Studies On Load-independent Unified Constraint Parameter Of In-Plane And Out-of-Plane Constraint And Its Application

In this paper,by using the finite element numerical calculations of J integral and crack tip opening displacement(CTOD)of a large number of specimens with different geometries and sizes(different in-plane and out-of-plane constraints),combining with experimental data of fracture toughness of materials and theoretical analysis,a load-independent unified constraint parameter Ad*of in-plane and out-of-plane constraint based on CTOD and its application have been mainly investigated.The research work and main conclusions obtained are as follows:(1)Based on the unified constraint parameter Ad,a load-independent unified constraint parameter Ad*of in-plane and out-of-plane constraint has been defined.Through the finite element calculation of Ad*of specimens with different geometries and sizes,the load independence and its ability of the parameter Ad*to simultaneously characterize in-plane and out-of-plane constraints have been verified.(2)Based on a large number of three-dimensional finite element analyses,the solutions of the parameter Ad*have been obtained for four types of specimens(C(T),SEN(B),SEN(T),and M(T))with a wide range of different sizes.The influences of specimen thickness ratio B/W,specimen width W and crack depth a/W on the constraint parameters Ad*of different specimens have been verified.It provides bases for designing standard and non-standard specimen with different constraint levels,establishing constraint-dependent fracture toughness of materials,realizing the conversion of fracture toughness data of specimens with different geometries and sizes,and comparison and correlation of constraint between specimens and structures.(3)A monotonous correlation curve between ductile and brittle fracture toughness Jc/Jref data of different specimens and alloy steels and the constraint parameter Ad*can be formed,which further indicating that the parameter Ad*is a load-independent unified constraint parameter that can simultaneously characterize both in-plane and out-of-plane constraints.The mathematical correlation formula between the fracture toughness Jo/Jref and constraint parameter Ad*for different alloy steels have been obtained by fitting the correlation data.(4)A monotonous correlation curve between the fracture toughness Jc/Jref of specimens with different constraints and the constraint parameter Ad*for cracks in dissimilar metal welded joints can also be formed,which indicates that the constraint parameter Ad*can simultaneously incorporate the in-plane and out-of-plane geometric constraint and material constraint.Mathematical correlation formula between the fracture toughness Jc/Jref and the parameter Ad*of cracks at different positions of welded joints have been obtained by fitting the correlation data.(5)Based on the parameters Ad*and the experimental J-R resistance curves of specimens with different constraints of nuclear pressure vessel A508 steel and X80 pipeline steel,a method for constructing constraint-dependent J-R resistance curves of materials have been proposed.The mathematical equations of the constraint-dependent J-R resistance curves of the two steels have been obtained,and the prediction accuracy from equations for the J-R resistance curves of specimens with different constraints has been verified.The constructed J-R resistance curve equations can be used in the assessments of ductile tearing and crack propagation stability of structures(such as pressure vessels,pipelines,etc.)to improve the accuracy of the traditional assessment without considering constraint effect.