Stress concentration factors in FRP-strengthened steel tubular KT-joints

Abstract

This paper presents a brand new application of the fiber-reinforced polymer (FRP) materials as a strengthening tool for reinforcing offshore steel tubular KT-joints to reduce the stress concentration factors (SCFs). The finite element (FE) approach was validated against the experimental data on the FRP-strengthened joints as well as the well-known standards on the analysis and design of SCFs in tubular KT-joints. Having been validated, the numerical modeling methodology was used for an optimization study, which was conducted to find the most practical and effective fibers’ orientation in the FRP layup. The study continued with 1458 FE models under three different axial brace loading conditions. Three different types of FRP materials, including glass/epoxy, glass/vinyl ester, and carbon/epoxy, were considered as reinforcing materials. Results of the parametric studies showed that apart from the loading scheme, by using CFRP as the strengthening material, more than 50 percent reduction in SCF values was achieved at the saddle position of the central brace. Based on the parametric study results, a parabolic trend was observed for relative SCFs (ψ) against β, in which the minimum values of ψ occurs at β=0.5. Increasing γ and τ decreased the FRP effectiveness while increasing θ enhanced the FRP efficacy in reducing the SCFs. Finally, practical parametric equations were presented for fatigue design of unstrengthened and FRP-strengthened steel tubular KT-joints. The performance of the proposed formulae was examined not only by the statistical indices and the recommendations of Fatigue Guidance Review Panel but also by an external validation analysis the performance of the new equations in predicting the SCF values in their range of applicability was investigated.