GARTEUR Structures & Materials Action Group 32 – An European Research Project On Damage Growth In Aerospace Composite Structures
The high specific strength and stiffness of composite materials make them suitable for use in aerospace structures. However, the high sensitivity of these materials to the presence of damage, arising after impact with foreign objects or caused by manufacturing defects and stress concentrators, makes designing with composites a very challenging task. The damage mechanisms in composites are very complex and can involve one or more constituents at a time. Delaminations, fibre breakage and matrix cracking can strongly reduce the load carrying capability of composite structures leading, in general, to a premature failure. Moreover, depending on the composite internal layout and on the adopted manufacturing technique, the damage mechanisms may interact with each other, making it difficult to predict the residual properties of composite components. In recent years, the inability in predicting the damage onset and its evolution in composite structures, has led to over-conservative designs, not fully realizing the composites promised economic benefits. Hence, in order to make the composites affordable in aerospace design, many research projects have been started in the last decade aimed to investigate the composites’ damage mechanisms and to promote damage tolerant design approaches. A number of GARTEUR (Group for Aeronautical Research and Technology in EURope) Action Groups together with several EU funded and MoDs funded research projects have been started on damage management in composite structures. All these research activities have contributed to increasing the knowledge of the composites structural response and failure mechanisms. However, in the last 20 years big steps forward have been made in the field of computational technologies and new “composites oriented” non-destructive inspection tools have been introduced. These innovative numerical/experimental features can be considered as relevant driving factors for the development of newer effective numerical approaches oriented to the prediction of damage on-set and growth in composites. Furthermore, fifteen years of research and growing applications in industries have brought to life new needs, to be addressed by Research and Development, for example related to the presence of new composites typologies (textile composites) and new manufacturing techniques (RFI, RTM, fibres placement). Finally the emerging tendency to adopt composite materials for primary structures opens new scenarios involving new safety issues which imply considering damage tolerance design approaches from the earlier phases of the design process (including optimisation) rather than limiting the use of damage on-set and growth numerical techniques to complex/expensive non-linear verification analyses. Taking into account the above considerations, the GARTEUR Action Group AG 32 carried out between year 2007 and year 2012 a joint research work focused on “DAMAGE GROWTH IN COMPOSITES”. The action Group was composed by C.I.R.A. (Centro Italiano Ricerche Aerospaziali - Italy), QinetiQ (United Kingdom), DLR (The German Aerospace Center - Germany), INTA (Instituto Nacional de Tecnica Aerospacial – Spain), SWEREA-SICOMP- Sweden, EADS-M – Germany, ALENIA AERMACCHI – Italy, Imperial College of London - United Kingdom, CNR (Centro Nazionale Ricerche – Italy), Lulea University of Technology – Sweden, SAAB – Sweden, ONERA (The French Aerospace Lab- France), University of Nantes – France and University of Naples “Federico II” – Italy. The main objective of the Group was: “to develop integrated numerical/experimental methodologies capable to take into account the presence of damage and its evolution in composite structures from the early phases of the design (conceptual design) up to the detailed FEM analysis and verification phases” addressing the following issues: Integration between numerical and experimental methodologies (oriented to the validation and to the interpretation of the most significant physical phenomena governing the damage mechanisms in composites) Prediction of generalised composite damage (delamination, fibre breakage, matrix cracking, etc.), onset and evolution Development of numerical methodologies oriented to the different design phases (fast methodologies for conceptual/preliminary design and detailed methodologies for analysis and verification) The new methodologies developed in the frame of AG-32, able to support the composites design process and able to drive it towards a damage tolerant philosophy, can surely imply relevant changes in the way composite structures are adopted in aerospace industry leading to the reduction of the overall composite design time and costs by improving the efficiency of simulation tools and by reducing the number of experimental validation tests. The project structure and the main results from the AG-32, in terms of methodologies developments, experimental methods developments and validation experimental activities are summarized in this paper.