Review Paper On Composites In Origami Structures
The appearance of origami structures has galvanized innovative and efficient ways of storing large objects in compact spaces and creating movable parts for automated machines. When most individuals consider the word “origami, ” they immediately think of folding paper; however, there are many other materials that can be utilized in origami and are extremely useful for real-world engineering applications. These applications are currently ranging from bulletproof shielding for police to satellite solar panels in the aerospace industry. Researchers are constantly establishing theories and providing empirical results to describe the behavior of composites within origami structures. This paper will concentrate on three scholarly articles each of which focuses on a particular aspect of origami structures. Paper focuses on proving the plausibility of reconciling foldable and stiff structures using Tachi-Miura-Polyhedron Origami (TMP) as well as the Miura-ori fold pattern. Since TMP is an application of the Miura-ori fold, it demonstrates rigid-folding behavior as well as other interesting phenomena.
The paper defines the geometry of both TMP and Miura-or in order to establish the equations related to force-displacement and Poisson’s ratio. Equations 1 and 2 are found using the assumption that all panels are rigid and connected by springs with a virtual work argument. Where α is the panel tilt angle, θM is half of the dihedral main-crease fold angle, θS half of the dihedral sub-crease fold angle. The initial folding angle and height of each layer control the stiffness of the structure. The author plots different formations of TMP to illustrate the fact that the force-displacement phenomenon includes either equilibrium states, negative stiffness, and strain hardening. For conciseness, one of the graphs is shown in Figure 3 for strain hardening, which can be very valuable when an increase in stiffness is required under specified loading conditions. Strain hardening, for the TMP case, will deliver a greater spring-back effect under axial loading.
For the experiment performed, different fiber and resin materials were tested; the origami structures were integrated into the layup and the whole laminate was made up of three plies. Dry fibers for the crest materials were first tested, and then resin was brought into the system to avoid unnecessary unthreading. To decrease bending stresses linearly proportional to the thickness of the laminate, they used the thinnest unidirectional carbon fiber prepreg, resulting in the crests being able to fold much better when compared to the fiberglass; despite this, they were ultimately also disposed to cracking. After testing for a usable TMP origami structure, the experiment finally found a successful ingredient: urethane epoxy.
Paper 1 does a great job of laying down the fundamentals of what structures can be implemented through the use of origami composites, but paper 2 takes those ideas even further. Paper 2 discusses the introduction of anisotropy into elastomers and how paper can be folded through origami principles to create 3D structures in robots. The structures increase the anisotropy and stiffness of elastomeric actuators while avoiding being too heavy in terms of weight. Unlike paper 1, paper 2 states that their materials involved a silicone elastomer (Ecoflex) and a polyester/cellulose blend paper. The materials are for most applications of the actuators but a variety of other polymers and flexible structures work just as well. According to the article, Ecoflex is a soft elastomer that can withstand bending a fractured over a max strain of 900%. Also, pneumatic artificial muscles (PAMs) are based on the pressurization of a flexibly thin tubular membrane with fiber reinforcement, and they boost strength and flexibility when applied in precision robotic duties and have also been utilized in other technologies. To characterize the relative change of length of the actuators upon pressurization, the authors use ε, defined in equation 2, which the authors of article did not consider. Where lPatm is the length of the device with no externally applied pressure and lp is the length of the device upon applied pressure. Using folded paper structures in elastomeric polymers makes it possible to create soft actuators where motion on pressurization is decided by the paper’s folding patterns. According to the article, these paper structures can strengthen the elastomeric matrix and support pressures around 300 mbar.
Furthermore, large positive ε values or large extensions can be accomplished by increasing the amount of folds the paper contains. This strategy was implemented by creating what paper 2 describes as a “bellows” structure that, when pressurized to 170 mbar, extends up to ε = 3. 61. It was discovered that these actuator structures can lift loads 120 times their weight.
It is imperative to note that this structure in Figure 5 is very similar to the TMP structure described in paper 1 since they are both polyhedron bellows and take advantage of the Miura-ori features. Both also have a certain foldability while filling a 3-D space and can be practical to various deployable structures and actuators. However, since the structure in Figure 5 is made of paper, it is not as rigid as the TMP structure described in paper 1 which is extremely rigid. The reason the authors of paper 2 use paper is most likely due to the face that paper is very flexible and easy to make crease lines on; thus, it will be much easier to study the folding behavior of the TMP. It will also be easier to discover more scenarios with paper as well, making for more reliable results. For example, paper 2 mentions that some bellow structures can achieve different movements when the pleats of its lateral faces are stuck together with Ecoflex, which results in a constricting force that creates an angular elongation of the actuator (Figure 6). Therefore, with pressurization, the length of the strip connected to the pleats of the bellows structure helps in defining the actuator’s radius of curvature.
Paper 3 emphasizes the use of self-folding origami, involving shape memory composites. Experiments have shown shape memory polymers (SMPs) to self-fold into specified structures with light absorption techniques, yet this approach has many limits. For example, it is challenging to create fold lines that are well defined by solely utilizing SMP. However, similar SMPs have been established and are stimulated by resistive heaters, which help to create self-folding structures. The authors created a process that stimulates folding of shape-memory composites activated with uniform heating; this process defers from 1 and 2 in that the origami structures for the previous two were never activated with any sort of thermal process.
There is a torque exerted by hinges in the self-folding structures that depend on the stress in the SMP from the thermal activation σx, thickness tsmp, the distance between SMP and point of rotation δ, and width W of the hinge, all of which are given by equation 3. The stresses along the SMP length are calculated with the assumption that there are no stresses in the z-direction. However, when the SMP is active, the x and y strains εx and εy are tantamount to the activation strain εa while the material is not constrained in the z-direction. Finally, the equation for torque can be expressed in equation (5) by combining equations 3 and 4. For the SMP layer, the authors used pre-strained polystyrene shrink film sheets, which contract to about 50% of their initial size when heated above a certain temperature. The layers used were bonded with silicone tape, which was used because it could withstand higher temperatures. The fact that they tested multiple adhesives to operate at different temperatures shows their dedication and reliability for the experiment. They also fashioned 30 mm wide test hinges to describe the folding torque of the self-folding composite, and the final angles are plotted. When calculating the torque, the Poisson ratio was assumed to be 0. 5 for the SMP.
As the length of the face approached the crossover point, the measure of the final angle slowly reduced, until it fell quickly at the crossover point. According to 3 this inconsistency is due to the viscoelastic characteristics of the SMP during its rubber state, allowing for plastic deformation by the resisting moment. Despite SMPs behaving elastically in general, extended external stresses result in disentangling and slipping of the polymer chains into new actively useful positions, which ultimately lead to plastic deformation. From these results, the authors of 3 were able to test multiple self-folding origami shapes: a cube, an icosahedron, a flower, and a Miura-ori pattern. Each structures’ self-folding composites were placed in the oven and usually completed their target frame in one to four minutes. All of the structures folded successfully, yet there were some differences with shapes that had many folds. This was most likely due to the limitations of the composites when they were manufactured and caused errors with the fold angles. The Miura-ori pattern has only one degree of freedom and its self-folding repeatability was improved because of the many folds that were acting parallel to one another.
Overall, this experiment was useful and provides dependable results that could be used to create improvements in composite designs; it is a unique take on origami composites because 1 and 2 did not account for a heating process, which could very well determine the feasibility of a structure when it needs to be used in a real-world application.
Despite the different approaches each of these papers took in analyzing origami composites, they all have a remarkably common design: Miura-ori structures. The reason this pattern is used in a lot of these experiments is because the pattern is very versatile and can be used for both rigid and non-rigid bodies. The Miura-ori is also unique in that is has a negative Poisson’s ratio such that when one applies a force on the sides of the structure, the top and bottom will tighten. Paper 1 showed that the TMP, which is a variation of the Miura-ori pattern, was able to be constructed with woven carbon-fiber pre-pegs, and compression tests can show that the mechanical behavior of the structure agrees with the theory of TMP. Paper 2 took the ideas of paper 1 further and also used the same Miura-ori variation during experiments and showed the magnitude of strength and resistance the pattern has. Paper 2 illustrated the viability of uniting an extremely stretchable elastomer (Ecoflex) with a non-stretchable but bendable sheet and subjected them to various pressures to model components useful in soft actuators and robots. Paper 3 used the Miura-ori fold as well but the experiment involved heating the structure to see if it would retain its targeted shape and ultimately demonstrated that future work could help achieve a decrease in the variability of the folded structures through improvements in the composites. The Miura-ori pattern, in the experiment of paper 3 also demonstrated that the self-folding repeatability was improved for the structure which completely contrasts the fact that the other structures has limitations that inhibited the repeatability of self-folding.
This topical review paper has performed extensive comparison and contrast of three papers: TMP Origami, Elastomeric Origami, and Self-Folding Origami. The rudimentary theories and equations are discussed along with the experiments performed. Each paper used a specific origami technique, which were remarkable to learn about. These origami structures are very useful because they can have many applications in aerospace, robotics, and architecture. For example, the Miura-ori fold could be very viable for a satellite solar panel requiring a compact space for launch but also requiring it to expand once it is deployed in space. This concept is already being used by NASA, and it will be exciting to see what new developments origami structures will have in the future of composites engineering.
