Fiberwork materials - sandwich construction

Fiberwork materials
*sandwich construction* sandwich principle
	A light and pressure resistant core material holds the distance between two thin layers. Because especially the outer layers take up the tension and pressure forces in the load case "bend", you can use lighter material for the core. It´s responsible for the transfer of shear forces only. With the sandwich construction especially in combination with high strength fibers you can realize minimal weight with a big wall thickness.
On which parts is this technique useful ?
	On parts with a big area, so e.g. fairing parts, seats, etc. The flatter the form and the less edges and narrow radii, the easier is the production as sandwich.
selection of the core material
	The with distance best shear and pressure strengths referring on weight has polyamid-honeycomb. It´s available e.g. at R&G in the thicknesses 1,5mm, 2mm, 3mm und 5mm. I´ve produced almost all parts with 2mm-honeycomb. Merely in exceptions (e.g. the seat) I´ve used 3mm or 5mm. Disadvantage of the honeycomb is the extreme high price. Alternatively with a little worse qualities you can use also balsa wood, polystyrene, thick cardboard or similar materials.
working steps

production of sandwich structures with fiberwork materials

measurements to optimize the relation of thicknesses between outer layers and honeycomb core
target	measurement

increase of dent stiffness	higher core thickness
increase of strength	higher laminate thicknesses
increase of shock strength	lower core thickness and higher laminate thicknesses

in hollow bodies like the fairing, the frame or similar parts you can build under circumstances the inner laminate weaker than the outer, especially when the area inside isn´t exposed to knock loads.

To avoid the draw through of the honeycomb structure you should take a top layer with at least 0,1mm thickness, e.g. 100g glass. But if the weight is more important than a perfect smooth surface you can also go down to 49g glass ( so at my tail fairing )

problems with sandwich parts
	As soon as there´s no adhesion any more between top and core layers it would come to extreme losses of stiffness and strength. Those detachments can arise e.g. by knock loads.
tips for vakuuming
	In spite of the high costs the purchase of a latex tarpaulin pays, because with PE films and especially complex forms the processing often gets a little arduous.
design of force introductions
	Forces should get introduced as shear load in plane direction into the sandwich with as big area as possible. In the area of the force introduction you should fill the honeycomb core with a thick mix of resin. In high load introduction areas you can use also metalic inlets. In screw connections you should use as big as possible washers.
general construction and dimensioning form building