Simulation in der umFormteCHniK

Simulation of a sheet forming process with using of the

Lemaitre damage model

 

Aleksandr Romanov, SYZRAN HYDROTURBINE PLANT, Syzran

Vladimir Skorokhod, JSC "TECHPROM", Syzran

Nowadays competitiveness of a production increasing is very important due to competition

propagation in the sheet forging market. Computer aided engineering methods can help to solve this

problem. It can reduce the time needed for preparing to production and market entry, especially if it is

a new part and if a plant has no experience in production parts the same type.

This paper shows how the new part production process was analyzed for a rapid preparing of

manufacturing process on the sheet forming process example. All simulations were executed in the

Simufact Forming 10.

The main problem in this process was to select proper fillet radius between the top surface of the

matrix and cylindrical hole in it. Incorrect radius was a cause of a crack occurrence in the part (Fig.1).

Fig.1 crack in the sheet.

Session : Blechumformung

Simulation in der umFormteCHniK

This problem was occurred because of reduction of plasticity after cold rolling manufacturing process

and wrong radius size. One of the aims was to find out a radius which will not lead to crack

occurrence in the punching of spherical protrusion process on sheet metal blanks. The part which has

been analyzed was a heat-insulating sheet with thickness of 0.3mm, with 296 spherical protrusions,

made of steel X10CrNiTi18-9(Fig.2). The problem was the occurrence of cracks in the transitional

area between spherical protrusion and flat sheet.

Fig.2 heat-insulating sheet: а) up view, в) 2 protrusions section.

Simufact Forming allows estimate the possibility of metal destruction in the deformation process.

From six damage models of Simufact the Lemaitre model has been chosen because of simplicity

determination of the model parameters.

Parameters of a model, has been taken from a tensile test diagram, but specimen has been made of

annealed steel. This data has been used for simulation. For minimum yield stress correction, tensile

tests of thin stripes have been performed. By the reason of low thickness of the sheet for the

simulation shell approach and solid-shell elements have been used.

The problem has been solved with 3 types of transitional radius: t, 2t, 3t (where “t” is the thickness of

the sheet).

The distribution of the equivalent plastic strain in spherical protrusion is shown at Fig.3. The

dangerous area is located on a transitional radius between flat sheet and a spherical protrusion as we

are expected. Maximum value of relative damage on this area is 1; it tells us that possibility of

destruction of the sheet is about 100%.

Session : Blechumformung

Simulation in der umFormteCHniK

Fig. 3 the equivalent plastic strain distribution in a section of the protrusion

The deformations in the dangerous areas were compared with the diagram of plasticity. Combining

results from tensile tests of thin stripes of cold rolled steel and the tests of annealed cylindrical

standard specimens we got the initial value of plastic strain, which, as we assume, is uniformly

distributed over the surface. The value of initial plastic deformation in combination with plastic strain

accumulated within the deformation process, also tells us, that destruction can happen.

Further analysis of this problem shows that increasing the value of the transitional radius till 3t

reduces value of damage to 0.

Second positive moment is that needed for stamping force, decreases. It decreases by 3 kN, after

radius was increased (Fig.4). It is a big difference especially if we punching many protrusions

simultaneously.

Session : Blechumformung

Simulation in der umFormteCHniK

Fig.4 the punching force, transitional radiuses: t, 2t, 3t.

Really invaluable help, computer simulation gives in case of optimization technological processes. It is

easier to solve many problems, than to remake dies many times.

For example, for this model we have solved many variants of the construction and analyzing the

contact spot between punch and sheet (Fig. 5) we have found, that punch could be modified, to

reduce punching force.

Also in case of that kind of stamping it is very important to consider axiality of punches and holes in

the matrix. If we don`t, we can get cracks in the workpiece.

Session : Blechumformung

Simulation in der umFormteCHniK

Fig.5 contact spot between the punch and the workpiece.

Taking in to account construction of the part and constraints applied to it, have been simulated some

variants, one of them is shown in the Fig.6.

This configuration of punch allows reducing possibility of damage of the workpiece, and also to

reduce stamping force (Fig.8).

Fig.6 comparison of two types of punch

Session : Blechumformung

Simulation in der umFormteCHniK

Fig.7 the distribution of equivalent plastic strain, in a section of the protrusion with a cone punch

Fig.8 the punching force, kN

Thus, numerical simulation in Simufact Forming helped us to define proper shape of stamp parts,

which can ensure the integrity of the workpiece and additionally to reduce force needed for punching

a single spherical protrusion. And also thanks to conical punch, to reduce needed axiality precision of

punches and matrix holes.

Session : Blechumformung