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The stress and strain state in the deformation zone of bending and tensile bending.

Bending is the process of bending metal materials (including plate, wire, pipe, profiles and blanks, etc.) into a certain Angle and shape along the cua curve. It is one of the basic processes of stamping and is widely used in production. According to the different mold and equipment used in bending forming, the method of bending can be divided into pressure bending, tensile bending, bending, rolling bending, winding, etc. The paper mainly introduces the process of pressing the bending machine and the design of bending mold.

The mold used by Li Qu is called bending die, which is an essential technological equipment in the bending process. Before the bending begins, the blank of the flat plate is first placed in the positioning plate 1, and then the punch goes down, the punch and the ejector rod, the plate is pressed down (to prevent the offset of the plate in the process of buckling), and the bending is implemented until the plate is completely close to the punch and the four molds 8, and finally the die is opened and the V-shaped drum ejector rod is ejector.

The stress and strain state in the deformation zone of bending and tensile bending

Stress and strain state in bending deformation zone

As mentioned above, the relative width blr of sheet metal directly affects the strain of sheet metal along the race-degree direction, and then influences its stress. Therefore, with the difference of bit, there will be different stress and strain states. The specific analysis is as follows.

(1) Strain state

Length direction (also called tangential or longitudinal) In the deformation zone of Kuaqu, the longitudinal fiber 56 outside the sheet is stretched, and its longitudinal strain is the tensile strain (50 200). The longitudinal fiber aa on the medial side is shortened under compression, and its longitudinal strain is compressive strain (6<0). And the strain becomes the principal strain with the largest absolute value.

Thickness direction (also known as radial) 6: because in bending deformation, the principal strain with the largest absolute value is the longitudinal strain. According to the law of constant volume of plastic deformation, longitudinal strain is bound to occur along the width and thickness direction. The spo of the strain with the opposite sign is on the outside of the sheet material, and the thickness becomes thinner because of the longitudinal elongation, and the thick strain is the happy strain (6<0). And on the inside of the board, because the longitudinal is shortened, so the thickness is thick. The thickness strain is the tensile strain (6,>0). However, because the thinning on the outside is more than the thickness on the inside, the total thickness of the whole sheet material in the bending process is still thin, from the original thickness of 1 thin to?, its ratio is 5-4

It's called the thinning factor. For a wide plate, the smaller the relative bending radius oh is, the more serious the thinning is, and the smaller the thinning coefficient and.

Width direction 5: the deformation of the width direction 8 is closely related to the width size 6 of the sheet material. For narrow plates (relative width 6/1<3).

Due to the small size of the material along the width direction, it can be deformed freely. On the outside, the width shortening is caused by longitudinal fiber elongation, and the width strain is compressive strain (6<0); on the inside, the width strain is tensile strain (8>0) due to longitudinal fiber shortening, and the width elongation is caused by longitudinal fiber shortening. And the whole section changed from a rectangle to a fan. For a wide plate (relative width blt>3), due to its large width size, its wide extension and shortening are subject to the mutual control between the fibers of each layer of the material, so it cannot be deformed freely. Therefore, it can be considered that there is no deformation in the width direction and its strain is zero (6=0). Its section remains rectangular

It can be seen that the strain state of the narrow plate is three-dimensional when it is bent, while the strain state of the wide plate is planar when it is flexed.

(2) stress state

Length direction (also known as tangential or longitudinal) ce: the longitudinal fiber 66 on the outside of the plate is pulled, and its longitudinal stress is tensile stress

(Go>0) : The longitudinal fiber aa is on the medial side, and its longitudinal stress is compressive stress (oo<0).

Thickness direction (also known as radial). : During plastic bending, due to the increase of curvature in the deformation zone and the mutual extrusion between the metal layers, the radial compressive stress (G,<0) is generated in the deformation zone, and in the sheet surface o,-0, gradually increased from the surface to the inside, to the neutral layer reached the maximum.

Width direction center. : The stress center in the width direction is closely related to the width size 6 of the sheet material. For narrow plates (relative width b/f<3)

Because the size of the material along the width direction is not large, can be free deformation, will not be subjected to obvious positive force, so it can be considered as the stress along the width

G,=0; For the wide plate (relative width 6/1>3), the wide stress G6 caused by the wide deformation is not zero because of the mutual restraint between the fibers in each layer of the material. The blocked lateral contraction is the tensile stress (50>0), and the blocked inner elongation is the compressive stress (56<0).

From the point of view of stress state, the stress state of narrow plate is plane when it is bent, while that of wide plate is three-dimensional.