Foam Core Materials in the Marine Industry
by Trevor Gundberg, Composite Materials Engineer with DIAB Inc.
For over 60 years foam cores have been utilized in marine applications to lighten, stiffen, and strengthen everything form hull bottoms to fly bridges. But what exactly is foam core? What type do I use, and where can I use it? Should I use it instead of balsa or plywood? With the seemingly endless variety of foam core materials on the market today, it can be frustrating for the average boating enthusiast to find the right foam core for his/her particular application. This article will attempt to inform boat builders about the properties and correct manufacturing procedures involved in constructing foam core sandwich laminates.
The first foam material specifically formulated for a marine environment was a poly vinyl chloride (PVC) and isocyanate blend (simply called PVC foam) created in Germany by Dr. Lindemann in the late 1930's and 40's. It has been rumored that this early version of PVC foam was used in the German E-boats and even in the famous 'Bismarck' battleship. After World War II, France acquired the formula as part of its war reparations. From there, the formula was licensed out to companies in Sweden, Switzerland, and Germany, who kept developing the original recipe in their own distinct ways. After many years of different formula offshoots and company consolidations two main suppliers of PVC foam remain, DIAB and Airex/Herex.
Other foams based on chemical components other than PVC have also been developed over the years, including: linear PVC (also originally formulated by Dr. Lindemann), polystyrene (PS), styreneacrylonitrile
(SAN), polyurethane (PUR), polyisocyanurate (PIR), polymethylmethacrylate (PMI), polyetherimide (PEI), and many others. Each type of foam has different physical and mechanical properties due to their chemical differences, but all are used for the same function: to lighten, stiffen, and strengthen by utilizing the sandwich principle.
The sandwich concept is based on two main ideas: increasing the stiffness in bending of a beam or panel and doing so without adding excessive weight. The general term for bending stiffness is flexural rigidity (D), which is the product of the material(s) elastic modulus, and the cross section moment of inertia (I). For a symmetric sandwich beam (both skins have the same thickness and material properties), the formula for flexural rigidity is:
Ef = Elastic Modulus of the Facings (Skins)
Ec = Elastic Modulus of the Core
b = Width of the Beam
d = Distance Between Facing Centroids
t = Thickness of a Facing
c = Core Thickness
If the skins are relatively thin compared to the core (d/t > 6) and the core material is considerably weaker than the skins (Ef/Ec . td2/c3 > 17), the equation can be reduced to:
From this equation, it is apparent that the core material does not directly contribute to the stiffness of the panel or beam, (at least in lower density cores) but it's the distance between the skins that is the overwhelming factor. Increasing the "d" variable will have a much greater effect on the flexural rigidity than any other component in the equation, since every other variable has a linear contribution. When dealing with higher density cores (usually > 5 lb/ft 3 ) and thicker skin laminates, the full equation must be used in order to properly predict the stiffness properties. This is due to the high-density core contributing stiffness in the first case, and the thick skins absorbing more shear stress.
While the core keeps the skins an equal distance apart from each other thereby increasing the stiffness, it also bears most of the shear loading. In bending, the lower skin is in tension, while the upper (or inner) skin is in compression thereby putting the core in shear (See Figure 1). In order for the sandwich to function correctly the adhesive layers between the skins and the core must be able to transfer the loads, and thereby be as least as strong as the core material. Without a proper bond, the three entities work as separate beams/plates and the stiffness is lost. This is why proper core/skin bonding is so critical.
Sandwich vs. Single Skin Fiberglass
Exactly how much stiffer is a sandwich structure versus a single skin laminate, and what are the weight savings? As noted above, the flexural rigidity of a structure is dependent on two factors: the material(s) stiffness or modulus, and the cross sectional geometry or moment of inertia. The material properties are often difficult to change (and sometimes expensive), so a change in the geometry can be done to increase stiffness while not compromising on strength or other properties of a single skin laminate. Figure 2 shows.the difference in stiffness, strength, and weight when a core material is placed between the plies of a single skin laminate (all attributes are approximately normalized).
From just increasing the cross sectional geometry, the stiffness increased 48 times, while the flexural strength increased 6 times, and all with a marginal increase in weight. The increase in strength and stiffness allows builders to use less skin materials, resulting in considerably lower weight structures. Decreased weight helps to increase top speed and acceleration, increases cargo capacity, and reduces fuel consumption. A sandwich construction is compared to a single skin laminate with relatively the same flexural rigidity in Figure 3:
Other advantages of the sandwich construction include: greater insulation, better impact/damage resistance, sound attenuation, and reduced labor. The core material, which is usually cellular in construction, provides a much lower thermal conductivity and higher R-value than a comparable single skin laminate. Labor is reduced since less plies of material are being used and the greater stiffness of the sandwich reduces the number of needed stiffeners. With less stiffeners, and consequently larger panel sizes, impact energy is dissipated more readily. The cellular core materials also reduce the "drum head" effect, thereby reducing noise, resulting in a quieter ride.
Some downsides to sandwich construction include the core material cost and the employee learning curve. Core materials are generally more expensive than the resin and glass that it is replacing, and in some cases the labor savings will not offset the cored laminate price. More care and attention needs to be taken when processing cored laminates. Employees need to be aware of all the possible problems that could occur if the core materials are not handled or bonded to the skins correctly.
Foam Core Materials in the Marine Industry
by Trevor Gundberg, DIAB Inc.