Fracture Assessment Procedures Of Mismatching Welded Joint With HAZ Cracks Based On Tri-material Model

Welding technology is extensively used for the key equipment in the modern plants which are generally operated in petrochemical and power generation industries. Many key equipment in service over along period of time due to the needs of production engineering. The failure cases of key equipment provoked by premature failures of weldments have been reported during the past few years. Welded structure is body of inhomogeneous of material and geometry, resulting from a smelted process of complex in heat physics and chemistry. The non-uniform of organization, mechanical property and geometric property is provoked for weldments, It brings many troubles for produce, process and actual application of welded structure. At present, for most fracture issues of welded joints, people have disregarded the role of HAZ and simplified the welded joint as a composed "idealized bi-material" model of weld and base material. There are only a few reports dealing with the effect of heterogeneity on welded joints with HAZ cracks. This thesis gives a review on the development and research progress of the rupture performance of mismatching welded joints and defect assessment procedures. Then it analyzes the effect of inhomogeneous on the fracture behaviour of HAZ cracks of welded joints establishes the theories of assessment technology used to the procedure of failure analysis of the welded joints containing the HAZ cracks.Several research progresses have been achieved in this thesis as follows:(1) Mechanics analysis model is built to describe the fracture of mismatching welded joint with HAZ crack. Taking into account of the non-uniform of organization, mechanical property and geometric property of weldments, the thesis introduces the theory of fracture mechanics parameter of mismatching welded joints, based on the"tri-material"structure assumption for the mismatching welded joints referring to an idealized "bi-material" model. A validation is carried out by FE analysis and experiments. Mechanics analysis model is built to describe the fracture of mismatching welded joint with HAZ crack.(2) Solution of limit load is obtained for mismatching welded joint with HAZ crack based on. the "tri-material"simplified model of mismatching welded joints. The effect of mechanical property and geometry size of heat affected zone is considered in this research throughout an elastoplastic finite element analysis. Some solutions of plastic yield loads are given for a middle crack tensile plate taken from the mismatching welded joint with crack in HAZ.(3) Effect of mismatching on HAZ crack driving force is acquired from the researches for various mechanical and geometric properties of welded joints. HAZ crack driving force is determined based on the "tri-material" structure assumption, and the distribution of crack driving force is give to show the effect of inhomogeneous condition on the fracture behaviour of crack in an accurately known heat affect zone characteristics.(4) An inhomogeneous dependent failure assessment diagram is created for the mismatching welded joints with HAZ cracks. The failure assessment diagram of welded structure is constructed on the foundation of R6. The non-dimensional parameters, Kr and Lr, and equivalent material curve of stress-strain are introduced for the mismatching welded joints. The inhomogeneous dependent failure assessment curves are based on an equivalent material, taking into account of the factors including mismatch of materials, geometry property of heat affect zone. The engineering assessment procedures are established for mismatching welded joints with HAZ crack.An engineering application is performed using the inhomogeneous dependent failure assessment diagram referring to R6 procedures, The inhomogeneous dependent failure assessment curves of mismatching welded joints with HAZ crack are used to failure assessment of pressure vessel with welded joints. The assessment procedures of a pressure vessel is carried our for the mismatching welded joints with HAZ crack. The structure intergrity assessment procedures are constructed to improve the structure intergrity assessment technology for mismatching welded joints.