Research On Fracture Analysis And Structural Integrity Evaluation Method For Embedded Crack Structure Under Multiaxial Loading

The embedded defects in the component are typical types in structure integrity assessment.At present,many evaluation criteria for structure integral have given evaluation method for plates containing embedded off-set ellipse or rectangle cracks under uniaxial load,such as R6,BS7910,API579-1/ASME FFS-1.However,some special equipments?such as pressure pipe,pressure vessel,boiler?often bear multiaxial loading in practical application.It is possible to get dangerous conclusions when they are evaluated by existing standards.Therefore,it is very urgent to carry out the research on the structural integrity evaluation of components containing embedded cracks under multiaxial load.Considering the common multiaxial loading conditions for components with embedded defect,it is simplified as plates with off-set embedded ellipse or rectangular cracks.The loads considered are an end tensile/compressive force,N₁,applied at the centroids of the end sections of the plate perpendicular to the crack plane,a tensile/compressive stress,?₂,parallel to the crack plane and a cross-thickness bending moment,M₁,applied at the end sections of the plate.Failure assessment diagram?FAD?is often used to evaluate the structural integrity for defective component,where limit load and J integral are the important inputs.The main work are as follows:1.For a plate with thickness t and width W,it contains an embedded off-set elliptical crack with depth 2a and width 2c.Off-set distance y_off is between the center of plate section and crack along thickness direction.Normalized treatment of geometric size and load of the model has been done under proportional loading.The limit load solution for plates containing embedded off-set ellipse cracks under above-mentioned multiaxial loading conditions has been deduced based on the lower bound limit load theorem and Von Mises yield criterion under a plane stress assumption.The limit load solutions are validated using 3D elastic-perfectly plastic finite element?FE?analyses results based on the twice elastic slope method.The results show that the predictions using the solution are close to the FE results and slightly conservative.Under the same loading conditions,the limit load solutions for plates containing embedded off-set rectangular cracks can be gotten by the same method.The solutions can be simplified as some other defect types in plates,such as through-thickness cracks,surface rectangular cracks and extended surface cracks.2.For embedded off-set crack plate with Ramberg-Osgood material.Models with geometric parameters of different crack shapes?a/c=0.3,0.5,1?,different crack depth?a/t=0.125,0.25,c/W=0.25?,different crack location(y_off/t=0,0.1,0.2)and different hardening exponent?n=5,10?of Ramberg-Osgood material have been established.The J-integral solutions have been gotten by FE analyses under different load cases of M₁/?tN₁?=0,0.2,?and2Wt?₂/N₁=-1,-0.5,0,0.5,1,2?N₁?0?or 3M₁?₂/?Wt²?=-2/3,-1/3,0,1/3,2/3?N₁=0?.Then fully plastic factor h₁ values have been obtained based on EPRI method.The results show that the stress?₂ and hardening exponent n have an influence on results.3.For embedded off-set crack plate with Ramberg-Osgood material,J values predicted by reference stress method have been analyzed based on linear elastic stress intensity and the corresponding limit load solution.The estimation of J integral based on the global/local limit load solution has been done following the given procedure of R6 option 2 reference stress method.The results show that a/c,a/t and?₂ affect the contrast results.Estimated J values based on R6 option 1 and R6 option 2 have been compared with J values based on R6 option 3 related to FE solutions.The errors between estimated J values and FE results have been analyzed.At last,a correction coefficient is added to supplement the formula of R6 option 2.4.For embedded off-set crack plate with Ramberg-Osgood material,embedded off-set crack will propagate as surface crack,through-thickness cracks or extended surface cracks under multiaxial load.Evaluation scheme for plates with off-set embedded cracks has been given.First,the scheme is direct against the plates with off-set embedded cracks.Then,the scheme is against the plates with surface cracks which extended by the off-set embedded cracks.At last,the scheme is against to through-thickness cracks and extended surface cracks which extended by the surface cracks.Aimed at the structures with off-set embedded cracks which meet the material model of Ramberg-Osgood,their structure integrity assessment have been given under proportional multiaxial loading.It is suitable for the evaluation of related defective components,such as large diameter pressure vessel and nuclear power pipe.