This proposal was for the structural analysis and design optimisation of the lower fuselage of an aircraft. The objective of the structural optimisation was minimum weight and the design constraints fall into two categories; firstly, the capability to withstand structural loads with sufficient margins, and secondly constraints relating to the manufacturing methods. The structure to be optimised was a thin-walled metallic structure with stiffening elements (stringers and frames) using three options for the manufacturing technology:
- Riveted stiffeners (conventional technology),
- Laser welded integral stiffeners,
- Load-adapted stiffeners with welded integral knots.
The first two options are reference designs that were analysed and compared with a new proposal based on the third manufacturing technology. Additionally, two types of aluminium alloy were considered for the weldable options. The work was carried out by experienced aerospace engineers at AOES Group BV, a Dutch SME with many years specialist experience in the structural design, analysis and verification of aircraft and spacecraft structures, including FEM-simulation of large components for aircraft fuselage structures. AOES used design optimisation and topology optimisation tools in NASTRAN for identifying the preliminary layout and sizing then standard stress and buckling analysis approaches for the accurate simulation of the load-bearing characteristics and detailed mass calculation. For seamless transmission of data and dissemination of results, data can be received or transmitted as engineering drawings, neutral CAD files or CATIA files. AOES also intended to use the resources of its Media lab department to provide attractive 2-D and 3-D rendering of the results of the work performed.
The purpose of the FusDeOpt project was to use detailed numerical simulations to investigate possible mass savings on aircraft fuselage structures using Laser Beam Welding (LBW) technologies. Analyses were conducted for stinger-reinforced aluminium panels with a number of different stringer configuration and material options. A reference panel using conventional riveted aluminium construction was analysed, as well as a number of LBW options.
Analyses were first undertaken firstly using the traditional ‘classical’ techniques, and then a variety of Finite Element Method (FEM) models were analysed.
2D FEM analyses were conducted of pull-test specimens for a variety of welded stringer configurations. 3D FEM analyses were conducted for panel compression specimens, which exhibited complex failure modes involving panel buckling and local plasticity at the root of the stringers. The 3D analyses included SHELL element models and SOLID element models.
All FEM models included the effects of material nonlinearity, geometric nonlinearity (i.e. large displacements) and, in the case of the riveted panel structures, contact and gapping. Initial imperfections were included in the SHELL element models for the compression panels however the results showed that the effect of imperfections was always less than 2% of the ultimate strength. Thus, for the 3D SOLID element analyses, considering the high computational efforts required, no attempt was made to model initial imperfections.
The 2D pull-test specimens showed failure modes with almost pure tension in the weld seam. This differs from the 3D panel compression specimens which showed failure by lateral bending of the stringer root at a load weakened by plasticity in the weld seam (see Figure 5 5 of this report). The difference between these local failure modes means that the results of the 2D pull-test specimens cannot be directly used to predict the strength of the 3D compression panels.
The 2D results do however permit an assessment of the relative strength of the different materials and heat treating options of the welded joint, and it is likely that a design that performs well in the pull-test loading will also show good resistance to stringer rotation in 3D panel compression loading.
The SHELL element models have relatively few elements and thus provide a solution time of only a few hours on the typical modern computers used for this project. The SHELL elements are suitable for modelling global effects in the large regions of the panels, but the representation of the weld seam does not give sufficient accuracy for local failures.
The SOLID element models can provide sufficient accuracy for local failures, but this comes at a high computational cost, since it is impossible to predict in advance where the failure will occur. The high computational costs also make a thorough investigation of the effects of initial imperfections very time consuming.