This part of ISO 12004 specifies the testing conditions to be used when constructing a forming-limit curve (FLC) at ambient temperature and using linear strain paths. The material considered is flat, metallic and of thickness between 0,3 mm and 4 mm.
For steel sheet, a maximum thickness of 2,5 mm is recommended.
A forming-limit diagram (FLD) is a diagram containing major/minor strain points.
An FLD can distinguish between safe points and necked or failed points. The transition from safe to failed points is defined by the forming-limit curve (FLC).
To determine the forming limit of materials, two different methods are possible.
1) Strain analysis on failed press shop components to determine component and process dependent FLCs:
In the press shop, the strain paths followed to reach these points are generally not known. Such an FLC depends on the material, the component and the chosen forming conditions. This method is described in ISO 12004-1.
2) Determination of FLCs under well-defined laboratory conditions:
For evaluating formability, one unique FLC for each material in several strain states is necessary.
The determination of the FLC has to be specific and it is necessary to use different linear strain paths.
This method should be used for material characterization as described in ISO 12004-2.
For this part of ISO 12004 (concerning determination of forming-limit curves in laboratory), the following conditions are also valid.
Forming-limit curves (FLCs) are determined for specific materials to define the extent to which they can be deformed by drawing, stretching or any combination of drawing and stretching. This capability is limited by the occurrence of fracture, localized necking. Many methods exist to determine the forming limit of a material; however, it should be noted that results obtained using different methods cannot be used for comparison purposes.
The FLC characterizes the deformation limit of a material in the condition after a defined thermomechanical treatment and in the analysed thickness. For a judgement of formability, the additional knowledge of mechanical properties and the material’s history prior to the FLC-test are important.
To compare the formability of different materials, it is important not only to judge the FLC but also the following parameters:
a) mechanical properties at least in the main direction;
b) percentage plastic extension at maximum force, according to ISO 6892;
c) r-value with given deformation range, according to ISO 10113;
d) n-value with given deformation range, according to ISO 10275.
