Jérôme Bonini

Combined implicit-explicit algorithms for non-linear structural dynamics

To solve fast dynamic problems, an explicit method is the most adapted. But for slower dynamics, an implicit method is more stable. The industrial problems are governed by high frequency (impact, …) during short time intervals and slower dynamics (spring-back, …) during other time intervals. The optimal solution is then to have both implicit algorithm and explicit methods readily available in the same code and to be able to switch automatically from one to another. Criteria that decide when to shift from a method to another have been developed here. Implicit balanced restarting conditions that annihilate numerical oscillations resulting for an explicit calculation are also proposed.

Prediction of transient engine loads and damage due to hollow fan blade-off

The loss of a fan blade causes serious damages on an engine and can endanger the aircraft integrity and the safety of passengers. Commercial aircraft engines must then meet the FAA (Federal Aviation Administration) and JAA (Joint Aviation Authorities) certification requirements concerning the fan blade containment. The certification is validated through a Fan Blade-Off (FBO) test on a whole engine. The success in this test requires destructive and expensive development tests performed at the different stages of the design process. To reduce the number of these experiments and thus, the costs and the time of development, the engine behaviour under FBO can be understood and even predicted thanks to finite element (FE) analysis. This paper shows a comparison between a FBO simulation of hollow blades, computed with an explicit integration FE code, and experimental data obtained during an intermediate FBO test carried out by Snecma Moteurs. The results of the load levels and the similarity on the sequence of events show good agreement.

Automatic time stepping algorithms for implicit numerical simulations of blade/casing interactions.

Abstract An automatic time stepping algorithm for non-linear problems, solved by implicit schemes, is presented. The time step computation is based on the estimation of an integration error calculated from the acceleration difference. It is normalised according to the size of the problem and the integration parameters. This time step control algorithm modifies the time step size only if the problem has a long time physical change. Additionally, the Hessian matrix can be kept constant for several iterations, even though the problem is non-linear. A criterion selecting if the Hessian matrix must be calculated or not is developed. Finally, a criterion of iterations divergence is also proposed. It avoids the determination, by the user, of a maximal iteration number. This minimises the total number of iterations, and thus the computation cost. Industrial numerical examples are presented that demonstrate the performances (precision and computational cost) of the algorithms.

Modélisation du contact en implicite: Application à I'interaction rotor/stator

ABSTRACT The two main contact methods in dynamics are firstly studied. The influence of the Newmark implicit time scheme parameter values is then analysed with a special attention to the contact compatibility conditions. A new contact stiffness is proposed to compute interaction forces in a finite element code with a Newmark implicit integration scheme. The contact model is finally used to simulate rotor/stator interaction in an aircraft engine, after a blade-off event.

A new formulation of internal forces for non-linear hypoelastic constitutive models verifying conservation laws

This paper proposes a new formulation of the internal forces for hypoelastic constitutive models ensuring that the elastic work of deformation can be restored by the scheme. Moreover, we demonstrate that the work of the plastic deformation is positive and consistent with the material model.

Implicit-explicit time integration algorithms for the numerical simulation of blade-casing interactions

In order to solve fast dynamic problems, an explicit method is the most adapted. But for slower dynamics, an implicit method is more stable. Typical industrial problems are governed by high frequency, e.g. impact, during short time intervals and slower dynamics, e.g. spring-back, during other time intervals. The optimal solution is then to have both implicit algorithm and explicit methods readily available in the same code and to be able to switch automatically from one to the other. Criteria that decide when to shift from a method to another have been developed here. Implicit balanced restarting conditions that annihilate numerical oscillations resulting for an explicit calculation are also proposed.

Search of Contact in Dynamic Finite Element Code: Presentation of an Analytical Method

The purpose of this communication is to develop a method of analytic contact search applied to a finite element code and to present the first numerical results. This method is based on the construction of geometrical entities being sufficiently continuous to get rid off the facetisation problems due to the spatial discretisation of the studied structure by finite elements. These geometrical entities are built up using the mathematical notion of spline applied to some nodes of the mesh. The search of contact is then realised with these entities, and then the obtained results are projected on the initial mesh.