Core Issues

Picking up where the last A Cat post left off, we were contemplating the relative merits of foam core and Nomex style aramid paper honeycomb.

Honeycomb is a very efficient structural solution because it concentrates material in effective load paths between the skins. 

Each cell is braced at the interface with other cells, and there is a lot of empty space within the thickness of the material.

By contrast, foam contains some voids in the form of random bubbles but needs to be much denser to achieve a given global rigidity.

Foam does however have some secondary advantages: It has toughness when loaded in directions such that the skins cannot work effectively (for example blunt impacts), and the ability to keep working when deformed (such as in the ubiquitous bruises caused by knees and trapeze hooks). 

It also does not allow water to travel through it as each empty bubble is closed and separate from the others.

Honeycomb core uses inherently strong shapes
to keep the skins from moving relative to each-other.
Image from http://www.rocketmaterials.org/ 

As a simplified example, a honeycomb core with half the density of foam could be twice the thickness for the same weight. 

With purely global structural considerations in mind, honeycomb gives the option to build a thicker laminate for a given weight. 

Even if the mechanical properties of the honeycomb were slightly inferior to the foam at such a reduced density, the laminate would still be much stiffer because thickness improves stiffness in a non linear relationship – a small increase in thickness yields a large improvement in stiffness.

Resin fillets at the skin/core interface shown in blue.
Getting these fillets right without filling the cells or
starving some areas of resin is critical to the manufacturing process

With both core types, there is a resin-rich layer between each skin and the core. 

Core bonding in the case of honeycomb relies on little fillets of resin forming along the edges of each cell where it touches a skin. 

Foam cores have greater contact area with the skins. Bubbles that are open to the cut outer face of the core often trap resin because there is no path out of the bubble. This provides additional ‘keying’ and bonding area but adds weight to the finished laminate. In a well bonded foam sandwich panel the core usually fails before the skin-to-core bond.

An exaggerated representation of foam core/skin bond
showing surface cells filled with resin (again in blue)

As is often the case, the trade-offs have implications beyond the inherent structural merits discussed so far. Each solution has different requirements with respect to construction method. The choice must take into account the effect each option has on the build process and related constraints such as complexity and cost…