Conventional hinge actuators often face limitations including excessive weight, large size and unpleasant noise. Shape memory alloys (SMAs) offer a solution to address these issues due to their favorable characteristics, such as lightweight, high actuation force and small form factor. However, most existing SMA-based hinge actuators rely on the tension loading mode. Achieving an ideal actuation angle thereby necessitates the inclusion of long SMA wires, which inadvertently constrains the actuator size. Notably, the full potential of SMAs’ deformation capacities, encompassing torsion and bending, remains largely untapped and underutilized. In this research, a reversible torsion SMA hinge actuator is studied, which can reversibly open 60° during heating and cooling. The actuator weighs 2 g, and can produce actuation forces of up to 5 N. The mechanical performances of nitinol at different temperatures are measured. Based on the measurements, a model which can predict the opening and closing angle is proposed, with deviations of 13.5 ± 8.2 %. Gripper and butterfly demonstrators constructed by the hinge actuators are given as application examples. The actuators hold potential in many fields like soft robotics, aerospace and medical instruments.

Emerging 4D printing techniques have enabled the realization of smart materials whose shape or properties can change with time. Two important phenomena play important roles in the 4D printing of shape memory polymeric materials. First, the anisotropic deformation of the printed filaments due to residual stresses can be harnessed to create out-of-plane shape transformations. Second, the unavoidable formation of micro-defects during the printing processes often affects the programmability of the printed object. Here, we propose a design approach that harnesses these two effects occurring during fused deposition modeling to create tailor-made curved geometries from initially 2D flat disks. We first determined the size and distribution of the imperfections formed within printed structures by varying two printing parameters namely the printing speed and the number of printed materials. Spatially varying the printing speed and combining polylactic acid filaments with a softer material without shape memory properties allowed us to cover a variety of shapes from negative to positive values of the mean and Gaussian curvature. We propose an analytical model to calculate the magnitude of the maximum out-of-plane deformation from the anisotropic expansion factor of the constituting microstructures. Furthermore, we develop computational models to predict the complex shape-changing of thermally actuated 4D printed structures given the distribution of rationally introduced imperfections and we demonstrate the potential applications of such defect-based metamaterials in drug delivery systems.

We developed a shape-changing constructive kit, named Mimosa1. A key component of the toolkit is the modular hinges, each of which is equipped with two antagonistic shape memory alloy (SMA) wires. One wire deforms the hinge to approach its predetermined angle at high temperature, and another wire drives the hinge back when it cools down. Hinge leaves are available in different materials including acrylic, cardboard and textile, which increases the versatility of the toolkit. Every hinge weighs 2.1-5.4 g, and generates up to 5.7 N actuation force. A Bluetooth control module was developed, enabling remote control of the shape-changing objects. Mimosa aims to inspire designers to explore and create interactive shape-morphing objects with SMAs. A few examples are given such as a gripper, a rolling robot, a butterfly, an airplane and a self-closing pocket. A workshop study with 6 participants showed that Mimosa indeed motivated and inspired the participants to create new ideas.

As an emerging technology, smart textiles have attracted attention for rehabilitation purposes or to monitor heart rate, blood pressure, breathing rate, body posture, as well as limb movements. Traditional rigid sensors do not always provide the desired level of comfort, flexibility, and adaptability. To improve this, recent research focuses on the development of textile-based sensors. In this study, knitted strain sensors that are linear up to 40% strain with a sensitivity of 1.19 and a low hysteresis characteristic were integrated into different versions of wearable finger sensors for rehabilitation purposes. The results showed that the different finger sensor versions have accurate responses to different angles of the index finger at relaxation, 45° and 90°. Additionally, the effect of spacer layer thickness between the finger and sensor was investigated.

As an emerging technology, smart textiles have attracted attention for rehabilitation purposes to monitor heart rate, blood pressure, breathing rate, body posture and limb movements. Compared with traditional sensors, knitted sensors constructed from conductive yarns are breathable, stretchable and washable, and therefore, provide more comfort to the body and can be used in everyday life. In this study, knitted strain sensors were produced that are linear with up to 40% strain, sensitivity of 1.19 and hysteresis of 1.2% in absolute values, and hysteresis of 0.03 when scaled to the working range of 40%. The developed sensor was integrated into a wearable wrist-glove system for finger and wrist monitoring. The results show that the wearable was able to detect different finger angles and positions of the wrist.

Plant root growth can be altered by introducing obstacles in the path of growth. This principle is used in design to produce planar grid structures composed of interweaving roots. The Engineered Plant Root Materials (EPRMs) grown with this method have the potential to serve as environmentally sensitive alternatives for conventional materials, but their applications are delimited by their material properties. To bridge the gap in the wider application of these materials, the role of plant root structure and an agar-agar matrix are explored in relation to the mechanical properties of the EPRMs. Tensile tests were performed on five root configurations, ranging from single roots to grids of varying sizes. Heterogeneities in each configuration suggest poor load distribution throughout the structure. Agar-agar was introduced as a biopolymer matrix to improve load distribution and tensile properties. Digital microscopy at the intersection of grid cells suggests a correlation between cell size, root tip density, and material strength. The largest cell size (2 cm) had the highest root tip density and yield strength (0.568 ± 0.181 roots/mm² and 0.234 ± 0.018 MPa, respectively), whereas the structure with the least root tips (1 cm) was 31 % weaker.

Devices delivering sophisticated and natural haptic feedback often encompass numerous mechanical elements, leading to increased sizes and wearability challenges. Shape memory alloys (SMAs) are lightweight, compact, and have high power-to-weight ratios, and thus can easily be embedded without affecting the overall device shapes. Here, a review of SMA-based haptic wearables is provided. The article starts with an introduction of SMAs, while incorporating analyses of relevant devices documented in the literature. Haptic and SMA materials fields are correlated, with haptic perception insights aiding SMA actuator design, and distinct SMA mechanisms offering diverse haptic feedback types. A design process for SMA haptic wearables is proposed based on material-centered approach. We show SMAs hold potential for haptic devices aiding visually impaired people and promise in immersive technology and remote interpersonal haptic communication.

Shape memory alloy (SMA) wires are excellent candidates for wearable actuators since they are thin, low weight and have a high actuation force. The main drawbacks are that the wire should be kept straight and needs to be relatively long to enable a large enough actuation stroke. Embedding the SMA wire in a flexible tube largely enhances its applicability since then the counter forces are transferred by the tube material and the tube can be rolled up or attached to flexible surfaces or clothing layers. The performance of such tube-guided SMA actuators is, however, more complicated since it not only depends on the SMA behaviour but also on the tube materials and the actuator construction. In this research, a simple end-state model for a tube-guided SMA actuator system is proposed. We measure and model both the SMA and tube material properties, including tube creep effects, and derive an approximate prediction for the actuator stroke. Validation experiments showed that the predicted stroke during the second heating and cooling experiments agreed well with the measurements and that the average deviation is 9.6%, even though the deviation is much larger (27.3%) for the maximum applied force.

In recent years, knitted strain sensors have been developed that aim to achieve reliable sensing and high wearability, but they are associated with difficulties due to high hysteresis and low gauge factor (GF) values. This study investigated the electromechanical performance of the weft-knitted strain sensors with a systematic approach to achieve reliable knitted sensors. For two elastic yarn types, six conductive yarns with different resistivities, the knitting density as well as the number of conductive courses were considered as variables in the study. We focused on the 1 × 1 rib structure and in the sensing areas co-knit the conductive and elastic yarns and observed that positioning the conductive yarns at the inside was crucial for obtaining sensors with low hysteresis values. We show that using this technique and varying the knitting density, linear sensors with a working range up to 40% with low hysteresis can be obtained. In addition, using this technique and varying the knitting density, linear sensors with a working range up to 40% strain, hysteresis values as low as 0.03, and GFs varying between 0 and 1.19 can be achieved.

Magnetic soft materials (MSMs) and magnetic shape memory polymers (MSMPs) have been some of the most intensely investigated newly developed material types in the last decade, thanks to the great and versatile potential of their innovative characteristic behaviors such as remote and nearly heatless shape transformation in the case of MSMs. With regard to a number of properties such as shape recovery ratio, manufacturability, cost or programming potential, MSMs and MSMPs may exceed conventional shape memory materials such as shape memory alloys or shape memory polymers. Nevertheless, MSMs and MSMPs have not yet fully touched their scientific-industrial potential, basically due to the lack of detailed knowledge on various aspects of their constitutive response. Therefore, MSMs and MSMPs have been developed slowly but their importance will undoubtedly increase in the near future. This review emphasizes the development of MSMs and MSMPs with a specific focus on the role of the magnetic particles which affect the shape memory recovery and programming behavior of these materials. In addition, the synthesis and application of these materials are addressed.

Compacted Graphite Iron is a suitable material for the engine cylinder heads of heavy duty trucks. In the base of the cylinder head, the Valve Bridge is subjected to Thermo-Mechanical Fatigue as a result of daily start-up and shut-down operational cycles. With the aim to investigate crack initiation, a semi in-situ experiment was conducted by employing a disassemblable dog-bone sample. The procedure consisted of iteratively interrupting a thermo-mechanical fatigue test to observe the evolution of the microstructure by Scanning Electron Microscopy, and Electron Backscatter Diffraction. Finishing the first thermo-mechanical fatigue cycle, edge delaminated graphite particles were observed to be connected by cracks. Cyclic plastic strain computation showed that edge graphite particles can interact by overlapping plastic zones that arise at the tip of the delamination cracks and thus provide a crack propagation path. Finally, the electron backscattered diffraction technique allowed to observe the competition between recovery and plastic strain during the first few thermo-mechanical fatigue cycles.

The cylinder head of heavy-duty fuel engines, made of compacted graphite iron, is sensitive to cracking as a result of a phenomenon called Thermo-Mechanical Fatigue (TMF) induced by subsequent start-up and shut-down cycles of the engine. Under laboratory conditions, various test setups were applied to reproduce the TMF behavior of the valve bridge areas, which are specifically prone to TMF. In these laboratory tests, various mechanical boundary conditions were applied including single and double constraints at low and high temperatures. The TMF lifetime is satisfactorily modeled based on the Paris Crack Growth Law. The reason why the law can accurately simulate the lifetime is due to the fact that this law allows for a description whereby plastically induced damage is gradually built up cycle by cycle, which eventually is reflected in the Cp parameter of the Paris equation. It was proven that the description is valid under partial constraint, full constraint, and over-constraint boundary conditions and even with varying constraint conditions at high and low temperature. Post-processing of the Paris Law model allowed defining an equivalent constraint value γ′, which is a single constraint that yields an identical lifetime as the experiment with double constraint at low and high temperature.

The present study deals with the effect of constituent phase (austenite and martensite) characteristics on the microstructure and mechanical properties of an advanced high strength steel obtained through the quenching and partitioning (Q&P) process. The thermomechanical processing route is purposefully employed to modify the microstructures, to produce better mechanical properties. The final microstructures include first and second (fresh) martensite and retained austenite with film or blocky morphology in a wide range of size. The martensite laths mainly contribute in increasing the strength, and the retained austenite positively affects the ductility. Presence of a high fraction of high angle grain boundaries in the martensite laths indicate that the prior austenite grains are fully recrystallized in the thermomechanical process preceding the Q&P treatment. Although a lower carbon content is observed in the initial martensite of the specimen that is deformed during annealing treatment before the Q&P process, nevertheless, a higher work hardenability is found, because the lower carbon content is compensated by increasing the volume fraction of retained austenite and decreasing the martensite islands average sizes. The latter also the great film-like retained austenite fraction besides the low amount of fresh martensite is found to improve the mechanical behavior significantly.

Compacted graphite iron is the material of choice for engine cylinder heads of heavy-duty trucks. Compacted graphite iron provides the best possible compromise between optimum mechanical properties, compared to flake graphite iron, and optimum thermal conductivity, compared to spheroidal graphite iron. The vermicular-shaped graphite particles, however, act as stress concentrators, and, as a result of delamination from the metal matrix, they are responsible for crack initiation during the thermomechanical fatigue cycles occurring through engine startup and shutdown cycles. Scratch tests driven over the matrix and into the graphite particles were performed in order to characterize the strength of the metal-graphite interface. Samples extracted from a cylinder head in as-cast condition were compared to samples subjected to a heat-treatment at 700 °C for 60 h. The former samples were composed of a primarily pearlitic matrix and graphite particles (~11.5 vol %), whereas, after annealing, a certain pearlite fraction decomposed into Fe and C, producing a microstructure with graphite-ferrite interfaces, exhibiting a partially spiky morphology. The scratch test revealed that the ferrite-graphite interfaces with spiky nature exhibited a stronger resistance to delamination compared to the ferrite-graphite interfaces with smooth morphology. One reason for the high interface strength is the mechanical interlocking between graphite spikes and ferrite, increasing the contact area between the two phases.