European aircraft industry demands a reduction in development and operating costs, by 20% and 50% in the short and long term respectively. COCOMAT aimed to contribute to this aim by reducing the structural weight by expanding the limits of safe design; it will exploit considerable reserves in primary fibre composite fuselage structures by an accurate and reliable simulation of collapse. Collapse is specified by that point of the load-displacement curve where a sharp decrease occurs, thus limiting the load-carrying capacity.
The main objective of COCOMAT is to accomplish a large step from the current to a future design scenario of stringer-stiffened composite panels. The current industrial design scenario is illustrated in a typical load-shortening curve, which is divided into three different regions. Region I covers loads allowed under operating flight conditions and is bound by the limit load; region II is the safety region and extends up to the ultimate load; region III comprises the forbidden area, which reaches up to collapse. There is still a large unused structural reserve capacity between the current ultimate load and collapse. In a future design scenario like the one this project aspires to realise, the ultimate load limit is shifted as close as possible towards collapse. Another main difference to the current design scenario is that the onset of degradation is moved from the forbidden region III to the safety region II due to a reliable simulation of collapse.
The project results comprised a substantially extended database on material properties and on the collapse of undamaged and pre-damaged statically and cyclically loaded structures, degradation models, improved slow and fast computation tools for statically loaded structures, as well as design guidelines. Although this project was orientated towards an application in fuselage structures, the results will be transferable to other airframe structures as well.
To reach this main objective, the project provided improved slow and fast simulation tools, experimental databases and design guidelines for stiffened panels, which took skin stringer separation and material degradation into account. The experimental database was indispensable for verification of the analytically developed degradation models, which could be implemented into the new tools, and for the validation of these tools as well. Reliable fast tools would allow an economic design process, whereas very accurate but unavoidably slow tools were required for the final certification.
The partners cooperated in the following six technical Work Packages:
- Benchmarking on collapse analysis of undamaged and damaged panels with existing tools: knowledge of the partners is compared and the deficiencies of existing software are identified.
- Material characterisation, degradation investigation and design of panels for static and cyclic tests: material properties are characterised, degradation models are developed and test panels are designed to the requirements of the research in order to overcome deficiencies.
- Development of improved simulation procedures for collapse: slow certification and fast design tools are developed and validated by the tests.
- Manufacture, inspection and testing by static and cyclic loading of undamaged panels: the experimental database is extended by testing undamaged panels.
- Manufacture, inspection and testing by static and cyclic loading of pre-damaged panels: the experimental database is extended by testing pre-damaged panels.
- Design guidelines and industrial validation: all project results are assembled and final design guidelines derived. The tools are validated by the industry.
Industrial partners brought in their experience with design and manufacture of real shells; research partners contributed knowledge on testing and on development of simulation tools. Design guidelines were defined in common, and the developed tools were validated by the industrial partners.
COCOMAT was fully based upon the results of the finished Fifth Framework Programme (FP5) project 'Improved postbuckling simulation for design of fibre composite stiffened fuselage structures' (POSICOSS) which developed improved, fast and reliable procedures for buckling and post-buckling analysis of fibre composite stiffened panels of future fuselage structures, created experimental databases and derived design guidelines.
The COCOMAT project extended the POSICOSS results and goes beyond by accounting for degradation. That requires knowing about degradation due to static as well as low cycle loading in the post-buckling range. It is wellknown that thin-walled structures made of carbon fibre reinforced plastics are able to tolerate repeated buckling without any change in their buckling behaviour. However, it has to be found out, how deep into the post-buckling regime one can go without severely damaging the structure, and how this can be predicted by fast and precise simulation procedures. This issue was dealt with by COCOMAT.
The project results comprise an experimental data base, improved slow certification tools, fast design tools and design guidelines.
The conference and the COCOMAT workshop (3-5 September 2008) were primarily organised as the final events of the project COCOMAT, demonstrating its results to that date.
Behaviour of composite laminated shell structures was presented in terms of:
- experimental methods and results;
- degradation models;
- failure criteria;
- advanced finite element tools for certification;
- fast design tools;
- influence of imperfections;
- structural optimisation;and
- design guidelines
The scientific community and industry in general were invited to attend. Both events aimed to promote discussion and an exchange of information between scientists and engineers in the field of buckling, post-buckling and collapse behaviour of composite structures. The conference presented scientific achievements from the COCOMAT project. In addition, papers with recent results on the same topic but from outside the project were also presented. Within the workshop new fast design tools and improved commercial tools, developed by COCOMAT, were demonstrated.