PSI - Issue 64

Yuan Xu et al. / Procedia Structural Integrity 64 (2024) 1865–1872 Author name / Structural Integrity Procedia 00 (2019) 000–000

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1. Introduction Due to excellent mechanical properties and convenient maintenance, tubular structures have been widely used in bridge engineering, building structures and offshore platforms. The most common connection between the tubular members are welded joints, which are recognized as the most crucial parts of tubular structures due to stress concentration. Many tubular joints under long-term fatigue loading are required to be strengthened. Traditional methods of tubular joints strengthening such as bolting and welding metallic stiffeners, may introduce additional fatigue defects and residual stress (Choo et al., 2005). Recently, carbon fiber-reinforced polymer (CFRP) composite has been an alternative material to strengthen the tubular structures, owing to its superior strength-to-weight ratio and flexibility. Studies on fatigue performance of CFRP-strengthened tubular joints have proved feasibility of this new strengthening method. The hot spot stress method is widely adopted to study fatigue behavior of welded members, in which the stress concentration factor (SCF) is used to evaluate stress concentration near weld toes. Tong et al. (2019) proposed a strengthening scheme for tubular gap K-joints using unidirectional and bidirectional CFRP sheets. It was found that the maximum SCF in CFRP-strengthened tubular gap K-joints decreased by 6% − 28% than it in un-strengthened K joints. Hosseini et al. (2020) investigated SCFs of circular hollow section T-joints strengthened with fiber-reinforced polymer (FRP) subjected to axial loading, and observed that the SCFs were more influenced by the FRP on the chord member than that on the brace. Nassiraei et al. established numerical models of FRP-strengthened complex space joints under various loading conditions, including compressive load (2021a), in -plane bending moment (2021b) and out-of-plane bending moment (2021c), based on the experimental data of simple T/Y and K planar joints. Parametric formulas were proposed for predicting the SCF at the saddle and crown points. Few fatigue tests on CFRP-strengthened tubular joints have been reported. Xu et al. (2022) found that fatigue life of CFRP-strengthened TT-joint was prolonged by 47% in comparison with the unreinforced joint. Tong et al. (2021) performed experiments on tubular gap K-joints strengthened with CFRP sheets and proved that the CFRP strengthening effectively decreased the hot spot stress range and consequently prolonged fatigue life of the joints. The failure modes of joints were not changed by CFRP-strengthening. Although prestressing CFRP has shown great promise in making full use of the material’s high strength and restraining development of fatigue cracks, application of prestressed CFRP in strengthening of tubular joints is less reported. In this paper, numerical analysis was carried out to investigate the effect of prestressing CFRP application on the hot spot stress distribution of tubular T-joints under brace axial tension loading. Finite element models of the unstrengthened T-joints were verified against experimental results and the predictions of Lloyd’s Register (LR) equations. Influence of strengthening parameters on the hot spot stress distribution was investigated, including the prestressing level ( p ), CFRP width ( w ), and CFRP layers ( n ). 2. Numerical modelling and verification A finite element model of prestressed CFRP strengthened T-joints was developed using the ABAQUS software package. The tubular T-joints modeled in this study were based on an experiment conducted by the authors. Dimensions, geometrical configuration and parameters definitions of the tubular T-joints are shown in Fig. 1 and Table. 1. The T-joint without CFRP strengthening was tested under axial loading to verify the finite element model. The complex geometry of the weld profile was modelled in accordance with AWS (2010) rules and recommendations, to obtain stress distribution along the chord-brace intersection.

Table 1. Dimensions of the tubular T-joint. D (mm) d (mm) T (mm)

t (mm)

L (mm)

l (mm)

β

γ

τ

θ

α

219 0.75 where D and T are the chord diameter and thickness, respectively; d and t are the brace diameter and thickness, respectively; L is the chord length; l is the brace length; θ, α , β, γ, τ are dimensionless parameters of the tubular T-joint. θ is the angle between brace and chord, α =2 L / D , β = d / D , γ = D /2 T , τ = t / T . 121 8 6 1660 600 90 15.16 0.55 13.69