Unified Characterization Parameter Of In-plane And Out-of-plane Constraint And Its Correlation With Fracture Toughness Of Materials And Welded Joints

As constraint significantly affect the fracture behavior and fracture toughness of materials and structures, it needs to accurately consider the constraint effect in the integrity assessment of the structures. Constraint can be divided into two types, i.e. in-plane and out-of-plane, and each of them has its own characterization parameter. However, there exists a compound of both in-plane and out-of-plane constraints for three-dimensional (3D) crack in actual engineering structures. How to quantify both types of constraints by using a unified constraint parameter and to establish its correlation with fracture toughness of materials and welded joints is one of the most important foundational problems in the structural integrity area.In this dissertation, by using the combined approaches of the numerical simulations, experiments and theoretical analyses, the unified characterization parameter of in-plane and out-of-plane constraint and its correlation with fracture toughness of materials and welded joints was investigated. The application of the unified constraint parameter in the elastic-plastic structural integrity assessment containing the constraint effect was preliminarily analyzed. The main work and results obtained are as following:(1) Based on the area surrounded by the ep isoline ahead of crack tips, a unified constraint parameter Ap which can characterize both in-plane and out-of-plane constraints is defined. There exists a sole linear relation between the normalized fracture toughness Jc/Jref and(?) regardless of in-plane or out-of-plane constraint or both. The J(?)line is independent on the selection of the εp isolines, and only dependent on material. It characterizes the effect of different in-plane and out-of-plane constraints on fracture toughness of a material.(2) Further studies show that, the Ap is also a unified constraint parameter which can characterize both geometry constraint (in-plane and out-of-constraint) and material constraint in the welded joints. Based on the fracture tests under different in-plane/out-of-plane and material constraints and the calculation of Ap, the unified J(?) lines of interface cracks located at the weakest region in the dissimilar metal welded joint are established. It characterizes the combining effect of in-plane/out-of-plane and material constraints on fracture toughness of welded joint. (3) The mechanism of both in-plane and out-of-plane constraints effects on fracture toughness and fracture rsistance of a dissimilar metal welded joint in nuclear power plant is investigated by fracture tests and microscopic observations. It is found that with increasing in-plane and out-of-plane constraints, the fracture mode of the dissimilar metal welded joint changes from ductile fracture through mixed ductile and brittle fracture to brittle fracture, and the corresponding crack growth resistance (J-R curve) decreases. The crack growth path generally towards the material sides with lower strength.(4) There exists interaction between the in-plane constraint and the out-of-plane constraint. The higher in-plane constraint strengthens the out-of-plane constraint effect, while the lower in-plane constraint is not sensitive to the out-of-plane constraint. The finite element simulations based on the GTN damage model can be used to obtain the J-R curve and ductile fracture toughness of a material under different in-plane and out-of-plane constraints. Thus the Jc/Jref-(?) line of a material can be obtained by a small number of tests combined with the numerical simulations based on GTN damage model.(5) A preliminary research in the application of the unified constraint parameter Ap in the structural integrity assessment is carried out, and it is found that the parameter (?) which is a compound of J and Ap can predict the fracture location along a3D crack front in the structures. A method combining the J-(?) two parameter with the J(?) line of a material for predicting the fracture toughness and failure load of3D circumferential surface cracks in pipes are preliminarily studied by finite element calculations.