Forming limit diagram

The mechanical test is performed by placing a circular mark on the work piece prior to deformation, and then measuring the post-deformation ellipse that is generated from the action on this circle.

With the availability and use of optical strain measurement system in combination with digital data processing forming limit curves can be acquired in a more automatic and productive way compared to the classic way as described above.

[4] In order to obtain a full forming limit curve, test pieces with different geometries are drawn by a punch (e. g. with a diameter of 100 mm) until fracture occurs.

It is recognized by some authors that the nature of fracture and formability is intrinsically non-deterministic, since large variations might be observed even within a single experimental campaign.

With the definition of the onset of local necking (e. g. membrane force reaches an extreme value) and the assumption of a hardening law according to Hollomon (σ = K εn) it can be shown that the corresponding theoretical plane strain forming limit is identical with the strain hardening coefficient, n. There is no thickness effect.

It assumes an inclined band in the investigated plane sheet piece with smaller thickness which denotes an imperfection.

Basically with this calculation method smooth forming limit curves are generated for materials for which only one experimental value exists.