Material failure is the most common problem in the design of a sheet metal forming process and makes an improvement in the method essential. Material failure can be predicted with a very high accuracy by simulation and the visual representation of how the failure occurs helps to improve the method. We distinguish the 4 types of failure:

• Splitting due to excessive thinning

• Edge cracking

• Bending failure

• Shear fracture

Splitting:

A split is caused by strong stretching of the sheet metal. At first the sheet metal thins out strongly and under further tensile load local necking occurs and the metal finally fails. Splittings are the most common type of failure in sheet metal forming processes. They typically occur in a drawn part near the punch radius. To avoid splitting, the material flow in the component must be improved, for example by:

• increasing the punch radius

• reducing the blank holder force

• reducing friction through improved lubrication

Edge crack:

The edge of a sheet of metal is damaged when the sheet is trimmed during the cutting process and is therefore more prone to material failure than other areas. The formability is particularly reduced by shear cutting but also by laser cut cutting. Typical countermeasures are the

• reduction of elongation along the edge

• avoidance of sharp concave radii

Bending failure:

A bended sheet metal is stretched on the outside and compressed on the inside. Bending failure occurs in the outer phase and progresses inwards. As a preliminary stage of the bending failure, the sheet metal roughens up and a surface damage is formed, the so-called orange skin. Bending failure is only detected in the solid simulation, since the good resolution of the solid simulation along the sheet thickness is required here.

Shear fracture:

If one part of the sheet metal is strongly compressed in one direction but stretched in the other direction, shear fracture can occur. In practice, shear fracture only occurs with high strength steels. A typical example is a failure in the infeed of the sheet during deep drawing of a square cup. Since the 3D damage criterion of the Stampack solid simulation, the so-called triaxial failure criterion, is required to detect shear fracture, shear fracture can only be shown in solid but not in shell simulation.