Charpy V-notch Data Research Articles

Algorithms to Estimate the Ductile to Brittle Transition Temperature, Upper Shelf Energy, and Their Uncertainties for Steel Using Charpy V-Notch Shear Area and Absorbed Energy Data

Toughness and the transition from ductile to brittle behavior are long-standing concerns for applications of ferritic steel such as line-pipe. Three algorithms have been developed to fit a hyperbolic tangent curve to any Charpy V-notch dataset and estimate the uncertainty for (1) the 85% shear appearance area transition temperature and (2) the upper shelf absorbed energy. To fit the hyperbolic tangent curve to the data the (I) first algorithm relied on iterative estimation of four-parameters; (II) the second algorithm on two parameters (after simplification based on physical assumptions); and (III) the third algorithm on only one parameter (after further simplification). The algorithms were written using the open-source programing language, R. The minimum input requirements for the algorithm are experimental data for shear appearance area and absorbed energy from at least four temperatures for the four-parameter algorithm, two temperatures for the two-parameter algorithm, and one temperature for the one-parameter algorithm. The test temperatures and quantity of tests at each temperature can vary. The algorithms are described in detail and demonstrated using a data set of 12 Charpy test results (shear area and absorbed energy) from one API-5L grade X52 pipe with 4.5 mm thick Charpy bars. A future paper will test and compare the algorithms using a wide variety of Charpy V-notch data sets to clarify their applicability and possible limitations.

Assessment of the Generalization Ability of the ASTM E900-15 Embrittlement Trend Curve by Means of Monte Carlo Cross-Validation

The standard ASTM E900-15 provides an analytical expression to determine the transition temperature shift exhibited by Charpy V-notch data at 41-J for irradiated pressure vessel materials as a function of the variables copper, nickel, phosphorus, manganese, irradiation temperature, neutron fluence, and product form. The 26 free parameters included in this embrittlement correlation were fitted through maximum likelihood estimation using the PLOTTER—BASELINE database, which contains 1878 observations from commercial power reactors. The complexity of this model, derived from its high number of free parameters, invites a consideration of the possible existence of overfitting. The undeniable goal of a good predictive model is to generalize well from the training data that was used to fit its free parameters to new data from the problem domain. Overfitting takes place when a model, due to its high complexity, is able to learn not only the signal but also the noise in the training data to the extent that it negatively impacts the performance of the model on new data. This paper proposes the resampling method of Monte Carlo cross-validation to estimate the putative overfitting level of the ASTM E900-15 predictive model. This methodology is general and can be employed with any predictive model. After 5000 iterations of Monte Carlo cross-validation, large training and test datasets (7,035,000 and 2,355,000 instances, respectively) were obtained and compared to measure the amount of overfitting. A slightly lower prediction capacity was observed in the test set, both in terms of R2 (0.871 vs. 0.877 in the train set) and the RMSE (13.53 °C vs. 13.22 °C in the train set). Besides, strong statistically significant differences, which contrast with the subtle differences observed in R2 and RMSE, were obtained both between the means and the variances of the training and test sets. This result, which may seem paradoxical, can be properly interpreted from a correct understanding of the meaning of the p-value in practical terms. In conclusion, the ASTM E900-15 embrittlement trend curve possess good generalization ability and experiences a limited amount of overfitting.

A Comparison of the tanh and Exponential Fitting Methods for Charpy V-Notch Energy Data

In the 1960s and 1970s, when the surveillance programs for currently operating commercial nuclear reactors were established, state of knowledge limitations resulted in the use of Charpy-V notch (CVN) specimens rather than fracture toughness specimens. Reasonable success has since been achieved in correlating CVN and fracture toughness parameters. Such correlations provide an important part of the technical basis for both current regulations and ASME codes. These correlations imply that trends manifest in CVN data must also appear in fracture toughness data, even though empirical evidence demonstrate that this is not always true. For example, the temperature dependence of CVN energy (CVE) in transition is thought to be a unique feature of each specific sample of ferritic steel that is tested, a view in sharp contrast with the now widely accepted view of a “master curve” for transition fracture toughness (KJc). Also, effects of product form on CVE temperature dependence and property correlations are widely reported, despite the fact that product form effects are absent from KJc properties. These observations suggest that the mapping of CVE behavior onto fracture toughness implicit to correlation-based regulations and ASME codes may produce erroneous trends in estimated values of fracture toughness. In this paper we investigate the hypothesis that the apparent differences between CVE and fracture toughness arise due to differences in how the temperature dependence of CVE and KJc data have historically been modeled. Our analysis shows that when CVE data are analyzed in a manner consistent with KJc data (i.e., transition and upper shelf data are partitioned from each other and analyzed separately rather than being fit with a continuous tanh function), the apparent differences between CVE and toughness characterizations are minimized significantly, and may disappear entirely. These findings demonstrate the differences between CVE and fracture toughness data to be an artifact of the tanh analysis method rather than an intrinsic property of CVE.

Effects of microstructure on inverse fracture occurring during drop-weight tear testing of high-toughness X70 pipeline steels

The effects of microstructure on inverse fracture occurring in the hammer-impacted region were analyzed after conducting a drop-weight tear test (DWTT) on high-toughness pipeline steels. Three kinds of steels were fabricated by varying the alloying elements, and their microstructures were varied by the rolling conditions. The pressed-notch (PN) or chevron-notch (CN) DWTT and Charpy V-notch (CVN) impact tests were conducted on the rolled steel specimens, and the results were discussed in comparison with the data obtained from CVN tests of prestrained specimens. In the hammer-impacted region of the DWTT specimens, abnormal inverse fracture having a cleavage fracture mode appeared, and the inverse fracture area correlated well with the upper-shelf energy (USE) obtained from the CVN test and with the grain size. The steel specimens having a higher USE or having coarse polygonal ferrite tended to have a larger inverse fracture area than those having a lower USE or having fine acicular ferrite. This was because steels having a higher impact absorption energy required higher energy for fracture initiation and propagation during the DWTT. These results were confirmed by the CVN data of prestrained steel specimens.

Influence of dynamic strain aging on the ductile tearing of C–Mn steels: modelling by a local approach method

C–Mn steels and associated welds can be susceptible to dynamic strain aging (DSA). In this case, fracture toughness passes through a minimum when the temperature increases from 20 to 300 °C. Since Charpy V-Notch data are not affected by the DSA phenomenon, the method for predicting the evolution of J 0.2 and d J/d a in the temperature domain where DSA occurs is still an open question. The purpose of the present study is the assessment of this decrease in fracture toughness using a local approach method. The results of J– R tearing resistance tests, characterized by crack initiation resistance J 0.2 and tearing modulus d J/d a relative to a base metal (A48 French standard steel) and manual metal arc deposited metals are presented and discussed. The local approach determination of J 0.2 was performed using the Rice and Tracey model. Comparisons between experimental results and the results of local approach modelling indicate that both J 0.2 and d J/d a decreases with DSA can be correctly predicted.

Estimating Charpy transition temperature shift using Weibull analysis

A primary consideration for operation of commercial nuclear light-water reactors is the fracture toughness of the reactor pressure vessel. One of the tests conducted for toughness determinations of the pressure vessel materials is the Charpy V-notch impact test. The tests are conducted over a range of test temperatures and the total impact energy required to break each specimen is determined and plotted vs test temperature. The impact energy values for a fixed temperature and radiation exposure are usually assumed to be Weibull distributed. This assumption was validated by a statistical selection process. An important parameter in the evaluation of reactor vessel materials is termed the ductile-to-brittle transition temperature shift (ΔT 4) and is the temperature shift between the unirradiated and irradiated Charpy V-notch curves at the 41-J (30-ft-lb) energy level. The purpose of this paper is to introduce a methodology for estimating the ΔT 41 value and to place limits on the error of estimation. This has been accomplished by assuming that the scale parameter in the Weibull distribution is a function of temperature (and neutron fluence for the irradiated Charpy V-notch data). An inverse regression technique is then used to solve for the temperature corresponding to an impact energy of 41 J. To obtain error bounds on this estimate, the parameters of the equation describing the scale parameter are resampled assuming a normal distribution and the inverse regression is repeated several times. This leads to several estimates of the temperature, and the variance of these values can be used to give bounds on the error of estimation.

Recent Progress in Subsized Charpy Impact Specimen Testing for Fusion Reactor Materials Development

The current approach to developing structural materials for fusion reactors requires a heavy reliance on the use of small irradiation specimens largely because of limitations in available irradiation volumes. The miniature Charpy-V-notch specimen (MCVN) is one of several techniques currently under development. It has particular potential application in addressing problems associated with fracture mode transition in ferritic/martensitic steels. The authors have previously reported a technique for testing MCVNs that are one-third the size in all dimensions of a standard Charpy-V-notch (CVN) specimen. This involves an instrumented drop tower and a computerized data acquisition/processing system. We have now tested a variety of ferritic steels using MCVN specimens and compared the data to CVN data obtained by ourselves or others. These materials include A302B correlation monitor material, A508 forgings, Ht-9, a high-copper A533B weld, and an A710 copper-bearing steel heat-treated to four conditions. These heat treatments result in age hardening by copper precipitation and correspond to underaged, peak-hardened, and overaged conditions. The MCVN test appears to provide data that are both qualitatively and quantitatively similar to CVN data. The most promising approach to applying MCVN tests, in our opinion, is to extract fundamental property data. Even so, there are a number ofmore » differences between MCVN and CVN tests that remain to be sorted out.« less

Some advances in fracture studies under the heavy-section steel technology program

Recent results are summarized from HSST studies in three major areas that relate to assessing nuclear reactor pressure vessel integrity under pressurized-thermal-shock (PTS) conditions. These areas are irradiation effects on the fracture properties of stainless steel cladding, crack run-arrest behavior under non-isothermal conditions, and fracture behavior of a thick-wall vessel under combined thermal and pressure loadings. Since a layer of tough stainless steel weld overlay cladding on the interior of a pressure vessel could assist in limiting surface crack extension under PTS conditions, its resistance to radiation embrittlement was examined. A stainless steel overlay cladding, applied by a submerged arc, single-wire, oscillating-electrode method, was irradiated to 2 × 10 23 neutrons/m 2 (> 1 MeV) at 288°C. Yield strength increases up to 27% and a slight increase in ductility were observed. Charpy V-Notch data showed a ductile-to-brittle transition behavior caused by temperature-dependent failure of the 8-ferrite phase. The type 308 cladding, microstructurally typical of that in reactor pressure vessels, showed very little degradation in either upper-shelf energy or transition temperature due to irradiation. Crack-arrest behavior of A533 grade B class 1 steel was examined for temperatures extending above the onset of Charpy upper-shelf. Crack-arrest experiments that use wide-plate specimens have shown crack arrest occurring prior to transition to tearing or tensile instability. High values of crack-arrest toughness have been recorded (static values above 400 MPa m that are well above the maximum value that safety assessment criteria assume such materials can exhibit. A validation experiment was performed by exposing an intentionally flawed HSST intermediate test vessel to combined pressure and thermal transients. The experiment addressed warm-prestressing phenomena, crack propagation from brittle to ductile regions, and crack stabilization in ductile regions. Test and analysis results are summarized.

Adequacy of estimates and variability of fracture-related properties for reactor pressure vessel materials

It is sometimes necessary to estimate K Ic , NDTT, or RT NDT based on other types of test data because actual toughness results are not available. Correlations used to estimate these toughness parameters are often based on minimal Charpy V-notch data. This paper evaluates the adequacy of correlating Charpy V-notch data with fracture toughness parameters for two pressure vessel materials and weldments. Statistically based correlations or ‘engineering judgments’ were used to establish a conservative prediction criterion.