Donald E. Mccabe

Technical Basis for the Master Curve Concept of Fracture Toughness Evaluations in the Transition Range

An American Society for Testing and Materials (ASTM) standard method (E 1921-97) has been developed that exclusively uses fracture mechanics test practices and advanced statistical methods to establish the ductile-to-brittle transition range of fracture toughness for structural steels. The development of suitably accurate analyses had been slowed in the past due to an incomplete understanding of the operational mechanisms that control the fracture toughness behavior of structural steels. New perspectives taken are (1) that dominant linear-elastic conditions need not be rigidly enforced in specimens and (2) that the effect of specimen size on fracture toughness performance is mostly controlled by a weakest-link mechanism instead of being completely controlled by crack tip constraint conditions. The weakest-link behavior is defined from local cleavage crack initiators such as precipitates, inclusions, and grain boundary embrittlement, namely, all microstructural features in steel. Statistical models can be built upon such mechanisms that result in defined fracture probability levels and, when coupled to a master curve concept, can more accurately define the true location of the ductile-to-brittle transition temperature. An integral part of the ASTM test standard development work has been the production of a supporting technical basis document. This document presents substantial background data and supporting theoretical aspects that have been used to justify the method development. The paper will include some of the salient features presented.

Applicability of the Fracture Toughness Master Curve to Irradiated Highly Embrittled Steel and Intergranular Fracture

The Heavy-Section Steel Irradiation Program at Oak Ridge National Laboratory has evaluated a submerged-arc (SA) weld irradiated to a high level of embrittlement and a temper embrittled base metal that exhibits significant intergranular fracture relative to representation by the Master Curve. The temper embrittled steel revealed that the intergranular mechanism significantly extended the transition temperature range up to 150°C above To. For the irradiated highly embrittled SA weld study, a total of 21 1T compact specimens were tested at five different temperatures and showed the Master Curve to be nonconservative relative to the results, although that observation is uncertain due to evidence of intergranular fracture.

Experimental Evaluation of Deformation and Constraint Characteristics in Precracked Charpy and Other Three-Point Bend Specimens

To enable determination of the fracture toughness reference temperature, T0 , with reactor pressure vessel surveillance specimens, the precracked Charpy (PCVN) three-point bend, SE(B), specimen is of interest. Compared with the 25-mm (1 in.) thick compact, 1TC(T), specimen, tests with the PCVN specimen (10×10×55 mm) have resulted in T0 temperatures as much as 40°C lower (a so-called specimen bias effect). The Heavy-Section Steel Irradiation (HSSI) Program at Oak Ridge National Laboratory developed a two-part project to evaluate the C(T) versus PCVN differences, (1) calibration experiments concentrating on test practices, and (2) a matrix of transition range tests with various specimen geometries and sizes, including 1T SE(B) and 1TC(T). The test material selected was a plate of A533 grade B class 1 steel. The calibration experiments included assessment of the computational validity of J-integral determinations, while the constraint characteristics of various specimen types and sizes were evaluated using key curves and notch strength determinations. The results indicate that J-integral solutions for the small PCVN specimen are comparable in terms of J-integral validity with 1T bend specimens. Regarding constraint evaluations, Phase I deformation is defined where plastic deformation is confined to crack tip plastic zone development, whereas Phase II deformation is defined where plastic hinging deformation develops. In Phase II deformation, the 0.5T SE(B) BxB specimen (slightly larger than the PCVN specimen) consistently showed the highest constraint of all SE(B) specimens evaluated for constraint comparisons. The PCVN specimen begins the Phase II type of deformation at relatively low KR levels, with the result that KJc values above about 70 MPa√m from precracked Charpy specimens are under extensive plastic hinging deformation. For the second part, about twenty specimens of each type and size (65 PCVN) have been tested to enable statistically reliable comparisons of T0 for the various cases, with tests completed at temperatures from −28 to −37°C. The KJc test data obtained for HSST Plates 13B and 13A show PCVN bias values from −30 to −40°C relative to 1TC(T), but the standard 1TSE(B) specimen shows a bias of about −10°C. However, the SE(B) specimens exhibit a tendency for decreasing T0 with decreasing specimen size (B and/or b). The results are compared with those from other materials and observations are noted regarding potential effects of test temperature and metallurgical factors.

Analysis of warm prestress data

Loading a cracked structure at elevated temperature, or warm prestressing (WPS), enhances its fracture resistance at a lower temperature. Five data sets, comprising 119 unclad pressure vessel steel specimens, were combined to derive correlations for WPS-enhanced fracture toughness (K I f r a c ) in the absence of ductile tearing. New WPS test results for 27 surface-flawed specimens, eight subclad-flawed specimens, and five strain-aged specimens are discussed. K I f r a c exceeded non-WPS fracture toughness, K I c , for all experiments. The WPS data showed that no specimens failed while K was decreasing, and that at least an additional 7% additional reloading from the minimum value of applied K 1 took place prior to final fracture. The data included complete and partial unloading after WPS prior to final fracture. Crack tip three-dimensional elastic-plastic finite element (3DEPFE) analysis was performed to support statistical analysis of the data. Regression models were compared with the Chell WPS model. Crack tip 3DEPFE analysis indicated that partially unloaded and completely unloaded data should be treated separately, and that the amount of unloading is unimportant for partially unloaded data. The regression models, which use K 1 at WPS (K I w p s ) and K I c as independent variables, better represented the WPS benefit than did the more complicated Chell model. An adequate accounting was made for constraint in the WPS experiments. The subclad flaw data support the use of the partial unload regression model provided that some care is taken to represent the effect of intact cladding if present. The effect of strain aging at or below 260°C (500°F) on the WPS benefit was of no consequence for the pressure vessel steels and WPS temperatures used to derive the regression models. The presence of ductile tearing precludes the use of the regression models. The regression model for partial unloading accurately predicted the behavior of full-scale pressure vessel WPS experiments. All but one of the 174 experiments considered lie above the lower 2σ estimate of the regressions. The experiments all supported Type I WPS, i.e., there was no fracture during cooling until reloading occurred. However, the regression equations apply to the reload and are inapplicable to Type I WPS.