Shallow Flaws Under Biaxial Loading Conditions—Part I: The Effect of Specimen Size on Fracture Toughness Values Obtained From Large-Scale Cruciform Specimens1

Abstract

A technology to determine shallow-flaw fracture toughness of reactor pressure vessel (RPV) steels is being developed. This technology is for application to the safety assessment of RPVs containing postulated shallow-surface flaws. It has been shown that relaxation of crack-tip constraint causes shallow-flaw fracture toughness of RPV material to have a higher mean value than that for deep flaws in the lower transition temperature region. Cruciform beam specimens developed at Oak Ridge National Laboratory (ORNL) introduce far-field, out-of-plane biaxial stress components in the test section that approximates the nonlinear stresses resulting from pressurized-thermal-shock (PTS) loading of an RPV. The biaxial stress component has been shown to increase stress triaxiality (constraint) at the crack tip, and thereby reduce the shallow-flaw fracture toughness enhancement. The cruciform specimen permits controlled application of biaxial loading ratios, resulting in controlled variation of crack-tip constraint. An extensive matrix of intermediate-scale cruciform specimens with a uniform depth surface flaw was previously tested and demonstrated a continued decrease in shallow-flaw fracture toughness with increasing biaxial loading. This paper describes the test results for a series of large-scale cruciform specimens with a uniform depth surface flaw. These specimens were all of the same size with the same depth flaw and were tested at the same temperature and biaxial load ratio (1:1). The configuration is the same as the previous set of intermediate-scale tests, but has been scaled upward in size by 150 percent. These tests demonstrated the effect of biaxial loading and specimen size on shallow-flaw fracture toughness in the lower transition temperature region for RPV materials. For specimens tested under full biaxial (1:1) loading at test temperatures in the range of 23°F (−5°C) to 34°F (1°C), toughness was reduced by approximately 15 percent for a 150-percent increase in specimen size. This decrease was slightly greater than the predicted reduction for this increase in specimen size. The size corrections for 1/2T C(T) specimens did not predict the experimentally determined mean toughness values for larger size shallow-flaw specimens tested under biaxial (1:1) loading in the lower transition temperature region.

In-plane and out-of-plane constraint effects under pressurized thermal shocks

“…Extensive experiments were also performed in the Oak Ridge National Laboratory to study the mixed effect of in-plane and out-of-plane constraint (McAfee et al, 2001;Williams et al, 2001). This work consisted of two sets of experiments on cleavage fracture of A533B steel.…”

Reduction of measured toughness due to out‐of‐plane constraint in ductile fracture of aluminium alloy specimens

“…The mixed effect of low in‐plane and high out‐of‐plane constraint has received some attention, for example notable experiments were performed in the Oak Ridge National Laboratory 22–26 . The Oak Ridge National Laboratory work consisted of two sets of experiments on cleavage fracture of A533B steel.…”

“…It is clear then that there is uncertainty in the effectiveness of biaxial loading on shallow cracks to demonstrate an effect of out‐of‐plane loading. Some researchers believe they found an effect of biaxial loading 22–28 whereas others observed no change 29,30,32 . Because most of the experimental investigations were performed using FPB cruciform specimens in bending, shallow cracks had to be introduced in the specimens which led to a mixed effect of low in and high out‐of‐plane constraint.…”

In‐plane and out‐of‐plane constraint for single edge notched bending specimen and cruciform specimen under uniaxial and biaxial loading

“…The effect of T z on 3D crack-front fields and fracture toughness were systematically studied by Guo. [25][26][27] In addition, the mixed effect of low in-plane and high out-of-plane constraint have received some attention 28,29 : it was found that the shallow crack effect is reduced if the specimen was tested under high outof-plane constraint conditions. Now, 3D effects has been received much attention.…”