Why Cored Composite Laminates
Spring, 2025
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Core is a valuable part of composite construction. Core materials add stiffness while adding very little weight and usually low additional material costs. The construction of an I-beam is a perfect example of how increasing web height (or distance between two load-bearing flanges) increases the stiffness of a beam in cored composite laminates.
Solid Laminate vs. Cored Laminate
Fig. 1. Fig. 2 shows a solid laminate made of chopped strand mat and woven roving. The drawing on the right shows a fiberglass laminate with a balsa core. Fig. 3 shows how a cored composite reacts to a bending load in cored composite laminates. The top fiberglass skins are loaded in compression while the bottom fiberglass skins are loaded in tension.
Compressive Force, Tensile Force, and Shear Strength
Another visual representation of this is if you have a sponge and you draw vertical lines on the side of it. If you bend the sponge to mimic Fig. 3, you will see the lines toward the top of the sponge get closer together. Conversely, toward the bottom of the sponge, the lines will get farther apart. We call the pushing force that causes the lines to get closer to each other a compressive force. Meanwhile, we call the pulling force that causes the lines to move farther apart tensile force. It is the core in the middle that has to control the change between the movement on the top laminate. We refer to this opposing force as shear force. One of the good things about end-grain balsa core is that it has high shear strength for a core material and is often used in cored composite laminates.
Core Thickness vs. Mechanical Properties
Using these principles, we ran some calculations to find the comparative data shown in Fig. 4. This comparative data shows how adding balsa core to a composite panel affects different properties of the composite panel in cored composite laminates.
In this table, 1t represents the thickness of a solid fiberglass laminate. For this comparative data, we kept the thickness of the fiberglass laminate the same and increased the overall composite’s thickness by adding balsa at varying thicknesses. As we would expect when considering the I-beam model, as the thickness of the composite increases, the stiffness significantly increases. Since we are using balsa as the core material to increase the thickness of the composite, we can see that the weight increase is very minimal in cored composite laminates.
Calculating Strength
When we moved on to calculating the strength of the composite, we realized we had to consider that the core may fail in shear before either of the fiberglass skins would fail. It is clear to see that the fiberglass laminates for the 4t sample would take about 9 times the load of the 1t laminate before it would fail. However, the overall cored composite of the 4t sample would only be able to handle about 2.5 times the load of the 1t laminate before the core would fail in shear. It is important to consider this because the core could have failed, and there would have been no visual indication of this failure when looking at the top and/or bottom skins. This is just one of the things engineers need to keep in mind when designing a composite structure, and now you can too with your knowledge of cored composite laminates.
Another possible failure mode that can happen in cored composite construction is crushing the core. This is most common in areas where fasteners are installed and the load is not distributed appropriately. This is why backing plates are critical when installing bolts through cored laminates. Fastener installation in cored composites is described in more detail in section 7 of our Fiberglass Boat Maintenance and Repair Manual.
Want more information about cored laminates? Learn more here!
