# Springback Prediction of Free Bending Based on Experimental Method
## 1 Introduction  
Bending components are widely used in construction, manufacturing and aerospace applications [1; 2; 3; 4]. With the continuous development of science and technology, the tube-bending process is facing more and more requirements. Traditional bending methods include press bending, stretch bending and so on [5; 6], The above methods require the use of mold made according to each method's profile. Therefore, these methods are difficult to apply in small batch productions. In order to solve this problem, free-bending technology was proposed. It was invented by Japanese scientists [PERSON], [PERSON] and [PERSON]. Originally, its name was composed of the initials of the three surnames, and was called MOS bending [7]. This technology can meet the requirements of bending tubes with different radii without changing the die [8]. In recent years, more and more researchers have conducted research on free-bending technology [8; 9; 10; 11; 12; 13] based on many methods. [PERSON] et al. [14] described in detail the theoretical development and technical status of free-bending technology in recent years and discussed its importance in tube forming. [PERSON] et al. [15] studied the influence of the mandrel on the forming quality of thin-walled tubes in free-bending forming, and optimized and determined the structure, size and installation position of the mandrel. [PERSON] et al. [16] studied the effect of the bending die offset \(U\) on the forming radius \(R\), which is a key factor in accurate tube construction. [PERSON] et al. [17] studied and analyzed tube formability under different wall thicknesses, and the results showed that when the relative wall thickness was 0.133, the ellipticity of the tube was the smallest, and a better forming quality could be obtained. At present, most of the research on free-bending technology focuses on forming accuracy and forming quality, among which forming accuracy is the focus of scholars' research. When the external load is unloaded, the stress in the tube returns to equilibrium, and the formed tube is different from the ideal shape. The springback phenomenon directly affects the final shape and forming quality of the forming parts. Therefore, it is necessary to study the springback phenomenon and find a solution. Many scholars have analyzedthe springback phenomenon at the theoretical level, [PERSON] et al. [18] studied the influence of different A values on the springback phenomenon through finite element analysis. The larger the A value is, the more obvious the springback phenomenon is. [PERSON] et al. [19], taking the AL6061 tube as a research object, predicted the springback based on finite element analysis, and enhanced the springback prediction model by considering the offset of the neutral layer. [PERSON] et al. [20] provided a method to quantitatively predict springback based on the constitutive equation and obtained a more accurate springback calculation model. [PERSON] et al. [21] proposed a springback prediction model for variable curvature tubes based on the theoretical formula for springback angle prediction of fixed curvature tubes, which has good performance in solving the springback phenomenon of variable curvature tubes. [PERSON] et al. [22] studied and solved the springback compensation problem through finite element numerical simulation and experimental verification. The results showed that the wall thickness and bending radius of the test tube had an important impact on the springback. With an increase in wall thickness, the amount of springback compensation decreased rapidly, and then showed a gentle trend. [PERSON] et al. [23] proposed a springback prediction method for an accurate, fully implicit elasto-plastic finite element analysis function and multi-body processing scheme based on a tetrahedral micro-mesh system, which had an average error rate and maximum error rate of 8.1% and 12.4%, respectively. [PERSON] et al.