What Is a Forming Limit Diagram?

Forming Limit Diagrams, also known as Forming Limit Curves, are strips composed of local instability limit engineering strains e1 and e2 or limit real strains 1 and 2 of the sheet under different strain paths Shaped area or curve, usually expressed by FLD or FLC.

In the early 20th century, the industrial production of Ford Motor in the United States greatly promoted the research and development of sheet metal forming technology. Basically, the research work has been carried out on both sheet forming technology and formability. The key issues are cracking, wrinkling and springback. They involve formability estimation, innovation of forming methods, and analysis and control of the forming process. The research focuses on sheet forming properties and forming mechanics, which are far from meeting the needs of the automotive industry.

The 1960s was an important period for the development of sheet metal forming technology, and various new forming technologies appeared one after another. In particular, the proposal of the forming limit diagram (FLD) has promoted the coordinated development of sheet performance, forming theory, forming process and quality control, and has become a milestone in the development history of sheet forming technology.

Due to the early development of the finite element method and CAD technology in the 1980s, the numerical simulation and computer application technologies centered on numerical simulation and simulation in the 1990s have rapidly developed and become practical, which has become a powerful research on material deformation behavior and process design. tool. Sheet metal forming technology has really entered the analysis stage, and traditional sheet metal forming technology has begun to become scientific from experience.

The forming limit chart is established by the provisions of the "Thin Steel Sheet Forming Limit Chart (FLD) Test Method" (JB 4409.8). The mold structure of the test energy is shown below:

The forming limit diagram can be used to evaluate the local forming performance of the sheet. The higher the strain level of the forming limit diagram, the better the local forming performance of the sheet. The forming limit diagram can be applied in the computer-aided design of the stamping forming process, and it can be used to judge whether the process formulation is reasonable, and it can also be used to solve practical problems in production.

1. Judging the dangerous points of forming

When stamping a large cover, the forming limit diagram and grid analysis technology can be applied to determine whether the stamping process can be performed normally.

The specific method is: make a grid in advance on the surface of the blank, measure the change of the grid after deformation, and mark the strain value of the dangerous point on the FLD of the corresponding material, as shown below:

Judgment of forming dangerous points

If it falls in the critical area, such as 4 points, there is a potential danger of rupture, and it is easy to produce scraps when punching. If it falls below the critical area, such as points B, C, and D, the parts can be successfully punched out, but close to the B in the critical area And point D, process factors and production conditions must be controlled to prevent cracking

2. Judging the deformation margin and selecting materials reasonably

The grid method is used to measure the strain values of representative points on the stamped part, draw the deformation state diagram (SCV line), and compare it with the blank material FLD, as shown in the figure below:

Judgment of deformation margin

If the strain value at the dangerous position of the blank reaches n points, and the break point of the same deformation path on FLD is b, then the distance between a and b is the deformation margin. The smaller the margin, the closer the deformation of the dangerous part is to the damaged state, and the production conditions (such as lubrication, mold state, operation, materials, etc.) change slightly, and waste products will appear. When the margin is large, in order to give full play to the deformation potential of the material, on the premise of meeting the performance requirements of the stamping parts, low-grade materials can be used to reduce costs.

3. Analyze the cause of rupture and improve process conditions

Die fillet, blank size, lubrication status and blank holding force directly affect the forming process. These process parameters are often used as controllable factors at the production site to adjust and optimize appropriately to improve the stamping deformation process.

As shown in the figure below, during the test punching, a chevron crack appeared between the fillet of the die at the front end and the impact line of the die.

Cracked electric iron cover

Using the blank with the grid copied, the stamping depth is 1/4, 1/2, 3/4, 7/8 and full depth of the part depth, as shown in the figure below:

Stamping at different depths

When the punching depth reaches 3/4 of the part's depth, the tensile strain at this part increases sharply. Causes the parts to crack after stamping. After inspection, the tip of the punch was not smooth, and there were local protrusions on the contour surface near the depth of 3/4 of the part. After re-punching, the part no longer cracks. ^[1]