there are several options for calculating tangential stresses in bent tubes, including analytical, mechanical, and numerical (FEM) models. The best method to use will depend on your specific task or process. If you need approximate stress values quickly, then analytical models may be the best choice. However, if you are interested in more precise stress values and time is not a major concern, you may want to consider creating a finite element model (FEM) using a program such as Abaqus.
For more information on tube bending modeling, you may want to refer to the following references:
Article Modeling and validation of residual stresses in tube bending...
Article Mechanical Modeling of Tube Bending Considering Elastoplasti...