Notes on Face Sheets and Adhesives

• They carry the major applied loads

• The stiffness, stability, configuration, and to a large extent, the strength of the part are determined by properties of the face sheets

• Face sheets sometimes have additional functions like providing a profile of aerodynamic smoothness, or having other surface properties, or sealing core from environmental effects such as moisture

• Any thin material can serve as a sandwich face sheet but usually they are chosen to impart certain structural properties

Purpose of face sheets

There are 3 basic types of face sheets, each is generally the method in which the sheet is incorporated into the sandwich

• Adhesively-bonded pre-fabricated face sheets

  • Examples would be all sandwich structures using metallic face sheets as well as pre-cured composite face sheets

  • Typically uses a film adhesive or paste adhesive (will talk about later in this document)

  • As of now, this is the method we are planning to use for this year, just because we may be using a secondary bonding epoxy (more info should be written in other docs)

• Co-cured or co-bonded face sheet w/ adhesive

  • Some composite face sheet sandwich configurations need adhesive between face sheet and core in addition to the resin in the face sheet

  • Sandwich co-cure is when both face sheets are assembled to core in an uncured state and the adhesive and face sheets are cured in a single cycle

    • We did this for testing Nomex core during Daybreak manufacturing year, it worked pretty well but we have no empirical data to back that claim up,

      • Also for this our face sheet size was larger than our core so we had some crushing on the corners, and in general, composite face sheet plus Nomex honeycomb under vacuum does cause the core to crush a bit, at least at the vacuum pressure we had

      • We also had issues where our core was getting saturated which defeats the point of core

    • We could try this for composite frame but I think its not worth it since we want a good surface finish on both sides of our panel when we finish to make it easier to bond stuff to it

  • Sandwich co-bond is when one face sheet is pre-cured and the other is cured in the same cure cycle as the the adhesive that bonds core to face sheet

    • This is good for when you want a nice surface finish on both sides, and when you want to have a defined thickness (how this helps define thickness I’m not too sure)

    • We used this method for most, if not all of Daybreak’s bulkheads, it works pretty well, however we did have some issue in terms of face sheet finish as we were not using the right peel ply

  • General notes about co-bond and co-cure

    • Curing face sheet directly to honeycomb may result in waviness and/or dimpling of face sheet plies adjacent to core

      • This can lead to weaker structural properties and should be tested before used for structural purposes

• Self adhesive face sheets

  • Prepreg stuff, doesn’t apply to us as we won’t be able to do this on any large scale when making frame as combustion’s oven isn’t large enough

Adhesives

• Primary purpose is to structurally attach face sheets to core

  • Allows materials to work together in a system

  • Reacts in shear and peeling between face sheets and core

• Are especially useful for bonding thin materials which makes it invaluable for sandwich panel construction

• Adhesives for sandwich construction are generally synthetic and polymer based

• Adhesives that we could use

  • Foaming adhesives

    • This is generally used in sandwich construction to splice core sections and fill gaps

  • Paste adhesives

    • Inexpensive, but it is hard to maintain a specific bond line thickness meaning strength isn’t exactly constant

    • Can be used to bond various parts of the assembly (such as inserts) or to stiffen/strengthen core in a certain area

  • Liquid resins

    • Can bond core materials to face sheets during cure process assuming the core is relatively solid such as balsa or foam

    • Liquid resins can also be used to bond face sheets to honeycomb only after the core has been sufficiently sealed such that cells are not filled with resins

      • This is what we are going to have to do, I think our best option may just be putting epoxy on the sheet and then placing honeycomb on top, bonding only one side at a time to ensure epoxy does not flow into the cells

• Types of resin we should look into

  • Epoxy resin

    • This is what we are