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.
This paper reviews work at the Aeronautical Research Institute of Sweden, and addresses major issues of importance in evaluating the effect of impact on composite structures. Some more extensive reviews of work by other researchers are referenced. The paper addresses impact response and damage formation, damage characterization, and residual strength and stability by combination of experiments and analysis. Studies showing that impact response type depends on impactor-plate mass ratio are presented. Small mass impact is generally more critical at a given configuration and energy. Analytical models for small mass impact and for damage initiation and growth during large mass impact are discussed. Rate dependency of matrix-dominated properties is briefly discussed. Geometric and constitutive characterization of impact damage zones is presented and the influence of degraded properties demonstrated. The use of an FE-based plate model to simulate delamination growth due to sublaminate buckling and panel skin buckling in stiffened panels after impact is described. Skin buckling causes a steep increase in delamination strain energy release rate and should be prevented.
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