In this paper, we implement the combined method of drilling stop hole and FRP reinforcement to repair cracked steel plates subjected to cyclic tension. The crack initiation and growth are experimentally investigated. The stress distribution at the stop hole and the residual fatigue life are evaluated by the FEM. The effects on prolonging residual fatigue life are analyzed. The results show that the effectiveness of only using the stop hole is limited, while the combined method has dramatically prolonged the residual fatigue life. The effect mainly owes to the increasing of the crack initiation life from the stop hole.

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In this paper, we experimentally studied the external surface crack growth in steel pipes reinforced with Composite System Repaired (CRS). CRS reinforced surface cracked API 5L X65 pipes were tested, containing three initial notch sizes and four reinforcement schemes. During the tests, the crack growth results, as well as the strain on the external mid-bottom composite laminate around the cracked area, and the vertical deflection of the specimens were recorded. The results showed that within the surface crack growth stage, the composite laminates were adequately bonded on the steel substrate, which significantly decreased the crack growth rate and prolonged the residual fatigue life. In addition, CRS reinforcement has increased the stiffness of the surface cracked pipes. Through the analysis on applying different reinforcement schemes, we indicated that increasing the amount of composite layers evidently facilitated the reinforcement effectiveness, while increasing the bond length did not; and the inversely diagonal wrapping pattern performance less effective.

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In this paper, we analyse the surface crack growth in the Fibre-Reinforced Polymer (FRP) reinforced steel plates subjected to tension by means of the finite element (FE) method. Following the experimental study, a three-dimensional FE model is developed to evaluate the Stress Intensity Factor (SIF) of the surface crack, and the crack growth rate is calculated by using the Paris’ law. Then the FE model is validated by the experimental results. Afterwards, on account of the validated FE model, a parametric study is developed in order to guide the optimization design of FRP reinforcement accounting for different reinforcing schemes and multiple influential parameters. The results indicate that the single-side FRP reinforcement on the cracked surface is the most efficient method, owing to the generated out-of-plane bending moment. In addition, the optimum bond length and number of layers are indicated. Besides, surface crack growth is sensitive to the influential parameters including aspect ratio of the surface crack and crack dimension, while less sensitive to the Carbon-FRP (CFRP) tensile modulus, and the adhesive thickness. The analysis is of instructive value to facilitate the application of FRP reinforcement on the surface cracked metallic structure repairing domain.

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In the present paper, we propose an analytical method to calculate the Stress Intensity Factor (SIF) of circumferential surface cracks in steel pipes subjected to bending. In light of pipe geometry and bending load case, the analytical formula is raised by introducing new bending correction factors and new geometry correction factors on the basis of the Newman-Raju’s method. The bending correction factors are deduced based on the bending stress gradient, while the geometry correction factors are determined by parametric studies for internal surface cracks and external surface cracks respectively. Owing to a large data set requirement by the parametric studies, three-dimensional finite element (FE) models of evaluating SIFs of circumferential surface cracks are developed. The FE method is validated to ensure that it could provide accurate SIF estimations. Analytical verification is conducted which shows that the SIF evaluated by the proposed analytical method match well with the results evaluated by the recommended analytical method. Then experimental investigations of external surface crack growth in offshore steel pipe subjected to fatigue bending are implemented to further validate the analytical method of predicting surface crack growth rate. The analytical results match well with the test results and the available experimental data from literature, indicating that the analytical method can be used for practical purposes and facilitate the crack growth evaluation and residual fatigue life prediction of cracked steel pipes. Previous article in issue@en

Rigid pipelines have been widely applied in offshore oil & gas operation and transmission industries on account of their structural simplicity, cost-effectiveness, ease of installation and maintenance. However, surface cracks frequently appear in the internal surface of rigid pipes due to dynamic loads or hydrogen embrittlement, etc. Under dynamic fatigue loads, surface cracks may continue to propagate and finally develop into penetrated cracks, which may cause leakage and serious accidents. Fiber-reinforced polymer (FRP) strengthening technology is already a reliable technique for structure maintenance in onshore pipelines and penetrated cracks in load-bearing circular hollow sections (CHS). Nevertheless, there are very limited systematic investigations of surface crack in rigid pipes reinforced with FRP, which has a remarkable significance for offshore rigid pipes. This paper aims to understand the mechanism of semi-elliptical surface crack growth in the internal surface of rigid pipes under fatigue bending moment reinforced with FRP. Stress intensity factors along the crack front are computed through finite-element (FE) models, which are validated by experimental data from references. The influence of wrapping orientation of CFRP are discussed as well. The numerical results show that under CFRP reinforcement, surface crack growth rate decreases significantly which ensures the safety use of rigid pipes in offshore industry.@en