Residual Stress in Composite Hardware Assemblies

When we drill holes and install hardware like screws, bolts, or metal inserts into composite materials, we often create "hidden" forces known as residual stresses. These are internal pressures that remain in the material even when no outside load is being applied. This study looks at how these stresses build up around holes located dangerously close to the curves and edges of a structure.

The research finds that installing a fastener is not a "neutral" act. Forcing a metal bolt or a press-fit insert into a composite bore creates a permanent "push" against the hole walls. When this hole is near a curved edge, these internal forces don't spread out evenly; instead, they bunch up, creating a high-pressure zone that can crack the resin or peel the layers apart before the part is even in use.

By using computer simulations, we measured how different hardware like tightly torqued bolts versus bonded inserts changes these hidden stress levels. Our results show that ignoring these stresses leads to overestimating the part's strength. We provide simple "safety zones" for engineers to ensure that hardware is placed far enough from edges to prevent the material from failing prematurely due to its own internal pressure.