Research On Fracture Mechanics Assessment Of Industrial Pipes Containing Defects Under Elastic-Plastic Conditions

Industrial piping plays an important role in industries such as petroleum,chemical industry,nuclear power.Defects arising from poor working environment and complex load conditions are main causes of industrial pipes fracture accidents.Accordingly,periodically making fracture mechanics evaluations of industrial pipes containing defects becomes an important way to avoid disaster.Development of elastic-plastic fracture mechanics makes fracture mechanics evaluation of industrial pipes containing defects possible,and how to deal with interaction between varied loads in the process of assessing procedure has attracted more and more researchers' attentions,however this issue has not been settled so far.In addition,the steep-like demand for the staff of fracture mechanics assessment hindered its popularizing in industry use,and more research is requested for the verification of the applicability of fracture mechanics evaluation method for specific important equipments as well.This thesis will focus on some of these issues,based on the combination of finite element numerical simulation and theoretical derivation,study on the fracture mechanics evaluation method for industrial piping containing defects.Given that brazed stainless steel pipes play siginificant roles in aero-engine,nuclear industry,etc…Fracture mechanics study on them are conducted.The main research contents and conclusions are as follows:(1)Combining the twice-elastic-slope(TES)method and local plastic collapse method,a new FE plastic limit load determination method for structures containing defects is proposed.Axisymmetric and 3D FE models are built to achieve the plastic limit inner-surface pressure solutions for pipes containing fully-circumferential and axial semi-elliptical internal surface cracks.Comparative study between these solutions and existing formulas verifies the accuracy of the proposed method and illustrates higher reliabity of the proposed method.(2)J-integrals under varied loading conditions for cracked pipes are precisely calculated using the universal finite element software ABAQUS with axisymmetric and 3D models,and a new elastic-plastic interaction factor g()versus primary load ratio Lr is set up.Results indicate that under different secondary stress levels,upper and lower bound of g()calculated with axisymmetric or 3D models are almost the same,although 3D models give smaller g()when subjected to secondary load and low primary load.Effects of strain hardening exponent on g()are learned.Several features are found: all curves of g()versus Lr have an intersection point at Lr?1.0,which shares with R6 FAC feature;at high Lr(Lr >1.0),g()decreases rapidly;When Lr >1.0,the higher the strain hardening exponent,the lower the g();When Lr <1.0,the higher the strain hardening exponent,the higher the g().(3)Two different brazing procedures(warming to cooling and directly cooling)for stainless steel 316 L pipe of varied filler metal(BNi2)thickness are simulated and brazed residual stresses are obtained respectively.Effects of filler metal thickness on residual stress and distribution laws of residual stress along different paths are studied.Weight function SIF solutions for pipes containing circumferential through-wall cracks are deduced.SIFs under residual stress only is calculated with API 579 and WFM respectively.Using R6 FAC option 1,option 2 and the promoted g()method,SIFs for brazed 316 L pipes under both residual stress and internal surface pressure are calculated.Results illustrate that R6 methods neglects the residual stress relief at high primary load(Lr >1.0),and are more conservative compared with the proposed g()method in this thesis.(4)A proper software ‘Software for fracture mechanics assessment of defected industrial pipes' is developed with programming language ‘visual basic'.The reliability of the software is verified through comparative stuty with evaluation reports from qualified organizations,which is believed to promote the practical use of fracture mechanics evaluation method into engineering.The main innovative points included are as follows:(1)A finite element method basing on “Combination of TES & local plastic collapse” to determine plastic limit load of structures is proposed;(2)A formulae of g()factors versus load ratio for characterising the interaction between primary and secondary stresses is established;(3)Weight function SIF solutions for pipes containing circumferential through-wall cracks are deduced;(4)A fitness-for-use software ‘Software for fracture mechanics assessment of defected industrial pipes' is compiled.