Composite Compression Zone Due To Fasteners

Composite Compression Zone Due To Fasteners

At Northern Lights Solutions, Inc. we have worked on numerous composite projects and fastening of composite panels is important and a bit complex.  Glass fiber and carbon fiber composites are layers of fibers and a central foam core are held together with a resin material.  When a small mechanical fastener like a bolt with washers and threads are used, care must be taken to evaluate stresses and strains caused by the material being crushed between the washers in fastener assembly.  Small details like oversized washers can spread the loads more evenly and reduce crushing of the resin, fiber material or deformation of the foam cores.  In some cases eliminating the foam core and having a solid laminate can be enough to distribute loads evenly.  In other cases, we suggest metal inserts are embedded in the composite structure to eliminate damage to the composite material.

The key when designing with composite materials is to evaluate the material as a composite material and not as a traditional bulk material with different properties in different directions.  The flange image above is of a composite aerospace product and we are evaluating the stresses, and strains at each fastener location.  We used a composite model where we defined the materials in each ply and the orientation of each ply and the volume fraction of material and resin.  The model is truly a model of the composite assembly and not just a solid model with non-isotropic properties.

Creating a custom layered material in a program like ANSYS with composite specific plugins that actually treat your material as a composite material is the only way to design composite products.