To solve the problem, this paper establishes a mechanical model of casing bend to explain the relative slipping and casing bend mechanism for some layer segments.

Based on lots of tube bending experiments,the stress-strain state along the tangential direction of bending line and the direction of the wall thickness is analyzed.

Based on the experiments,the stress-strain state along the tangential direction of outer side of tube bending and the direction of the wall thickness is analyzed and related mean strain calculating formula is also derived.

This article analyses the state and the distribution of the stress and the strain in the deformed region of steel tube,and then introduces the calculation equation of tube bending moment derived,and also how to apply it to the design of a tube bender.

The numerical control (NC) tube bending process used as a method of thin walled tube bending has been widely applied in fields of aeronautics and astronautics.