FLC-Test according to ISO 12004
FLC-Test - forming limit curve
-according to ISO 12004-
Testing according to the FLC principle is based on the Forming Limit Diagram (FLD) with the Forming Limit Curve (FLC).
This method offers the possibility to determine the upper and lower limit values of sheet metal materials with the help of a defined forming process and to compare them with the drawn parts in the press shop.
Application
Forming limit curves are used in method planning and tool making to optimise forming tools and component geometries. With the help of the deformation analysis at critical points of a component and the comparison with the FLC, it is possible to reliably assess the failure behaviour of a sheet metal material.
Before commissioning press tools in sheet metal forming, forming processes can thus be specifically optimised with regard to blank and tool geometries, blankholder pressure, lubrication and material in order to avoid rejects in production.
Another important area of application for the FLD is the computer-aided simulation of forming processes. The FLC of the sheet metal material used is included in the calculation as an important material parameter.
Testing principle
Forming limit tests specified as standard
> ISO/WD 12004:2004 E - Determination of forming limit curves < is the name of an international standard for determining forming limit curves for semi-finished sheet metal products from 0.4 mm to 4.0 mm thickness. To comply with this standard, the forming limit curves are usually determined by the material manufacturer using the Nakazima or Marciniak method. This can be used to clearly define the quality of the material and thus make it easier for the user to choose the right material.
Nakazima test
The most common method of determining the forming limit curve of a sheet material is the Nakazima test.
The principle is based on forming sheet metal blanks of different geometries to failure with a hemispherical punch. By varying the width of the specimen (at least five test specimens), widely differing deep and stretch forming conditions are established, ranging from uniform bi-axial to uni-axial deformation on the sheet surface. The characteristic, maximum achievable changes in shape (just before cracking) of the various specimen shapes are determined and thus define the FLC of the material.
Sample sheet
Marciniak test
The Marciniak test described in the standard differs from the Nakazima test only in that a cylindrical instead of a hemispherical drawing punch is used and that a different tool geometry is used.
In addition, a perforated support plate is used instead of sliding foils made of plastic (PE and PTFE) in order to reduce friction.
Sample sheet
Optical measuring systems replace magnifying glass and microscope
Up to now, the recording of a forming limit curve has generally been done by marking the sheets with circular or line nets before forming. In the course of loading the sheet metal, the applied circles deform into ellipses whose main axes represent the strains on the component surface in the main and secondary directions. After forming, the >deformed< line patterns are measured by hand with the aid of measuring magnifiers, measuring microscopes and flexible measuring strips. This method is limited by the contour sharpness of the formed grid, by the lengthy evaluation, by the low spatial resolution and by the subjective, user-dependent measurement value recording. The time required for the conventional creation of an FLD can be up to 45 min per material quality.
Specimen preparation, forming process and determination of the characteristic values can be done much easier and faster with optical measuring systems. They thus clearly meet today's requirements, also in terms of accuracy, reproducibility and efficiency. Due to the great time savings, the testing costs per forming limit curve decrease considerably. The determined measured values are displayed as a 3D section graph and in a deformation diagram.
When determining accurate, practical material parameters, it is not only a matter of selecting the appropriate test method. Equally important is the selection and configuration of the appropriate test equipment. Here it is particularly important to electronically control or regulate the forces applied by the hydraulic system according to the desired test parameters. Furthermore, it is necessary to record, process and evaluate the measurement data with a PC system designed for this purpose. The deformations of the specimens are preferably determined with the aid of optical image recognition or special laser methods.
Testing machines
Directly to our Bulge-/FLC-Tester Model 161.
