Panel Stiffness
Composites structures have requirements for stiffness to provide support and stability. Tests can be completed to identify the stiffness of a given area on a composites structure, i.e. how much it will bend for a given force.
The required stiffness of a section of a composites part depends upon the overall design and service expectations. Several factors, including the life expectancy of the object, the load rating of the composites surface, the inter-laminar bond strength, will help determine the threshold requirements. Testing is very important to determine the life expectancy of the part and whether it meets the requirements of its’ job.
Panel stiffness can be modified to meet these requirements using two basic methods. One way to increase the stiffness of an unsupported composites panel is to reduce the size of the panel through additional support structures. The other way is to increase the panel thickness across the same area. Choosing which method to use depends upon the engineering of the part and determining which method is acceptable with the surrounding part layout. If there is room to add supports, this is likely a quick and easy option. If there is room to increase the thickness, adding new or additional coring materials may be a simple solution.
Building a strong and stiff composites structure can be accomplished with the extremes of building a robust “skeleton” with small open areas and a thin skin, or having a basic, limited “skeleton” with a heavy duty cored laminate that supports itself.
A combination of the two usually works out best.
Stiff to the core
The thickness of the laminate will affect its stiffness. Stiffness can be increased by adding structural supports to the backside of the laminate, such as bulkheads and stringers in a boat. Another way to increase stiffness while minimizing the weight of carrying a solid glass and resin cross section is to use a core material.
Core materials are fairly wide ranging. They include wood; especially end-grain balsa and plywood. Plastics such as foamed PVC, foamed polyurethane, honeycombed polypropylene, and several others can be used with success. Paper honeycomb and cardboard can also be used with success in the proper application.
The biggest key to successful core use is successful adhesion and capture of the coring material. It needs to be part of the laminate in order to be beneficial. Each material has their own downfall, and these must be considered for each application. Wood and paper rot if exposed to water. Some of the thermoplastics materials melt and deform under high temperatures. Some are too expensive. Plywood and Balsa have density ranges across the sheets, while plastics are much more controlled and consistent.
Ideally the core material is placed directly in the center of the cross-section of the laminate so that the neutral axis passes in the center of the core material. This balances the loading forces of compression and tension under loading conditions from either side. The thickness of the coring can be determined through laboratory testing of panels differing by only the one variable of thickness.
Sponsor
