Foaming adhesive Core splicing Use in a repair Table 2 2t 4t t Thickness Flexural Strength Weight 1.0 1.0 1.0 7.0 3.5 1.03 37.0 9.2 1.06 Solid Material Core  Thickness t Core  Thickness 3t Figure 7-18.  Strength and stiffness of honeycomb sandwich material  compared to a solid laminate. Figure 7-16.  The use of foaming adhesives.  Figure 7-17.  Honeycomb sandwich construction. Figure 7-19.  Honeycomb core materials. Prepreg skin Honeycomb (or foam) Adhesive film (optional) Prepreg skin 7-11 Sandwich construction has high bending stiffness at  minimal weight in comparison to aluminum and composite  laminate construction. Most honeycombs are anisotropic;  that is, properties are directional.  Figure 7-18  illustrates  the advantages of using a honeycomb construction.  Increasing the core thickness greatly increases the stiffness  of the honeycomb construction, while the weight increase  is minimal. Due to the high stiffness of a honeycomb  construction, it is not necessary to use external stiffeners,  such as stringers and frames.  [Figure 7-18] Each honeycomb material provides certain properties and  has specific benefits.  [Figure 7-19]  The most common core  material used for aircraft honeycomb structures is aramid  paper (Nomex$^{®}$ or Korex$^{®}$). Fiberglass is used for higher  strength applications.  laminates may be precured and subsequently bonded to core,  co-cured to core in one operation, or a combination of the  two methods. Examples of honeycomb structure are: wing  spoilers, fairings, ailerons, flaps, nacelles, floor boards, and  rudders.  [Figure 7-17] Most honeycomb structures used in aircraft construction have  aluminum, fiberglass, Kevlar$^{®}$, or carbon fiber face sheets.  Carbon fiber face sheets cannot be used with aluminum  honeycomb core material, because it causes the aluminum to  corrode. Titanium and steel are used for specialty applications  in high-temperature constructions. The face sheets of many  components, such as spoilers and flight controls, are very  thin—sometimes only 3 or 4 plies. Field reports have  indicated that these face sheets do not have a good impact  resistance. Facing Materials Properties Core Materials Honeycomb •  Kraft paper—relatively low strength, good insulating  properties, is available in large quantities, and has a  low cost.