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.     

Probabilistic failure assessment of ferritic steels using the master curve approach including constraint effects

The present study is concerned with an enhancement of the master curve approach for the probabilistic failure assessment of ferritic steel structures considering constraint effects. In an experimental program based on a variety of different specimens, distinct effects of the specimen geometry on the reference temperature T₀ are observed. The experimental data base is examined in terms of the K-T_stress-, J-A₂-, J-Q- and J-h-concepts. Based on the results, a constraint enhancement for the master curve concept is suggested consisting of a constraint dependent temperature shift or an alternative constraint dependent scaling of the stress intensity factor.     

Master curve analysis of the “Euro” fracture toughness dataset

Brittle fracture in the ductile to brittle transition regime is connected with specimen size effects and - more importantly - tremendous scatter of fracture toughness, which the technical community is currently becoming increasingly aware of. The size effects have the consequence that fracture toughness data obtained from small laboratory specimens do not directly describe the fracture behavior of real flawed structures. Intensive research has been conducted in the last decade in order to overcome these problems. Different approaches have been developed and proposed, one of the most promising being the master curve method, developed at VTT Manufacturing Technology.For validation purposes, a large nuclear grade pressure vessel forging 22NiMoCr37 (A508 Cl.2) has been extensively characterized with fracture toughness testing. The tests have been performed on standard geometry CT-specimens having thickness 12.5, 25, 50 and 100 mm. The a/W ratio is close to 0.6 for all specimens. One set of specimens had 20% side-grooves. The obtained data consists of a total of 757 results fulfilling the ESIS-P2 test method validity requirements with respect to pre-fatigue crack shape and the ASTM E-1921 pre-fatigue load. The master curve statistical analysis method is meticulously applied on the data, in order to verify the validity of the method. Based on the analysis it can be concluded that the validity of all the assumptions in the master curve method is confirmed for this material.     

Determination of constraint limits for cleavage initiated toughness data

The proper constraint limits for cleavage initiated toughness data within the ductile-to-brittle transition regime have been studied extensively using both numerical analysis and analysis of experimental data. Historically, the experimentally based constraint limits have supported less conservative limits. This study conducts analysis of existing and new experimental data developed using data sets targeted to exhibit constraint loss toughness enhancement. Constraint herein is quantified in terms of the scaled specimen deformation level, more commonly known as M. It is expected that data with low M values will exhibit the greatest affect of constraint loss. Large data sets are therefore developed and extracted from the literature that include data with a large range of M levels and at least the required number of uncensored results with M > 30 for valid T₀ measurement as per ASTM E1921-02. Differences in the calculated T₀ values using censoring limits of 5-500 are then determined. The onset of T_lim differences due to constraint loss is examined by simply increasing the censoring limit, M_lim, utilized in determining the indexing temperature, T_lim, and evaluating differences between T₀ and the T_lim values obtained using higher constraint limits. Bias resulting from this censoring procedure is examined using a Monte-Carlo analysis and shown to be small.Measurement of a high constraint T₀ in bend specimens is shown to require M_lim > 200. As M_lim increases from 30 to 200 in bend specimens, the corresponding T_lim can increase by approximately 15 °C. Further increases in M_lim do not result in substantial increases in T_lim. This evolution buttresses previous numerical findings by Dodds and co-workers [Specimen size requirements for fracture toughness testing in the ductile-to-brittle transition regime, J Test Eval 1991;191:123-34; Size and deformation limits to maintain constraint in K_Ic and J_c testing of bend specimens. In: Kirk M, Ad Bakker, editors. Constraint effects in fracture theory and applications: second volume. ASTM STP 1244, 1995; Numerical investigation of 3-D constraint effects on brittle fracture in SE(B) and C(T) specimens, Int J Fract 1995;74:131-61] and provides a strong justification for changes to ASTM E1921-02 if a conservative, geometry insensitive, and transferable reference temperature, T₀, is to be determined using this standard. Possible short-term changes to ASTM E1921-02 could include raising M_lim and requiring an upward shift of SE(B) T₀ values. A more desirable solution is to adjust individual K_Jc toughness values before evaluation of T₀ to eliminate specimen geometry bias.     

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.     

Influence of statistical and constraint loss size effects on cleavage fracture toughness in the transition—A single variable experiment and database

This paper, which is the first of two parts, describes a comprehensive experimental study to systematically measure constraint loss and statistical size effects on fracture toughness of a typical low alloy pressure vessel steel in the brittle-to-ductile transition. Both of these size effects lead to higher toughness in smaller specimens. Previous studies primarily involved specimens that differed in size, but with self-similar standard geometric configurations, making it difficult to separate constraint loss from statistical size effects. The focus here is on a single-variable database developed for a homogeneous plate section of the decommissioned Shoreham Reactor vessel steel, tested at the 3/4-plate thickness position. Tests were carried out at a common set of conditions and covered a full matrix of specimens with six thicknesses (B) ranging from 7.9 to 254 mm and four widths (W) ranging from 6.4 to 50.8 mm. The variations in B were designed to provide readily observable statistical sampling effects, while variations in W result in fracture at deformation conditions ranging from small scale yielding to those involving extensive loss of constraint. A detailed analysis of this database is presented in a companion paper, where physically based models are used to quantify the relative contributions of constraint loss and statistical size effects. The models also provide a general approach to adjusting measured toughness for both testing and applications. The B-W database and analysis also provides a comprehensive independent assessment of some key aspects of the master curve method in the ASTM E 1921 Standard discussed in detail elsewhere. In this paper, we present an empirical analysis that shows a high constraint toughness scaling with B that is reasonably consistent with ASTM E 1921, as well as effects of loss of constraint that occur at much lower deformation levels than reflected in the data censoring limit in ASTM E 1921.     

Estimation and comparative study of JIC using different methods for 20MnMoNi55 steel

Fracture toughness is one of the key input variables to compute critical load of the structural components. The resistance against ductile fracture can be quantified either by the initiation value or by the entire resistance curve. Different standard methods like J_SZW, JSME and ASTM: E1820 etc. are mainly used to estimate the critical crack initiation value from the resistance curve developed by the J-integral test. However, the results vary from method to method and are even inconsistent for the same method. Pehrson and Landes suggested a simple method for estimation of the critical fracture toughness by identifying the critical point corresponding to the maximum load on load–displacement curve. In the present study, different standard methods along with the one suggested by Pehrson and Landes are used to find out the critical fracture toughness using 1T–CT and ½T–CT specimens of the material 20MnMoNi55 steel for varying temperatures and crack size. The results are analyzed to compare the merits of the different methods of estimation of fracture toughness.     

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.     

DAMAGE MECHANICS APPROACH TO REMOVE THE CONSTRAINT DEPENDENCE OF ELASTIC–PLASTIC FRACTURE TOUGHNESS

It is now generally agreed that the applicability of a one-parameter J-based ductile fracture approach is limited to so-called high constraint crack geometries, and that the elastic-plastic fracture toughness J_1c, is not a material constant but strongly specimen geometry constraint-dependent. In this paper, the constraint effect on elastic-plastic fracture toughness is investigated by use of a continuum damage mechanics approach. Based on a new local damage theory for ductile fracture(proposed by the author) which has a clear physical meaning and can describe both deformation and constraint effects on ductile fracture, a relationship is described between the conventional elastic-plastic fracture toughness, J_1c, and crack tip constraint, characterized by crack tip stress triaxiality T. Then, a new parameter J_dc (and associated criterion, J_d=J_dc) for ductile fracture is proposed. Experiments show that toughness variation with specimen geometry constraint changes can effectively be removed by use of the constraint correction procedure proposed in this paper, and that the new parameter J_dc is a material constant independent of specimen geometry (constraint). This parameter can serve as a new parameter to differentiate the elastic-plastic fracture toughness of engineering materials, which provides a new approach for fracture assessments of structures. It is not necessary to determine which laboratory specimen matches the structural constraint; rather, any specimen geometry can be tested to measure the size-independent fracture toughness J_dc. The potential advantage is clear and the results are very encouraging.     

A review of fracture toughness transferability with constraint and stress gradient

In this review paper, only constraint and stress gradient approaches to transferability of fracture toughness are examined. The different constraint parameters are defined and discussed, and one example is given in each case. Factors that influence the constraint are studied. Special attention is given to the actual trends to use the plastic constraint in the material failure master curve and the material transition temperature master curve. The paper also deals on the influence of T stress on the crack path and out‐of‐plane constraint and on the influence of thickness on fracture toughness. The uses of plasticity with gradient and the relative stress gradient in local fracture approaches are also examined.     

Characterization of stable crack extension in aluminium sheet material using the crack tip opening angle determined optically and by the δ5 clip gauge technique

Tests standards aimed at deriving fracture toughness data and crack resistance curves under low constraint condition have recently been finished by ASTM and ISO. These standards cover various experimental methods for determining critical crack tip opening angles, CTOA, for characterising stable crack extension in sheet material. In this paper, some key items of these standard methods are validated, namely the experimental determination of the crack tip opening angle by optical observation and using the δ₅ clip gauge method. When applying such standard methods to material characterization it is of particular interest to know how CTOA-data derived by different methods compare with each other. This paper compares CTOA-data as derived by the optical method with those derived by using the δ₅ clip gauge method. In order to study possible specimen size and geometry effects the methods have been applied to a wide range of specimen geometries. The results demonstrate that CTOA-data derived by the optical method are well suited to provide a specimen size and geometry independent characterization of stable crack extension where the thus obtained CTOA-data are constant over a large amount of stable crack extension. In contrast to this result, CTOA-data obtained from the δ₅ clip gauge method revealed a complex pattern of size and geometry effects, and only in case of compact specimens with a selected size the two CTOA-methods provide nearly identical CTOA-data over a large amount of crack extension.     

Fracture toughness of high strength seamless pipe steel from SE(T) and SE(B) specimens evaluated by different standards

Recently released, BS 8571:2014 standard offers an alternative for the experimental determination of fracture toughness and resistance curves of metallic materials from SE(T) specimens. Similar in stress conditions and constraint to cracked tubes, specimens in SE(T) geometry should yield less conservative toughness values than conventional high‐constraint specimens, such as SE(B) geometry. However, the convention to determine the J‐integral fracture toughness proposed in the new BS standard is different from ASTM standards. In this work, SE(T) and SE(B) specimens of similar dimensions of a tough high‐strength seamless pipe steel were tested following the BS 8571:2014 and ASTM E1820‐16 standards, respectively. Because of the different standardized definitions, SE(T) specimens yield lower fracture toughness than SE(B) specimens, which could lead to more conservative results in structural integrity analysis. This investigation also suggests the introduction of the blunting line concept in the BS 8571:2014 standard in order to minimize this problem.     

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.     

Warm pre-stress based pre-cracking criteria for fracture toughness testing

There are presently a magnitude of different fracture toughness testing standards that have different criteria for fatigue pre-cracking specimens prior to testing. The reason for the criteria is that too high pre-fatigue load may influence the subsequently measured fracture toughness value. The criteria have to a large extent been developed specifically for each standard in question and this has lead to the considerable variability in the criteria. The basic reason for the pre-fatigue having an effect on the fracture toughness is the warm pre-stress (WPS) effect. Here, existing data relating to pre-fatigue load levels are examined with the help of a newly developed simple WPS correction and a criteria and correction procedure for too high pre-fatigue loads are proposed. The new criteria focuses on brittle fracture, but is equally applicable for ductile fracture, thus enabling a unification of pre-fatigue criteria in different fracture toughness testing standards.     

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.     

Experimental T33-stress formulation of test specimen thickness effect on fracture toughness in the transition temperature region

This paper describes a study of the test specimen thickness effect on fracture toughness of a material, in the transition temperature region, for CT specimens. In addition we studied the specimen thickness effect on the T₃₃-stress (the out-of-plane non-singular term in the series of elastic crack-tip stress fields), expecting that T₃₃-stress affected the crack-tip triaxiality and thus constraint in the out-of-plane direction. Finally, an experimental expression for the thickness effect on the fracture toughness using T₃₃-stress is proposed for 0.55% carbon steel S55C. In addition to the fact that T₃₃ (which was negative) seemed to show an upper bound for large B/W, these results indicate the possibility of improving the existing methods for correlating fracture toughness obtained by test specimen with the toughness of actual cracks found in the structure, using T₃₃-stress.     

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.