Ein Ingenieurmodell für den rißfortschritt in hochleistungsgefrästen (HPC) Panels (English shop)

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Today, aircraft manufacturers are required to deliver cost-efficient and more environmentally friendly aircraft which at the same time are to be more efficient than previous ones. Alongside many other innovations based on the latest research findings, particular attention is being paid to reducing take-off mass while maintaining performance.

Conventional aircraft are created by riveting individual components into an overall structure. Through the use of new manufacturing processes and the advancement of new production technologies, components that used to be produced with rivets can now be manufactured, for example, by milling from a solid block. In addition to a design better matched to the loads, minimum wall thicknesses can be avoided, manufacturing times reduced and weight saved, since the joining elements are no longer required. Nevertheless, these parts must meet the high, aviation-specific requirements. Particular attention is paid to the damage tolerance of these components, i.e. how the component behaves when damage occurs and, specifically, how a crack propagates in such an integral structure.

In this work, the analysis methods previously used for differentially manufactured components have been further developed so that they can also be applied to integral structures. A skin panel with several integral stiffeners of differing geometry is considered.

Chapter 2 briefly presents once again the theoretical fundamentals required for the calculations. Subsequently, chapter three presents those for the conventional design of skin panels stiffened with stringers. Here a distinction must be made between an intact, a partially cracked and a broken stiffener. This is shown for the stiffened skin without bending; a solution including bending, however, is only available for intact and broken stiffeners when these are bonded onto the skin.

In chapter four, these existing solution approaches are further developed so that integral structures under additional bending can also be calculated, taking into account a distinction between the stiffener conditions (intact, partially cracked or broken).

The methods presented and further developed in chapters 2 to 4 are validated using results from finite element calculations and experimental results.

For this purpose, chapter 5 briefly presents the fundamentals that must be observed in fracture mechanics investigations using the finite element method. In addition, the procedure for generating the model is briefly explained and the principle of the mesh generator described.

Chapter 6 introduces the three specimen geometries used for the tests. Furthermore, the test set-up, the test procedure and the particular aspects of introducing the crack into the structure are described in detail. The test results are presented separately according to specimen form and specimen geometry, and initial conclusions about the crack growth behaviour are drawn. In addition, it is pointed out which aspects require particular attention during specimen preparation in order to provide input values for the calculations so that these can be compared with one another. For the documentation of the tests and the determination of crack growth, notes and suggestions are given in order to obtain the results more reliably and more easily, so that comparability can be improved.

In chapter 7, all results are presented, compared with one another and conclusions drawn as to how an integrally stiffened structure should be designed with regard to damage-tolerant behaviour. It was possible to identify clearly in which ranges and for which configurations the analytical model yields very good results, i.e. where crack propagation can be well predicted. These are, on the one hand, configurations with stiffeners that all have the same physical properties, in particular the same height, and, on the other hand, those in which the central stringer is also broken. With stiffeners of differing heights the results become poorer and are not suitable for predicting crack growth. Furthermore, it is shown which particular aspects must be observed in the analytical modelling.

The test results of the configuration with intact stringers can be reproduced with the results of the finite element calculations and the analytical calculations, whereby the differences in the FE calculation can be explained. The FE analyses for the configuration with a broken central stringer yield only inadequate results and are unsuitable for predicting crack growth. The reason for this lies in the insufficient number of elements across the component thickness for this type of structure, which is confirmed by the literature.

For the analytical calculation, several input parameters were varied and their effects on the results investigated. In doing so, statements in the literature were confirmed and findings obtained which must be taken into account in further investigations.

For the prediction of crack growth in integrally stiffened structures with different initial configurations and simple stiffener geometry, it was possible to create an analysis tool which was validated by tests on relatively small specimens. These results should, if possible, be substantiated by further specimens, whereby particular importance must be attached to the introduction of the crack and the documentation of crack propagation in the test.

Furthermore, it must be investigated whether and to what extent the analysis tool can be applied to more complicated stringer geometries, such as double-T or C-profiles, and how such geometries can be implemented in the tool. For this, a sufficient number of test results as well as their input parameters of appropriate quality are of course necessary. The same also applies to larger components. In this context it is important that the material properties and boundary conditions are available to an appropriate extent.