Charpy-size Specimens Research Articles

Inicijacija i rast prsline u zavarenom spoju čelika za rad na povišenim temperaturama

In a paper given, experimental investigations have included the analysis of crack initiation and growth in welded joint of steel for operation at elevated temperatures. For better understanding of the phenomenon of crack initiation and propagation in welded joints of A-204 Gr. A steel, designed for high-temperature and high-pressure application, it is necessary to determine the effect of heterogeneity of microstructural and mechanical properties on fracture toughness and fatigue crack initiation and propagation in welded components. Based on the tests conducted with pre-cracked CT and Charpy size specimens, the effect of heterogeneity of microstructural and mechanical properties of welded joints on fracture toughness and fatigue-crack growth parameters was determined.

New Equation for the Plastic Correction Factor η for J-Integral Determination from Test Results of Three-Point-Bend Specimens

Abstract When material yielding occurs, the stress intensity factor, K, no longer correctly characterizes the magnitude of the stress field around the crack tip. For significant amounts of yielding, the J-integral approach is applied as an advanced tool. In practice, for many engineering applications, the non-linear plasticity effects are of importance and therefore material behavior beyond yield needs an accurate description for input to tools for assessment. This work presents J-integral values of two different steel grades (1006 and 4340) using a newly developed analytical approach for the correction factors ηpl, which takes into account the elastic–plastic properties of the material. The evaluation approach is based on absorbed energies in a Charpy-sized specimen during the elastic and plastic deformation phases. Values of these energy terms were obtained via numerical simulation of 1006 and 4340 steel Charpy-sized specimens loaded in three-point-bending. This work highlights the effect of materials plastic properties on the J-integral. Different steel grades show different amounts of plasticity defined by the strain-hardening exponent and the strain-hardening constant and these influence the fracture parameters. Application of the plastic correction factor ηpl to Charpy-sized specimens, considering the respective plastic properties of the materials, leads to values of ηpl equal to 2.286 for 1006 steel and 2.621 for 4340 steel.

Analysis of Material Inhomogeneity in the European Round Robin Fracture Toughness Data Set

Abstract The European fracture toughness dataset was developed by ten European laboratories in order to provide an experimental data base sufficiently large to study specimen size and temperature effects on cleavage fracture toughness in the ductile-to-brittle transition regime. The initial Master Curve analysis of this data set was presented by Wallin [Eng. Fract. Mech., Vol. 69, 2002, pp. 451–481. showing a range of T0 reference values ranging from −80°C to −110°C, with −90°C as an average and a standard deviation of 6.7°C. The initial round robin data set included all scaled C(T) specimens ranging from 1/2T to 4T in scale, but more recently additional specimens have been tested using this material including deep and shallow crack bend specimens (SE(B)) [Link, R. E., unpublished], precracked Charpy size specimens [Wallin, K., Fracture, Fatigue, and Residual stress—PVP, Vol. 393, J. Pan, Ed., ASME, New York (1999)], and even some biaxially loaded cruciform specimens [Lidbury, D. P. G., Fatigue Fract. Eng. Mater. Struct., Vol. 29, 2006, pp. 829–849]. These specimens are being analyzed to develop and test constraint analysis tools essential in the application of the Master Curve method to structural applications in the commercial nuclear power industry. The present authors have also developed fracture toughness data on this material over a range of loading rates to investigate the sensitivity of the calibrated Weibull coefficients used in the constraint correction models of Gao and Dodds [Eng. Fract. Mech., Vol., 68, 2001, pp. 263–284] to material loading rates in typical nuclear pressure vessel steels. Clear material inhomogeneity characteristics have developed in this data, and it was felt that a more complete investigation of the inhomogenity in the Euro forging data set should be conducted. Following the proposal of Wallin [Eng. Fract. Mech., Vol. 71, No. 16, 2004, pp. 2392–2346], software was written to investigate whether the subject data set could best be expressed as a bimodal distribution of two combined Master Curve distributions, and this software was then applied to the various individual data sets in the European round robin data. The output of each analysis (e.g., the 2T data set) was two Tx values and a probability value pa, where Ta and Tb are the T0 values for the two constituents and pa is the probability of Ta. Using a procedure being developed as an annex to ASTM E1921 which is based on suggestions of Wallin, standard deviations of each of these quantities are estimated and used to judge if the dataset represents inhomogeneous material. The results show that several of the data sets obtained in the original round robin test program are better described by bimodal probability distributions than by the single Weibull distribution of ASTM E1921. Both single specimen datasets, and multi-temperature analysis of the various size specimens appear to show a preference for bimodal probability distributions. Additional sets of precracked Charpy size specimens were then machined from slices of the cross section and the results of testing these specimens shows that some cross sections of the Euro forging demonstrate a clear pattern of toughness variation across the forging cross section, while other cross sections show nearly constant fracture toughness with crack position. A major conclusion of this work is that making “mechanics based” conclusions comparing constraint effects using the Euro forging material is going to be more difficult given the apparent presence of material inhomogeneity in this material as demonstrated very convincingly by the data and analysis presented in this paper.

Static and impact crack properties of a high-strength steel welded joint

In order to gain the benefits of weldable high-strength steels in pressurized equipment applications, satisfactory toughness and crack properties of the welded joint, both in the weld metal and the heat-affected –zone (HAZ), are required. Experimental investigations of toughness and crack resistance parameters through static and impact tests of a high-strength, low-alloy steel (HSLA) with a nominal yield strength of 700 MPa and its welded joint, were performed on Charpy-sized specimens, V-notched and pre-cracked, of the parent metal, weld metal and HAZ. The selected electrode produced slight undermatching and enabled the welded joints to be manufactured without cold cracks. The impact energy and its parts responsible for crack initiation and propagation were determined by toughness evaluation. Crack sensitivity, defined as the ratio of the impact energy for V-notched and for pre-cracked specimens, enabled a comparison of the homogeneous microstructure of the parent metal and the weld metal, and of the heterogeneous microstructure of the heat-affected-zone (HAZ), which indicated a better crack toughness behaviour of the HAZ. The results obtained showed that the toughness and crack resistance of the weld metal were significantly lower than those of the parent metal and the HAZ. The fracture mechanics parameters, J Ic integral, and plane strain fracture toughness, K Ic, as well as J resistance curves expressed the degradation less.

Corrected Procedure for Crack Length Calculation by the Unloading Compliance Technique for Charpy-sized Specimens

Abstract The problems with crack length determination by the unloading compliance method are well known for Charpy-sized specimens. The final crack lengths calculated for bend specimens do not fulfill ASTM E 1820 accuracy requirements. Some investigations have been performed to resolve this problem by correcting the compliance value, but satisfactory results were not obtained. In the present work the measured specimen compliance was taken as a correct value and the calculation procedure was modified. On the basis of experimentally obtained compliances of bent specimens and optically measured crack lengths, a calculation procedure enabling accurate crack length calculation up to 5 mm of plastic deflection was developed. Applying the new procedure to a sample of 238 measured crack lengths, more than 80 % of the values fulfilled the ASTM E 1820 accuracy requirement, while the presently used procedure provided only about 30 % valid results. The newly proposed procedure also can be used prospectively in modified form for specimens other than Charpy-sized.

Dynamic fracture toughness test and master curve method analysis of IAEA JRQ material

IAEA conducted a round-robin fracture test program to test and verify the Master Curve method. One of the materials selected for the round-robin is a A-533B1 plate designated as reference material JRQ. Unnotched Charpy-size specimens were fabricated and distributed to a number of testing laboratories. The three US Owners Groups received specimens for both Charpy impact and three-point bending tests to establish fracture toughness master curves. The B&W Owners Group elected to perform a dynamic fracture toughness test under a high loading rate using the JRQ specimens. The master curve method was successfully applied to numerous fracture toughness data sets of pressure vessel steels. Joyce [Small Specimens Test Technique, ASTM STP 1329, 1997, ASTM] applied this method to high loading rate fracture toughness data for A-515 steel and showed the applicability of this approach to dynamic fracture toughness data. This paper presents the data and the resulting reference temperature shift in the Master Curves from static to dynamic fracture data. Based on earlier PTS analyses performed in 1985, an appropriate T 0 shift value is selected for nuclear power plant applications. This shift in T 0 is compared with the temperature shift between K Ic and K Ia curves in ASME Boiler and Pressure Vessel Code.

Applicability of miniature size bend specimens to determine the master curve reference temperature T0

The master curve method enables characterisation of the brittle fracture toughness based on a few relatively small specimens. Presently the general view is that pre-cracked Charpy-V specimens constitute, effectively, the smallest specimens that can be used with the master curve. However, even though the method includes a specific measuring capacity limit for the specimen, it does not specify a minimum specimen size to be used. In this work, the applicability of miniature specimens, smaller than the normal Charpy size bend specimen, are investigated by comparing the test results of miniature and normal Charpy size specimens. Furthermore, the possible differences in estimates from CT- and 3PB-specimen tests are examined.

Development of the T0 reference temperature from precracked Charpy specimens

The master curve approach specified within the new ASTM E1921 Test Standard is a significant advance in defining an indexing temperature, T 0, the median fracture toughness, and associated failure probability bounds for ferritic steels in the ductile-to-brittle transition regime. An objective in developing this standard test procedure has been that it should, if possible, allow the use of precracked Charpy-size specimens to measure T 0 and the associated fracture performance. However, the supporting technical basis document for E1921 [NUREG/CR-5504, US Nuclear Regulatory Commission, Washington, DC, November 1998] presents no experimental evidence which demonstrates the accuracy of T 0 estimates from precracked Charpy specimens. There was also little independent experimental verification presented [NUREG/CR-5504, US Nuclear Regulatory Commission, Washington, DC, November 1998] to support the limiting deformation criterion ( M=30) contained in the standard. Computational and analytical support [NUREG/CR-5504, US Nuclear Regulatory Commission, Washington, DC, November 1998] of the deformation criterion for precracked Charpy specimens is also lacking. The most applicable recent computational work by Ruggieri and Dodds [Engng. Frac. Mech. 60 (1) (1998) 19] supports a deformation criterion that is almost twice as stringent as the E1921 standard to ensure that constraint loss does not occur. Historically, constraint loss has been evaluated for individual specimens, while E1921 implicitly only requires that constraint loss does not significantly alter the median toughness. This work experimentally evaluates the use of precracked Charpy specimens to estimate T 0 by comparing results obtained on both precracked Charpy and 1T (or larger) fracture toughness measurements. The data demonstrate that the Charpy-size specimens tend to have a non-conservative bias, even when the data fall within the allowable E1921 deformation criterion. This effect may be a function of the degree of crack tip constraint loss, which strongly depends on the material flow properties. An attempt is made to correct for any constraint loss in the Charpy-size specimen and remove any bias in the measured T 0 using a simple constant stressed volume cleavage failure criterion [Int. J. Frac. 74 (1995) 131]. While this method does decrease the difference between T 0 values measured using Charpy and conventional 1T specimens, more detailed cleavage initiation models which can be independently calibrated will be required to rigorously account for any constraint differences between Charpy and larger specimens.

Evaluation of the Fracture Toughness Transition Characteristics of RPV Steels Based on the ASTM Master Curve Method Using Small Specimens

Fracture toughness of five different reactor pressure vessel steels was characterized in the transition temperature region by the ASTM E1921-97 standard method using Charpy-sized small specimens. T he predominant fracture mode of the tested steels was transgranular cleavage in the test conditions. A statistical analysis based on the Weibull distribution was applied to the interpretation of the scattered fracture toughness data. The size-dependence of the measured fracture toughness values was also well predicted by means of the Weibull probabilistic analysis. The measured fracture toughness transition curves followed the temperature-dependence of the ASTM master curve within the expected scatter bands. Therefore, the fracture toughness characteristics in the transition region could be described by a single parameter, so-called the reference temperature (T。), for a given steel. The determined reference temperatures of the tested materials could not be correlated with the conventional index temperatures from Charpy impact tests.

The studies on fracture toughness of nuclear pressure vessel steels by using a small round compact tension specimen and a comparison with a Charpy-size specimen

This paper presents a single-specimen method of evaluating the fracture toughness of nuclear pressure vessel steels by using a small round compact tension (RCT) specimen with deep side-grooves. The method has been used to estimate the fracture toughness parameters of the nuclear pressure vessel steels A508CL3-A, A508CL3-B and A508CL3-A′ produced by two plants. A comparison of the measured fracture toughness results has been done among the small RCT specimen, the Charpy-size specimen and the large specimen, which GB2038 and ASTM E813 require. The research results indicate that the fracture toughness values obtained from the RCT specimens with the double-size deep side-grooves have little difference when compared with the valid fracture toughness results. Finally, this paper discusses the possibility of using the RCT specimens in nuclear pressure vessel steel embrittlement surveillance.

How to obtain a complete dependence curve of impact toughness or fracture toughness vs temperature on nuclear pressure vessel steels by using only one Charpy-size specimen

Considering the re-utilization of parts of the broken Charpy-size specimens used in nuclear pressure vessel fracture toughness surveillance programs, this paper has investigated how to evaluate the dependence curve of impact toughness or fracture toughness vs temperature for a nuclear pressure vessel A508CL3 steel by using one standard Charpy-size specimen and the eight reconstituted Charpy-size specimens which are manufactured from the two broken halves of the tested standard Charpy-size specimen. The research results indicate that by this means a reliable and complete dependence curve of impact toughness or fracture toughness vs temperature on nuclear pressure vessel steels can be obtained by using only one standard Charpy-size specimen. This, in a nuclear surveillance program, is equivalent to getting the complete dependence curve of material property vs temperature from only one Charpy-size specimen that is taken out of a reactor core; this would be very valuable for improving the reliability of surveillance programmes on nuclear pressure vessel embrittlement due to neutron irradiation. Especially this method is extremely important for the life-time prediction and safety surveillance of a nuclear pressure vessel which has been operated for a long-time and for which there is left only a few surveillance specimens in it.

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.

The reasonable design of Charpy-size specimen for fracture toughness test in nuclear surveillance

Use of a precracked and side-grooved Charpy-size specimen is an economical and convenient single-specimen method of evaluating the elastic-plastic fracture toughness of nuclear pressure vessel steels. This paper has studied the influence of side-groove depth on fracture toughness and stable increment of crack on several pressure vessel steels in detail. Test results are compared with those of large-size specimens which the National Standard of China (i.e. GB 2038) requires. The research results indicate that using a precracked Charpy-size specimen with side-groove depth 30% and adopting the energy before the maximum load of a three-point bending test curve, the elastic-plastic fracture toughness of materials can be evaluated conservatively when the crack begins to propagate, and the reasonable design of a Charpy-size specimen for fracture toughness tests in nuclear surveillance has been established. At the same time, the constraint effect and thickening action of the side-groove on a Charpy-size specimen have also been discussed, and theoretically explained.

Fracture toughness of Charpy-size compound specimens and its application in engineering

The use of a pre-cracked Charpy-size specimen with a side-groove to evaluate the fracture toughness of materials has been researched and considered. This method not only satisfies the demand for small-size specimens in surveillance tests of fracture toughness but also avoids using complicated physical methods to monitor the initial conditions of crack propagation. For most materials this method has solved the problem in which the small-size specimen did not satisfy the valid conditions of a fracture toughness measurement. In order to obtain more information from neutron-irradiated sample specimens and raise the reliability of fracture toughness surveillance tests, it has been considered more important to repeatedly exploit the broken Charpy-size specimen tested in the surveillance test, and to make it renewable. In this work, on the renewing design and utilization of Charpy-size specimens, nine data on fracture toughness can be obtained from one pre-cracked side-grooved Charpy-size specimen, while at present usually only one to three data on fracture toughness can be obtained from one Charpy-size specimen. Thus, it is found that the new method would improve the reliability of fracture toughness surveillance testing and evaluation. In addition, some factors that affect the optimum design of pre-cracked deep side-groove Charpy-size compound specimens have also been discussed.

Application of the advanced instrumented impact testing facility for the irradiation damage assessment

A new test method suitable for the dynamic fracture toughness evaluation of irradiated specimens has been developed. The method is based on the application of the new pendulum type impact tester equipped with a self-adjusting IR-laser extensometer and fully computerized data acquisition and processing. The testing system contains several unique features, which make the impact testing more appropriate from the fracture mechanics point of view. These features include, for example, the inversion of the test geometry to reduce inertia and specimen oscillation effects, the utilization of larger than normal Charpy-size specimens, and the application of the IR-extensometer to evaluate ductile fracture initiation enabling the determination of the energy-based fracture toughness parameters. In this paper the testing procedure will be described and its application on the characterization of the unirradiated and irradiated IAEA phase III correlation monitor material for surveillance programmes will be presented.

Comparison of Charpy-V and [formula omitted] transition temperature shifts in CrMoV pressure vessel steels

Both Charpy-V and precracked Charpy-size specimens were included in the original surveillance programme of the Loviisa nuclear power plant pressure vessel made of CrMoV steel. With precracked specimens the J- value at the onset of brittle fracture was determined as a function of temperature and irradiation dose. The irradiation induced transition temperature shifts measured with Charpy-V and precracked three-point bend specimens are correlated by using indexed temperature values. The shapes of brittle fracture transition curves measured with the fracture mechanical specimens both for the irradiated and non-irradiated material state are compared using a statistical brittle fracture model.

Constraint of side-groove and its influence on fracture toughness parameter in Charpy-size specimens

The effect of a side-groove constraint on a pre-cracked Charpy-size specimen, and the influence of a side-groove on fracture toughness parameters are studied. The results indicate that when side-groove depth approaches a critical value, the maximum load toughness J m values are nearly coincident with the initiation toughness J i , values. The thickening action of the side-groove may be expressed by calculating the extra thickness of the specimen. For different materials the additional thickness due to the side-groove is distinct. When the side-groove depth is 33% of the specimen thickness the specimen obtains the maximum additional thickness. The side-groove constraint coefficient ( C) presented by this paper can quantitatively evaluate the level of plastic constraint at the crack tip, and explains the experimental results well.

Void growth at ductile crack initiation of a structural steel

This investigation deals with the critical void growth in a BS4360 Grade 50D structural steel. The tests are carried out on four variations of the Charpy size specimens, namely a standard geometry; side grooved; fatigue cracked; side grooved and fatigue cracked. The geometry variations are intended to vary the level of hydrostatic stress or constraint in the region of fracture initiation. The void growth is measured on mid-sections of specimens at onset of crack growth. It is known that there are two micromechanistic concepts regarding initiation of the ductile failure processes. One is the critical void volume fraction, and the other is the critical void growth rate. The experimental studies indicate that the critical void volume fraction (ƒ v) i , at onset of crack growth is not a material constant. The volume fraction is dependent on the geometry constraint. However, the critical void growth rate R i / R 0 is a material constant within a first approximation. Thus, it is probable in engineering to evaluate the crack initiation in a cracked specimen by using the critical void growth rate measured on smooth tensile specimen with lower constraint to take it easy.

Application of a convenient JIC test method using a charpy-size specimen to welded joints.

Application of a convenient JIC test method using a charpy-size specimen to welded joints.

Determination of the ductile-brittle transition temperature by theJ-integral test using small bend specimens

The toughness of 2 1/4 Cr-1 Mo steel is evaluated in the ductile-brittle transition range. It is shown thatJmax, the value of theJ-integral at maximum load in three-point bend tests, is a consistent measure of toughness. Statistical analysis indicates that although they do not provide a valid estimate of upper shelf toughness, precracked Charpy-size specimens can be used to determine the characteristic parameters of the transition.