Increasing Strength of Graphite/Epoxy Bolted Joints by Introducing an Adhesively Bonded Metallic Insert

In this paper a method to increase the failure load in graphite/epoxy bolted joints where the failure mode is bearing strength is proposed by bonding a metallic insert in the hole. In order to design test specimens with significantly reduced compressive stresses at the hole boundary, finite element (FE) analysis was performed. The FE analysis revealed the possibility to reduce the compressive stresses at the hole boundary in quasi-isotropic laminate by approximately 50%, if a 10 mm hole is enlarged to 14 mm and a 2 mm thick metallic insert is adhesively bonded in the hole. Experimental results showed an increased failure load of up to 55% when using an aluminum insert and 30% when using steel insert, the steel insert also giving a 13 % stiffer joint. The adhesively bonded insert altered the failure mode to failure in the adhesive combined with net tension of the composite member.

Further studies of FE-results explained the difference in failure load when using a steel or aluminum insert respectively by the fact that the stiffer steel insert causes higher tension stresses in the adhesive, which apperently leads to a primary tension failure in the adhe sive.