The following page outlines the most critical aspects of the design and manufacturing of composite systems.

Composites

The most important aspects of composites are outlined below:

Matrix (Resin)

Fibre

Different fibres can have wildly different strengths in out and in plane compression, tension, and shear. To give a brief example the below table gives the properties of a 60% fibre/epoxy unidirection ply. Note these properties are for an arbitrary matrix and are not exactly what all composite materials will exhibit.

The following graph shows the point of micro buckling where a composite will permanently become damaged from microscopic tears in either the resin or the fibre. The point of "microcracking" will occur in very rigid/brittle resins much earlier than in a more ductile material. If microcracking in resins occurs during normal use the ultimate strength will greatly decrease, it is therefore critical to ensure the maximum yield of your respective composite does not exceed the strain to first micro-crack. The easiest way of doing this is to use a more ductile resin, ie an epoxy based resin.

Weaves

Prepreg & Dry Fabrics

Stack-Up

The above figure details the impact of ply angle on Youngs modulus, shear modulus, and poisson's ratio.

Core Materials - Sandwich Construction

To increase the flexural stiffness of a material a lightweight core material can be added to the centre of a laminate stack up, in the same way ham is added to bread creating a sandwich. This is done by increasing the thickness of the material, as stiffness is proportional to the cube of the thickness. To increase the thickness without greatly increasing the weight a lightweight core material is placed in the centre of the composite.

There are a variety of sandwich materials, from Balsa wood, to PVC foam, to Aluminum or Nomex honeycomb. However for the purposes of this document we will only talk discuss honeycomb cores. I highly encourage the reader to investigate other types of foam as outlined in section 5.4 of the attached document.

During bending the sandwich panel will put the outer skins into compression or tension while the core will take the vast majority of the shear force. Therefore during calculations special attention must be given to the shear stress in the core, otherwise the core will break causing delamination from the laminate skin, drastically decreasing the strength of the laminate and potentially causing failure. Special attention must also be given to the compressive strength of the core to ensure during buckling the skin does not warp or wrinkle into the core causing premature buckle failure.

Honeycomb

Honeycomb cores can be made from various materials including aluminum, kevlar, paper, or card. Honeycombs provide exceptional stiffness and strength to a material while adding the least amount of weight for any other core material. Due to the small bond lines only epoxy resins should be used between a honeycomb and the laminate skin. An additional adhesive layer may be required for kevlar to honeycomb skins due to the properties of kevlar. Due to the large holes in the honeycomb, resin must be applied to the laminate before it is placed on the honeycomb, otherwise the honeycomb will fill with resin.

The Honeycomb midnight sun will exclusively use on MSXIV is called Nomex, a kevlar (aramid) paper. Nomex is anticlastic, meaning if you bend it about one axis it will also deform about the perpendicular planar axis. ie imagine how a pringle chip bends about both x and y. Therefore care must be taken when forming nomex about curvature. There are different kinds of nomex you can use to reduce the anticlastic properties including OX (overexpanded) and Flex-Core. Further information can be found in the attached document, in the pdf library under resources, or on the following site https://www.hexcel.com/Products/Honeycomb/.

Resins

Three main types of resins are used in the composite realm, however epoxy resins have superior strength, bonding, and yielding properties and will therefore be the main topic.

Manufacturing

There are a myriad of manufacturing techniques however the following three are the most relevant for our design processes

Prepreg - Autoclave

• Preimpregnated fabric is placed into a mold, a vacuum is pulled across the mold, and the part is placed in an autoclave that applies heat and pressure to cure the resin under "ideal" conditions. The autoclave will apply up to 6 atmospheres to the laminate.
• Autoclaves are difficult to gain access to and are limited in size
• Parts cured in an autoclave will have extremely close mechanical properties to the design

Prepreg - Non-Autoclave

• Prepreg is placed into a mold, a vacuum is pulled, and the part is placed in a heated chamber. The part is then cured under the pressure of the vacuum and the heat of the chamber.
• You do not require an autoclave, however you will require a lower temperature and viscosity resin since you do not have the same pressure and you may not be able to reach the same temperatures as the autoclave.
• Parts may have slightly worse properties than designed due to air pockets that would be removed in the elevated pressure of an autoclave.

Infusion

• Dry fabrics are placed in the mold with or without a foam core and resin is then pulled through the material through a vacuum.
• Infusion layups do not require prepreg nor elevated temperature and can be done anywhere with sufficient ventilation
• Infusion layups do not provide consistent material properties due to the inconsistency of resin flow and are therefore too dangerous or overweight (due to excess resin) for lightweight structural applications.  
• No honeycomb cores can be used as the space would be filled with resin.
• Resin flow design is extremely difficult to complete succesfully

Adhesives

To join two parts we will most commonly use adhesives. There are a variety of available adhesives however for our purposes we will only look at structural epoxies. The key points to remember when designing for the use of adhesives are as follows.

Design Considerations

• Adhesives are strong in compression and shear but relatively week in tension.
• Peel will cause extreme premature failure in adhesives and must be avoided in all designs. An example of incorrect and correct designs are found below:
• 

Epoxy Characteristics

• Either 2 component room temperature curing or one component heat activated (normally 130-180C for 20-60 min).
• 25-40Mpa shear resistance
• Excellent adhesion to most metals, composites, many plastics, glass, and wood
• High chemical resistance
• Long term durability
• High rigidity, therefore poor resistance to peel and cleavage

Manufacturing

• Surface preparation of the adhesive-material bond is absolutely critical to its performance. An unprepared or incorrectly prepared surface will drastically reduce the strength of the bond.
  • To ensure your design will perform as expected research what surface preparation technique to use for your specific adhesive - material bond. ie dp-420 - carbon fibre will require different surface prep than super glue - plastic
• A uniform bond line is important for the desired performance and will reduce the amount of unnecessary weight.  Painters tape should be placed where adhesive is not desired and then a Popsicle stick can be used to create a uniform bond before gel and after application.
  • Photo to be inserted

Testing

Basic Equations

For further details read the following Gurit guide: