Many studies have shown that the linear elastic fracture mechanics (LEFM) method based on the stress intensity factor range (ΔK) has limitations that cannot be ignored. Due to neglecting the influence of plastic deformation near the crack tip, LEFM shows apparent deviations in evaluating the fracture behaviour. Therefore, in this study, the crack tip opening displacement range (ΔCTOD) is chosen as an alternative to ΔK and based on the elastic–plastic fracture mechanics (EPFM) to develop a new fracture behaviour assessment approach for marine structures. Firstly, a ΔCTOD model based on the HRR (Hutchinson, Rice, and Rosengren) solution is proposed considering the crack closure effect. Secondly, a series of compact tension (CT) specimen crack growth experiments under constant amplitude loading is carried out. According to the experimental results, the prediction accuracy of the HRR model and traditional Irwin and Dugdale models is compared and analysed. The rationality of ΔCTOD as an alternative to ΔK is verified. The results show that ΔCTOD can describe the crack propagation behaviour well. The proposed HRR model shows better accuracy and a more comprehensive application range than the traditional models, which has a guiding significance for studying fracture behaviour for marine structural applications.

The welding residual stress re-distribution behavior during fatigue crack propagation in butt-welded high-strength steel plates for ship construction is investigated based on experimental test results and numerical analyses. The specimens’ initial welding residual stresses are obtained from X-ray for middle tensile (MT) specimens cut from butt-welded high-strength steel plates. Then, fatigue crack propagation experiments on MT specimens are conducted, and a strain gauge is used to measure the residual stress re-distribution field around cracks. A practical fatigue crack propagation simulation procedure is developed with a dynamic update of in-situ welding residual stress, where the residual stress intensity factor K_res of the MT specimen is deduced. The stress ratio effect on K_res during fatigue crack propagation is analyzed and a good agreement between experimental and numerical results is achieved.

Central spherical detector is one of the most essential and complicated support systems for the large-scale underground neutrino observatory. If a central spherical detector accident happens, the consequences may be disastrous with huge economic loss. However, there is very few published research works which focus on its quantitative risk analysis. In this paper, an improved fuzzy fault tree analysis (FFTA) method incorporated with the weakest t-norm (T_ω) algorithm, the confidence level concept, and the analytic hierarchy process (AHP) approach is proposed to perform its risk assessment. By carrying out the identification of failure modes and failure reasons, fault tree (FT) model of central detector is constructed based on logical relationship among subcomponents. Fuzzy set theory is applied to obtain failure data, and T_ω algorithm is exploited to eliminate fuzzy accumulation in the aggregation process. In addition, a confidence level coefficient and AHP approach are employed to enhance the reliability of the evaluation. Both importance and sensitivity analysis have been conducted to identify the critical basic events and provide improvement measures. Finally, the comparison of the occurrence possibility of detector failure is used to verify the applicability and the feasibility of proposed method. The calculated results indicate that the improved approach is more consistent with real situation and can provide a more effective engineering reference for the risk decision of central spherical detector.

Keywords: Fatigue crack propagation; Welding residual stress; Plastic re-distribution; Stress ratio effect

Multiplanar tubular KK-joints are one of the most popular joint types in offshore structures, which are exposed to cyclic loads and fatigue damages. In this paper, a fatigue prediction method based on scaled model test is proposed. First, a scaled KK-joint, including deviations for model simplification, was designed based on sensitive analysis and similarity analysis. Then, static and fatigue tests on the scaled model under axial loading were performed, by which hot spot stress (HSS) distributions and the maximum HSS were recorded. From the test, the fatigue crack initiates from the location of the maximum HSS and propagates along the weld toe. Finally, the maximum HSS of original KK-joint was deduced by finite element analysis (FEA), and then, the fatigue life was predicted accordingly and compared with the rule-based result.