The stamping process of bending flat blanks, profiles, or pipes into a certain curvature and a certain angle to form parts of a certain shape is called bending.
When an external bending moment M acts on the blank, the curvature of the blank changes. The part where the curvature of the blank changes (A, B, C, D in Figure 1). The metal on one side of the center of the zone (hereinafter referred to as the inner layer) produces compression deformation under the action of tangential compressive stress, and the metal on the far center (hereinafter referred to as the outer layer) produces elongation deformation under the action of tangential tensile stress. The distribution of tangential stress in the blank deformation zone is shown in Fig. 1
The bending process of sheet metal can be divided into three stages
- In the elastic bending stage, when the value of the external bending force is not large, the stress induced on the inner and outer surfaces of the deformation zone of the hair is smaller than the yield point of the material, and elastic deformation is only caused inside the hair. The deformation zone in this stage The tangential stress distribution is shown in Fig. 1a.
- Elastic-plastic bending stage. When the value of the outer bending moment continues to increase, the radius of curvature of the rough becomes smaller, and the inner and outer surfaces of the deformed area of the rough first transition from an elastic deformation state to a plastic deformation state, and then the plastic deformation is from the inner and outer faces to the center Gradually expand. The stress distribution is shown in Fig. 1b.
- Pure plasticity stage. When the external bending moment continues to increase, the material in the hairy deformation zone is completely in a state of plastic deformation, and its stress distribution is shown in Figure 1d
The stress on the metal plate section transitions from the tensile stress of the outer layer to the compressive stress of the inner layer, and there must be a layer of metal in the middle whose tangential stress is zero, called the stress neutral layer. Its radius of curvature is p. express. Similarly, the distribution of strain also transitions from the tensile strain of the outer layer to the compressive strain of the inner layer, and there must be a layer of metal whose strain is zero, and its length does not change during bending deformation, which is called the strain neutral layer. Its radius of curvature is denoted by P. When the degree of elastic bending or bending deformation is small, the stress-neutral layer and the strain-neutral layer coincide and are located in the center of the plate thickness; when the degree of bending deformation is large, both the stress-neutral layer and the strain-neutral layer start from the thickness The center moves to the inner layer, and the displacement of the stress-neutral layer is greater than that of the strain-neutral layer, that is, p:>P.
When the narrow plate is bent, its cross-section is greatly distorted: the thickness of the plate at the bend decreases, the inner layer becomes wider, the outer layer narrows, and the inner and outer layers are both star-shaped (Figure 2). A wide plate also becomes thinner in bending, but there is little distortion in the cross-section because transverse deformation is prevented by the resistance of the wide material.
Four types of the plate bending process
Sheet bending can be divided into four categories according to the equipment and tools used.
- Press bending: the bending of a sheet on a press or plate bender
- Stretch forming: For parts with a large bending radius (small curvature), bending is performed under tension to obtain plastic deformation
- Roll bending: Use 2~4 rollers to complete the rolling plate with a large radius of curvature
- Roll forming: During the continuous longitudinal movement of the strip material, it is gradually bent into the required shape by several sets of die wheels