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 ship hull is generally subjected to asymmetric cyclic loads under various and complex oceanic environments. Especially, under extreme sea conditions, the loads suffered by ship structures conform to the characters of low-cycle fatigue loads. Consequently, the large-scale plastic yield may occur at the locations of stress concentration part of the ship structure due to structural discontinuity or initial defects and result in low-cycle fatigue damage. However, the commonly used methods for low-cycle fatigue life prediction do not premeditate the influence of large range of yield and asymmetric load. Therefore, the effect of mean stress on the crack initiation life of the stiffened plate, the basic unit of ship structure, is investigated in this paper. The crack initiation experiments of welded stiffened plates under different stress amplitudes and stress ratios were carried out. Moreover, the modified Neuber rule considering the large-scale plastic yield and several strain-life models considering the influence of mean stress were used to estimate the life under experimental loading conditions respectively. Different life results were predicted by various strain-life models and compared with the results of experiments. The comparison shows that the modified Neuber rule can accurately describe the local stress-strain at the root of notch in the state of large-scale plastic yield than the Neuber rule, and the Walker model is the most recommended to predict the low cycle fatigue crack initiation life.

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.

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