Aeroelasticity and Structural Dynamics
Methods and Simulation Codes for Inelastic Analysis, Damage, Failure and Crash
Simulation of the forming-tempering process for optimizing the dimensioning of an aerospace structure
The final purpose of this study is to completely model the “forming-tempering” process so that Sabca can master the industrialization for the shaping of self-stiffened panel in the Ariane 5 structure. This work stems from close collaboration between Onera, Sabca (Société Anonyme Belge de Constructions Aéronuatiques), Péchiney and the LTAS in Liège.
The “forming-tempering” process is a metal shaping method that uses the phenomena of stress relaxation that occurs in the course of commonly practiced heat treatments to improve the mechanical strength of the structure. The particular feature of this process is that it uses the thermal and pressure capacities of an autoclave to give the piece a desired geometry while bringing it to the desired metallurgical state. This tool is very promising for the manufacture of aeronautical and space structures compared with the more costly and non-reproducible press forming. Mastery of this process resides in determining the initial shape of the mold, of the pressure parameters (for pressing the plate into the tooling), of autoclave temperature (internal stress relaxation and heat treatment) and time to keep to the dimensional tolerances of the piece in the end. The material may, moreover, be the seat of metallurgical transformations that have to be integrated for a fine modeling of the process.
Fig. 1 – Tensile-compressive tests with thermal cycling and ageing periods.
Onera’s work concerned the development of new behavior laws in cyclic viscoplasticity for aluminum alloys, which include the effects the changing metallurgical state of the material (germination, growth and coalescence of the precipitates) has on its response to a thermo-mechanical loading representative of the process (Fig. 2).
Fig. 2 – Model/experiment comparison:
tensile tests at two rates for two metallurgical states.
Fig. 3 – Effect of time on the mechanical properties under complex loading. Series 2000 alloy at 150°C .
Fig. 4a – Comparison of simulation and tests for two metallurgical states.
Fig. 4b – Comparisons on relaxation tests (two deformations) in metallurgical state T35.
Fig. 5 – Variation of the yield strength for artificial ageing at 150°C .
The material’s behavior laws having been determined, mastering the process next calls for a complex numerical simulation using a finite element code with:
- 3D discretization of the mold and panel and definition of the boundary conditions;
- numerical simulation of the forming-tempering treatment;
- automatic optimization of the mold geometry and/or of the cycle time, temperature, and pressure parameters by minimization of a functional.
Figure 6 gives an example of mold shape optimization (the tempering cycle being set) for obtaining a 3D conical structure from a panel.
Fig. 6 – Example of mold shape optimization in the case of a perforated conical panel (ZéBuLoN code).
