Composite Lattice/Grid Structures Design and Manufacturing method
An Italian start-up designed, realised and tested a composite grid structure and proposes it to replace monocoque, skin-stringer, and honeycomb sandwich structures. The benefits associated to this advanced lattice structures are the environmental robustness, the low-cost manufacturing and the structural efficiency. The proposed technology can be applied as the structural element of, e.g. wing structures, fuselage, rudder panels, antenna booms, etc.
Grid stiffened structures are shells supported by a grid lattice of stiffeners used as a possible replacement to monocoque, skin-stringer, and honeycomb sandwich structures. The Italian provider has developed this technology within an ESA contract which aim was the development of a small scale main primary cylinder grid/lattice structure for spacecraft with thermosetting resin matrix by using Automated Tape Laying (ATL) / Automated Fiber Placement (AFP) technology. The project leads to the manufacturing of a prototype of the VEGA launcher central cylinder’s realised with an anisogrid structure instead of the usual sandwich structure. The robustness, low-cost manufacturing and structural efficiency make the composite grid structures herein proposed the suitable solution for various applications, also beyond the space sector. Cost savings and performance improvements can be guaranteed by composite lattice structures in the following applications:
• Wing structures
• Fuselage and Rudder panels
• Payload and cargo Floor
• Tail Cone & Tail Boom
• Rocket’s interstages and fairings
• Payload adapters
• Antenna booms
The possibility to vary the number and change the positioning of the ribs, rings and vertical hoops allows the creation of five different regular and symmetric grid pattern:
• the orthogrid made by the intersection of horizontal rings and vertical hoops (Fig.1-1);
• the diamond grid made by the intersection of helicoidal ribs (Fig.1-2);
• the isogrid made by the intersection of helicoidal ribs and horizontal rings (Fig.1-3);
• the triangle & rectangle grid made by the intersection of helicoidal ribs, horizontal rings and vertical hoops (Fig.1-4);
• the kagome grid, similar to the isogrid, having the horizontal rings does not intersect the helicoidal ribs in their point of intersection (node), in this way the pattern is a hexagon with 6 triangles build on his sides (Fig.1-5).
The company has a long experience in the design and development of aircraft structural items mainly realised in light alloys and composite materials with carbon fibre, fibreglass and Kevlar. The Engineering Department analyses, plans and develops all design stages: Design (CATIA V5), Stress Analysis (Nastran/Patran), Configuration Management (Team Centre, ENOVIA) and Material & Process selection. The Italian provider has also a long experience in the industrialisation of the aeronautical products. Furthermore, the criteria of Concurrent Engineering and the integration of Designing team with that of Industrialisation can reduce the time of market for each item.
Innovations & Advantages
The benefits of Composite grid structures are numerous:
• ENVIRONMENTAL ROBUSTNESS
o Significantly higher damage tolerance than honeycomb sandwich
o Tendency to contain delamination to within one cell (open cell feature)
o Do not absorb and retain water over their lifetime, unlike honeycomb sandwich structures.
• LOW-COST MANUFACTURING
o Automated and single cure process (not hand operations are involved)
• STRUCTURAL EFFICIENCY
o Stiffer in plane and less stiff out-of-plane: a grid structure design tends to be preferable when deflection or fundamental frequency requirements govern the design space. Sandwich structure designs are preferable when global buckling is the driving factor
o From 20% up to 61% lighter, 300% stronger, and 1000% stiffer than the aluminium structure it replaces
o Thermal stability
Furthermore, the company proposes to merge the design concepts of Multi-Functional Structure (MFS) and isogrid structures to develop the Mutifunctional Composite 3D Stitched Isogrid Structure. A Multi-Functional Structure (MFS) combines the functional capabilities of one or more subsystems with that of the load bearing structure, thereby reducing the mass and volume of the total system. This is achieved by integrating functional components of e.g. battery, thermal control systems, electronic sub-systems, power connections, optical fibres and electrical harnesses in the structure, thereby improving the overall efficiency of the total system. By taking advantage of isogrid architectures, electronics embedded and sub-systems can be integrated into MF panels (tile concept), then being mounted into the grid’s cells. The isogrid is made up of composite material 3D stitched then undergone resin infusion process. The new and innovative structure is basically a frame that allows the integration of subpanels embedding printed circuit boards (PCBs), Multi-Chip Modules (MCMs) and electronic harnesses as a whole, just by stitching them together with structural fibres (aramidics, carbon). Natural fibres (hemp), silica, silicon, copper and silver are also stitchable material in a stand-alone or blended way. The proposed MFS technology herein proposed is suitable for satellites or spacecraft but can be also applied in any system where the mass and volume constraints are stringent (e.g. automotive, crafts etc.)
Current and Potential Domains of Application
The technology can be currently applied to realize lighter, stronger and stiffer structures of, e.g., Payload and cargo Floor, Rocket’s interstages, Spacecraft central cylinders etc. Nevertheless thanks to its features, it can be applied in any system in which the mass and volume constraints are stringent e.g. automotive sector, manned/unmanned aviation, helicopters, crafts, trains etc.