitutive Relations

The shape memory alloy fiber is subjected to an elastic limit, and stress increases for the relation expressed in terms of deformation as follows:dεs and αA are stress-induced martensite sites at the initial stage and thermal expansion, respectively [42].(7)dσf=Efdεf−εLEfdεs−EfσfdT.Ef and αf are nonconstant materials functions.(8)Efξ=1−ξEA+ξEM,αfξ=1−ξαA+ξαM.For shape control, the expressions used are as follows:(9)dεf=1Efdσf−εLdεs−αfdT.The volume fractionξs can be expressed as subjected to martensite with stress influenced factors.Substitutedξs in the above deformation equation:(10)dσf=Ef1+εLEf∂ξs/dσfdεf−εL∂ξs∂T+αfdT.The strain can be replaced bydξs.(11)dεf=1Ef+εL∂ξs∂σfdσf+εL∂ξs∂T+αfdT.


### 1.11. Heat Transfer Formulation

For actuator applications, shape memory alloys could be actuated during the application of voltage for Joule heating. Primarily, thermal resistance by heat linked to enthalpy (H), heat capacity (Cp), and convective heat transfer (h) for the transformation of the phase from the phase of martensite to the phase of austenite was reflected in the temperature equations of the modeling of the shape memory alloys as [43, 44](12)V2Rw=hAsurTt−Ta+mCpdTdt+m∆H,where m, Ta, and Rw represent the mass, ambient temperature, and wire’s electrical resistance, respectively. The austenite’s volume fractions and martensite’s volume fractions are used to determine the resistance of shape memory alloy wire.The cooling expressions are as follows:(13)hAsurTt−Ta+mCpdTdt+m∆H=0.


### 1.12. SMPs Applications

This review article focused on polymer shape memory alloys and their reinforced composite materials. In 1960, the first large-scale application of PE thermal contraction tubes gained the shape memory polymers. Currently, nylon and polystyrene materials are used to fabricate the contraction tubes [45, 46]. A thermal contraction tube consists of good tolerance, and temperature properties enhance erosion prevention and better insulation flame retardancy properties. There is a huge scope for the use of shape memory polymers, and consequently, they are pulled into recent research activities.Researchers have to understand the behavior of polymer-based composites, such as SMA, to design applications that rely on them. Initially, we shall look at two-phase shape memory polymers, which have a lot of beneficial characteristics and are closely examined. The most important fact is it exhibits standard recovery that is load releasing process, and shape memory polymers must only shift from transitional shapes to their native shapes unless they are reconfigured. Although the stimulations during the recuperation process are extensive, another motion limits their uses. For such a reason, mainly one-way deformations are considered while fabricating the shape memory polymers. An important form of these kinds of technologies is the solution of transport difficulties by the flexibility of transport and restoration of morphologies required for operations at certain periods, such as geographically growing structures and lightly obtrusive operational equipment. Self-arrangement combined with cold programming under operating conditions is another important aspect of shape memory polymer. In this scenario, form restoration is being used to enable instrumentation and objects to recover from accumulated plastic deformations throughout usage, thus increasing the life duration. Natural polymers are also elastic adhesive elastic materials, which means they have normal viscoelastic characteristics without vibration and adhesive resilience when activated. This is something that should be considered in industrial applications. As biomolecule materials, SMPs’ programming and restoration too are creeping phenomena, keeping their shape memory characteristics over time. Similarly, the reaction time confers low impulse to smart structures made of shape memory polymers, however, it also limits applications requiring instantaneous reactions.Although shape memory polymers were also constrained to lesser thermal properties and a specific treatment mechanism, shape memory polymer matrix composites become especially beneficial. SMPs are used in the aerospace sector because of their structural qualities, such as appreciable ductility, low weight, and extensive fastening rate. The toughness, durability ratio, healing forces, and other thermal aspects of biocomposite materials might have been considerably improved by keeping the above-mentioned features. Shape memory polymers are mostly employed in appliances, brackets, and specular highlights in biomedical device applications. Inside-the-body applications of SMP smart structures, such as endoscopic instruments, necessitate stimulation avoiding direct touch, which is frequently achieved through the passive overheating of modifying compounds. Substrates, sensors, energy collectors, and soft electronic accessories could all be made from shape memory polymers. Smart fibers and other applications benefit from shape memory polymer matrix composites.


### 1.13. Aviation Applications

Shape memory materials have gained a lot of interest because of excellent stable standard capabilities of deformations. An aerospace component which has expanding designs with computer locking structures are notable example [47]. Previous investigation of new structures featured novel materials inside aviation and various applications of hinges, which are spatially expanding. They are primarily using shape memory alloys as driving factors [48]. Shape memory alloys have been challenging to use as structural parts, given their high density, and they had to be supported through lightweight boards or wire materials, which made the design quite difficult and choosing the associated structural components troublesome.If the shape memory characteristics are found insufficient for SMAs or their composites results, SMPs become a prominent figure in the field of conflict resolution. Researchers effectively obtained concepts on the fundamental variations in design concepts by evaluating the characteristics of fiber-reinforced SMPs that is SMA. Shape memory polymer composites may be formed into the matrix of major expansion designs, thereby accomplishing the integration of controlling mechanisms and structural materials because of low density, high hardness, and massive distortion [49]. The usage of polymer-based SMP composites in the aircraft and aerospace areas has been closely researched, and their considerations should be taken into account. In this post, we shall go over the design ideas in further detail and explain several common uses, such as a large-scale extension.The atmosphere is even worse than the conditions in space. Polymers and other biological molecules will be degraded more severely by atomic oxygen and UV radiation than metals and ceramics and would be exposed to severe changes in temperature, resulting in polymer mass loss, degradation in dynamic characteristics, or maybe even full-impaired function [50, 51]. As concluded, space-specific SMPs are to be carefully chosen and confirmed. Such studies for shape memory polymers must compensate by not just standard concerns, including shrinkage, constituent changes, and elasticity modification, but also shape memory voltage drops. It has already been demonstrated that glass fiber-reinforced polymer and cyanate acylation shape memory polymers can withstand the brutal environments of space. Polyimide-based shape memory polymers were also recognized as potential alternatives for the possible development of SMPs in the environment because of their remarkable thermodynamic characteristics and chemically flexible properties. When manufacturing shape memory polymer composites, additives with great space sensitivity, such as carbon material and fiberglass, should be used. Special design considerations are required when doping with materials that have dynamic physicochemical characteristics, such as chemical admixtures, to avoid certain dynamic qualities from ever being impacted by cosmic rays.


### 1.14. Biomedical Applications

Vernon et al. [52] reported in the fourth decade of the last century that SMPs are highly suitable for applications in biomedical instruments and implants. SMPs were first introduced by the invention denitrify. Polymers, in contrast with metallic alloys, have a better restoration strength and biological flexibility. Several elastic polymers, such as polythene, polyurethanes, polyesters isocyanate, and acrylic acid, can be destroyed in the organism. The controlling methodologies of shape memory polymers can be enriched by developing lightweight structures. Light navigation and magnetic navigation are two of them as they allow for greater control. Shape memory polymers have a lot of potential applications in medical tools that are deployed within the organ because of such two qualities. As reported in technology demonstrator and experiment reports, shape memory polymers and related structural components are utilized in various biomedical devices. Orthodontic technicians, medical threads, cardiovascular stents, aneurysm specular highlights, and thrombus cleaners are all examples of their applications. Buckley et al. [53] invented new plant-shaped and deflated ball-shaped surgical devices from SMP and SMP foam with electromagnetic performance-enhancing drugs that might be forbiddingly expanded in the organism by magnetostrictive stimulation. To achieve IR-stimulated form recovery, it was proposed that the shape memory polymer could be wrapped in a layer around fiber [54]. Those intermediate controlling technologies that paved the way for SMPs should be utilized in surgical devices. A ferromagnetic shape memory material was developed for drug dissolution mechanism based on conventional heating activation using double-Cu-coated polyimide.Lendlein et al. [55] developed a surgical thread consisting of shape memory polymers that transformed at a temperature close to that of the human body. Animal experiments showed that as the temperature climbed to 410°C, the suture recovered its shape and closed the medical wounds. A stent might be stretched to solve the issue of regular form metal, restricting medicine transportation by narrowing arteries. Shandas created a permeable stent that improved the quality of the modification required and made microsurgical cardiology procedures more convenient. Decomposable SMPs described for a urinary stent may enable guided administration and disintegration. However, they developed a directive loading structure built of polymer-based composites, with a single-step and multistep shape recovery and a consistent release of such an efficient payload. Cho et al. used melt spinning to manufacture SMP threads for utilizing polymeric orthodontics base elastic polymers. In orthodontic experiments, it was concluded that the strength of SMPs was enough to straighten uncorrected teeth as it has a balance recovery force of nearly 50 gf [56].Aneurysm coils were developed by Hampikan et al. [57] using SMP to supplement standard platinum bands and prevent aneurysms actually because of physiological resistance. The SMPs are mixed with a tantalum filler to improve their X-ray isolative properties, allowing the X-ray images continuously for a period of SMP coil to be obtained. According to the experimental tests, the blood flow is not impacted by the coil. In the medical stent device, the dosing of magnetite nanoparticles is used to improve the transparency of magnetic resonance (MR) spectroscopy [58]. It was described that a urethane foam could efficiently occlude aneurysms in animal studies. The creation of a thick internal membrane mostly on top of the foaming explosions sealed most aneurysms necks. Small and colleagues created a novel filtration design using stents and polyurethane [59]. The arrangement was developed to identify and heal the challenging wide-necked arteries, and both portions were made of SMPs. Using the expandable SMP foam developed a new form of aneurysm embolization architecture. Anchorage was provided by platinum coils, and the shape memory polymer foam is exaggerated to approximately 150 times its initial volume. Hernandez et al. [60] created a new hemostatic device composed of SMP foam, which could quickly fill injuries and accomplish occlusion and disinfection. The device’s expansion force is relatively low. Thus, there will be no secondary wound injury [61].


### 1.15. Active and Passive Vibration Control

To reduce the vibration, shape memory alloys are predominantly solicitated in composite structures. The vibration control uses the shape because of the hysteresis effect. The vibrational modes of SMA hybrid composite structures and the specific damping capacity (SDC) are hugely affected by SMA composite design parameters [62]. Vibrations can be controlled by changing the structure’s stiffness and adjusting the frequency response. The thermo-elastic transformation of the martensitic transformation is perfect. Various irreversible processes, such as crystal defect and dislocation mobility, dissipate the energy. When poor inherent damping is found in the austenite phase, the growth of the austenite-martensite interface causes flaws, and it is an important source for thermomechanical interacting.The following are the descriptions of the passive mode and active mode of the vibration control. Vibration is regulated using characteristics that could be adjusted by multiple systems in an active technique of vibration control. Carried out by an individual parameter is the heating of SMA, employing heat or externally heated surroundings. Natural frequency is obtained by the material’s rigidity. As the temperature rises beyond the achievement temperature of austenite, the rigidity of the shape memory alloy improves and the low stiffness martensite state transitions into specific strength austenite. As a result, in an aggressive state, the rigidity and free vibration of the SMA composite rises. Active property tuning (APT) that would be utilized for changing the natural frequency is covered by this technique [63]. By incorporating prestrained SMAs throughout the composite, the natural frequency would be significantly altered. Lau et al. [64] used a balanced glass fiber (0/90)/epoxy beam (25 × 200 × 1.5 mm3) to embed 0.5 mm Nitinol SMA. The natural frequency did not enhance considerably as the number of SMA wires increased, although the damping concentration rose in the dual case. As a result, the transition phase occurs when martensite transforms to austenite or vice versa. Furthermore, when prestressed SMA wires were actuated, the resulting extensive tensile stress increased the carbon and glass fibers’ natural frequency.The frequencies and amplitudes of low-amplitude vibrations are indeed a measure of the recovered stress and toughness. It can be altered by adjusting the restoration stress using SMA heating. Bidaux et al. [65, 66] looked at the vibrational characteristics of a composite (polymer/epoxy) integrated with a percentage of the tonnage of Nickel-Titanium fibers and prestrain of 5 vol. %. Rogers et al. [67] used a graphite-epoxy laminate with fundamental sensors mounted in a hybrid composite material. The graphite/epoxy/Shape memory alloy hybrid composites of the clamped beam (2.03 × 82.2 × 0.01 cm3) subsonic structural-acoustic transmissions were stabilized.Chandra [6] investigated the role of sleeves in the composite beams of graphite/epoxy by implanting the shape memory alloy. Shape memory alloys were attached to the ends of the sleeves. The sleeves ends are energized through resistive heating. In the result, the first natural frequency was noticed at the implanting of Shape memory alloy wires (2 vol. %) and substantial growths of 22%. With less than 8% volume percentage of SMA embedded, a raise of fundamental frequency 23 per cent was observed in the testing of a composite shaft made for the helicopter. NiTiCu shape memory alloy was employed to shift the natural frequency from 360 Hz at NTP of composite shaft of Kevlar/epoxy with SMA to 450 Hz at 100°C [68].


### 1.16. Passive Vibration

This method is used to manage vibrations in the absence of an external source of power to control operating parameters. When the applied external stress exceeds the critical stress, it can be accomplished using the hysteretic characteristic of superelasticity SMA. Hysteretic loops with a large loop area have a lot of inherent material damping. To avoid composite failure because of vibrations, polymer composite systems require a lot of material dampening. The dampening of SMA composites is affected by design parameters, such as intermolecular interactions and volume fraction of constituent materials. Gupta et al. [69] investigated the damping capabilities of composite beams of shape memory alloy-embedded pseudoelastic fiber optics and matched them to a steel wire placed inside the position of the shape memory alloy. The damping ratio of GFRP embedded with a steel wire increased nearly twice that of a steel wire embedded with GFRP.


### 1.17. Health Monitoring

The prolonged formulation of SMA, including smart materials, such as polyvinylidene fluoride and piezoceramics, are used to take care of the health of composites using sensing properties. These sensing variables are used to evaluate the health of polymer composite structures, whether explicitly or implicitly. The use of SMA as a sensor is dependent on fracture toughness changes caused by part transformation. To test the sensing features of such material, Cui et al. explored the inter-relationship among electrical resistance fluctuation and strain in a shape memory alloy [70]. To identify the indemnities in the SMACs, certain unique properties are measured. Nagai et al. [71] demonstrated by monitoring the fluctuations of electrical resistance that the embedded shape memory alloy may be used to assess the extent of damage in a hybridized GFRP. Pseudoelastic SMA is commonly used for monitoring. Changes in resistance are sometimes assessed as a purpose for a combined strain. NiTiNOL’s high resistivity makes it ideal for use as a sensor with a Wheatstone bridge support. As NiTiNOL would sense both temperature and strain, it can be utilized as a dual-mode sensor. Embedding sensors inside the composite, including fiber optics, can lead to early failure. The utilization of cavalries as sensors and a garnish of the composite’s structural properties is one of the design requirements for intelligent composite structures. A self-sensing system is a term used to describe these types of devices. The sensor is a crucial component of the whole.Damage detection and tracking can be accomplished by resistance-based self-sensing. Damage assessment and location are required for health monitoring. The location of damage can be easily shown by graphing the conductivity values of more than one region. SMA wires come in a variety of colors. The transformation temperature is low from −30°C to +10°C, and strain sensors have been employed [72]. The creation of adaptive systems composite material is with self-sensing lamina which leads to good result. In comparison to traditional strain gauges, the gauge factor is higher. Then, the glass fiber polymer composite embedded with pseudoelastic wires, for resistance monitoring, depends on the locations of damage [73]. Pinto et al. [74] employed imaging techniques to detect damage in SMA composites caused by impact or other dynamic workloads. The external magnetic field sensing is provided as a capacity to monitor the structural health by implanting Fe-based wires of magnetic shape memory alloy (MSMA). In MSMA wire, limited phase transformation occurs at the damage sites. The fields going through the damaged spots are modified by these phase-transformed places [75–79]. The detection of damage is done by the use of a magnetic field line [80].


## 1.1. Overview of Shape Memory Polymer Composites

The complex and hazardous gold-cadmium alloy was the first of this group of metal materials to also be identified in the twentieth century. Nitinol alloys are nontoxic and a little more expensive, and they became the dominant class of SMAs in the 1960s and figured prominently in a diverse range of products [12, 13]. The prepared shape memory alloys are characterized based on the following three main properties:(1)
Reshaped from the state of martensitic deformation while heating a shape memory alloy(2)
Damping and vibrational characteristics in the martensitic state(3)
Austenitic state super elastic nature in SMAsShape memory polymers composite (SMPC) of mono-way (one-way) and dual-way (two-way) transformations are utilized in their crystal structure from Austenite to martensite (Refer to Figure3). The change in the crystal structure is linked to the intersection of the atomic layers above the distance in the atomic layer. As a response, thermodynamically stable martensitic variations occur within the morphology since SMA is chilled in the phase of cubic austenitic to its transformation temperature under no stress. There is no macroscopic structural change because the variations are broken in all directions. Self-accommodated martensite (SAM) is the term applied to the resulting structure.Figure 3
Mono- and dual-way shape memory effects.At the time of the material experiencing external stress, the maximum favorably orientated variations rise without a regard for the smallest favorably directed, generating detwinned martensite (Figure4). The phase transition could be examined using thermal mechanics since such a phase transition is initiated by a bit of Gibbs free energy. The martensitic phases become established when the primary phase GA possesses higher Gibbs free energy than the martensitic GM. The Gibbs free energy diagram clearly shows the equilibrium state for GA and GM in martensite transformation below T0. The cooling process initiated the nucleation and was accomplished by martensite at a lower temperature. The reverse transition occurs during heating, commencing at austenite start and terminating at Af (austenite finish).Figure 4
Stress-strain diagram of shape memory alloy (SMA).In particular, composite materials are significant because various components supplement each other, promote collaboration, or enhance the performance of polymer composite. Composites for SMAs are developed with a couple of objectives in consciousness, which are as follows: strengthening by reinforcement and the establishment of novel and efficient stimulation techniques. Versatile and adaptive shape memory polymer materials, which have self-healing [14–16], drag reduction [17], and refractive features [18–22], are other intriguing research fields. The shaped memory can be used as a substrate for polymer matrix compounds, a matrix, binding agent, or compounds, which are discussed in detail. The simulation model plays a vital role in predicting the reinforcement effects in particularly self-healing functions and advanced subfunctions, which help to examine the SMA composite structures in multiple views, such as different dimensional views and sectional views, and isolate the views for examining the layers of ceramics, films, and metals. To make a versatile composite material, SMP can be employed as the form of fibers. Two-way recovery, functional filler or sensitive materials, and more helpful functionalities can be achieved with a good composite material. The investigation of SMPC is motivated by such appreciable demand to determine the class and quantity of filler in an appropriate manner. The intergraded from shape memory alloy for the shape memory polymer was illustrated in Figure 5. Figure 6 comprehensively demonstrates the various possibilities of SMPC. Excessive filler reduces the thermal qualities of the material, resulting in a decreased transformation temperature. Some of the fillers, instead of enhancing the shape memory effects on polymer composites, degrade the final shape memory effects (refer to Figure 5).Figure 5
Shape memory polymer integrated from a shape memory alloy.Figure 6
Shape memory polymer composites.


## 1.2. Stimulation Methods

Stimulation methods are frequently employed in the recovery of shape memory effects and some programming operations, apart from enhancing mechanical and thermal properties. There seems to be a necessity for nonthermal responses in a diverse variety of polymeric composite materials, including biomedical devices, actuators, and aerospace structures. Figure6 shows different new features, such as the reduction in the image memory effect and the effect of the shape memory polymers because of the addition of smaller fillers. Electromagnetic, ferromagnetic, and optical responses and more precise and quasi-stimulation approaches are crucial stages in the implementation of polymer-based shape memory composites. The alternative option is to use a filler to provide secondary stimulation that is simple and efficient by doping with adsorbing materials and magnetically conductive SMPC, and it can also be stimulated with the use of ultrasonic, solvents, and other methods.


## 1.3. Electric Stimulation

Electric stimulation (ES) is one of the very exact and convenient methods of stimulation. The utilization of the current-driven form has resulted in a massive growth in shape memory composite materials. Implicitly conductive polymers still have not been discovered, leading to limited electrical stability and low permeability. When constructing an ES of EMPC, several difficulties have been taken into consideration [23, 24]. The modified filament is first constructed into something of a three-dimensional active network structure. For example, under a magnetic field, Nickle powder forms a chain [25, 26], and connecting short carbon fibers with carbon black can improve conductivity [27]. Secondly, surfactant [28] and chemical modification are commonly utilized for making the loaded conductor more consistently and permanently absorb into the polymer matrix. The sample of electrically resistive shape memory polymer composite of Ag/CCF/H-EP with 5.4 wt% Ag/CCF activated by electricity under 60 V [29] was presented.


## 1.4. Light Stimulation

Light transmission is also a noncontact stimulation, and it is one of the most common and practical methods. It is ideal for medical instrumentation and biological applications in which regular contact stimulation is problematic. There are two types of approaches to these two modes of transportation: direct steering and indirect steering. Spontaneous redox linkages are common in light-reactive shape memory polymers, such as the cinnamic acid’s cycle-addition reaction [28, 30]. Nevertheless, as the polymer has no intrinsic increase in temperature, the electromagnetic thermal transfer can only be accomplished inadvertently. Magnetic drives are analogous to electrical machines in that they address the doping and matrix connection issues. Other magnetic driving mechanisms have not been documented so far.


## 1.5. Magnetic Stimulation

The magnetic stimulation principle is opposite to the light stimulation method as it directly gets in touch with SMA. By adding soft and hard magnetic materials to shape memory polymers, we observed indirect heating principles. This kind of heating is ideal for medical devices like implanted shape memory polymer scaffolds. Also, this technology is strengthened by a heating technique that combines electromagnetic fields and direct heating [31]. The requirement for ambient heating is variable since magnetic heating is changeable. The material’s perceived switching temperature is thereby altered in this manner. They also created magnetic memory effects to form memory polymer nanoscale composites by adding magnetic elements to SMPCs with temperature memory effects [32]. Smoukov et al. synthesized a thin membrane with Fe3O4 and Nafion material for magnetic driving with the adjustable realization of up to four predefined shapes [13]. Magnetic stimulation is like magnet-induced deformation and shape recovery measurement [33]. The surface temperature is maintained around body temperature (38–400°C) even when the area ambient approaches 800°C, which suggested tremendous promise in clinical uses.


## 1.6. Wetting Stimulation

Solvents can also be used to speed up the shape recovery process. An SMP is with either a hydrogen relationship or a glass state transformation. SMPs are among the modified solvents discovered to govern shape memory polymers. Plasticization is the essence of solution actuation for an SMP with a glass new system implementation. Because of the swelling and polarization effects, the polymer network portions progressively soften as moisture levels rise, and the heat of the glass phase transitions drops. The rate at which solvents permeate utmost plastics, such as phenyl ethylene, epoxy resin, etc., is extremely slow. Leng et al. showed in 2014 that an epoxy resin SMP loaded with sodium dodecyl sulfate may be driven by moisture [34].The effect of form memory is a kind that could be generated by scratching with the help of composites. A nanotechnology polymeric composite material of Poval and oxidized graphite took on transitory forms when heated to a specific temperature and returned to their initial form (shape) when exposed to moisture [35]. The transition period of this SMP has been dependent on the bonds of hydrogen. Adsorbing and heating on the surface might dissolve the chemical bonds, allowing form recovery by unfreezing macromolecular chain sections frozen at the oxidized graphene surface. The elastic thermoplastic polyurethane with microsized cellulose whiskers is included in the polymer structure to enhance shape memory effects [36]. The SMP with manually created cracks obtains tunable hydrophobicity. A new SMP composite (SMPC) was synthesized by [37] that clings to a coating of collagen nanocrystal reinforced carbon black in the polyurethane microsized skeleton, thereby imposing a shape freeze. It can be accomplished by the use of heat, wetness, and a variety of other stimuli. Polyurethane is an entropic elasticity system in the three cases above. However, it did not exhibit shape memory effects. That system remained frozen after the inclusion of nanosized structures capable of forming the bonds of hydrogen, and it exhibited the effects of shape memory at that time, which indicates the development of shape functional decline to a certain measure.The fundamental concepts of SMP are discussed, although no details were given. As a result, most of this article is devoted to a common topic that could be observed in every SMP analysis. The simulation methodology, a crucial asset of the SMP, is not discussed in this section. The shape memory polymers, which are extensively employed in the industry, are based on phase transition. The driving mechanism is limited if the composite material is created. Through the process of switching and SME classification, SMP can be classified into molecular/supramolecular switching and physical bonding. SMPs can be classified as mono-way or dual-way based on the directionality of the SME, such as dual-way shape memory effect, which has only been documented in shape memory alloys, or as dual shape memory effect or multishape memory effect, based on the number of constant configurations (Figure4). There seem to be numerous subtle variations to be made. Their emphasis is on the multiple shape memory polymer-based physical bonding. It is efficient and well-established in industrial applications. Based on the thermal mechanics, the SME actuation method is shown in Figure 7.Figure 7
Shape memory effect on polymer composites.


## 1.7. Principles of SME

The SME in an SMP-based glass transition is predominantly compared with the following configuration of the material: the continuous stage to sustain the natural structure of the structural properties and the bidirectional stage of hardening and softening, according to molecular dynamics. The shape of the bidirectional stage could be maintained whenever the temperature is below the Tg region. Alternatively, thermodynamic elasticity causes the polymeric matrix to return toward its initialization stage. The solid phase assures that the circuit displacement is unidirectional and that external pressures do not cause the flow to be viscous. Physical bonding and chemical bonding in general correlate to thermoplastic and thermosetting shape memory plastics, correspondingly. The retention of such natural morphology could be assured whenever the Tg band of the solid stage is high and softness and loosening need not happen in the ambient temperature with the use of the element. Softening and hardening can happen as the temperature varies because of the low range of bidirectional component Tg, and at higher temperatures, it has a high deformation capacity.


## 1.8. The Course of Action on SMPC

As a result, the shape memory effect encompasses both product and manufacturing impacts. The phase transformation principles of SMPs permit them to appropriately illustrate with the help of a standardized linear solid model (SLS model) for the cycle of SMPs. The presented stress-strain-temperature curves for each state position are most relevant. Researchers demonstrate a dual-SME with rapid programming, short recovering, and no-load [31]. The most noticeable mechanical features of shape memory polymers are identified. The polymer exhibits viscous optimum activity temperatures and general elastic behavior for relatively low temperatures, demonstrating that viscosity has strong temperature dependence. Viscosity fluctuates among the maximal and a smaller quantifiable value throughout the stage transformation period. Initially, the dashpot’s viscosity is decreased to almost undetectable levels after heating, and creep occurs under an impact field to get the desired shape.The viscosity of the fluid increases dramatically, while the shape is preserved by an applied stimulus, and the temperature is reduced underneath the transition ambient temperature. Once the external force is withdrawn, the programmed shape remains the same, except for a minor general elastic response, and the shape construction operation was already complete. The viscosity of the dashpot diminishes when reheated far beyond the crossover range of temperature, and the solution returns to its natural characteristics. This definition describes the shape memory effect’s important mechanical mechanisms, and it is challenging to get an accurate description of the program’s details, thus rendering improving the viscoelastic modeling difficult.As the shape memory polymer’s process of recovery might transmit an external force, the SMA seems to have a propelling performance. However, energies were stored in this process by mechanical deformation during the programed stage, and external stimulation is needed to open the switch. As a result, when we discuss the stimulus afterwards, we avoid using the term “driving technique,” which is susceptible to contradiction. Above that, the shape memory cycles of hot programming, followed by hot recovery, dual-SME without load, and hot programming, have been employed to determine if the SME is positive or negative, which is generally accomplished by defining various indicative shape memory cycle metrics, such as the fastening and retrieval rates. The reduction of the fastening rate is made of two parts: generalized elasticity structure restoration owing here to the elimination of stress factors after that procedure and cold restoration during extended storage. Cold recovery is usually affected by the nature of the material and its loading circumstances, such as chemical environment external force and humidity. The faster cold restoration occurs the larger the external pressures and greater the energy that moves to the previous shape [24, 38, 39]. Recovery can also be triggered by the surrounding environmental conditions, such as moisture. The rate of recuperation is affected by the temperature. Furthermore, the attachment and recovery rates show the material’s effectiveness during a certain shape memory cycle. They are process-dependent quantities that are influenced by strain, temperature, period, and often even the strain rate.


## 1.9. Modeling

The stress-temperature phase diagram for the Brinson model is depicted, [40] in which the transition temperature at zero stress is represented by the intersection of boundaries in the temperature direction. Here on a graph, there are regions that reflect the pure phase and regions that demonstrate the coexistence of many phases [D]. The transition temperature is considered to vary continuously with tension. The stress contribution factors of CA and CM are representing the slopes of the austenite and martensite segment lines, appropriately, as well as the consequences of stress and overall temperature changes. The finish and start of critical stresses are, respectively, denoted as σfcr and σscr. The SMA function may vary with respect to the volume fraction. The temperature-induced twinned martensite and stress-induced detwinned martensite classifications are used for slope behavior (refer to Figure 8).Figure 8
Brinson model.The final volume of martensite is calculated by the following:(1)ξ=ξs+ξT.Stress influenced volume fraction during the detwinned martensite, the terminologiesξs, σ0,ε0,ξ0sand T0 represents the initial state of martensite.(2)σ−σ0=Eε−ε0+Ωξs−ξ0s+θT−T0.θ− represents the E-Young’s modulus and thermal expansion estimated value from the rule of mixture.(3)ξEM−EA+EA=Eξ.Transformation coefficients are as follows:(4)Ω=−εLEεL−Maximumresidualstrain.The condition for detwinned martensite conversion is as follows:Ms < T [41].(5)σscr+T−MsCM<σ<σfcr+T–MsCM,ξs=1−ξ0s2×cosπσscr−σfcrσ−σfcr−CmT−Ms+1+ξ0s2,ξT=ξ0T−ξ0T1−ξ0sξs−ξ0s,ξs=1−ξ0s2×cosπσscr−σfcrσ−σfcr+1+ξ0s2,ξT=∆Te−∆Te1−ξ0sξs−ξ0s,Mf<T<MsandT<T0,∆Te=1−ξ0s−ξ0T2cosam(T−Mf+1−ξ0s+ξ0T2,am=πms−mf,∆Te=ξ0T.The condition for austenite conversion is as follows:As<T(6)T−AfCA<σ<T–AsCA,ξ=ξ02cosaA(T−As−σCA+1,aA=πAs−Af,ξs=ξ0sξ0ξ−ξ0−ξ0s,ξT=ξ0Tξ0ξ−ξ0−ξ0T,where ξ0s and ξ0T are the volume fractions of martensite stress-induced.


## 1.10. Differential Form for Constitutive Relations

The shape memory alloy fiber is subjected to an elastic limit, and stress increases for the relation expressed in terms of deformation as follows:dεs and αA are stress-induced martensite sites at the initial stage and thermal expansion, respectively [42].(7)dσf=Efdεf−εLEfdεs−EfσfdT.Ef and αf are nonconstant materials functions.(8)Efξ=1−ξEA+ξEM,αfξ=1−ξαA+ξαM.For shape control, the expressions used are as follows:(9)dεf=1Efdσf−εLdεs−αfdT.The volume fractionξs can be expressed as subjected to martensite with stress influenced factors.Substitutedξs in the above deformation equation:(10)dσf=Ef1+εLEf∂ξs/dσfdεf−εL∂ξs∂T+αfdT.The strain can be replaced bydξs.(11)dεf=1Ef+εL∂ξs∂σfdσf+εL∂ξs∂T+αfdT.


## 1.11. Heat Transfer Formulation

For actuator applications, shape memory alloys could be actuated during the application of voltage for Joule heating. Primarily, thermal resistance by heat linked to enthalpy (H), heat capacity (Cp), and convective heat transfer (h) for the transformation of the phase from the phase of martensite to the phase of austenite was reflected in the temperature equations of the modeling of the shape memory alloys as [43, 44](12)V2Rw=hAsurTt−Ta+mCpdTdt+m∆H,where m, Ta, and Rw represent the mass, ambient temperature, and wire’s electrical resistance, respectively. The austenite’s volume fractions and martensite’s volume fractions are used to determine the resistance of shape memory alloy wire.The cooling expressions are as follows:(13)hAsurTt−Ta+mCpdTdt+m∆H=0.


## 1.12. SMPs Applications

This review article focused on polymer shape memory alloys and their reinforced composite materials. In 1960, the first large-scale application of PE thermal contraction tubes gained the shape memory polymers. Currently, nylon and polystyrene materials are used to fabricate the contraction tubes [45, 46]. A thermal contraction tube consists of good tolerance, and temperature properties enhance erosion prevention and better insulation flame retardancy properties. There is a huge scope for the use of shape memory polymers, and consequently, they are pulled into recent research activities.Researchers have to understand the behavior of polymer-based composites, such as SMA, to design applications that rely on them. Initially, we shall look at two-phase shape memory polymers, which have a lot of beneficial characteristics and are closely examined. The most important fact is it exhibits standard recovery that is load releasing process, and shape memory polymers must only shift from transitional shapes to their native shapes unless they are reconfigured. Although the stimulations during the recuperation process are extensive, another motion limits their uses. For such a reason, mainly one-way deformations are considered while fabricating the shape memory polymers. An important form of these kinds of technologies is the solution of transport difficulties by the flexibility of transport and restoration of morphologies required for operations at certain periods, such as geographically growing structures and lightly obtrusive operational equipment. Self-arrangement combined with cold programming under operating conditions is another important aspect of shape memory polymer. In this scenario, form restoration is being used to enable instrumentation and objects to recover from accumulated plastic deformations throughout usage, thus increasing the life duration. Natural polymers are also elastic adhesive elastic materials, which means they have normal viscoelastic characteristics without vibration and adhesive resilience when activated. This is something that should be considered in industrial applications. As biomolecule materials, SMPs’ programming and restoration too are creeping phenomena, keeping their shape memory characteristics over time. Similarly, the reaction time confers low impulse to smart structures made of shape memory polymers, however, it also limits applications requiring instantaneous reactions.Although shape memory polymers were also constrained to lesser thermal properties and a specific treatment mechanism, shape memory polymer matrix composites become especially beneficial. SMPs are used in the aerospace sector because of their structural qualities, such as appreciable ductility, low weight, and extensive fastening rate. The toughness, durability ratio, healing forces, and other thermal aspects of biocomposite materials might have been considerably improved by keeping the above-mentioned features. Shape memory polymers are mostly employed in appliances, brackets, and specular highlights in biomedical device applications. Inside-the-body applications of SMP smart structures, such as endoscopic instruments, necessitate stimulation avoiding direct touch, which is frequently achieved through the passive overheating of modifying compounds. Substrates, sensors, energy collectors, and soft electronic accessories could all be made from shape memory polymers. Smart fibers and other applications benefit from shape memory polymer matrix composites.


## 1.13. Aviation Applications

Shape memory materials have gained a lot of interest because of excellent stable standard capabilities of deformations. An aerospace component which has expanding designs with computer locking structures are notable example [47]. Previous investigation of new structures featured novel materials inside aviation and various applications of hinges, which are spatially expanding. They are primarily using shape memory alloys as driving factors [48]. Shape memory alloys have been challenging to use as structural parts, given their high density, and they had to be supported through lightweight boards or wire materials, which made the design quite difficult and choosing the associated structural components troublesome.If the shape memory characteristics are found insufficient for SMAs or their composites results, SMPs become a prominent figure in the field of conflict resolution. Researchers effectively obtained concepts on the fundamental variations in design concepts by evaluating the characteristics of fiber-reinforced SMPs that is SMA. Shape memory polymer composites may be formed into the matrix of major expansion designs, thereby accomplishing the integration of controlling mechanisms and structural materials because of low density, high hardness, and massive distortion [49]. The usage of polymer-based SMP composites in the aircraft and aerospace areas has been closely researched, and their considerations should be taken into account. In this post, we shall go over the design ideas in further detail and explain several common uses, such as a large-scale extension.The atmosphere is even worse than the conditions in space. Polymers and other biological molecules will be degraded more severely by atomic oxygen and UV radiation than metals and ceramics and would be exposed to severe changes in temperature, resulting in polymer mass loss, degradation in dynamic characteristics, or maybe even full-impaired function [50, 51]. As concluded, space-specific SMPs are to be carefully chosen and confirmed. Such studies for shape memory polymers must compensate by not just standard concerns, including shrinkage, constituent changes, and elasticity modification, but also shape memory voltage drops. It has already been demonstrated that glass fiber-reinforced polymer and cyanate acylation shape memory polymers can withstand the brutal environments of space. Polyimide-based shape memory polymers were also recognized as potential alternatives for the possible development of SMPs in the environment because of their remarkable thermodynamic characteristics and chemically flexible properties. When manufacturing shape memory polymer composites, additives with great space sensitivity, such as carbon material and fiberglass, should be used. Special design considerations are required when doping with materials that have dynamic physicochemical characteristics, such as chemical admixtures, to avoid certain dynamic qualities from ever being impacted by cosmic rays.


## 1.14. Biomedical Applications

Vernon et al. [52] reported in the fourth decade of the last century that SMPs are highly suitable for applications in biomedical instruments and implants. SMPs were first introduced by the invention denitrify. Polymers, in contrast with metallic alloys, have a better restoration strength and biological flexibility. Several elastic polymers, such as polythene, polyurethanes, polyesters isocyanate, and acrylic acid, can be destroyed in the organism. The controlling methodologies of shape memory polymers can be enriched by developing lightweight structures. Light navigation and magnetic navigation are two of them as they allow for greater control. Shape memory polymers have a lot of potential applications in medical tools that are deployed within the organ because of such two qualities. As reported in technology demonstrator and experiment reports, shape memory polymers and related structural components are utilized in various biomedical devices. Orthodontic technicians, medical threads, cardiovascular stents, aneurysm specular highlights, and thrombus cleaners are all examples of their applications. Buckley et al. [53] invented new plant-shaped and deflated ball-shaped surgical devices from SMP and SMP foam with electromagnetic performance-enhancing drugs that might be forbiddingly expanded in the organism by magnetostrictive stimulation. To achieve IR-stimulated form recovery, it was proposed that the shape memory polymer could be wrapped in a layer around fiber [54]. Those intermediate controlling technologies that paved the way for SMPs should be utilized in surgical devices. A ferromagnetic shape memory material was developed for drug dissolution mechanism based on conventional heating activation using double-Cu-coated polyimide.Lendlein et al. [55] developed a surgical thread consisting of shape memory polymers that transformed at a temperature close to that of the human body. Animal experiments showed that as the temperature climbed to 410°C, the suture recovered its shape and closed the medical wounds. A stent might be stretched to solve the issue of regular form metal, restricting medicine transportation by narrowing arteries. Shandas created a permeable stent that improved the quality of the modification required and made microsurgical cardiology procedures more convenient. Decomposable SMPs described for a urinary stent may enable guided administration and disintegration. However, they developed a directive loading structure built of polymer-based composites, with a single-step and multistep shape recovery and a consistent release of such an efficient payload. Cho et al. used melt spinning to manufacture SMP threads for utilizing polymeric orthodontics base elastic polymers. In orthodontic experiments, it was concluded that the strength of SMPs was enough to straighten uncorrected teeth as it has a balance recovery force of nearly 50 gf [56].Aneurysm coils were developed by Hampikan et al. [57] using SMP to supplement standard platinum bands and prevent aneurysms actually because of physiological resistance. The SMPs are mixed with a tantalum filler to improve their X-ray isolative properties, allowing the X-ray images continuously for a period of SMP coil to be obtained. According to the experimental tests, the blood flow is not impacted by the coil. In the medical stent device, the dosing of magnetite nanoparticles is used to improve the transparency of magnetic resonance (MR) spectroscopy [58]. It was described that a urethane foam could efficiently occlude aneurysms in animal studies. The creation of a thick internal membrane mostly on top of the foaming explosions sealed most aneurysms necks. Small and colleagues created a novel filtration design using stents and polyurethane [59]. The arrangement was developed to identify and heal the challenging wide-necked arteries, and both portions were made of SMPs. Using the expandable SMP foam developed a new form of aneurysm embolization architecture. Anchorage was provided by platinum coils, and the shape memory polymer foam is exaggerated to approximately 150 times its initial volume. Hernandez et al. [60] created a new hemostatic device composed of SMP foam, which could quickly fill injuries and accomplish occlusion and disinfection. The device’s expansion force is relatively low. Thus, there will be no secondary wound injury [61].


## 1.15. Active and Passive Vibration Control

To reduce the vibration, shape memory alloys are predominantly solicitated in composite structures. The vibration control uses the shape because of the hysteresis effect. The vibrational modes of SMA hybrid composite structures and the specific damping capacity (SDC) are hugely affected by SMA composite design parameters [62]. Vibrations can be controlled by changing the structure’s stiffness and adjusting the frequency response. The thermo-elastic transformation of the martensitic transformation is perfect. Various irreversible processes, such as crystal defect and dislocation mobility, dissipate the energy. When poor inherent damping is found in the austenite phase, the growth of the austenite-martensite interface causes flaws, and it is an important source for thermomechanical interacting.The following are the descriptions of the passive mode and active mode of the vibration control. Vibration is regulated using characteristics that could be adjusted by multiple systems in an active technique of vibration control. Carried out by an individual parameter is the heating of SMA, employing heat or externally heated surroundings. Natural frequency is obtained by the material’s rigidity. As the temperature rises beyond the achievement temperature of austenite, the rigidity of the shape memory alloy improves and the low stiffness martensite state transitions into specific strength austenite. As a result, in an aggressive state, the rigidity and free vibration of the SMA composite rises. Active property tuning (APT) that would be utilized for changing the natural frequency is covered by this technique [63]. By incorporating prestrained SMAs throughout the composite, the natural frequency would be significantly altered. Lau et al. [64] used a balanced glass fiber (0/90)/epoxy beam (25 × 200 × 1.5 mm3) to embed 0.5 mm Nitinol SMA. The natural frequency did not enhance considerably as the number of SMA wires increased, although the damping concentration rose in the dual case. As a result, the transition phase occurs when martensite transforms to austenite or vice versa. Furthermore, when prestressed SMA wires were actuated, the resulting extensive tensile stress increased the carbon and glass fibers’ natural frequency.The frequencies and amplitudes of low-amplitude vibrations are indeed a measure of the recovered stress and toughness. It can be altered by adjusting the restoration stress using SMA heating. Bidaux et al. [65, 66] looked at the vibrational characteristics of a composite (polymer/epoxy) integrated with a percentage of the tonnage of Nickel-Titanium fibers and prestrain of 5 vol. %. Rogers et al. [67] used a graphite-epoxy laminate with fundamental sensors mounted in a hybrid composite material. The graphite/epoxy/Shape memory alloy hybrid composites of the clamped beam (2.03 × 82.2 × 0.01 cm3) subsonic structural-acoustic transmissions were stabilized.Chandra [6] investigated the role of sleeves in the composite beams of graphite/epoxy by implanting the shape memory alloy. Shape memory alloys were attached to the ends of the sleeves. The sleeves ends are energized through resistive heating. In the result, the first natural frequency was noticed at the implanting of Shape memory alloy wires (2 vol. %) and substantial growths of 22%. With less than 8% volume percentage of SMA embedded, a raise of fundamental frequency 23 per cent was observed in the testing of a composite shaft made for the helicopter. NiTiCu shape memory alloy was employed to shift the natural frequency from 360 Hz at NTP of composite shaft of Kevlar/epoxy with SMA to 450 Hz at 100°C [68].


## 1.16. Passive Vibration

This method is used to manage vibrations in the absence of an external source of power to control operating parameters. When the applied external stress exceeds the critical stress, it can be accomplished using the hysteretic characteristic of superelasticity SMA. Hysteretic loops with a large loop area have a lot of inherent material damping. To avoid composite failure because of vibrations, polymer composite systems require a lot of material dampening. The dampening of SMA composites is affected by design parameters, such as intermolecular interactions and volume fraction of constituent materials. Gupta et al. [69] investigated the damping capabilities of composite beams of shape memory alloy-embedded pseudoelastic fiber optics and matched them to a steel wire placed inside the position of the shape memory alloy. The damping ratio of GFRP embedded with a steel wire increased nearly twice that of a steel wire embedded with GFRP.


## 1.17. Health Monitoring

The prolonged formulation of SMA, including smart materials, such as polyvinylidene fluoride and piezoceramics, are used to take care of the health of composites using sensing properties. These sensing variables are used to evaluate the health of polymer composite structures, whether explicitly or implicitly. The use of SMA as a sensor is dependent on fracture toughness changes caused by part transformation. To test the sensing features of such material, Cui et al. explored the inter-relationship among electrical resistance fluctuation and strain in a shape memory alloy [70]. To identify the indemnities in the SMACs, certain unique properties are measured. Nagai et al. [71] demonstrated by monitoring the fluctuations of electrical resistance that the embedded shape memory alloy may be used to assess the extent of damage in a hybridized GFRP. Pseudoelastic SMA is commonly used for monitoring. Changes in resistance are sometimes assessed as a purpose for a combined strain. NiTiNOL’s high resistivity makes it ideal for use as a sensor with a Wheatstone bridge support. As NiTiNOL would sense both temperature and strain, it can be utilized as a dual-mode sensor. Embedding sensors inside the composite, including fiber optics, can lead to early failure. The utilization of cavalries as sensors and a garnish of the composite’s structural properties is one of the design requirements for intelligent composite structures. A self-sensing system is a term used to describe these types of devices. The sensor is a crucial component of the whole.Damage detection and tracking can be accomplished by resistance-based self-sensing. Damage assessment and location are required for health monitoring. The location of damage can be easily shown by graphing the conductivity values of more than one region. SMA wires come in a variety of colors. The transformation temperature is low from −30°C to +10°C, and strain sensors have been employed [72]. The creation of adaptive systems composite material is with self-sensing lamina which leads to good result. In comparison to traditional strain gauges, the gauge factor is higher. Then, the glass fiber polymer composite embedded with pseudoelastic wires, for resistance monitoring, depends on the locations of damage [73]. Pinto et al. [74] employed imaging techniques to detect damage in SMA composites caused by impact or other dynamic workloads. The external magnetic field sensing is provided as a capacity to monitor the structural health by implanting Fe-based wires of magnetic shape memory alloy (MSMA). In MSMA wire, limited phase transformation occurs at the damage sites. The fields going through the damaged spots are modified by these phase-transformed places [75–79]. The detection of damage is done by the use of a magnetic field line [80].


## 2. Conclusions

SMAs are highly relevant in this study to discuss memory polymer composites and their effects. It is determined by the material’s properties and external factors. This principle would be used to drive their design, particularly in the event of active deformation. Because of the diverse emphasis in engineering and sciences related to the material, this manuscript focused on exploring shape memory behaviors and their competence in preparing the shape memory polymers that satisfy the static and dynamic properties of composites. In this direction, modeling for SMA and SMA-reinforced composites was examined. Shape regulation, stiffness adjustment, vibrational control, damage prevention, consciousness, and health monitoring applications have received significant attention. The following are the conclusions:Shape memory polymers’ performance and application range have been substantially broadened primarily to the production of shape memory polymer composite materials. The growth of SMPC materials has been examined with respect to the application and chemical function. Fixing methods have been discovered to rectify or avoid the flaws in SMP, such as little force acting and constrained stimulation methods, allowing SMPCs to be used in a range of sectors, such as aerospace and medical [81–87]:(i)
The proposed application of sample composites and matrix material’s elastic modulus should be considered when choosing a matrix material in the form of shape memory polymer composites.(ii)
For regulating customized stiffness and delivering the needed actuation capability, interfacial bonding is critical.(iii)
To achieve the same effect, the suitable location of wires of shape memory alloys within the structure of composite is preferred.(iv)
The electric stimulation of SMPCs is the stimulation approach that has the largest and a potential application, and that is widely utilized.(v)
Despite significant and positive advances, we must acknowledge that electric driving is still in its development.(vi)
There are no findings reported about the fabrication of SMP nanocomposites in electrically stimulated devices and embedded resistance film for heating.(vii)
Inserting SMAs inside the laminate, for example, can improve the damping qualities. Shape morphing can be accomplished by adjusting bending characteristics or controlling simple strain (positioning the wires of shape memory at the neutral axis or plane of SMPC). SMAs can be placed as a continuous stitch through the reinforcing thickness to effectively close a crack.(viii)
The temperature control (maintaining less than that of transformation temperature of the shape memory alloy that is included) is mandatory while synthesizing the SMPC for avoiding specific fixtures.(ix)
The temperature must be maintained lower than the shape memory alloy’s transformation temperature for avoiding the use of specific fixtures while synthesizing SMPC.(x)
The discontinuous shape memory alloys are effective for increasing the properties of the material for either active mode or the passive mode under a magnetic or electric field.(xi)
The entire matrix must be highly conductive for operating in active mode with heating by the resistive method. However, to avoid the degradation of the polymer matrix because of hotness, the transformation temperature might not be too high.



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*Source: 6988731-2022-10-03.xml*