Rapid Assessment Of Fatigue Life For Q420R Steel T-joints With Embedded Cracks

Fatigue assessment of welded structures with defects is an important aspect of fatigue life assessment research in pressure vessels.Many mountries have also formulated standards or specifications for fatigue assessment of welded structures with defects.With the development of pressure vessels in China,the structure is becoming more complicated,the working environment is becoming worse,and the size is becoming larger.The corresponding inspection requirements are also increased accordingly.In some special cases,a fast and accurate evaluation of the structure containing defects is required.At present,the corresponding domestic standards or specifications have not yet met this requirement,but the British standard BS7910 has achieved a rapid assessment of the fatigue life of defective structures based on the principle of fit-for-purpose.However,the equivalent basis of cracks and the origins of crack size equivalent drawings and the quality category chart still lack a detailed introduction.In this paper,the T-welded joint is taken as the research object.The fatigue crack growth life of the joints with embedded cracks and surface cracks at the weld toe is discussed.The equivalent basis of the surface cracks at the weld toe and the propagation model were analyzed and calculated,which provides a theoretical basis for formulating relevant evaluation standards in China.Q420 R steel is a material that is newly incorporated into the special steel plate for pressure vessels.The applicability of fatigue assessment based on the quality category method needs further study.In this paper,the fatigue test of T-welded joint of Q420 R steel is also carried out,the fatigue performance of the material is analyzed and studied.The applicability of the fatigue life assessment of the material based on the quality category method were studied by comparing the P-S-N curve of the joint with the curve in the quality category method.In order to provide theoretical and data support for the establishment of relevant assessment standards in China,the following basic research work has been carried out around the research object of T-welding joint:(1)The fatigue crack growth of the surface crack at the weld toe under the action of axial force is studied.The infinite crack equivalent of the surface crack at the weld toe is carried out based on the equivalent fatigue life and equivalent fatigue strength by VBA programming,and the fatigue crack growth at the weld toe is calculated under different parameters.The results are compared with the standard values in BS7910.The results show that there is still a certain gap between the equivalent infinite crack size calculated by the two equivalent methods and the standard value in the standard;under the same life condition,the calculation of the initial crack size of the surface infinite long crack corresponding to different fatigue grades and the standard value in the standard also have a greater effect on the specific wave motion;but under the same fatigue level,the relative error between the calculation of the initial crack size of the infinite crack corresponding to different fatigue life conditions and the standard value in the standard is relatively small.(2)The propagation behavior of the embedded crack is analyzed.Two propagation modes of the embedded crack are calculated by programming based on the equivalent fatigue strength method.The initial crack depth corresponding to the same fatigue level with different thickness under the two modes is calculated respectively,and the failure criterion of thin edge penetration is determined.On the basis of the above,the initial depth of the infinite crack corresponding to different fatigue grades and the allowable crack depth corresponding to the initial crack with different size parameters are calculated under different plate thicknesses,and compared with the standard value in BS7910,the relative error is small,all of which are within 10%,and the equivalent diagram of the embedded crack size and quality category chart are drawn.The results show that the propagation mode of the thin edge penetration of the embedded crack can accurately reflect the basic principle of drawing the size equivalent diagram and the quality category chart of the embedded crack in BS7910.(3)The fatigue test of Q420 R steel T-joint was carried out,and the P-S-N curve with high survival rate and high confidence level was drawn according to the group method,which compared the quality category curve in the double logarithmic coordinate system;the macro and micro observation of the fatigue fracture were carried out,and the typical characteristics of the fatigue fracture were analyzed.The results show that the P-S-N curve of T-welded joint is basically parallel to the curve in the quality category under the survival rate of P=97.7% and the confidence level of γ =95%,which indicates that the fatigue performance of the T-welded joint is good.At the same time,it also reflects that the fatigue resistance of the material is better.The general welded structure of the material is suitable for the rapid evaluation of the quality category method,which verifies the applicability of the material.(4)By comparing the two methods to evaluate the fatigue life of welded structures with embedded cracks,compared with the traditional fatigue life evaluation method of fracture mechanics,the quality category method can realize the rapid evaluation of welded structures with cracks,and the evaluation results are conservative.