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.     

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.     

An investigation of the loading rate dependence of the Weibull stress parameters

This paper examines the dependence of the Weibull stress parameters on loading rate for a 22NiMoCr37 pressure vessel steel. Extensive fracture tests, including both quasi-static and dynamic tests, are conducted using deep- and shallow-cracked SE(B) specimens. The fracture specimens are carefully prepared to ensure the crack fronts are placed at the location where the material is homogeneous. Three dynamic loading rates (in terms of the stress intensity factor rate, in the low-to-moderate range are considered. The load-line velocities for the dynamic tests are chosen so that the resulted values for the deep- and shallow-cracked specimens are the same. Independent calibrations performed at each loading rate (quasi-static and the three dynamic loading rates) using deep- and shallow-cracked fracture toughness data show that the Weibull modulus, m, is invariant of loading rate. The calibrated m-value is 7.1 for this material. Rate dependencies of the scale parameter (σ_u) and the threshold parameter (σ_w-min) are computed using the calibrated m and the results indicate that σ_u decreases and σ_w-min increases with higher loading rates. The demonstrated loading rate invariant of m, when combined with the master curve for dynamic loading, can provide a practical approach which simplifies the process to estimate σ_u as a function of loading rate.     

The use of master curve method for statistical re-evaluation of surveillance test data for WWER-1000 reactor pressure vessels

The surveillance fracture toughness test data for WWER-1000 reactor pressure vessel materials from Ukrainian nuclear power plants were re-evaluated using the Master curve methodology. It has been shown that experimental temperature dependence of fracture toughness parameters and a scatter of K_Jc values are in a good agreement with a Master curve shape and 5 and 95% tolerance bounds for materials in unirradiated condition and after neutron irradiation up to fluence 41. 2·10²² n/m² (E > 0.5 MeV). For the Khmelnitsky nuclear power plant unit 1 reactor pressure vessel an analysis has shown that normative approach PNAé G-7-002-86 underestimates essentially the measured fracture toughness of unirradiated weld metal. The reference temperature T₀ calculated according to the Master curve method was compared with a critical brittleness temperature T_K0 for reactor pressure vessel materials in unirradiated condition. It has been found that temperature T₀ is much lower than T_K0 . Furthermore a difference between T₀ and T_K0 values varies essentially from one material to another. A correlation between temperatures T_{28 J} defined from Charpy energy curve and T₀ values calculated from precracked Charpy specimens test was obtained. The analysis has shown that the results based on precracked Charpy specimens can provide nonconservative assessment of fracture toughness for WWER-1000 reactor pressure vessel materials.     

Crack arrest testing of high strength structural steels for naval applications

The crack arrest fracture toughness of two high strength steel alloys used in naval construction, HSLA-100, Composition 3 and HY-100, was characterized in this investigation. A greatly scaled-down version of the wide-plate crack arrest test was developed to characterize the crack arrest performance of these tough steel alloys in the upper region of the ductile-brittle transition. The specimen is a single edge-notched, 152 mm wide by 19 mm thick by 910 mm long plate subjected to a strong thermal gradient and a tensile loading. The thermal gradient is required to arrest the crack at temperatures high in the transition region, close to the expected service temperature for crack arrest applications in surface ships. Strain gages were placed along the crack path to obtain crack position and crack velocity data, and this data, along with the applied loading is combined in a “generation mode” analysis using finite element analysis to obtain a dynamic analysis of the crack arrest event. Detailed finite element analyses were conducted to understand the effect of various modeling assumptions on the results and to validate the methodology compared with more conventional crack arrest tests.Brittle cracks initiation, significant cleavage crack propagation and subsequent crack arrest was achieved in all 15 of the tests conducted in this investigation. A crack arrest master curve approach was used to characterize and compare the crack arrest fracture toughness. The HSLA-100, Comp. 3 steel alloy had superior performance to the HY-100 steel alloy. The crack arrest reference temperature was T_KIA = −136 °C for the HSLA-100 plate and T_KIA = −64 °C for the HY-100 plate.     

The effect of aluminum alloying on ductile-to-brittle transition in Hadfield steel single crystal

The ductile-to-brittle transition (DBT) in Fe-13Mn-1.3C (Hadfield steel, I) and Fe-13Mn-2.7 Al-1.3C (Hadfield steel, II) (wt.%) single crystals oriented along [011], [[`1]44]{[011], [{\bar{{1}}}44]}, and [[`1]11{\bar{{1}}11}] directions was investigated under tension in the temperature interval of 77 to 673 K. The DBT temperature interval was found to be independent of single crystal orientation. The DBT temperatures were estimated (1) as the mean value between the temperature corresponding to the minimum crystal ductility and the one coinciding with the onset of the plateau of the e{\varepsilon}(T)-dependence (T_DBT1); and (2) as the temperature where the volume fraction of brittle failure on the fracture surfaces was 50% (T_DBT2). The DBT temperatures estimated this way, do not coincide for both steels. Mechanical twinning has been reported as the primary reason for the occurrence of the DBT in austenitic high-carbon Hadfield steel and appears to account for the difference in DBT temperatures as well. Alloying with aluminum partially suppresses twinning in steel (II). Twinning sets in only after a certain amount of dislocation slip, but still influences the fracture mechanism of steel (II).     

An experimental investigation of the effect of biaxial loading on the master curve transition temperature in RPV steels

During the 1990s considerable work was conducted to characterize the effect of biaxial loading on the ductile to brittle transition temperature. The work centered on a series of tests using large cruciform bend specimens from an experimental A533B test plate denoted as HSST Plate 14 (Heavy Section Steel Technology Plate 14). Recently a series of similar biaxial cruciform tests has been conducted on the steel used for an extensive European Round Robin that investigated the ductile-to-brittle transition master curve and associated T₀ reference temperature. The results of these tests have been used to promote the concept of a “Biaxial Effect” which corresponds to a shift in the shallow crack transition master curve of +20 °C or more when biaxial stresses are present, in comparison with the master curve for uniaxially loaded shallow crack specimens. A comprehensive analysis of the all of the available HSST Plate 14 data and data from two other structural steels was performed to investigate the extent of a biaxial effect on the reference temperature, T₀. The analysis included many additional biaxial cruciform test results on three different materials. The results of all three materials discussed in this paper fail to clearly demonstrate that biaxial loading, imposed through the use of a cruciform specimen geometry, has an effect on the fracture toughness, characterized using a master curve approach and reference temperature T₀. The analysis utilized in this paper assumes that the toughness distribution and temperature dependence of shallow cracked specimens can be modeled by using the master curve approach. This assumption has not been rigorously validated and would benefit from further study. Additional detailed stress analysis of the constraint evolution in the cruciform specimens may better define the precise conditions under which a biaxial effect on the fracture toughness could be realized.     

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.     

Correlation between quasi-static and dynamic crack resistance curves

The correlation between the crack resistance measured from dynamic and quasi-static J-R tests has been investigated based on test data from instrumented Charpy impact tests and quasi-static single edge notched bend (SENB) J-R tests.The method originally proposed by Aurich et al. [Analyse und Weiterentwicklung Bruchmechanischer versagenskonzepte--Lokales Risswachstum, Ermittlung des Risswiderstandsverhaltens aus der Kerbslagarbeit. BAM Forschungsbericht 192, Berlin 1993, ISBN 3-89429-329-2], to develop a correlation coefficient between the net fracture resistance of Charpy V-notch (ISO) and quasi-static compact tension tests, was extended to establish correlations between so-called ductile-brittle transition fracture resistance curves obtained from instrumented Charpy V-notched (ISO) and fatigue precracked impact tests and quasi-static SENB J-R fracture resistance tests.The correlation between the dynamic and quasi-static crack resistance with regard to practical application to fracture assessments appears to be reasonably consistent, bearing in mind the inherent scatter in crack resistance data in general. The findings of the present research project from testing of ship grade NVE 36 and pressure vessel steel to ASTM A516 Gr. 70 as well as two weldments in the NVE 36 steel, are in excellent agreement with the results of the research conducted by Aurich et al. [Analyse und Weiterentwicklung Bruchmechanischer versagenskonzepte--Lokales Risswachstum, Ermittlung des Risswiderstandsverhaltens aus der Kerbslagarbeit. BAM Forschungsbericht 192, Berlin 1993, ISBN 3-89429-329-2] for structural and pressure vessel steels to the German DIN standard steels including St 52-3, StE 355 and StE 460.     

Fracture toughness of coal under quasi-static and impact loading

The fracture toughness of specimens of Canadian coal, expressed as k_q, was determined under both quasi-static and dynamic loading conditions. In all tests, a wedge loaded compact tension (WLCT) specimen was used. The quasi-static tests were carried out in a servohydraulic testing machine while in the dynamic testing, a Split Hopkinson Bar was used. Also studied was the effect of orientation of the crack with respect to the coal seam. Loading rates ranging from 0.1 MPa √m/sec. to 6 × 10⁴ MPa √m/sec. were investigated. The results show a large increase of over one order of magnitude between k_q in the quasi-static and that obtained at high loading rates.     

Electrospun nanofibre toughened carbon/epoxy composites: Effects of polyetherketone cardo (PEK-C) nanofibre diameter and interlayer thickness

Polyetherketone cardo (PEK-C) nanofibres were produced by an electrospinning technique and directly deposited on carbon fabric to improve the interlaminar fracture toughness of carbon/epoxy composites. The influences of nanofibre diameter and interlayer thickness on the Mode I delamination fracture toughness, flexure property and thermal mechanical properties of the resultant composites were examined. Considerably enhanced interlaminar fracture toughness has been achieved by interleaving PEK-C nanofibres with the weight loading as low as 0.4% (based on weight of the composite). Finer nanofibres result in more stable crack propagation and better mechanical performance under flexure loading. Composites modified by finer nanofibres maintained the glass transition temperature (T_g) of the cured resin. Increasing nanofibre interlayer thickness improved the fracture toughness but compromised the flexure performance. The T_g of the cured resin deteriorated after the thickness increased to a certain extent.     

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.     

Fracture toughness variability of structural steel

The weakest-link model of brittle fracture initiation has had substantial success in describing the inherent variability (scatter) in fracture toughness values for steel samples failing by cleavage. The model predicts a Weibull distribution of fracture toughness with slope 4 when plotted in the conventional fashion [E 1921-02. Standard test method for determination of reference temperature, T₀, for ferritic steels in the transition range. Annual Book of ASTM Standards, vol. 3.01. PA, USA: American Society for Testing and Materials; 2002]. However, the Weibull slope for samples of a structural steel tested at CANMET has been found to be 1.86, significantly less than the expected value of 4. Possible reasons for the discrepancy are discussed.     

Loading rate effects on parameters of the Weibull stress model for ferritic steels

This study investigates the effects of loading rate on parameters of the Weibull stress model for prediction of cleavage fracture in a low strength, strongly rate-sensitive A515-70 pressure vessel steel. Based on measured, dynamic fracture toughness data from deep- and shallow-cracked SE(B) specimens, the calibrated Weibull modulus (m) at shows little difference from the value calibrated previously using static toughness data. This newly obtained result supports the hypothesis in an earlier study [Gao X, Dodds RH, Tregoning RL, Joyce JA. Weibull stress model for cleavage fracture under high-rate loading. Fatigue Fract Engng Mater Struct 2001;24:551-64] that the Weibull modulus likely remains rate independent for this material over the range of low-to-moderate loading rates. Additional experimental and computational results for higher rates show that a constant m-value remains applicable up to the maximum loading rate imposed in the testing program . Rate dependencies of the scale parameter (σ_u) and the threshold parameter (σ_w-min) are computed using the calibrated m, and the results indicate that σ_u decreases and σ_w-min increases with higher loading rates. The predicted cumulative probability for cleavage fracture exhibits a strong sensitivity to small changes in σ_u. Consequently, σ_u must be calibrated using dynamic fracture toughness data at each loading rate of interest in an application or selected to make the Weibull stress model predict a dynamic master curve of macroscopic toughness for the material.     

Time-dependent behaviour of steel/CFRP double strap joints subjected to combined thermal and mechanical loading

Degradation of structural adhesives at elevated temperatures makes the time-dependent behaviour of adhesively-bonded steel/CFRP joints a critical issue for safety considerations of CFRP strengthened steel structures. This paper reports the examination of specimens at different load levels (i.e. 80%, 50%, and 20% of their ultimate load measured at room temperature) and constant temperatures from 35 °C to 50 °C (i.e. temperatures below and above the glass transition temperature T_g, 42 °C of the adhesive). Furthermore, a scenario of cyclic thermal loading between 20 °C and 50 °C was included to represent more realistic exposure. Joint time-dependent behaviour was demonstrated by the stiffness and strength degradation as a function of not only temperature but also time. At the same temperature level close to or above T_g, a higher load level corresponded to a shorter time-to-failure. In addition, up to 47% of strength recovery was found for the specimens subjected to cyclic temperatures compared with those under constant 50 °C which failed at the same load level. Based on the proposed temperature and time-dependent material property models, the time-dependent failure time of steel/CFRP double strap joints was well described and validated by the experimental results.     

Synthesis and characterization of a novel multiblock copolyester containing poly(ethylene succinate) and poly(butylene succinate)

Multiblock copolyester (PBS-b-PES) containing poly(butylene succinate) (PBS) and poly(ethylene succinate) (PES) was successfully synthesized by chain-extension of dihydroxyl terminated PBS (HO-PBS-OH) and PES (HO-PES-OH) using 1,6-hexmethylene diisocyanate (HDI) as a chain extender. The chemical structures, molecular weights, crystallization behaviors, thermal and mechanical properties of the copolyesters were characterized by proton nuclear magnetic resonance spectroscopy (¹H NMR), gel permeation chromatography (GPC), differential scanning calorimetry (DSC), thermogravimetry analysis (TGA), wide-angle X-ray diffraction (WAXD), tensile testing and hydrolytic degradation. High-molecular-weight copolyesters with M_w more than 2.0 × 10⁵ g mol⁻¹ were easily obtained through chain-extension. The copolyesters showed a single glass transition temperature (T_g) which increased with PES content. The melting point temperature (T_m) and relative degree of crystallinity (X_c) of the copolyesters decreased first and then increased with PES content. The copolyesters manifested excellent mechanical properties, for example, PBS₅-b-PES₅ had fracture stress of 61.8 MPa and fracture strain of 1173%. The chain-extension reaction provided a very effective way to produce high molecular weight multiblock copolyesters.     

An experimental investigation of heat transfer in thermally unstable polymer systems

An experimental study is made into the process of heat transfer from a pulse-superheated probe to a solidified polymer system at probe temperatures above the temperature T _d∞ of the beginning of thermal destruction of matter in a quasi-static process. Use is made of the procedures of thermal stabilization of the pulse-superheated probe (with the characteristic time of constancy of superheated probe temperature of 1 ms) and of shock heating (with the characteristic time of increasing the probe temperature of 1 μs and that of monitoring its cooling-down of 1 ms). The effect of short-term thermal stability of polymers in the region of T > T _d∞ is revealed. A procedure is developed for identifying the signs of thermal destruction of polymers in the pulsed process. The maximal values are estimated of the density of heat flux through samples without their thermal destruction. __________ Translated from Teplofizika Vysokikh Temperatur, Vol. 44, No. 3, 2006, pp. 465–471. Original Russian Text Copyright ? 2006 by D. V. Volosnikov, V. P. Efremov, P. V. Skripov, A. A. Starostin, and A. V. Shishkin.     

The use of pre-cracked charpy specimens to determine dynamic fracture toughness

The use of an instrumented impact hammer on pre-cracked Charpy V specimens has led to an inexpensive method for determining the dynamic fracture toughness, K_ld. Measurements were made on A-533 steel over a range of temperature (−125 to + 75°F) at a loading rate K ≈ 10⁶ksi √in/sec. The data were found to be in excellent agreement with those obtained by other workers on much larger specimens. Analysis of the data leads to a new method for estimating the NDT temperature, which may be of practical value in nuclear reactor surveillance programs.     

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.     

Comparative studies of several methods to determine the dynamic fracture toughness of a nuclear pressure vessel steel A508 CL3 with Charpy-size specimen

In this investigation, five estimation methods have been adopted to estimate the dynamic fracture toughness of a nuclear pressure vessel steel A508 CL3 by using pre-cracked Charpy-size specimens on an instrumented impact test machine. Furthermore, the merits and the demerits of the five methods have also been compared. The experimental results indicate that the maximum load energy method based on the curve of load versus load-point displacement overestimates the dynamic fracture toughness J _Id , especially above room temperature. The method of compliance changing rate underestimates the dynamic fracture toughness. The method of measuring the critical stretch zone width (SZW _c ) at the crack tip by means of SEM fractography and then converting the SZW _c into J _Id has a relatively large error. In addition, it is expensive and difficult to measure the SZW _c . The method of energy revised at the maximum load may be considered a better single-specimen method for determining the dynamic fracture toughness. Furthermore, the results indicate that although the dynamic resistance curve method can exactly estimate the dynamic fracture toughness of the material, this method needs several specimens. Moreover, the test procedure is complicated. Thus, it is not suitable for nuclear reactor pressure vessel embrittlement surveillance.