Combined implicit/explicit time-integration algorithms for the numerical simulation of sheet metal forming

In order to simulate stamping processes, an explicit method, which is conditionally stable, is generally thought to be the most adapted. Such an algorithm presents the advantage of being noniterative while, the contact configuration evolves rapidly, and the conditional stability is not a disadvantage since time steps must be small enough anyway for an accurate computation. But during the springback simulation, an implicit method, which is iterative, presents the advantage of unconditional stability. The optimal solution is then to have both implicit and explicit methods readily available in the same code and to be able to switch automatically from one to the other. Criteria that decide to switch from a method to another, depending on the current dynamic, have been developed. Implicit restarting conditions are also proposed that annihilate numerical oscillations resulting from an explicit calculation.