Composite materials are more and more used in the field of transport, such as aeronautics, automotive or railway, in order to reduce the mass of structures. But composite structures are relatively brittle, in particular when subjected to impact, crash or holed. Moreover the damage developing in the laminate is particularly complex; it is composed of a complex mixture of three main damage types, i.e. matrix cracking, delamination and fibre failure. Then in order to numerically design this type of structure it is necessary to correctly simulate these different damage types, and in particular their interaction. For example it is well establish that the complex morphology of the damage developing during an impact test is intimately linked to the interaction between matrix cracking and delamination, with some effect of the fibre failure. In order to take into account the interaction between the intra and the interlaminar damage, i.e. between matrix cracks and delamination, the Discrete Ply Model (DPM) has been developed about more than 10 years ago in the ICA, Toulouse, France. The basic idea of the DPM is to account for the discontinuity of the matrix cracks using interface elements, and that this discontinuity can allow to simulate this interaction.
Composites impact analysis techniques used today are largely based on models and methods established over 25 years ago. Early non-interactive intra-ply failure criteria have evolved with many enhancements, so that the most recent models now include coupled matrix and fibre failure for tension and compression, including laws for plasticity and damage evolution. Inter-ply delamination is largely treated as a decoupled event with an energy-based criterion to control crack initiation and crack propagation. These two principle failure mechanisms are usually combined by stacking elements to represent individual plies, or sub-set of plies, that are then tied with the delamination model. This approach can be computationally expensive in an explicit FE scheme, but it is reliable and has proven to be effective.
Despite these advances and many reported validation studies there are still challenges. Only limited work has investigated failure criteria for bi-axial woven composites or considered the effect that fabric shear from draping has on failure behaviour. Some models use fracture toughness to alleviate mesh dependency problems, but damage propagation that is truly mesh independent is still an issue. Modelling techniques for axial crushing are simplistic and depend on properties from appropriate testing. Indeed, concerns could be raised regarding the accuracy of failure data obtained from several standardised test procedures. This presentation will briefly review some of these issues and discuss possibilities for improved composites impact and crash simulation.
The overall objective of the presentation is to show the analytical approach and experimental tests to predict the behaviour of aircraft structures subjected to a collection of survivable high-velocity impact scenarios. The implementation of such an approach will promote enhanced safety through damage-tolerant aircraft design and the development of crashworthy aircraft concepts. The impact scenarios considered include:
The analysis of aircraft structures subjected to the impact scenarios mentioned above is complex due to the high number of variables involved. Such variables include the material characteristics of the impacted media, impactors and surfaces, and the interaction between the aircraft structure and the impactors or surfaces. For these reasons, it is necessary, at the beginning of the project, to obtain material properties, including the effect of high strain rate, for modelling the impacted media (metal & composite) and impactors (rubber/tyre, hail/ice, stone/run-way concrete). This is essential for the development of FE methods and improving predictions of the response of aircraft structures subject to such high-velocity impact scenarios. The Split-Hopkinson bar technique was used to determine experimentally the dynamic properties at high strain rate. Surface models and methodologies have also been developed including fluid/structure interaction formulations (either Lagrange/Eulerian Hydrodynamics (SPH) ) to simulate impacted surfaces (rigid and water). Material models and failure criteria for structural composites have also been taken into account in this type of analysis.
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