Critical fracture stress and stress modified fracture strain models are utilized to describe the variation of lower and upper shelf fracture toughness with temperature and strain rate for two alloy steels used in the manufacture of nuclear pressure vessels, namely SA533B-1 (HSST Plate 02) and SA302B (Surveillance correlation heat). Both steels have been well characterized with regard to static and dynamic fracture toughness over a wide range of temperatures (-190 to 200~ although valid JIc measurements at upper shelf temperatures are still somewhat scarce. The present work utilizes simple models for the relevant fracture micromechanisms and local failure criteria to predict these variations in toughness from uniaxial tensile properties. Procedures are discussed for modelling the influence of neutron fluence on toughness in irradiated steel, and predictions are derived for the effect of increasing fluence on the variation of lower shelf fracture toughness with temperature in SA533B-1.

Based on a large body of test and power reactor data, empirical relationships between irradiation strengthening and embrittlement are derived. It is shown that the Charpy V-notch (Cv) 41-J indexed transition temperature increases and the upper-shelf energy decreases systematically with increases in the yield stress. The transition temperature shifts are related to two mechanisms: (1) increases in the maximum temperature of elastic-cleavage fracture, and (2) decreases in the slope of the Cv energy versus test temperature curve associated with reductions in the upper-shelf energy. The cleavage shift contribution, which is usually dominant, can be predicted from the initial temperature of fracture at general yield and the change in ambient temperature static yield stress. In developing this simplified cleavage fracture model, it is shown that: (a) yield stress changes are independent of temperature and strain rate; (b) the increase in yield stress with decreasing temperature is independent of the strain rate, irradiation, and metallurgical state; and (c) the microcleavage fracture stress is independent of irradiation and temperature. A semi-empirical procedure for estimating the shift contribution due to upper-shelf energy decreases and the total temperature shift at 41 J, based on the observation of an approximately constant temperature interval of the transition regime, is proposed, along with a method for forecasting the entire irradiated Cv curve.

No abstract

The problem of obtaining a meaningful value of toughness from small Charpy-size surveillance specimens, tested at temperature corresponding to the upper shelf where ductile fracture predominates, is investigated. Following the procedures of Green and Knott for measurement of crack opening displacements at initiation of ductile fracture, a test procedure is adopted in which small precracked Charpy-size bend specimens are side-grooved to increasing depths and tested to failure under both quasi-static and dynamic loading rates. Values of the J-contour integral at maximum load (Jmax) for specimens side-grooved in excess of 30 percent are found to agree, within acceptable limits, with “valid” initiation JIc fracture toughness values determined independently using multi-specimen resistance-curve techniques. Three nuclear pressure vessel materials (two base metals, SA533B-1 and SA302B, and a submerged arc weld metal) were evaluated at temperatures between 71 and 177° C, which correspond to upper shelf temperatures. The test procedure described offers a simple, inexpensive, small specimen compromise for estimating the fracture toughness at the onset of ductile fracture from a single Charpy-size bend test piece for both quasi-static and dynamic loading rates. This approach could be readily adopted in nuclear surveillance programs for toughness evaluation of unirradiated and neutron irradiated pressure vessel steels.

No abstract