Shape Memory Effect

In this groundbreaking study, Polyethylene Terephthalate Glycol (PETG)-Fe3O4 nanocomposites were developed for 4D printing, incorporating iron oxide (Fe3O4) nanoparticles into PETG matrix. The research contribution lies in its innovative approach to enhancing the shape memory effect (SME) through thermo-magnetic responsiveness, positioning PETG-Fe3O4 as a revolutionary material in smart additive manufacturing. The composites were synthesized using a melt mixing method, followed by 3D printing into specimens for comprehensive evaluation through dynamic mechanical thermal analysis (DMTA), scanning electron microscopy (SEM), and uniaxial tensile tests. The findings revealed that the incorporation of Fe3O4 nanoparticles significantly boosts the composites’ storage modulus and glass transition temperature, indicative of improved stiffness and thermal properties. Notably, the 15 % Fe3O4 composite emerged as the optimal blend, exhibiting the highest tensile strength and a favourable balance between mechanical integrity and flexibility. A key result was the enhanced SME under both thermal and magnetic stimuli, with recovery efficiency and speed escalating with nanoparticle concentration. This advancement underscores the potential of PETG-Fe3O4 nanocomposites in fabricating smart structures capable of environmental adaptability, paving the way for impacts in biomedical, aerospace, and robotic devices. Through this work, a new paradigm in material functionality for 4D printing has been established, demonstrating the viability of magnetic nanoparticle integration for added smart capabilities.

This study presents a 4D-printed shape memory alloy (SMA)-thermoplastic polyurethane (TPU) soft composite actuator. Fabricated by co-extruding two independently addressable Nitinol wire circuits (0.3 mm ⌀) within a compliant TPU matrix, producing a single monolithic structure capable of reversible bidirectional bending. A constrained thermomechanical training protocol was used to induce two-way shape memory behaviour. Response surface methodology was applied to evaluate the effects of electrical current (1-3 A) and training cycles (20-40) on actuation performance. Applied current was found to be the dominant parameter (R² = 0.96) controlling normalised curvature difference, which reached a maximum of approximately 36% at 2.5 A, while the number of training cycles was not statistically significant within the investigated range. Under repeated actuation at 2.5 A, the normalised curvature difference decayed logarithmically over 50 cycles (R² = 0.99). The embedded wires were also assessed as intrinsic resistance-based sensors. Cycle-resolved analysis showed that the resistance-curvature relationship is unstable during the initial cycles but stabilises after approximately 10-15 conditioning cycles, with the median Spearman correlation exceeding 0.9 in both actuation directions. The proposed manufacturing-integrated approach demonstrates the potential of additively manufactured SMA-polymer composites for compact actuating and feedback-capable soft structures.

There has been a rapidly growing interest in the use of shape memory alloys (SMAs) in defense and aerospace applications. Shape memory alloys that are well known to exhibit a highly non-linear thermoelastic response involving such unique phenomena as superelasticity and shape memory effect. These fascinating phenomena are highly dependent on the chemical composition and heat treatment applied to the material. Among shape memory alloys, NiTi SMAs are potential candidate for many defense and aerospace applications. NiTi SMA based actuators offer many functional advantages over their conventional counterparts providing higher strokes, reduced weight, smaller size, and simple mechanism. In addition to the actuation, NiTi SMA based composite structures provide self-healing, sensing, and damping for the future composite systems. Besides the chemical composition, the power-to-weight ratio and the operation cost, the operational temperature range should also be carefully considered. This work is focused on investigating the effect of the long-term heat treatment on the thermomechanical response in NiTi SMA for the purpose of shifting its martensitic transformation temperature to the desired operational temperature range. The fundamental thermomechanical behavior of SMAs explained. Defense and aerospace applications of the SMAs are presented. The state of the art technology for the SMAs for the future potential defense and aerospace applications are also stated in the conclusion.

There has been a rapidly growing interest in the use of shape memory alloys (SMAs) in defense and aerospace applications. Shape memory alloys that are well known to exhibit a highly non-linear thermoelastic response involving such unique phenomena as superelasticity and shape memory effect. These fascinating phenomena are highly dependent on the chemical composition and heat treatment applied to the material. Among shape memory alloys, NiTi SMAs are potential candidate for many defense and aerospace applications. NiTi SMA based actuators offer many functional advantages over their conventional counterparts providing higher strokes, reduced weight, smaller size, and simple mechanism. In addition to the actuation, NiTi SMA based composite structures provide self-healing, sensing, and damping for the future composite systems. Besides the chemical composition, the power-to-weight ratio and the operation cost, the operational temperature range should also be carefully considered. This work is focused on investigating the effect of the long-term heat treatment on the thermomechanical response in NiTi SMA for the purpose of shifting its martensitic transformation temperature to the desired operational temperature range. The fundamental thermomechanical behavior of SMAs explained. Defense and aerospace applications of the SMAs are presented. The state of the art technology for the SMAs for the future potential defense and aerospace applications are also stated in the conclusion.

The use of shape memory alloys (SMA) in an increasing number of applications in many ¯elds of engineering, such as biomedical engineering, is leading to a growing interest toward an exhaustive modeling of their macroscopic behavior in order to construct reliable simulation tools for SMA devices. In this paper we review a robust three-dimensional model able to reproduce both pseudo-elastic and shape-memory behaviors and we report numerical studies where it is used for the simulation of SMA-based biomedical devices.

The existence of a magnetic moment at the nickel sites in ferromagnetic shape-memory alloys (FSMA) has long been speculated. Using a state-of-the-art X-ray magnetic circular dichroism (XMCD) technique we found that Ni 3d moments indeed exist in Ni-Mn-Ga-based FSMA. Band-structure calculations were used to simulate the XMCD spectra and estimate this moment to be $0:33 m B . Such a Ni moment plays a critical role in the dispersions of non-collinear spin structures, which determine the finite-temperature magnetic properties.

An algorithm to model the one-dimensional cyclic behavior of NiTi strings is addressed. The NiTi alloy belongs to the shape memory alloy class of materials, therefore it presents both shape memory effect, for thermally-induced cycling, and superelasticity, for stress-induced cycles. The superelastic property has been the basis of some devices designed to mitigate the earthquake hazard level in structures. Throughout this paper the implementation of a one-dimensional cyclic behavior algorithm to model the NiTi constitutive relation is presented, supported by the thermomechanical formulation developed by Lagoudas and co-workers. The model was set up in MatLab environment and it accounts for isothermal superelastic behavior, incorporating minor hysteretic transformation loops. The definition of the transformation hardening function allowed for a better adjustment of the numerical model weighted against experimental results. Especial emphasis was given to the process of calibration of t...

Shape memory alloys (SMA), because of their unique mechanical characteristics and shape memory effect (SME), have been widely used as force and displacement actuators in many fields [ Duering et al, 1990]. In the industrial applications, it is necessary not only to calculate the mechanical response of the actuator in terms of recovery force or deformation, but also to evaluate its temporal characteristics, i.e., the actuation and reset times. This paper presents the fundamental characteristics of SMA and a complete design model, which requires a close connection between three models: a mechanical model to predict the response of the actuator to a given temperature increment, a thermal model to compute the temperature change in the device, and a continuum-mechanical model to predict the martensite fraction on the SMA. The methodology is applied to a linear wire actuator.

We report the effect of decrease in the grain size on the structural, magnetic and exchange bias ͑EB͒ behavior in ball milled Ni 50-x Co x Mn 38 Sb 12 ͑x = 0 and 5͒ Heusler alloys. The existence of a wide range of grain sizes in the ball milled samples results in dramatic changes in the structural and magnetic properties. For x = 0, a large EB field of 3.2 kOe is observed in the ball milled sample, compared to a value of 245 Oe of the bulk sample. This increase is attributed to the enhanced exchange coupling between the soft and hard magnetic particles.

We report the effect of decrease in the grain size on the structural, magnetic and exchange bias (EB) behavior in ball milled Ni50−xCoxMn38Sb12 (x=0 and 5) Heusler alloys. The existence of a wide range of grain sizes in the ball milled samples results in dramatic changes in the structural and magnetic properties. For x=0, a large EB field of 3.2 kOe is observed in the ball milled sample, compared to a value of 245 Oe of the bulk sample. This increase is attributed to the enhanced exchange coupling between the soft and hard magnetic particles.

The shape memory effect in stainless shape memory alloys is a physical phenomenon related to the fcc(γ)↔hcp(ε) transformation and reversion. The morphology of the martensitic phase is a function of such factors as chemical composition, number of training cycles and microstructure refinement; these are very important because they influence strongly the shape recovery properties. In this work, the martensitic phase was induced in a stainless shape memory alloy by applying training cycles (deformation by compression followed by heating) to samples with average austenitic grain sizes varying between 75 and 129µm. It will be shown using optical microscopy that the martensitic morphology is a function of the grain refinement and of the training. Width of the martensite plates and their distribution within the prior austenite grains will be correlated with the data already gotten by the shape memory group (DEMA/FEM/UNICAMP) and presented in international literature.

We have investigated the response of amorphous silicon (a-Si), in particular crystallization, to external mechanical shear deformations using classical molecular dynamics (MD) simulations and the empirical Environment Dependent Inter-atomic Potential (EDIP) [Phys. Rev. B 56, 8542 (1997)]. In agreement with previous results we find that, at low shear velocity and low temperature, shear deformations increase disorder and defect density. At high temperatures, however, the deformations are found to induce crystallization, demonstrating a dynamical transition associated with both shear rate and temperature. The properties of a-Si under shear deformations and the extent at which the system crystallizes are analyzed in terms of the potential energy difference (PED) between the sheared and non-sheared material, as well as the fraction of defects and the number of particles that possess a crystalline environment.

The behaviour of Ni-Ti and Copper Ni-Ti orthodontic wircs was tcsted undcr induccd torsion in controlled conditions of moment and temperature. The results clearly demonstrate a great diffcrcncc in the behaviour of thcsc wires. The loading and unloading curves and plateau rcgions were found to bc closcly mlatcd to tcmpcraturc. Experiments carried out in conditions of identical toision revealed significant variations in stiffness duc to changcs in mouth temperature. Divcrsity of reaction to stress is linked to the crystalline struclurc of the alloys.

Cu-Sn shape-memory microwires were fabricated by a glass-coated melt spinning method. Effects of Sn content on the microstructure and mechanical property of microwires were investigated. The phase transforms from martensite to austenite with Sn content increasing from 14.0 atomic percent (at.%) to 16.5 at.%, and highly ordered intermetallic phase, δ, formed with higher Sn content. The fracture stress, σf, and the critical stress for martensitic transformation, σMs, increases with Sn content increasing. The mechanical property as well as superelasticity were improved greatly by high cooling rate of the glass-coated melt spinning method.

A rigorous experimentation and validation program is being undertaken to create constitutive models that elucidate the fundamental mechanisms controlling plasticity in ura nium-6 wt.% niobium alloys (U-6Nb). The first, "wrought", material produced by processing a cast ingot I'ia forging and forming into plate was studied. The second material investigated is a direct cast U-6Nb alloy. The purpose of the investigation is to detennine the principal differ ences, or more importantly, similarities, between the two materials due to processing. It is well known that parameters like grain size, impurity size and chemistry affect the deformation and failure characteristics of materials. Metallography conducted on these materials revealed that the microstructures are quite different. Characterization techniques like tension, compression, and shear were pelfonned to find the principal differences between the materials as a function of stress state. Dynamic characterization using a split Hopkinson pre.ssure bar in conjunction with Taylor impact testing was conducted to derive and thereafter validate constitutive material models. The Mechanical Threshold Strength Model is shown to accurately capture the constitu tive response of these materials and Taylor cylinder tests are used to provide a robust way to verify and validate the constitutive model predictions of deformation by comparing finite ele ment simulations with the experimental results. The primary differences between the materials will be described and predictions about material behavior will be made.

Abstract: An access device using a combination of a compression coil spring made of NiTi shape memory alloy (SMA) and NiTi superelastic strips has been developed. Particular consideration is given to the application of the concept in tran-sanal endocopic microsurgery (TEM), but it could be used in other procedures where tissue expansion is required. In TEM, it could realize the gasless dilation of the rectum with safety and reliability, and facilitate surgeons to perform TEM. The theoretical design procedure to determine the specifications of the NiTi SMA spring and superelastic compo-nents is established. The feasibility and the possible problems associated with this application are discussed generally.

An algorithm to model the one-dimensional cyclic behavior of NiTi strings is addressed. The NiTi alloy belongs to the shape memory alloy class of materials, therefore it presents both shape memory effect, for thermally-induced cycling, and superelasticity, for stress-induced cycles. The superelastic property has been the basis of some devices designed to mitigate the earthquake hazard level in structures. Throughout this paper the implementation of a one-dimensional cyclic behavior algorithm to model the NiTi constitutive relation is presented, supported by the thermomechanical formulation developed by Lagoudas and co-workers. The model was set up in MatLab environment and it accounts for isothermal superelastic behavior, incorporating minor hysteretic transformation loops. The definition of the transformation hardening function allowed for a better adjustment of the numerical model weighted against experimental results. Especial emphasis was given to the process of calibration of the model, regarding the definition of material parameters. The validation process consisted of the comparison between the results achieved with this algorithm and experimental tests performed at the Pacific Earthquake Engineering Research Center at the University of California at Berkeley. Quasi-static tensile tests and shake table tests of a small-scale steel structure with NiTi cross-bracing systems were used as reference. The model was able to simulate the experimental performance. This formulation can be implemented in more robust finite element analysis software, in order to perform studies in more elaborate structures.

New investigation results of N + , Mo, W, N and Ni high dose ion implantation of 10 18 cm -2 influence on alloys physical and mechanical properties are presented in this article. Increasing of alloys wear resistance by abrasion of the surface layer, increasing of nanohardness, corrosion resistance, and changes of surface NiTi morphology are obtained. The correlation between the change of the elemental composition (after implantation) and the increase in mechanical properties of the alloy is observed. In other words, a nanostructure of oxycarbides and oxynitrides is formed in the surface layer as a result of highdose ion implantation, which improves wear resistance, corrosion resistance and nanohardness of material. At the same time, the bulk properties of NiTi after implantation (pseudoplasticity and shape memory effect) do not change, which makes ion implantation an effective tool for improvement materials properties.