Cohesive Zone Modeling Approach for Crack Extensions in Small Laboratory Specimens of Conventionally Forged 21-6-9 Stainless Steels, Additively Manufactured 304 Stainless Steels and Irradiated Zr-2.5Nb Pressure Tube Materials
Creators: name: Wu, Shengjia
Publisher: name: My University
Year: 2021
Subjects: subject: cohesive zone modeling
subject: small laboratory specimen
subject: hydrogen effect
subject: varying cohesive parameters
subject: crack extension
subject: Mechanical Engineering
subject: FOS: Mechanical engineering
subject: Engineering

description: Due to the limited number of small laboratory specimens and high costs of specimen preparation, computational simulations of the fracture toughness test or JIC test for the small laboratory specimens should be carried out for correlation and validation of the experimental data for future computational prediction models. The crack extensions in the small laboratory specimens are three-dimensional due to the specimen sizes do not satisfy the linear elastic fracture mechanics (LEFM) plane strain requirement and ASTM fracture toughness test requirement. Therefore, cohesive zone modeling approach with varying cohesive parameters should be adopted for two-dimensional finite element analyses to fit the experimental data. Crack extensions in small laboratory specimens, arc-shaped tension (A(T)) specimens of conventionally forged (CF) 21-6-9 stainless steels, single edge bend (SE(B)) specimens of additively manufactured (AM) 304 stainless steels and curved compact tension (CCT) specimens of irradiated Zr-2.5Nb pressure tube (PT) materials, are investigated by finite element analyses with the cohesive zone modeling (CZM) approach and the nodal release method. A user-defined cohesive element subroutine is developed with the smooth trapezoidal traction-separation law. A procedure for the selection of fixed and varying cohesive parameters for small laboratory specimens is developed based on the stationary crack model and growing crack model with the nodal release method, respectively. The computational results of the two-dimensional finite element analyses with the CZM of A(T) specimens of uncharged, hydrogen-charged, and tritium-charged-and-decayed CF 21-6-9 stainless steels and SE(B) specimens of uncharged and hydrogen-charged AM 304 stainless steels suggest that the cohesive parameters should vary as functions of the crack extension to account for the three-dimensional effect and the change of the constraint conditions along the growing crack front. The maximum near-tip opening stresses and separation work rates obtained from the two-dimensional finite element analyses with the nodal release method provide references for the varying cohesive parameters. The three-dimensional finite element analyses with the nodal release method also reveal the change of constraint conditions with increasing crack extension based on the change of stress-triaxiality and out-of-plane stress along the growing crack front for the SE(B) specimens. For CCT specimens of irradiated and hydrided irradiated Zr-2.5Nb PT materials, the orthotropic Hill yield function is adopted to account for the plastic anisotropy of the pressure tubes due to the fabrication process. The results of the two-dimensional and three-dimensional finite element analyses with the nodal release method suggest that three-dimensional effect also exists for crack extension in the CCT specimens. Varying cohesive energies are needed for the two-dimensional plane stress and plane strain finite element analyses with the CZM to account for the three-dimensional effect. The computational load-displacement, load-crack extension, crack extension-displacement, and J-R curves with the selected varying cohesive parameters compare very well with the experimental data from the small laboratory specimens in this investigation. The lower best-fit varying cohesive parameters of the hydrogen-charged specimens compared with those of the uncharged specimens indicate the hydrogen effect on the lower J-R curves. The lower best-fit varying cohesive energies of the hydrided irradiated specimens compared with those of the irradiated specimens indicate the effect of fractured circumferential hydrides on the lower J-R curves.