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Cellular Structures for Impact Performance

Final Report Summary - CELPACT (Cellular Structures for Impact Performance)

The main objective of the CELPACT project was the development and design of cellular materials and twin skinned sandwich structures made from hybrid composites and metals. CELPACT is progressing fabrication technology for cellular metals based on selective laser melting and developing new fabrication concepts for hybrid composite sandwich structures with folded cellular composite cores. Computational methods were developed based on micromechanics cell models with multiscale modelling techniques for understanding progressive damage and collapse mechanisms and used for structural analysis. Impact performance is critical for sandwich aircraft structures and the simulation tools are being used to design efficient impact resistant aircraft structures. Structural integrity of these advanced cellular structures were assessed by testing generic cellular beam and shell structures under high velocity impact conditions relevant to aircraft structures.

Current aircraft sandwich structures are made from CFRP skins with Nomex paper honeycomb or polymer foam cores for low weight secondary structures such as radomes, wing flaps, internal bulkheads, luggage containers, etc. More critical structures with improved energy absorption may have aluminium skins with aluminium honeycomb cores. In all cases the main function of the core is to be low weight, low cost, and stiff enough in shear and compression to maintain the separation distance between the load bearing skins. The developments being considered in CELPACT are new low weight structural cellular cores with enhanced properties made of composites and metals.

The CELPACT proposal focused on basic research, for implementation by industry in an 8 to 12 year time scale. The research included next generation manufacturing techniques for both composite hybrid and metal cellular materials and structures. There was a wide range of candidate materials and geometries considered: cellular hybrid composites (CHC) with folded composite core structures, cellular metal (CM) with closed cell cores and selected laser melted lattice cores.

Main objectives:
- The development of new classes of materials for aeronautical structures based on: cellular metallic core structures and cellular hybrid composite core structures.
- The development of new manufacturing processes based on:
a. laser forming for manufacturing complex three dimensional metallic lattice structures;
b. brazing and bonding technology for making periodic closed and open cell metallic structures;
c. folded composite cell structures from aramid paper and carbon prepreg;
d. fabrication of twin-walled sandwich structures with innovative cores.
- The development of numerical tools to:
a. predict the mechanical behaviour of the cellular materials;
b. optimise the cellular and sandwich concepts with regard to applications;
c. develop design tools for predicting impact damage and failure in sandwich structures.
- Technology validation studies to demonstrate viability of these new materials for future aircraft structures:
a. validated design procedures for novel cellular and sandwich structures;
b. impact design tools validated by low and high velocity impact test programmes on sandwich panels;
c. road map for dissemination to aircraft industry design teams.

The project was structured into five work packages (WPs) as follows:

WP 1: Manufacture of cellular structures.
The main aim of this work package was to define the cellular materials and to manufacture CM and CHC materials. In the first phase of the project, a three-ply carbon composite laminate with unidirectional plies and a stacking sequence of [0 degrees / 90 degrees / 0 degrees] was used for the cell wall material. Based on the knowledge gained from these first specimens, some enhancements were implemented in the second phase foldcore structures. First of all, woven fabric prepreg material was used for the cell walls, as it is easier to handle during manufacturing compared to the unidirectional material. The unit cell geometry was optimised in a numerical study in order to obtain increased compressive properties while maintaining the same global density of around 110 kg/m^3. Furthermore, a dual-core configuration was investigated, using two separate foldcore layers - one carbon foldcore and one aramid foldcore - and an aramid plate in-between. This concept may allow for a two-phase energy absorption behaviour. All sandwich structures consisted of quasi-isotropic carbon / epoxy skins, which were bonded onto the foldcores with an epoxy-based pasty adhesive. The increased performance of these second phase structures could be verified during the compression and impact test series, demonstrating that the foldcore structures offer the designer certain measures to tailor the core's properties for specific requirements (like the compressive behaviour). These measures are primarily the choice of cell wall material, cell geometry and single core / multi-core configuration. However, it has to be stated that the discontinuous manufacturing process, which was used for the production of these carbon foldcores, is more or less limited to the prototype level, because the final foldcore plate size is of limited dimensions, which is insufficient when it comes to the manufacturing of large quantities on an industrial level.

WP 2: Modeling and simulation.
The WP objectives were:
- to develop models for predicting the mechanical behaviour of CM and CHC cellular cores and their sandwich structures in terms of core cell geometry, core materials and sandwich skins;
- to develop numerical FE techniques for simulating damage and failure in cellular core sandwich structures under high and low velocity impact loads from hard and soft projectiles.

The micro strut finite element model developed has been shown to be robust and reliable. The skin model use has been shown to give realistic results, which gives us confidence in closely modelling experimental panel tests. In term of virtual material testing, CELPACT partners achieved the same results as in laminates i.e. to be able to predict the behaviour of a core with respect to the good degradation mechanisms. The main additional difficulties was the modelling of defects as it is difficult to distinguish geometrical from material defects as well as their statistical distribution. Based on the microscale modelling, a first optimisation is possible by comparison between potential solutions. In term of virtual structural testing, the use of a homogenised scale is not straightforward. In fact, the coupling between the homogenised and the microscale descriptions is still an open problem. Concerning the associated numerical solvers, implicit solvers still lead to difficult treatment in case of complex instable behaviours. In the case of non-linear homogenisation or multiscale computational strategies, the major issue is to deal with localisation. This point was not properly addressed in CELPACT.

WP 3: Materials and structures tests.
The objectives of WP 3 were to provide the experimental data on the selected materials' behaviour in order to enable the development of numerical models capable of accurate simulation of experimentally observed and measured behaviour. This is aimed to be achieved in two phases:
a) by performing small scale tests on constituent materials and micro- and meso-scales to provide intrinsic information on the materials' behaviour, and
b) by performing medium and large scale experiments to validate the developed numerical modelling methodology.

The main achievements of the WP3 were:
- completed evaluation of composite core material at LMT, DLR, CRC-G, USTU and RWTH;
- completed evaluation of cellular metals at EADS IW F, ULIV, UOXF, DLR and UPAT.
% WP 4: Technology validation.
The objectives of work package WP4 'technology validation' were:
- to bring together the simulation results from WP2 with the experimental data from WP3 in a validation study and
- to assess the manufacturing methods, simulation tools and core performance of the cellular sandwich structures with respect to their potential application for future aircraft structures.

A-D reviewed all test and simulation results in CELPACT with respect to proven performance regarding impact resistance and specific energy dissipation and compared it with conventional honeycomb sandwich applications. According to a given zoning of an aircraft for impact statistical threats and by distinguishing between low and high velocity and low and high energy impact scenarios, a list of structural components was compiled for potential application of new cellular structures. CM core structures have been tested according a multiscale approach to assess the quasi-static and dynamic properties of the materials, the understanding of the materials behaviour, the robustness of the testing and modelling.

WP 5: Road map to applications.
The aim of this task is to focus on the uptake of the CELPACT CM and CHC technologies by the aircraft industry; in particular, the new fabrication routes for composites and metals, the novel twinwalled structural concepts and improved design analysis tools. The main activity of the 'road map' working group was to review the progress and achievements made with CM and CHC technologies with respect to readiness level requirements defined by the three industrial partners: A-D, EADS IW F and CRC-G. The critical assessment of the technology readiness level (TRL) reached was performed for the various cellular materials separately also taking into account the level of validation achieved for the corresponding analysis techniques.

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