Anwendung des instrumentierten Kerbschlagbiegeversuchs zur Ermittlung von Referenztemperaturen nach dem Master‐Curve‐Konzept

Application of the Instrumented Impact Test for the Determination of Reference Temperatures Using the Master Curve Concept The instrumented impact test is suitable for the determination of fracture mechanical parameters. In this paper the determination of the dynamic fracture toughness values in the lower ductile‐to‐brittle transition region is presented. The fracture toughness is determined at the onset of cleavage fracture and evaluated by the Master Curve (MC) concept. The MC concept allows to quantify the variation of fracture toughness with the temperature within the lower ductile‐to‐brittle transition region. Limit curves of fracture toughness for defined failure probabilities and a reference temperature can be determined using this method. This paper presents the application of the master curve concept to the reference temperature determination through the thickness of reactor pressure vessel (RPV) steel plate. The reference temperatures determined dynamic fracture toughness values (T₀^dy) are compared with quasi‐static reference temperatures (T₀^st) and Charpy‐V transition temperatures (TT). T₀^dy, T₀^st and TT increase from the surface to the middle of the RPV steel plate. Compared with T₀^st, the T₀^dy values are higher approximately 70 to 90 K.     

Comparison of ASME K IR curve and dynamic master curve in determining ductile–brittle transition temperature of A48P2 steel

Abstract

The ductile–brittle transition temperature (DBTT) of grade A48P2 steel is characterised based on the American Society of Mechanical Engineers (ASME) fracture toughness K _IR curve and dynamic master curve approaches. The indexing parameter for the K _IR curve, reference temperature RT_NDT, is determined from drop weight and Charpy tests to be ?45°C. The dynamic master curve is constructed following ASTM standard E1921 guidelines; however, instead of precracked tests, the dynamic fracture toughness K _Jd is determined from Charpy V notch tests using a modified Schindler's procedure. A Weibull plot is constructed using the K _Jd data, and it is found that the points comply reasonably with the forced fit line of slope 4. The reference temperature for constructing the dynamic master curve, termed T^dy,Sch₀, thus determined is ?56°C. The ASME K _IR curve is shown to be conservative compared with the dynamic master curve constructed using T^dy,Sch₀.     

Radiation-Induced Embrittlement of WWÉR-440 Reactor Pressure Vessel Steel under Loading

The results of evaluating the reference temperature T ₀ are presented and Master Curve is constructed from the experimental data for steel 15Cr2MoVA (base metal of WWÉR-440 reactor pressure vessel) in three states: unirradiated, irradiated, and irradiated under stress. It is shown that mechanical load, which simulates the coolant pressure, accelerates radiation embrittlement, whose contribution is comparable to that of neutron irradiation.     

Correlation between Charpy impact energy and J fracture toughness for thermally embrittled reactor pressure vessel steel

Abstract

The Charpy impact energies of a reactor pressure vessel steel in the as received and several thermally embrittled conditions have been tentatively correlated to three J fracture toughness parameters derived under quasi-static loading regimen. Very good correlation has been achieved over the whole fracture resistance range of the structural steel, as obtained by the application of special heat treatments. It has been established that the parameters controlling the impact energy absorption capacity of the materials are the equivalent grain size of dual phase (ferrite/bainite) annealed microstructures and the bainite packet size of single phase quenched and tempered materials. The dependence of the Charpy impact energy of precracked, side grooved bend bars on the representative grain size of the microstructures tested has been disclosed as a Hall-Petch relationship.     

Measurement of fracture initiation toughness and crack resistance in instrumented Charpy impact testing

A new method has been developed involving direct measurement of the load-line displacement during instrumented Charpy testing. The method uses a laser interferometer to measure displacement in addition to the load-line displacement derived from the load signal. Tests were conducted using fatigue precracked and V-notched test pieces in the temperature range +23°C to −80°C on a conventional ship grade steel, a pressure vessel steel and two welded joints. Good correlation was found between the J_0.2 initiation fracture toughness determined by the multi-specimen method and the J_i fracture toughness determined from single specimens using the new method to detect ductile fracture initiation.     

Fracture characterisation of a nuclear vessel steel under dynamic conditions in the transition region

The Master Curve (MC) methodology, originally proposed by Kim Wallin, is a standardised engineering tool for analysing the fracture toughness of ferritic steels in the ductile to brittle transition (DBT) region by means of the reference temperature T₀. This temperature is normally estimated from quasi-static fracture toughness tests, nevertheless, it has been recently extended to the determination of dynamic fracture toughness. The aim of the present contribution is to characterise the fracture resistance in the DBT region under high strain rate conditions by applying the MC methodology to the steel of the Santa María de Garoña Spanish nuclear power plant (NPP). In this sense, 15 Charpy instrumented tests were performed on pre-cracked specimens from the surveillance program of the plant. The dynamic reference temperature, T_0,dyn, was obtained and compared with the quasi-static reference temperature, T_0,sta. The reliability of a semi-empirical formula proposed by Wallin to obtain T_0,dyn from T_0,sta has been analysed for this material.     

Local approach to fracture-based prediction of reactor pressure vessel lifetime

New version of the local approach to fracture is presented. Within the framework of this approach a new methodology is developed, which provides prediction of radiation life time of a reactor pressure vessel not by ultimate shift of the Charpy critical temperature, ΔT_K, or by reference temperature, ΔT₀, but by the condition of brittle fracture initiation of irradiated metal ahead of a crack tip in reactor pressure vessels. __________ Translated from Problemy Prochnosti, No. 1, pp. 53–60, January–February, 2009.     

Optimum use of broken Charpy specimens from surveillance programs for the application of the master curve approach

Broken Charpy specimens can be used in various ways to determine the reference temperature, T ₀, as defined in the Master Curve approach. Reconstitution and machining of subsize specimens are two solutions for direct toughness evaluation. The optimum technique and specimen geometry preferentially: use a minimum number of broken half Charpys, have a small uncertainty on the reference temperature, have the widest valid test temperature window, have a reasonable cost, and are easy to perform into hot cells. In this study, the use of various miniature specimen types such as Compact (C(T)), Single Edge notch Bend, (SE(B)), or Miniature Precracked Charpy (MPCCv), and Circumferentially Cracked Round Bars (CRB), is investigated. As this study deals with small specimens, the problem of loss of constraint is also addressed. Taking all constraints into account, a 0.16T–C(T) specimen is found to be the more promising geometry. This work is supported by experimental data generated on various specimen types and sizes on the well-characterised reactor pressure vessel steels 22NiMoCr37, 20MoNiCr55 and JRQ.     

Study of the beltline weld and base metal of WWER-440 first generation reactor pressure vessel

We present experimental results of the circumferential core weld SN0.1.4 and the base metal ring 0.3.1 of the reactor pressure vessel from the Unit 1 of the Greifswald WWER-440/230. The investigated trepans represent the irradiated–annealed– reirradiated (IAI) condition. The working program is focused on the characterization of the reactor pressure vessel steels through the reactor pressure vessel wall. The key part of the testing is aimed at the determination of the reference temperature T0 following the ASTM Test Standard E1921 to determine the fracture toughness in different thickness locations.     

Analysis of dynamic conditions during thermal transient events for the structural assessment of a nuclear vessel

The possibility of dynamic loading conditions in a reactor pressure vessel (RPV) has been investigated in this paper. For this purpose, finite element (FE) numerical simulations of several thermal transients were performed including a normal shut down and two accidental thermal shocks, namely a loss of coolant accident (LOCA) and an extreme postulated pressurised thermal shock (PTS). The aim of the present contribution is to evaluate the influence of the loading rate on the fracture properties of the vessel steel of the Santa María de Garoña Spanish nuclear power plant (NPP) in the ductile to brittle transition (DBT) region. To describe the fracture behaviour of the steel in the DBT region, the master curve (MC) reference temperature, T₀, was used. This temperature is normally used for quasi-static conditions; however, it has been recently extended to the determination of dynamic fracture toughness by means of a phenomenological model proposed by Wallin. The dynamic reference temperature, T_0,dyn, was obtained for the loading rates corresponding to the three studied situations numerically simulated and compared with the quasi-static reference temperature, T_0,sta. From these results, conclusions about the importance of loading rates in nuclear vessels were established.     

Quantifying Tstress controlled constraint by the master curve transition temperature T0

Specimen size, crack depth and loading conditions may effect the materials fracture toughness. In order to safeguard against these geometry effects, fracture toughness testing standards prescribe the use of highly constrained deep cracked bend specimens having a sufficient size to guarantee conservative fracture toughness values. One of the more advanced testing standards, for brittle fracture, is the master curve standard ASTM E1921-97, which is based on technology developed at VTT Manufacturing Technology. When applied to a structure with low constraint geometry, the standard fracture toughness estimates may lead to strongly over-conservative estimate of structural performance. In some cases, this may lead to unnecessary repairs or even to an early “retirement” of the structure. In the case of brittle fracture, essentially three different methods to quantify constraint have been proposed, J small scale yielding correction, Q-parameter and the T_stress. Here, a relation between the T_stress and the master curve transition temperature T₀ is experimentally developed and verified. As a result, a new engineering tool to assess low constraint geometries with respect to brittle fracture has been obtained.     

SCK-CEN Contribution to the IAEA Round Robin Exercise on WWER-440 RPV Weld Material: Irradiation,Annealing, and Re-Embrittlement

Mechanical properties of WWER-440 RPV weld joints have been studied with account of different states of the material: baseline (unirradiated), irradiated up to the average fast neutron fluence of 6.5 10¹⁹ n/cm², irradiated and eventually annealed, re-irradiated with the accumulated fast neutron fluence of 1.3 10²⁰ n/cm², Tensile, impact fracture, and fracture toughness tests were performed for each state of the material with the use of Charpy specimens (standard, reconstituted, and pre-cracked).     

On Improvement of Methods for Experimental Determination of Strength Characteristics of NPP Component Materials

The methods for experimental determination of strength characteristics as per applicable standards have been reviewed. For some structural steels used in NPP facilities, the influence of loading parameters and specimen geometry are allowed for during the assessment of static fracture toughness (K_Ic, J_Ic). Recommendations are given on the setting of cycling conditions for fracture toughness testing of standard specimens with and without crack-guiding lateral grooves. The authors substantiate the applicability of the Master Curve method to determination of fatigue strength of small specimens with subsequent use of the results for calculating brittle fracture resistance of reactor pressure vessel materials in the welded joint.     

Fracture toughness of grade D ship steel

Results from fracture mechanics tests on 15 mm thick grade D ship steel and weld are organised into a toughness distribution indexed to the Charpy 27 joule temperature, T_27J. The tests are carried out at 300 MPa√m/s to simulate the strain rate appropriate to a long (≈1 m) through thickness crack in the deck of a ship under storm conditions. Most of the data are in the brittle to ductile transition region and end in cleavage fracture. A best fit to the data is found using the exponential curve fit (ECF) method. Lack of censoring of invalid results means that the trend line is not a true ‘plane strain’ fit. It is argued that inclusion of ‘plane stress’ data makes the resultant toughness distribution more relevant to ship fracture predictions. Equations are presented which allow the toughness to be plotted at any chosen probability level as a function of temperature relative to T_27J. A safe lower bound to the data is given by the 0.1% probability trend assuming that T_27J for grade D plate and weld is no higher than −20 °C. The data are also used to propose that it is impossible to generate an elastic ductile tearing instability in ship steel with Charpy upper shelf values of 100 J or more.     

Anwendung des instrumentierten Kerbschlagbiegeversuchs zur Abschätzung der Rissauffangzähigkeit

Application of Charpy V‐notch testing to estimate the crack‐arrest toughness Modern structural integrity assessment relies upon fracture mechanics, thus utilizing fracture mechanical parameters describing the material fracture resistance against crack initiation and crack propagation as well as the material crack‐arrest behaviour. However, crack‐arrest fracture toughness values are usually difficult and expensive to determine. In this paper correlations are proposed for estimating the nil‐ductility temperature (T_NDT) and the crack‐arrest fracture toughness (K_Ia) from a transition temperature, based on instrumented Charpy‐V crack‐arrest load information. The transition criteria used are the 4 kN crack‐arrest force and the mean crack‐arrest fracture toughness of 100 MPa√m according to the master curve approach. Correlations between transition temperatures, T(F_a = 4 kN), T(K_Ia), and T_NDT, which were proposed for various structural steels, work very well for the 18Ch2MFA material.     

Determination of Reference Temperature T0 for Steel JRQ in an Unirradiated State and Construction of a Master Curve

This paper considers the results of testing reactor steel JRQ involving determination of the fracture toughness and the reference temperature T₀, construction of a Master Curve, and evaluation of the boundaries of regions with different failure probability. The tests were conducted on small specimens (1/2T) made from blocks of steel JRQ supplied by the IAEA within the framework of the Round Robin program.     

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.     

Calibration of the Weibull stress scale parameter, σu, using the Master Curve

This work proposes that the Weibull stress scale parameter, σ_u, increases with temperature to reflect the increasing microscale toughness of ferritic steels caused by local events that include plastic shielding of microcracks, microcrack blunting, and microcrack arrest. The Weibull modulus, m, then characterizes the temperature invariant, random distribution of microcrack sizes in the material. Direct calibration of σ_u values at temperatures over the DBT region requires extensive sets of fracture toughness values. A more practical approach developed here utilizes the so-called Master Curve standardized in ASTM Test Method E1921-02 to provide the needed temperature vs. toughness dependence for a material using a minimum number of fracture tests conducted at one temperature. The calibration procedure then selects σ_u values that force the Weibull stress model to predict the Master Curve temperature dependence of K_Jc values for the material. At temperatures in mid-to-upper transition, the process becomes more complex as fracture test specimens undergo gradual constraint loss and the idealized conditions of high-constraint, small-scale yielding assumed in E1921-02 gradually degenerate. The paper develops the σ_u calibration process to incorporate these effects in addition to consideration of threshold toughness effects and the testing of fracture specimens with varying crack-front lengths. Initial illustrations of the calibration process for simpler conditions, i.e. 1T crack-front lengths, use the temperature dependent flow properties and a range of toughness levels for an A533B pressure vessel steel. Then using the extensive fracture toughness data sets for an A508 pressure vessel steel generated recently by Faleskog et al. [Engng. Fract. Mech., in press], the paper concludes with calibrations of both m and σ_u over the DBT region and assessments of the Master Curve calibration approach developed here.     

Dynamic J-R curves and tension-impact properties of AISI 308 stainless steel weld

In this paper, instrumented tension-impact (dynamic tensile) and instrumented Charpy impact test results for AISI 308 stainless steel welds at room temperature are reported. A few Charpy specimens precracked to a/W (crack length to width ratio) ratios of 0.42 to 0.59 were also tested. Dynamic yield strength obtained from tension-impact test agrees well with that from Charpy V-notch specimens. The strain rates obtained during the tension-impact test are compared with the various estimates of strain rates for V-notch and precracked Charpy specimens. A variation of the compliance changing rate method was necessary for determining the crack initiation point while crack growth was determined by power law key-curve procedure. J-R curves thus obtained from Charpy (precracked and V-notch) specimens are compared with those computed using handbook procedures using dynamic tensile results. Key words: Tension-impact testing, 308 stainless steel weld, Charpy V-notch, dynamic fracture toughness, dynamic yield strength, J-R curve, strain rate, key-curve.