Variation of contractile strain ratio of Ti-3Al-2.5V tubes and its effects in tubes numerical control bending process

The rapid development of high-technology fields, such as aviation and aerospace engineering, leads to the increased demand for lightweight and high-strength bent tubes due to their desirable properties. However, these tubes are of clear anisotropy. The anisotropy of tubes is usually described in terms of the contractile strain ratio (CSR). The current method to determine CSR can only be applied to cases of small uniform plastic deformation. This makes it difficult to obtain the variation in CSR during the plastic forming processes in which large plastic deformation usually occurs. Considering that the circumferential strain and axial strain can be directly and continuously obtained using the Digital Speckle Correlation Method (DSCM), this study proposed a new method based on the DSCM for determining CSR values over a large deformation range. Using this method, small gage lengths can be flexibly adopted to obtain a uniform plastic deformation as large as possible. To improve the precision of the method, elastic deformation was compensated theoretically, and varied values of Young's modulus with deformation were adopted. Using this method, the CSR of Ti-3Al-2.5V (ASTM Gr. 9) titanium alloy seamless tubes, which are typical anisotropic tubes, was observed varying with plastic deformation in that it decreased rapidly in the initial stage, then decreased slowly until stabilizing in the final stage. The variation can be expressed approximately as a second-order decay equation. The results of this study also show that the variation of CSR is sensitive to the values of Young's modulus and Poisson's ratio, therefore, varied values of Young's modulus and reliable Poisson's ratio should be adopted to improve precision. By embedding the variation in CSR with deformation into the finite element (FE) simulation for the numerical control (NC) bending of Ti-3Al-2.5V seamless tubes, the predication accuracy for the thinning degree of wall thickness, the flattening degree of the cross-section, and the springback angle can all be improved.