ASTM E1921 Standard Research Articles

Effect of material inhomogeneity on constraint loss measured with subsized C(T) fracture specimens of a reactor pressure vessel steel

In this study, the fracture behavior of the low-alloy reactor pressure vessel JRQ steel was investigated in the ductile-to-brittle transition region with two different subsized compact tension fracture specimens: 0.18T C(T) and 0.5T C(T). The specimens were extracted from two thin plates (surface and middle) of the large JRQ steel plate, which is well-known to be macroscopically inhomogeneous in terms of brittleness. In particular, the reference temperature T0 as defined in the ASTM-E1921 standard was calculated for the two plates with the 0.18T C(T) and 0.5T C(T) specimens. A significant difference in the reference temperature T0 of more than 90 °C as determined with the 0.5T C(T) data was found between the two plates indicating a clear macroscopic inhomogeneity between the two depths. The measured toughness of 0.18T C(T) specimens and corresponding T0 evaluation showed that the effect of constraint loss was more pronounced for the middle plate due to its lower strength and that the constraint loss effect can be confounded with inhomogeneity. A local approach to fracture model was applied to account for the constraint loss by calibrating a criterion based on critical stressed volume V* and critical stress σ* obtained by finite element simulations. A big difference in the local fracture stress σ* between the two plates was found and based on the deduced local criterion it was shown that the measured 0.18T toughness can be effectively predicted within the temperature range of the ductile-to-brittle transition.

Influence of fatigue precracking and specimen size on Master Curve fracture toughness measurements of EUROFER97 and F82H steels

EUROFER97 and F82H steels are two leading reduced-activation ferritic-martensitic (RAFM) steels for fusion first wall and blanket applications. Exposure to the harsh environment of fusion reactors can result in severe degradation of fracture toughness. Thus, the post-irradiation evaluation of fracture toughness is critical for understanding the material behavior. Due to the space constraints of irradiation facilities and challenges in controlling a uniform irradiation condition for large size specimens, the development of small specimen test techniques (SSTT) is indispensable to evaluate the performance of irradiated materials. In this study, we evaluated specimen size effects on the Master Curve fracture toughness of EUROFER97 and F82H steels. A wide variety of specimens, including 0.5 T compact tension (C(T)) specimens, 0.16 T mini-compact tension (miniC(T)) specimens, and 1.65 mm miniature bend bar specimens, were tested. The testing methodology was based on the Master Curve method in the ASTM E1921 standard. No specimen size effect was observed in 0.5 T C(T) and 0.16 T miniC(T) specimens on the Master Curve reference temperature T0, while 1.65 mm miniature bend bar specimens yielded a higher T0Q. A strong effect of fatigue precracking on T0 for 0.5 T C(T) and 0.16 T miniC(T) specimens was observed, such that testing on specimens with skewed fatigue precrack fronts resulted in lower T0 than for specimens with ASTM standard qualified straight fatigue precrack fronts. The results highlight the importance of experimental quality control in developing SSTT for Master Curve fracture toughness testing. Lastly, we also evaluated and provided recommendations on the minimum number of specimens needed for each specimen type for yielding reliable T0Q values.

A method for directly measuring fracture toughness and determining reference temperature for RPV steels by Charpy impact test

Accurately realizing the use of Charpy impact test to measure fracture toughness and determine reference temperature of low alloyed steel with ferritic microstructure has important practical engineering significance for the structural integrity assessment of reactor pressure vessels. In this paper, a methodology for measuring fracture toughness and determining reference temperature for RPV steels by Charpy impact test was proposed. The method referred to the ASTM E1820 and E1921 standard. By analyzing the physical meaning of the Charpy impact specimen’s fracture, load-time curve and size parameters, the fracture toughness of the material was directly calculated according to the linear elastic fracture mechanics. The reference temperature was calculated using the multi-temperature procedure formula. The reference temperature and fracture toughness were corrected according to two factors, such as the difference in fracture toughness caused by the difference in the loading rate of the stress intensity factor and the specimen shape and size between Charpy impact test and compact tension test. And the validity of the fracture toughness and reference temperature was evaluated. The corrected two are basically consistent with the data obtained by the compact tension test in accordance with ASTM E1820 and E1921 standards. This method may be used to solve the problem that the nuclear power plant needs to use the impact test data to evaluate the fracture toughness and reference temperature of the reactor pressure vessel in the structural integrity assessment of the reactor pressure vessel operation and life extension process.

Fracture toughness evaluation of 20MnMoNi55 pressure vessel steel in the ductile to brittle transition regime: Experiment & numerical simulations

In this work, fracture behaviour of 20MnMoNi55 reactor pressure vessel (RPV) steel in the ductile to brittle transition regime (DBTT) is characterised. Compact tension (CT) and single edged notched bend (SENB) specimens of two different sizes were tested in the DBTT regime. Reference temperature ‘T0’ was evaluated according to the ASTM E1921 standard. The effect of size and geometry on the T0 was studied and T0 was found to be lower for SENB geometry. In order to understand the fracture behaviour numerically, finite element (FE) simulations were performed using Beremin's model for cleavage and Rousselier's model for ductile failure mechanisms. The simulated fracture behaviour was found to be in good agreement with the experiment.

A study on the ASTM E1921 standard in determining the fracture toughness of ferritic steels

ABSTRACTOne of the most important aims of the fracture mechanics is to determine the fracture toughness of a material. Various methods were developed for this purpose and have been still used nowadays. In the J‐integral method that is one of them, providing of a dominant linear elastic condition on the specimen is not required. However, in ferritic steels, the fracture toughness values (KJC) obtained by the J‐integral method show some inconsistencies. Therefore, the ASTM E1921 standard was developed on ferritic steels, which are instabilities in the values of elastic or elastoplastic fracture toughness. In this study, a new method was used to determine the fracture toughness (KIC) of ferritic steels, and it was compared with the standard. Three steels with different mechanical properties and average grain size were investigated in this study.

Procedure for Uncertainty Estimation in Determining the Master Curve Reference Temperature1

The master curve reference temperature, T0, characterizes the fracture performance of structural steels in the ductile-to-brittle transition region. For a given material, this reference temperature is estimated via fracture toughness testing. A methodology is presented to compute the standard error of an estimated T0 value from a finite sample of toughness data, in a unified manner for both single temperature and multiple temperature test methods. Using the asymptotic properties of maximum likelihood estimators, closed-form expressions for the standard error of the estimate of T0 are presented for both test methods. This methodology includes statistically rigorous treatment of censored data, which represents an advance over the current ASTM E1921 methodology (“E1921-10, Standard Test Method for Determination of Reference Temperature, T0, for Ferritic Steels in the Transition Range,” ASTM International, West Conshohocken, PA, 2010). Through Monte Carlo simulations of realistic single temperature and multiple temperature test plans, the recommended likelihood-based procedure is shown to provide better statistical performance than the methods in the ASTM E1921 standard.

A new method for determining the fracture toughness of main pipeline steels

ABSTRACTOne of the fundamental aims of fracture mechanics is to define fracture toughness KIC of a material. Hence, the ASTM E399 standard was developed. However according to the standard, large‐sized specimens are required to determine the fracture toughness of low alloy carbon steels. ASTM E1921 standard was developed on the fracture toughness of ferritic steels. In this study, a new method was proposed to determine the fracture toughness of ferritic steels. The purpose of the present paper is to compare the results of the method with the experimental results. Two steels that are used in gas and oil main pipelines were investigated in this study.

Prediction of Temperature Dependence and Scatter in Fracture Toughness of Pressure Vessel Steel using Nonlocal Damage Models

Abstract The pressure vessel and piping components may be subjected to operation in the ductile-to-brittle transition regime of the material due to increase in the transition temperature. This increase can be due to irradiation embrittlement, vario s other material aging and degradation mechanisms taking place in a nuclear reactor environment. From fracture mechanics point of view, fracture toughness of the material must be adequate to prevent the failure. However, there is considerable scatter observed in the fracture toughness data and the analyst must account for this in the safety analysis. Master curve approach according to ASTM E1921 standard is popularly employed for this purpose. However, it requires the data for transition temperature T0, which is dependent upon the specimen geometry and loading configurations employed in the laboratory tests. Its transferability to safety analysis of components is questionable. Towards this objective, the authors have recently developed a nonlocal formulation for the Rousselier's damage model and combined this with the Beremin's model to predict the variation of fracture toughness and its scatter in the DBTT regime for the standard compact tension specimens. In this work, application of this new procedure to different types of geometry and loading conditions has been explored.

Evaluation of fracture toughness and its scatter in the DBTT region of different types of pressure vessel steels

Safety analysis of critical components and the overall plant is a major and important task in the field of mechanical engineering. Therefore, it is essential to know the allowable loads and the corresponding failure behaviour (e.g., crack initiation, growth and instability) of the component. Most of the safety critical components, such as pressure vessels of nuclear reactors, are made of ferritic grade low-alloy steels. When these components operate in the ductile-to-brittle transition (DBTT) regime of the material (due to irradiation embrittlement and other material aging and degradation mechanisms taking place), the two mechanisms that compete with each other in the failure process are: ductile and cleavage fracture. From fracture mechanics point of view, fracture toughness of the material must be adequate to prevent the failure. However, there is considerable scatter observed in the fracture toughness data and the analyst must account for this in the safety analysis. Master curve approach according to ASTM E-1921 standard is popularly employed for this purpose. However, it requires the data for transition temperature T 0, which is dependent upon the specimen geometry and loading configurations employed in the laboratory tests. Its transferability to safety analysis of components is questionable. In this work, a combined model for ductile and cleavage fracture is used to predict the fracture toughness scatter and its variation with temperature in the DBTT range. It is demonstrated that the above data for fracture toughness can be predicted once we know the material stress–strain data at different temperatures and a single set of Weibull statistics parameters for cleavage fracture. Extensive experimental investigations have been carried out on two types of pressure vessel steels in the DBTT region using different kinds of specimens to validate the predictions of the model.

Fracture toughness master-curve analysis of the tempered martensitic steel Eurofer97

We report fracture toughness data for the reduced activation tempered martensitic steel Eurofer97 in the lower to middle transition region. The fracture toughness was measured from tests carried out on 0.35 T and 0.87 T pre-cracked compact tension specimens. The data were first analyzed using the ASTM E1921 standard. The toughness–temperature behavior and scatter were shown to deviate from the ASTM E1921 standard predictions near the lower shelf. Using the method of maximum likelihood, the athermal component of the master-curve was calculated to better fit the data from the lower to the middle transition region. We showed that these master-curve adjustments are necessary to make the T o values obtained near the lower shelf with 0.35 T C( T) specimens consistent with those obtained in the middle transition region with 0.87 T C( T) specimens.

Predicting reference temperature from instrumented Charpy V-notch impact tests using modified Schindler procedure for computing dynamic fracture toughness

Reactor pressure vessel (RPV) steels are increasingly being characterised in terms of the reference temperature T 0 and the associated Master Curve (MC) Procedure, following the ASTM E-1921 standard. Though correlations have been proposed to predict the T 0 from Charpy transition temperature T 28J or instrumented impact test parameters like T 4kN, none can be taken as a universal correlation. Here we are proposing a new correlation of T 0 with T 0Sch dy, where T 0Sch dy is the reference temperature corresponding to a median K Id=100 MPa√m evaluated by the ASTM E1921 procedure applied to K Id vs T data, and K Id has been calculated from instrumented CVN impact test data using modified Schindler relations. This will provide a reliable method for determining T 0 from instrumented CVN tests alone. T 0Sch dy provides a conservative alternative to T 0 dy for application of the ASTM E 1921 MC procedure in dynamic situations. Since the above procedure depends only on instrumented CVN data, it will be less costly to apply (no precracking is necessary) and will also obviate the difficulties associated with determining T 0 dy from precracked CVN testing (because of severe size limitations, associated scatter and signal oscillations from the mechanics of the test, there needs to be precise control over test temperatures and test velocity for obtaining valid data from limited number of specimens). The RT NDT(est) from the suggested procedure (or its modifications based on future work) will provide an acceptable alternative to RT NDT for application of the ASME K IR curve based on instrumented CVN tests alone. For low-uppershelf steels, the new reference temperature estimate T 0.075 and its correlation to T 0Sch dy will provide a methodology for application of MCs to such steels. Further comprehensive work is needed to validate the procedures and correlations suggested in this paper.

On the Conversion of K - to J -Characterization in Linear Elastic 3D Configurations

An equation for converting characterization of 3D fields in cracked plates in terms of the stress intensity factor to the one in terms of J-integral is developed. It can be used in applications such as finite element codes and identification of the adequate “effective” elastic modulus in the ASTM E1921 standard. We consider standard specimens — plates that can be thin or thick as compared to the in-plane dimension. It is found that the actual specimen compliance lies in-between the ones corresponding to the 2D plane stress and 2D plane strain approximations. However, the near tip field that governs the K to J conversion is predominantly plane strain for the studied cases. Another finding is that the geometrical factor in 2D, f(a/W), for standard specimens is different from the one obtained on the basis of a 3D analysis.

A compendium of sources of fracture toughness and fatigue crack growth data for metallic alloys. III: Hudson, C.M. and Seward, S.K. Int. J. Fract. Apr. 1989 39, (4), r43–R63

Accurately realizing the use of Charpy impact test to measure fracture toughness and determine reference temperature of low alloyed steel with ferritic microstructure has important practical engineering significance for the structural integrity assessment of reactor pressure vessels. In this paper, a methodology for measuring fracture toughness and determining reference temperature for RPV steels by Charpy impact test was proposed. The method referred to the ASTM E1820 and E1921 standard. By analyzing the physical meaning of the Charpy impact specimen’s fracture, load-time curve and size parameters, the fracture toughness of the material was directly calculated according to the linear elastic fracture mechanics. The reference temperature was calculated using the multi-temperature procedure formula. The reference temperature and fracture toughness were corrected according to two factors, such as the difference in fracture toughness caused by the difference in the loading rate of the stress intensity factor and the specimen shape and size between Charpy impact test and compact tension test. And the validity of the fracture toughness and reference temperature was evaluated. The corrected two are basically consistent with the data obtained by the compact tension test in accordance with ASTM E1820 and E1921 standards. This method may be used to solve the problem that the nuclear power plant needs to use the impact test data to evaluate the fracture toughness and reference temperature of the reactor pressure vessel in the structural integrity assessment of the reactor pressure vessel operation and life extension process.