Results on Attenuation of Neutron Embrittlement Effects

A carefully designed irradiation experiment was conducted in which a 180-mm thick reactor pressure vessel (RPV) wall has been simulated using eighteen 10-mm slices of some key RPV steels and irradiated under test reactor conditions to investigate the through wall attenuation of neutron embrittlement. Results from two of the irradiated materials (a low copper content plate and a high copper content Linde 80 flux weld) were reported in 2006. Another RPV plate, the international reference steel, JRQ, was also extensively irradiated in the simulated RPV wall. Comparisons of predicted attenuation changes in toughness properties using measured Charpy V-notch results are presented for the JRQ steel and compared to the results from the low copper content plate and the high copper content Linde 80 weld metal. Also, Charpy V-notch and Master Curve fracture toughness test results are compared for the low copper plate and the high copper weld. Predictions are made of through-wall attenuation following the practice defined in ASTM E 900-02 and Regulatory Guide 1.99, Revision 2, in which the attenuation of high energy neutron fluence (E > 1 MeV) is projected based upon an approximate displacements per atom (dpa) change through the wall thickness.. The resultant degree of material damage using this dpa-based fluence change is estimated using the ASTM E 900–02 embrittlement correlation model and compared to the experimental data.

Technical Basis for Proposed Code Case of Using a Master Curve in Lieu of the Code KIc Curve in ASME Boiler and Pressure Vessel Code

The Master Curve method for determination of fracture toughness in the transition range in ASTM standard E1921 [1] brought an opportunity for the ASME Code to adopt a much better fracture toughness curve based on directly measured fracture toughness data. This also enables obtaining statistically based fracture toughness data. The industry, through PVRC Task Group (subsequently Section XI Task Group on Master Curve Fracture Toughness), took a two-phase approach to implement the adoption of the Master Curve method in the ASME Code. First, Phase I was completed with the issuance of ASME Code Cases N-629/N-631 [9, 10], published in 1999 which allowed the existing Code KIc curve to be used by means of an alternate indexing reference temperature RTT0. This provided an important new approach to allow material specific, measured fracture toughness curves for ferritic steels in the code applications. However, this only rectified part of the shortcomings of the present Code KIc curve. In Phase II, it is intended to develop a direct means to utilize a tolerance bound of the Master Curve itself in place of the ASME KIc curve. This paper summarizes a proposal for such a procedure whereby a Master Curve fracture toughness tolerance bound is made usable in the ASME flaw evaluation processes, i.e. in Appendix A and Appendix G to Section XI of the ASME Boiler and Pressure Vessel Code. A draft code case is presented in Appendix in this paper.

Experiment Studies on Fracture Energy of Cement Paste and Mortar

As cement-based composite, concrete can be properly represented by three phases in microstructure: cement paste, aggregate as well as interfacial transition zone between them. As the matrix compositions of concrete, fracture properties of cement paste and mortar have great influence on fracture performance of concrete, and fracture energy is an important parameter for concrete non-linear fracture mechanics research. Therefore, three-point bending beams of cement paste and mortar with different sizes and strengths were tested. A complete load-deflection (P-δ) curve was directly obtained, and a fit was made for the tail of the P-δ curve using power and exponential function, respectively. And then the fitting results of the two function were compared. It was found that with the increase of specimen size the influence of tail curve on fracture energy is decrease，and considering the influence of tail curve, fracture energy of cement paste and mortar are size-independent. The discussion on the obtained results was made.

Effect of storage in aqueous environments on polymer–metal interfacial fracture

The effect of environmental exposure on the fracture characteristics of two polymer–metal interfaces was measured using a four-point bend delamination test. Films of high-molecular-weight polymethylmethacrylate (PMMA) resin were spin-cast on metal-coated silicon wafer substrates, for which the metal was either titanium or aluminum. Sandwich beam specimens were fabricated and stored in one of the following conditions prior to the bending tests: vacuum desiccator at 25 °C, vacuum desiccator at 65 °C, distilled water at 25 °C, or distilled water at 65 °C. Load–displacement behavior measured during bending revealed significant differences between the delamination characteristics of the two PMMA–metal interfaces after vacuum exposure which were eliminated after the degrading effects of water exposure. Three distinctive load–displacement behaviors were observed: plateau, fracture at a single load; R-curve, fracture at increasing load with crack extension; and stick-slip, cycles of gradual load increase and sudden load drop with crack extension. PMMA–Al samples stored in vacuum desiccator at 25 °C exhibited R-curve fracture and the largest average crack driving force, GC, 12.2 (±0.5) J/m2, of all samples tested. After storage in 25 °C water, these PMMA–Al samples exhibited stick-slip fracture and GC decreased to 7.1 (±2.6) J/m2; storage in 65 °C water further decreased GC to 2.1 (±0.7) J/m2. PMMA–Ti samples exhibited stick-slip fracture after storage in vacuum desiccator at 25 °C, with an average GC of 8.0 (±2.6) J/m2; storage in 65 °C water resulted in a transition to plateau fracture and a decrease in GC to 1.6 (±0.3) J/m2. The initial difference and subsequent similarity are interpreted in terms of surface roughness and hydrolysis, respectively.

Application of Master Curve Data for Reactor Vessel Steels

The Master Curve method has been developed to determine fracture toughness of a specific material in the brittle-to-ductile transition range. This method is technically more descriptive of actual material behavior and accounts for the statistical nature of fracture toughness properties as an alternative to the current ASME Code reference toughness curves. The Master Curve method uses a single temperature, To, as an index of the Master Curve fracture toughness transition temperature. This method has been successfully applied to numerous fracture toughness data sets of pressure vessel steels contained in the Master Curve database, including the beltline materials for the Kewaunee reactor pressure vessel. The database currently contains over 5,500 toughness data records for vessel weld, plate and forging materials, and it is currently being updated to include more recent fracture toughness data. Application of Master Curve fracture toughness data to reactor pressure vessel (RPV) integrity evaluations requires some assumptions relative to the degree of constraint in the fracture toughness test specimens versus the actual assumed RPV flaw. An excessive degree of conservatism can be introduced if the constraint levels are substantially different. In performing a Master Curve evaluation, the analysis may be restricted by the type of fracture toughness data available. Any excess conservatism should be appropriately considered when the overall safety margin is applied. For example, the precracked Charpy three-point bend specimen actually has some advantages over the compact tension specimen when the application involves a shallow surface flaw in a RPV wall. This paper analyzes some key fracture toughness results from several weld data sets containing both unirradiated and irradiated data to evaluate constraint effects in fracture toughness and pre-cracked Charpy specimens. The evaluated To values were compared to determine if there is any difference in bias from specimen geometry between the unirradiated and irradiated data.

Development of Fracture Toughness Reference Curves

A large base of KIC, KId and JIC (R-curve) fracture toughness data has been used to develop reference toughness curves. The most successful results were obtained when a sigmoidal function was fitted to data from which the heat-heat variation in both the temperature and fracture toughness had been reduced by referencing. Several referencing procedures have been studied, but the only one found to be ‘successful in this work was based upon the precracked instrumented Charpy V-notch test. The tanh function K=A+BtanhT−T0C (K = toughness, T = temperature, and A, B, T0 and C are coefficients which give the best fit between curve and data) fitted to precracked instrumented Charpy V-notch test data provided suitable referencing quantities. Using the coefficients A and B to reference fracture toughness, and T0 and C to reference temperature, lower bound reference curves were developed. Weighted, nonlinear regression procedures were used to define lower bound reference toughness curves for each of three stress intensification rates. The lower bound was the statistical global tolerance bound to the referenced data. The reference curves can be readily used to define a lower bound relationship between fracture toughness and temperature for nuclear pressure vessel steel on the basis of a set of precracked instrumented Charpy V-notch tests.