During the last few years, a set of innovative manufacturing methods have been developed, or further developed, which promise large savings in manufacturing costs in the area of aircraft manufacturing.
These methods are High Speed Cutting, Laser Beam Welding and Friction Stir Welding. One of the main drawbacks of these methods is the fact that the damage tolerance of the resulting structures is not as clear as in the case of the conventional differential manufacturing method.
In order to allow the industry to use the newly developed manufacturing methods of (High Speed Cutting (HSC), Laser Beam Welding (LBW) and Friction Stir Welding (FSW) - which all promise high efficiency - a good damage tolerance capability under certain circumstances must be improved upon. The objective of this project was to develop new methods to assess of the damage tolerance capacity of such structures. All three methods led to a type of structure that is close to an integral structural design. This design offers benefits, such as cost savings, but there are concerns from the damage tolerance capacity point of view.
As stated above, the entire project was focused on the development of reliable tools for the assessment of the damage tolerance of integrally stiffened structures and damage tolerance characteristics.
The theoretical task, as well as the experimental task, inevitably needed to at least comprise of subtasks on crack growth and residual strength of the structures. This was reflected in all of the Work Packages.
In the theoretical area, methods of different theoretical sophistication were used by different partners. This had the big advantage that engineering tools could be checked, and that more sophisticated methods, such as finite elements or boundary elements, could be used to interpret results of the tests in a more phenomenological way.
The structure of the project followed an almost classical route to carry out a project, having the objective to develop theoretical/engineering models. It consisted of an introductory task in Work Package 1. The Work Packages 2 and 3 were dedicated to the development of the models themselves, and the manufacturing and testing, were run in parallel, interacting from the very beginning. New theoretical ideas were learnt from the experimental results and the work benefitted from insights discovered.
Apart from the continuous exchange between Work Packages 2 and 3, a real validation of the methods was required. This validation took place in the latter part of the development phase. It was of special relevance that this was, to a certain extent, done by means of a 'Round Robin' procedure, i.e. different partners could use the same input data but different models to predict theoretical results, which was also found by one partner via an experiment.
As a consequence of the work performed in the first four Work Packages, Work Package 5 aimed to put the results of these into a common guideline and gave appropriate advice on better designs of integrally stiffened structures.
From the experimental point of view, nearly 120 stiffened panels, manufactured from three different aluminium alloys, namely 6056-T6 (to be tested as welded), 6056-T4 welded and then aged to -T6, and 2024-T3, were tested.
From the methodological point of view, fracturemechanical methods have to include primarily the ability to take this influence into account. It could be shown that different methods of calculation (finite element methods (2D/3D), boundary element methods, quasi-analytical methods etc.) all yield good results. Apart from the influence of the residual stresses, the question of anti bending guides and their representation in the models was found to be essential, too.
The Definition of Critical Parameters for the Optimum Design of Integrally Stiffened Metallic Structures which reflects the experiences made by the partners during the project have been collected in the final workpackage of the project and Guidelines for Performing a Damage-Tolerance Substantiation of Integrally Stiffened Panels has also been summarised.
A quite extensive experimental and theoretical programme has been launched.
Both, experiments and theoretical approaches seem to bewell on the way toward meeting the planned goals:
- to have engineering tools of different level of sophistication;
- to have a good experimental background on the differences between the manufacturing methods.