The bending of pipes by pipe bending machines is the same as the bending of sheet metal. In the case of pure bending, when a pipe with an outer diameter of D and a wall thickness of t is bent by an external moment M, the outer wall of the neutral layer is subjected to tensile stress σ1, and the wall becomes thinner; the inner wall of the neutral layer is subjected to compressive stress σ1, and the wall becomes thicker. Moreover, the shape of the cross section changes from a circle to an approximate ellipse due to the combined forces F1 and F2. When the deformation is too large, cracks will appear on the outer wall and wrinkles will appear on the inner wall.
The degree of deformation of the pipe depends on the relative bending radius R/D and the relative thickness t/D. The smaller the R/D and t/D values, the greater the degree of deformation. In order to ensure the quality of pipe forming, the degree of deformation must be controlled within the permitted range. The bending forming limit of the pipe depends not only on the mechanical properties of the material and the bending method, but also on the use requirements of the pipe. The forming limits of pipe fittings should include the following contents:
1) The maximum elongation deformation in the tensile deformation zone outside the neutral layer does not exceed the plastic allowable value of the material and cause rupture;
2) In the compression deformation zone inside the neutral layer, the thin-walled structure part subjected to tangential compressive stress does not exceed instability and wrinkle;
3) If the pipe fitting has ovality requirements, control the distortion of its cross section;
4) If the pipe fitting has strength requirements to withstand internal pressure, control the forming limit of its wall thickness thinning.
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