Effect of Plastic Constraint and Cladding on Semi-Elliptical Shaped Crack in Fracture Toughness Evaluation for a Reactor Pressure Vessel Steel

In the structural integrity assessment of a reactor pressure vessel (RPV), the fracture toughness (KJc) should be higher than the stress intensity factor at the crack tip of a semi-elliptical shaped under-clad crack (UCC), which is prescribed in JEAC4206-2016. However, differences in crack depth and existence of cladding between the postulated crack and fracture toughness test specimens would be affected to the plastic constraint state and KJc evaluation. In this study, we performed fracture toughness tests and finite element analyses to investigate the effect of plastic constraint and cladding on the semi-elliptical shaped crack in KJc evaluation. The apparent KJc value evaluated at the deepest point of the crack exceeded 5% fracture probability based on the Master Curve method estimated from C(T) specimens, and the conservativeness of the current integrity assessment method was confirmed. Few initiation sites were observed along the tip of semi-elliptical shaped crack other than the deepest point. The plastic constraint state was also analyzed along the crack tip, and it was found that the plastic constraint at the crack tip near the surface was lower than that for the deepest point. Moreover, it was quantitatively showed that the UCC decreased the plastic constraint. The local approach suggested higher KJc value for the UCC than that for the surface crack, reflecting the low constraint effect for the UCC.

Constraint Effect On Fracture Behavior of Underclad Crack in Reactor Pressure Vessel

According to JEAC4206-2016, in the structural integrity assessment of a reactor pressure vessel (RPV), the fracture toughness (KJc) should be higher than the stress intensity factor at the crack tip of a postulated underclad crack (UCC) near the inner surface of the RPV during a pressurized thermal shock event. Previous analytical studies show that the plastic constraint for UCC is lower than that for surface crack. Consequently, the apparent KJc for UCC is expected to be higher than that for surface crack. In this study, we performed three-point bending fracture toughness tests and finite element analyses (FEAs) for RPV steel containing a UCC or a surface crack to quantitatively investigate the effect of cladding on the plastic constraint and subsequent KJc evaluation. From the tests, we found that the apparent KJc for the UCC was considerably higher than that for the surface crack. Such a high KJc could be explained by the lower plastic constraint parameters, such as T-stress and Q-parameter, of the UCC compared with those for the surface crack. Additionally, local approach analysis showed that the KJc for the UCC was significantly higher than the master curve estimated from the fracture toughness tests using compact tension specimens.

Plastic Constraint-Matched Pin-Loaded SENT Specimen for Fracture Analysis of Radially Growing Longitudinal Cracks in Thin-Walled Piping

In recent years increased attention has been given to the transferability of standardized fracture test data to thin-walled pressurized equipment. For smaller diameter piping, for example, standard test specimens loaded in “hoop” tension are not possible due to pipe curvature and thinner wall thickness (often less than 1/3 inch). Further, it is not clear that standardized fracture specimens produce transferrable results to the seam weld fracture problem due to low plastic constraint, which is exacerbated in thin wall pipe. Single-edge notched tension (SENT) specimens were fabricated by extracting a segment pipe wall and finishing to a parallel piped incorporating nearly all the wall thickness. These segments were electron beam welded to low carbon steel grip ends to produce only localized heat affected zone without affecting the test section. The result is a nearly ideal pin-loaded SENT specimen for radial fracture testing. This geometry was evaluated with 2D and 3D nonlinear finite element analysis in Abaqus, and plastic constraint matching was evaluated compared to a thin-walled pipe under internal pressure with a radially growing crack. A nearly ideal plastic constraint match between the pin-loaded SENT and radial seam weld crack was confirmed up to about 500 psi-in, which is substantially beyond a typical JIc in the X52 steel used in the analysis. Thus, pinned SENT testing is recommended for evaluation of radial fracture in thin-walled piping.

Validation on the Relationship Between J Integral and CTOD for Offshore Structural Steel Weldments by Experimental and Numerical Analyses

Elastic plastic fracture mechanics (EPFM) is the domain of fracture analysis which considers extensive plastic deformation at crack tip prior to fracture. J integral and crack tip opening displacement (CTOD) have been commonly used as parameters for EPFM analysis. The relationship between these parameters has been extensively studied by industry and academia. The plastic constraint factor can serve as a parameter to characterize constraint effects in fracture involving plastic deformation. Therefore, the characteristics of plastic constraint factor are important in EPFM analysis. In this study, the relationship between J Integral and CTOD was investigated by conducting fracture toughness tests using single edge notched bend (SENB) specimens. Also, plastic constraint factor was investigated by using finite element analysis. Numerical analysis was carried out using ABAQUS elastic-plastic analysis mode.

Estimation of Constraint Factor on the Relationship Between J Integral and CTOD for Offshore Structural Steel Weldments

As ships and offshore structures become larger than before the usage of high strength steel and ultra thick plate has been increased. However, the thick plates have disadvantages with respect to brittle fracture and fatigue strength. Elastic plastic fracture mechanics (EPFM) is the domain of fracture analysis which considers extensive plastic deformation at crack tip prior to fracture. The J integral and crack tip opening displacement (CTOD) have been commonly used as parameters for EPFM analysis. The relationship between these parameters has been studied by industry and academia. The plastic constraint factor can serve as a parameter to characterize constraint effects in fracture. Therefore, the characteristics of plastic constraint factor are important in EPFM analysis. In this study, the relationship between J Integral and CTOD was investigated by conducting 3-point bending tests using single edge notched bend (SENB) specimens. Two types of specimens were fabricated. One is API 2W Gr.50 welded by means of flux cored arc welding (FCAW) with heat input 15kJ/cm, and the other is API 2W Gr.50 welded by means of submerged arc welding (SAW) with heat input 45kJ/cm. The plastic constraint factor was estimated considering heat input, temperatures at weldment and heat affected zone (HAZ).