## 3 Finite Element Model of STACER
### Finite Element Model of STACER  
The STACER consists of three parts, the top rod, the spring reel, and the storage barrel, as shown in Figure 4. The spring reel and the top rod are riveted. The top rod and the storage barrel are rigid bodies, and the R3D4 rigid body element is selected without deformation. The model is applied with a displacement on the top rod, which only has a freedom to move downwards. The purpose of the displacement is to push the spring reel to be gathered. The top rod pulls the spring reel and controls its extension length. The size is 300 mm in length with a radius of 9.3 mm. The storage barrel is a storage device for STACER with a length of 130 mm and a radius of 25 mm. The structural parameters of STACER are shown in Table 1. Since its wall thickness is only 0.15 mm with a width of 127 mm, it is much smaller than the width dimension (generally less than 1/10). The thickness direction stress can be neglected and the shell element is selected to simulate this structure. In the modelling process, the spring reels are divided with three mesh sizes of 0.99075, 2.06864, and 3.03555 mm. These three sizes of mesh are applied for the mesh sensitivity study, as shown in Figure S1. The simulation results are shown in Figure S2. The variation trend of the force exerted on the top rod is similar for all three cases. It proves the model is not quite sensitive to the mesh size and presents high reliability for the simulation with various mesh sizes. Therefore, a mesh size of 2.06864 is selected with consideration of both the calculation accuracy and computational efficiency. In this work, a larger mesh is selected for the less concerned area, and the meshes which are near the area  
Figure 3: Geometric model of STACER (**a**) strip; (**b**) coordinates; (**c**) top view after curling and (**d**) spring reel. between the spring reel and the top rod are refined in order to achieve both efficiency and precision for the model. When selecting the element type, the meshes of the spring reel and the top rod connected with the concentrated force are refined, and the S3R (3-node triangular general-purpose shell) element is selected. The remaining part is set as a S4R (4-node thin shell) element. The total number of elements in this model is 52,349, which includes 13,419 R3D4 (3-node 3D rigid triangular facet) elements and 38,930 S3R and S4R elements. The development of STECER is based on its special materials property and materials processing technology. The material used for the STACER is Co\({}_{40}\)CrNiMo alloy. The Co\({}_{40}\)CrNiMo alloy is a kind of high elastic alloy, which has been widely used in elastic components because of its high strength, high elastic limit, low elastic aftereffect, and acid and alkali corrosion resistance. The Co\({}_{40}\)CrNiMo alloy strips used in this study are produced by hot rolling, annealing, and cold rolling. The structure of Co\({}_{40}\)CrNiMo alloy strip is FCC at room temperature. Due to the existence of Co element, the stacking fault energy of the alloy is relatively low, and a large number of extended dislocations and twins are easily produced during deformation. The extended dislocation is composed of two Shockley dislocations and a layer of stacking fault sandwiched in the middle. The stacking fault zone of the FCC Co\({}_{40}\)CrNiMo alloy is the HCP structure [38]. Some of the annealing twins are staggered, and the network structure formed by the fine deformation twins hinder the dislocation movement, which can significantly improve the strength and provide conditions for the aging strengthening of the alloy. Cold deformation makes the strength of the alloy increase significantly, and twinning is easy to occur during cold rolling [39]. At the same time, high dislocation density can strengthen the alloy. All these microstructures contribute is responsible for the production of a required STACER.