A forming limit diagram (FLD) shows how sheet metal behaves under different levels of strain. The forming limit curve (FLC) shows the maximum stress the metal can handle before cracking or pinching. FLDs are constructed using sheet metal test strips and measuring strain. The FLC is an irregular parabolic curve, and different types of sheet metal have their own unique FLD. The critical strain zone between two curves is where the material may be safe or may fail. FLDs are developed using a series of tests, and computerized methods can also be used to measure strain.
A forming limit diagram (FLD), also known as a Keeler-Goodwin diagram, is a graph that illustrates the behavior of sheet metal under different levels of strain. The line describing the behavior of the metal is called the forming limit curve (FLC). A forming limit diagram provides information on the maximum stress the metal can experience before cracking or pinching. The diagrams are constructed using sheet metal test strips and measuring strain.
Forming limit diagrams are graphed in a two-dimensional coordinate system, with the largest strain plotted on the y-axis and the smallest strain plotted on the x-axis. Strain is a measure of strain: the largest strain is defined as being in the direction with the largest strain, while the smallest strain is in the direction with the least strain. Different types and different thicknesses of sheet metal each have their own unique forming limit diagram.
A FLC is an irregular parabolic curve, with the minimum occurring at or near the major strain axis. A material subject to strains above the curve will fail, while strains below the curve can be applied to metal. FLDs are usually represented graphically with two curves: the area between the curves is a critical strain zone or safety zone, where the material may be safe or may fail, so in practice it is best not to apply such strains. The critical deformation that is likely to occur in this area is called gouging, which is when the metal is stretched thinner in some areas.
A forming limit diagram is developed using a series of tests. During the tests, stresses are applied to metal strips of different widths. The different widths of the strips simulate different deformation conditions. Each strip is marked with a circular grid pattern which is used to measure strain.
Tension is usually applied to the strips using a ball punch. A metal strip is stretched until the choke is observed. The strain values for the major and minor axes can be obtained by measuring the strain of the circular grid previously marked on the strip.
Computerized methods can also be used to measure strain. Computer images taken during the deformation process can be compared with a reference grid comparable to the circular grid on metal. The computer can calculate the logs using these images. Another method compares the before and after images of the circular grid to calculate the deformation.
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