Research On Damage Evolution And Failure Assessment Of Welded Structures Based On BS 7910

S355 weathering steel has the advantages of high strength and corrosion resistance and is widely used in ship hulls and high-speed train bogies.Welding process is an important way to connect hull steel structures.The service ti me of ships is long.Fatigue cracks are a major problem in welded structures.UK’s BS7910 standard enables rapid assessment of defective welded structures.Crack size(crack length 2c and crack depth a)are important parameters in the evaluation process for this standard.There is a lack of systematic discussion on the theoretical basis of crack defects and the relationship between crack length 2c and crack depth a in the standard.This standard is commonly used for safety assessment of crack characteristics with a fixed value of a/c.Therefore,this paper aims to monitor the damage evolution and failure process of S355 weathering steel by means of ultrasonic phased array and observe the characteristics of fatigue crack size in each stage of expansion.In this paper,the relationship between a and c will be analyzed and discussed,and the analytical formula of crack length 2c and crack depth a will be fitted,which will provide the empirical value of crack characteristics for BS 7910 evaluation and lay the foundation for the formulation of safety evaluation standards for welded structures in my country.First,the welding residual stress of S355 weathering steel butt joint is calculated by finite element numerical simulation.The obtained results are compared with the blind hole experiment method to verify the accuracy of welding numerical simulation.On this basis,Marc finite element software is used to calculate the welding residual stress release before and after the extraction of full-scale fatigue specimen.In this paper,the optimal welding sequence scheme of S355 weathering steel cross joint is determined by comparative analysis of post-weld temperature field and residual stress.It turns out that: The welding residual stress of the large plate specimen retained in the dog bone fatigue specimen after cutting is about 70%,which meets the definition of a full-size fatigue specimen.Different welding sequences have little effect on the residual stress distribution of cross joint welding.The cross joint is divided into two T joints,and the welding sequence of a complete discontinuous plate is welded first to minimize the structural deformation.Then,the fatigue test of butt joint and cross joint steel structure is carried out in this paper,and the fatigue crack propagation is monitored by ultrasonic phased array.The crack size(crack depth a and crack length 2c)in crack initiation,crack propagation and fracture are observed.Crack tip K factor is calculated based on BS 7910 standard.It is found that the relative error of the K factor calculation result between the actual crack feature monitored by the phased array and the crack feature with a fixed value of a/c=0.5 is about 15%,indicating that the crack size has a great influence on the analytical formula calculation result of the K factor.Finally,the crack depth a and the length c are fitted into an analytical formula.The study found that the relationship between c and a is not clear when the crack propagates,and the ratio between the two ratios a/c and the crack depth a conforms to a certain law.The fitting formula is extended to the analytical formula of the BS 7910 specification,which provides a basis for quantitatively describing the crack growth and provides a reference for the later improvement of the BS 7910 specification.The K-factor at the crack tip is calculated by the XFEM extended finite element method to verify the accuracy of the BS 7910 analytical formula for calculating the K-factor.Based on the optimized BS 7910 specification,the service safety analysis of weathering steel is carried out,and the critical crack size under different calculation methods of crack characteristics is compared.The results show that the error between the K factor calculated by the analytical formula a/c and the actual a/c settlement result is small,about 5%,which verifies the reliability of the empirical value of crack characteristics obtained by the analytical formula for FAD safety assessment.The crack characteristic size has a great influence on the BS 7910 evaluation results,and the critical crack size is too high for the fixed value a/c.