Construction

Cored Construction

Sandwich Composites

There are many advantages to using sandwich cored composites, but of greatest importance to us is how it contributes to the durability and longevity of the boats we make. The below diagrams will briefly explain how the loads are distributed in this method of construction. Explanation and diagrams courtesy of Vaclav Stejskal, Mechanical Engineering and Material Science, Harvard University.

The Skin

This drawing illustrates a profile cross section through a sandwich core panel. The bending causes the sandwich to stretch above the 'Neutral Axis' and to compress below the axis. The neutral axis or neutral 'plane' in real material experiences zero stress and strain.
The original length of the relaxed panel is "L".

As the panel bends, both the core and the skin elongate and shrink linearly (for simplicity) from the neutral axis. The thick black line represents the new section of the panel after bending. (very exaggerated)

Since the skins are firmly glued to the core, both the core and skin will stretch the same amount where they bond together.

Now, the important thing to keep in mind is that even though the materials stretch equally at the skin/core boundary, they both have completely different physical properties and therefore will react differently to this elongation.

In engineering terms the ratio of the elongation to the original length is expressed as 'strain'.

Knowing the strain, it is now possible to find the stresses in both the core and the skins.

It is important to realize that the 'stress' in a three dimensional panel applies to the entire face of the section. The drawings here represent only 2D 'side view'. This is OK since stress can also be defined as 'force per area', which means that we can think of the arrows in the drawing below as force applied to individual strands or a slice of the sandwich.

The equation below shows that Strain multiplied by the material's Modulus of Elasticity equals Stress.

Now, you will notice that the force (arrows) acting on the skin are far larger than on the core. This is because the fiberglass skin has a large Modulus (E) but the core such as foam has Modulus much smaller. So, equal strain at the boundary multiplied by larger Modulus will produce larger stress in the skin. The discontinuity of the stress at the skin/core boundary is a clear indication that the fiberglass is absorbing far more tension and compression than the core. The same applies to the simple 'I' beam.

So far, it has been only shown that bonding a strong, tensile material to the core will relieve it of a lot of stress yet make the entire sandwich core stronger.

The real advantage of the sandwich core can be only made clear by comparing it to a single skin fiberglass, subject to the same bending force.

In order to do this comparison, one more principle must be clarified:

The Applied bending force (Moment) produces internal reaction forces in the panel (the blue and red arrows) that counteract such bending. Since there is no acceleration in statics problem like this, it just happens that the sum of the internal forces(moments) must be equal to the applied bending force(moments) . In engineering terms, the forces are in 'equilibrium' or balance. This sum of internal forces is also graphically equivalent to the area covered by the 'force arrows'. Moments are defined as force x distance (arm) and represent bending only, as in this case.

Now, let's take this single skin fiberglass below and compare it to the sandwich core. The thickness of the panel is equivalent to three skins of the sandwich. In reality, the solid fiberglass panel would have to be far thicker to have acceptable thickness and stiffness but for the sake of the example let's say it is three layers. The main point to take into account is that this lay-up will be as heavy or heavier than the sandwich. This is because the density (lb/in^3) of the skins can be 3 to 6 times that of the core.

Since the fiberglass is subject to the same bending moment as the sandwich, the area covered by its force arrows is also the same. It may be of different shape because the entire panel is made of 'homogenous' material but the sum of the internal reaction forces is identical!

Finally, compare the length of the longest arrows on the sandwich and the fiberglass panel. The fibers in the surface of the fiberglass panel are stressed far more (max. stress) than in the sandwich.

The conclusion:

  • Even though both the sandwich core and the fiberglass are subject to the same bending, the surface fibers in the single skin fiberglass are stressed more thus they will also stretch more. The skin will mostly bend or break. The panel will be far less stiff and strong than the sandwich.
  • Conversely, if the single skin fiberglass is assumed stiff enough, it can be substituted by much lighter sandwich core panel. In order for the maximum stresses (longest arrows) to be equal, far thinner and lighter skin is required in the sandwich.


For more information about cored construction from Vaclav please visit his site at: http://www.oneoceankayaks.com/Sandcore.htm.

So what does all of this mean for a sailing dinghy? Well, let’s assume that, regardless of construction, these boats will be experiencing the same amount of loads when in use. It is clear that when single skin construction is utilized, the loads experienced by the fiberglass are maximized. The loads on a sandwich cored boat, however, are better distributed, and much smaller throughout the thickness of the laminate. Fiberglass is similar to most materials in that it can only be worked so hard and so often before it fails. The less it’s worked, the longer it will last. Let’s use the analogy of curling weights in a gym. You will be able to complete far more repetitions before exhaustion with a 10 lb weight than you will with a 100 lb weight.

Laminate Comparisons

We completely core all panels throughout our decks and hulls on all of our models to ensure maximum performance. Depending on the section of the boats we will utilize 6.4mm Corecell foam, 6mm LRC Soric, or 3mm LRC Soric. Each of these cores absorbs a minimal amount of resin to ensure a light weight structure. Below are drawings of each of these laminates:

Most dinghies typically use either a single skin laminate, utilizing foam ribs for localized stiffening, or a laminate using Coremat as a bulker material to give the panel thickness, and therefore stiffness. While Coremat does accomplish this, the tradeoff is that the material is essentially a sponge, absorbing large quantities of resin. This makes the final laminate both unnecessarily heavy as well as excessively brittle. Below are drawings of each of these laminates:

It is visually clear that the first three laminates will be stiffer, and as we’ve just learned, that will result in a more reliable and longer lasting boat. Ironically, the first three laminates will also be of the same weight or lighter than the last two laminates (discussed in more detail in the “infusion” section). This means your boat will be stiffer, lighter AND last longer. What more could you want?