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CELPACT
Cellular Structures for Impact Performance

Tags: Air

Background

The use of composite materials and new metal alloys in aircraft structural components has grown steadily with each generation of aircraft. The development of a complete pressurised fuselage in composites or hybrid metal/composites represents a big challenge, particularly because of increased vulnerability of these materials to impact threats. The big jump in technology needed for the realisation of a new fuselage for the next-generation Airbus requires new production technologies and materials for integrated structural concepts. Current concepts for fuselage structures introduce composites in the fuselage barrel with conventional frame/stringer concept monolithic skins. In the longer term the aircraft industry sees a potential for twin-walled sandwich structures due to their much higher shell-bending stiffness than single skin designs and far higher strength/weight ratios. They would allow novel highly efficient fuselage concepts without stringers and with much larger frame spacings, and would also be appropriate for next-generation aircraft concepts such as the blended wing. Current aircraft sandwich structures are particularly vulnerable to impact damage, due to their thin composite skins and low-strength honeycomb or polymer foam cores. Thus for efficient lightweight future aircraft structures there is a requirement now to develop new sandwich materials concepts with improved impact resistance.

Objectives

The scientific and technological objectives of CELPACT are the development of new sandwich material concepts for primary aircraft structures with higher performance low-weight cores designed to enhance impact resistance. The new core materials to be investigated include folded composite elements, low-weight metal honeycombs and lattice structures. Sandwich structures with composite skins are expected to be used in fuselage barrels; however, in highly critical impact-loaded regions, such as wing leading edges and front cockpit panels, there is interest in metal-skinned sandwich structures.

To meet these technological objectives, CELPACT will employ advanced manufacturing techniques for novel cellular material designs at the micro scale, in order to improve structural performance for strength/weight, fatigue resistance, damage tolerance and crashworthiness. Physical phenomena associated with impact damage and progressive collapse of such structures are complex, and theoretical models and simulation tools for design and analysis will be developed. CELPACT will develop improved design techniques for sandwich structures based on advanced computational tools. Prototype structures will be fabricated and tested under high-velocity impact conditions in order to validate design concepts, and material parameters will be experimentally derived as input to the simulation software.

Cellular metal lattice structure by selective laser melting
Cellular metal lattice structure by selective laser melting
University of Liverpool

Description of work

CELPACT will develop new manufacturing techniques for both composite hybrid and metal cellular materials and structures. Candidate materials and geometries defined are cellular hybrid composites (CHC) with folded composite core structures, and cellular metal (CM) with closed cell and lattice cores. Open lattice geometry CM cores will be fabricated by selective laser melting (SLM) using aluminium, stainless steel and titanium for the core materials. Folded composite core structures will be fabricated in a new continuous folding process using aramid fibre paper pre-impregnated with epoxy resin. Initial folding patterns to be investigated will be V-form zigzag geometry, which gives an open cellular structure.

The key to the design of improved cellular materials is modelling and simulation. Methods will be developed based on cell micromodels for optimising cell geometries, together with homogenised models and multiscale code developments for impact modelling in larger sandwich structures.

The structural case selected for study is foreign object impact damage from impactors such as bird strikes on CM panels and tyre rubber or runway debris impacts on CHC twin-skinned structures. Structural tests will focus on gas gun and drop tower impact tests on generic structures. Validation studies will assess the technology developments, verify the simulation and design methods by detailed comparison of impact damage predictions with test results, and finally make recommendations for aircraft designers. CELPACT will conclude with final assessment reports and design guides for aircraft industry users, the ‘Road map to application’ reports and a final workshop.

Results

CELPACT will provide new technologies for airframes which will facilitate novel aircraft configurations. They will be based on advanced twin-walled structural concepts, using advanced composite and metallic cellular structures as core materials and leading to structural weight reduction in airframes.

New advanced manufacturing processes will be developed and refined for cellular metal structures using selected laser melting technology, and for continuous fabrication of folded hybrid composite core structures. Concepts will be demonstrated for lightweight structures with improved impact resistance. These material developments have strong potential outside the aerospace industry such as filter technology and medical implants for CM structures, and boat hull and vehicle structures for CHC materials.

New design methods for advanced sandwich structures will be validated, based on new simulation tools which combine multiscale modelling with microscale cell models. They may be used to optimise structural concepts for improved behaviour under complex load systems such as impact and crash loads.

Folded core composite structural element
Folded core composite structural element
University of Stuttgart

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