Unified In-Plane And Out-of-Plane Constraint Parameter Based On Crack-Tip Opening Displacement And Its Application Study

To develop new fracture assessment methodology for structures to incorporate both in-plane and out-of-plane constraints,it is desirable to develop unified constraint parameters convenient for engineering calculation and application.Based on finite element numerical calculation combined with experimental data and theoretical analysis,in this paper,a new constraint parameter Ad based on crack-tip opening displacement has been investigated.The main work and conclusions are as follows:(1)The new constraint parameter Ad based on crack-tip opening displacement is the same as unified constraint parameter Ap based on crack-tip equivalent plastic strain.They can both qualify in-plane and out-of-plane constraints,and the unified correlation line between ductile fracture toughness Jc/Jref and parameter Ad can be established.The correlation line Jc/Jref-Ad is independent on the specimen type and the selection of reference specimen.The parameter Ad based on CTOD has clear physical and geometric meaning,and it can be easily measured and determined in FEM analyses.Therefore,the Ad may be an appropriate engineering unified constraint parameter.(2)In a wide range of in-plane and out-of-plane constraints under brittle fracture,parameter Ad and brittle fracture toughness KJc/Kref or Jc/Jref data can form a correlation line with very small data scatter.This indicates Ad is the same as Ap,and they can characterize both in-plane and out-of-plane constraints under brittle fracture.(3)The nonnalized fracture toughness Jc/Jref and Ad data can form a correlation line for cracks in welded joint specimens with different thickness and crack depth.This indicates Ad can characterize both geometry constraint(including in plane and out-of-plane constraints)and material constraint.Compared with Ad taken from middle plane,a better correlation line of Jc/Jref with average Ad along specimen thickness can be established.Because average Ad along specimen thickness can better reflect the entire constrain level of a specimen.(4)Constraint parameter Ad for specimens with various constraints increases with the increase of hardening index n,and decreases with the increase of yield strength σ0.In the range of lower n and lower yield strength σ0,n and σ0 influence the value of Ad remarkably.However,in the range of higher n and higher yield strength σ0,n and σ0 have a little influence on the value of Ad.(5)The research provides two basic methods incorporating J-Ad parameters,crack drive force(CDF)and failure assessment diagram(FAD),to predict the fracture toughness of specimens with different constraints.Based on the CDF and FAD methods incor:porating the unified constraint parameter Ad,specimen fracture toughness can be predicted more accurately.By incorporating the unified constraint parameter Ad in the fracture assessment of actual structures,excessive conservatism in the traditional assessment using single parameter can be reduced.