Correlation Of In-plane And Out-of-plane Constraints With Brittle Fracture Toughness And Fracture Assessment Of Structures Incorporating Constraint

The brittle fracture is the most dangerous failure mode of structural components which needs to be avoided.In cracked mechanical structures,both in-plane and out-of-plane constraints exist simultaneously.In order to conduct integrity assessment of the structures accurately,it needs to consider theeffects of the two constraints on fracture behavior of the structures.Thus,the unified correlation of the in-plane and out-of-plane constraints with brittle fracture toughness of materials needs to be investigated and the method of structural integrity assessment incorporating both in-plane and out-of-plane constraints needs to be developed.In this dissertation,based on a large amount of experimental data of specimens with different in-plane and out-of-plane constraints in the literature and combining extensive three-dimensional finite element calculations of different constraint parameters,the capabilities of the new constraint parameter A_p for characterizing in-plane and out-of-plane constraints and establishing a unified correlation of in-plane and out-of-plane constraints with brittle fracture toughness of materials have been investigated by comparative analyzes with the conventional constraint parameters?T,A₂,Q,T_z and h?based on crack-tip stress fields.On this basis,by conducting extensive three-dimensional finite element analyses of constraints in pressurized pipes with circumferential inner surface cracks,the fracture assessment methodology incorporating the unified constraint parameter A_p has been investigated.The correlation of constraint parameter A_p with the reference temperature T₀ in master curve approach was explored.The main work and results obtained are as followings:?1?The conventional constraint parameters?T,A₂,Q,T_z and h?can mainly describe the in-plane constraint or out-of-plane constraint separately.The new constraint parameter A_p based on crack-tip equivalent plastic strain can characterize a wide range of in-plane and out-of-plane constraints under brittle fracture condition.The unified correlation lines(K_Jc/Kref-???and J_c/J_ref-???relation lines)can be formed between the A_p and brittle fracture toughness Kjc or Jc of different alloy steels,and they are independent on the selections of the ep isolines and reference specimen.By fitting the unified correlation lines,the K_Jc/Kref-???and J_c/J_ref-???relation formulae have been obtained.?2?For the circumferential inner surface cracks in pressurized pipes,the distributions of constraint level along crack fronts described by different constraint parameters are different.The maximum T-stress constraint for the most pipe cracks occurs at the locations near the free surfaces,but the maximum Q and A_p constraints for the most pipe cracks occur at the deepest part along crack fronts.With increasing crack depth a/t and length c,the constraint levels of the pipe cracks increase,but after a/t>0.3,they are insensitive to a/t and a/c.?3?Based on the K-A_p two-parameter concept,by calculating the K factor and the unified constraint parameter A_p of the pipe cracks and adopting the constraint-dependent fracture toughness,the fracture assessment methodology based on parameter A_p for incorporating both in-plane and out-of-plane constraints has been proposed on the basis of the FAD-based fracture assessment.The results show that the fracture assessment based on the unified constraint parameter A_p can reduce the degree of conservat:ism in conventional assessments,and the benefits from the unified constraint assessments increase with decreasing crack depth and length.?4?The studies of correlations of different constraint parameters with the master curve reference temperature T₀ show thatgood T/?0-T₀ and QBLA-T₀ correlations cannot be established due to larger data scatter.Good correlation between the A_p and T₀ can be established,and the???-T₀correlation lines and formulae of three different alloy steels for making nuclear pressure vessels have been obtained.