Conservatism of ASME KIR-reference curve with respect to crack arrest

The conservatism of the RT_NDT temperature indexing parameter and the ASME K_IR-reference curve with respect to crack arrest toughness, has been evaluated. Based on an analysis of the original ASME K_Ia data, it was established that inherently, the ASME K_IR-reference curve corresponds to an overall 5% lower bound curve with respect to crack arrest. It was shown that the scatter of crack arrest toughness is essentially material independent and has a standard deviation (S.D.) of 18% and the temperature dependence of K_Ia has the same form as predicted by the master curve for crack initiation toughness. The ‘built in’ offset between the mean 100 MPa√m crack arrest temperature, TK_Ia, and RT_NDT is 38°C (TK_Ia=RT_NDT+38°C) and the experimental relation between TK_Ia and NDT is, TK_Ia=NDT+28°C. The K_IR-reference curve using NDT as reference temperature will be conservative with respect to the general 5% lower bound K_Ia(5%)-curve, with a 75% confidence. The use of RT_NDT, instead of NDT, will generally increase the degree of conservatism, both for non-irradiated as well as irradiated materials, close to a 95% confidence level. This trend is pronounced for materials with Charpy-V upper shelf energies below 100 J. It is shown that the K_IR-curve effectively constitutes a deterministic lower bound curve for crack arrest The findings are valid both for nuclear pressure vessel plates, forgings and welds.     

Interpreting the high KIa values obtained in thermal shock tests

A theoretical analysis shows that, as regards the two crack arrest events in the Oak Ridge pressurised thermal shock event PTSE I, the measured arrest K values should not be significantly larger than the arrest K_Ia value expected for a deep crack, and could indeed be smaller than K_Ia. This is due to the arrest crack depths being small, and becuase ligaments are associated with crack arrest in the transition temperature regime.     

Environmentally assisted crack growth in a low alloy boiler steel in high temperature water containing oxygen

The crack growth behaviour of the higher strength steel 17 MnMoV 6 4 in deionized high temperature water containing oxygen was investigated with respect to static loading. The tests were performed in an autoclave testing facility at an oxygen content of 8 ppm, a pressure of 70 bar and a temperature of 240°C under quasi-stagnant flow conditions. The stress intensities at the beginning of the tests were 17, 20, 27, 34, 40 and 58 MPa√m. In contrast to the higher loaded specimens no evidence of environmentally assisted cracking was found at stress intensities of 17 and 20 MPa√m.The maximum crack growth rate in the range where only environmetally assisted cracking occured amounted to about 4 x 10^-5 mm/s. The critical stress intensity K_IJ for the onset of stable ductile crack growth in air is 100 MPa√m. On the basis of fractographic studies the crack development found can be ascribed to the “Strain Induced Corrosion Cracking (SICC)” mechanism. This mechanism, used as a working hypothesis, gives a satisfactory explanation for the occurence of stress corrosion cracking of unalloyed and low alloyed steels in high temperature water. SICC is particularly characterized by aggravated corrosive attack occuring as soon as the magnetite/haematite protective layer has been locally disturbed. The stress concentration then just becomes so great that in the region of the resulting crack tips, yield/creep deformation within the critical range of strain rates occurs.     

Results and conclusions from the first pressurized-thermal-shock experiment

The first pressurized-thermal-shock test of a 148 mm thick steel pressure vessel with a 1 m long flaw was performed to investigate fracture behavior of a vessel under conditions relevant to a flawed nuclear reactor pressure vessel during an overcooling accident. The objectives were to observe crack arrest and stability on the ductile upper shelf and the effects of warm prestressing on crack initiation. Three coordinated pressure and thermal transients were imposed on the vessel, which was preheated to 290°C. Two episodes of crack propagation and arrest occurred. The thermal transients were induced by coolant at −29 to 15°C. Pressure transients were as high as 94.4 MPa. The experimental objectives were attained. The inhibiting effects of warm prestressing were definitely demonstrated. Crack propagation was nearly pure cleavage, and arrest at 30 K above the onset of the Charpy upper-shelf was experienced in a positive K₁ gradient and with K₁ = 300 MPa√m. Fracture-mechanics analysis of brittle fracture based on small-specimen toughness measurements was reasonably accurate. Flaw evaluation by procedures of the ASME Boiler and Pressure Vessel Code conservatively predicted vessel failure, which did not occur. No ductile tearing occurred after each crack arrest, although some stable tearing had been predicted on the basis of tearing resistance data.     

Crack arrest toughness of nodular iron

Duplex specimens were used to measure the crack arrest toughness (K_Ia) of a nodular cast iron. Over the temperature range from −40 to +23°C, K_Ia was close to published values of the dynamic initiation toughness (K_Id) of nodular irons with similar microstructures. However, K_Ia appeared to be somewhat more temperature dependent than K_Id. The temperature dependence of K_Ia is also somewhat greater than would be consistent with dependence of K_Ia is also somewhat greater than would be consistent with fractographic observations, which show almost no variation in fracture appearance over the temperature range investigated.When referred to the ductile/brittle transition temperature, the nodular iron K_Ia values are similar to those of reactor-pressure-vessel steels. A detailed comparison of the raw data with previous results on ferritic steels, combined with finite element analysis, suggests that the crack-arrest-toughness values reported here are either accurate or slightly conservative.     

Use of the Master Curve methodology for real three dimensional cracks

At VTT, development work has been in progress for 15 years to develop and validate testing and analysis methods applicable for fracture resistance determination from small material samples. The VTT approach is a holistic approach by which to determine static, dynamic and crack arrest fracture toughness properties either directly or by correlations from small material samples. The development work has evolved a testing standard for fracture toughness testing in the transition region. The standard, known as the Master Curve standard is in a way “first of a kind”, since it includes guidelines on how to properly treat the test data for use in structural integrity assessment. No standard, so far, has done this. The standard is based on the VTT approach, but presently, the VTT approach goes beyond the standard. Key components in the standard are statistical expressions for describing the data scatter, and for predicting a specimens size (crack front length) effect and an expression (Master Curve) for the fracture toughness temperature dependence. The standard and the approach, it is based upon, can be considered to represent the state of the art of small specimen fracture toughness characterization. Normally, the Master Curve parameters are determined using test specimens with “straight” crack fronts and comparatively uniform stress state along the crack front. This enables the use of a single K_I value and single constraint value to describe the whole specimen. For a real crack in a structure, this is usually not the case. Normally, both K_I and constraint vary along the crack front and in the case of a thermal shock, even the temperature will vary along the crack front. A proper means of applying the Master Curve methodology for such cases is presented here.     

Strain-induced corrosion cracking behaviour of low-alloy steels under boiling water reactor conditions

The strain-induced corrosion cracking (SICC) behaviour of different low-alloy reactor pressure vessel (RPV) and piping steels and of a RPV weld filler/weld heat-affected zone (HAZ) material was characterized under simulated boiling water reactor (BWR)/normal water chemistry (NWC) conditions by slow rising load (SRL) and very low-frequency fatigue tests with pre-cracked fracture mechanics specimens. Under highly oxidizing BWR/NWC conditions (ECP +50 mV_SHE, 0.4 ppm dissolved oxygen), the SICC crack growth rates were comparable for all materials (hardness <350 HV5) and increased (once initiated) with increasing loading rates and with increasing temperature with a possible maximum/plateau at 250 °C. A minimum K_I value of 25 MPa m^1/2 had to be exceeded to initiate SICC in SRL tests. Above this value, the SICC rates increased with increasing loading rate dK_I/dt, but were not dependent on the actual K_I values up to 60 MPa m^1/2. A maximum in SICC initiation susceptibility occurred at intermediate temperatures around 200–250 °C and at slow strain rates in all materials. In contrast to crack growth, the SICC initiation susceptibility was affected by environmental and material parameters within certain limits.     

European experiences of the proposed ASTM test method for crack arrest toughness of ferritic materials

The proposed ASTM test method for measuring the crack arrest toughness of ferritic materials using wedge-loaded, side-grooved, compact specimens was applied to three steels: A514 bridge steel tested at −30°C (CV30–50°C), A588 bridge steel tested at −30°C (CV30–65°C), and A533B pressure vessel steel tested at +10°C (CV30-12°C) and +24°C (CV30+2°C). Five sets of results from different laboratories are discussed here; in four cases FOX DUR 500 electrodes were used for notch preparation, in the remaining case HARDEX-N electrodes were used. In all cases, notches were prepared by spark erosion, although root radii varied from 0.1–1.5 mm. Although fast fractures were successfully initiated, arrest did not occur in a significant number of cases.The results showed no obvious dependence of crack arrest toughness, K_a, (determined by a static analysis) on crack initiation toughness, K₀. It was found that K_a decreases markedly with increasing crack jump distance, Δα/W. A limited amount of further work on smaller specimens of the A533B steel showed that lower K_a values tended to be recorded.It is concluded that a number of points relating to the proposed test method and notch preparation are worthy of further consideration. It is pointed out that the proposed validity criteria may screen out lower bound data. Nevertheless, for present practical purposes, K_a values may be regarded as useful in providing an estimate of arrest toughness — although not necessarily a conservative estimate.     

Three-dimensional interpretation of cleavage fracture tests of cladded specimens with local approach to cleavage fracture

Electricité de France has conducted during these last years an experimental and numerical research programme in order to evaluate fracture mechanics analyses used in nuclear reactor pressure vessels integrity assessment, regarding the risk of brittle fracture. Two cladded specimens made of ferritic steel A508 Cl3 with stainless steel cladding, and containing shallow subclad flaws, have been tested in four point bending at very low temperature to obtain cleavage failure. The crack instability was obtained in base metal by cleavage fracture, without crack arrest. The tests have been interpreted by local approach to cleavage fracture (Beremin model) using three-dimensional finite element computations. After the elastic–plastic computation of stress intensity factor K_J along the crack front, the probability of cleavage failure of each specimen is evaluated using m, σ_u Beremin model parameters identified on the same material. The failure of two specimens is conservatively predicted by both analyses. The elastic–plastic stress intensity factor K_J in base metal is always greater than base metal fracture toughness K_1c. The calculated probabilities of cleavage failure are in agreement with experimental results. The sensitivity of Beremin model to numerical aspects is finally exposed.     

HSST crack-arrest studies overview

An overview is given of the efforts underway in the Heavy-Section Steel Technology (HSST) Program as of October 1985 to better understand and model crack-arrest behavior in reactor pressure vessel steels. The efforts are both experimental and analytical. The experimental work provides K_Ia data from laboratory-sized specimens, from thick-wall cylinders which exhibit essentially-full restraint and from nonisothermal wide-plate specimens. These data serve to define toughness-temperature trends and to provide validation data under prototypical reactor conditions. The analytical efforts interpret and correlate the data, plus provide LEFM, elastodynamic and viscoplastic analysis methods for analyzing crack run-arrest behavior in reactor vessels. The analysis methods are incorporated into finite element computer programs which are under development at three separate laboratories.     

Energy transformation during dynamic crack initiation and arrest

This contribution deals with the experimental determination of fracture mechanics parameters concerning dynamic crack initiation, crack propagation and crack arrest demonstrated on reactor pressure vessel steels 20 MnMoNi 5 5 and 22 NiMoCr 3 7. Appropriate measuring methods are available to determine the impact fracture toughness K_Id for CT specimens and CCP specimens. However, for small scale specimens there are still experimental and theoretical problems to be met with when determining the fracture heat of a propagating crack and ascertaining the parameters of arrest.     

A study on the behavior of a crack located at the striping zone in a thermally stratified pipe

The behavior of a small crack located at the thermal striping zone in a thermally stratified piping is numerically investigated. The effects of the parameters such as fluctuation frequency/ amplitude of the density interface, heat transfer coefficient near the thermal striping zone, and crack depth on the behavior of the crack are examined to identify the governing parameters. The effect of the contact surfaces on the stress intensity factor range is also examined for several levels of the internal fluid pressure. The stress intensity factor range, ΔK_I, is used to describe the crack behavior due to the thermal striping. The findings of the numerical investigation are as follows: (1) the value of ΔK_I increases up to a specific Fourier number and then decreases beyond the specific Fourier number. (2) With the increasing Biot number the value of ΔK_I increases and the Fourier number(Fo) at which the ΔK_I vs. Fo curve has a peak decreases. This means that the behavior of a crack located at the thermal striping zone has a large dependence on the oscillating frequency of the density interface and the heat transfer coefficients. (3) The value of ΔK_I increases up to a specific crack depth and then decreases beyond the specific crack depth. (4) If the fluid pressure is lower than a specific value, the value of ΔK_I decreases because of the crack surface contact.     

New observations on the crack growth rate of low alloy nuclear grade ferritic steels under constant active load in oxygenated high-temperature water

Within the scope of reactor safety research attempts have been made over several decades to determine corrosion-assisted crack growth rates. National and international investigations have been performed on both an experimental and an analytical basis. A compilation of internationally available experimental data for ferritic steels exhibits a scatter of crack growth rates of up to 5 decades. This was one of the reasons for commencing further experimental investigations focused on the evaluation of corrosion-assisted crack growth rates. These experimental studies were performed under constant, active, external load on 2T-CT specimens of the materials 20 MnMoNi 5 5 with 0.009 and 0.020% S (similar to A508 Cl.3), 22 NiMoCr 3 7 with 0.006% S (similar to A508 Cl.2) and 17 MnMoV 6 4 with 0.017% S. The tests were carried out in deionized oxygenated high-temperature water (240°C; 0.4 and 8.0 ppm O₂). For K_I values up to 60 MPa m^1/2, the experimental results showed no significant dependence between corrosion-assisted crack growth rates and the stress intensity factor, the oxygen content of the medium or the sulphur content of the steel. Here it is important to note, that in this K_I region the high crack growth rates after the onset of cracking due to loading are decreasing and finally come to a standstill after a short period of time as compared with operational times of plants. Consequently, the determination of crack growth velocities as corrosion-assisted crack advance divided by the test duration, so far practised worldwide, results in wrong crack growth rate values in the above-mentioned range of loading up to 60 MPa m^1/2. Based on a test duration of 1000 h, the average crack growth rates are below 10⁻⁸ mm s⁻¹ for K_I ≤ 60 MPa m^1/2. When applied to a single start-up and service period of one year, this would formally lead to an average crack growth rate of 2·10⁻⁹ mm s⁻¹ (equivalent to 0.06 mm per year). At K_I values between 60 and 75 MPa m^1/2 the average corrosion-assisted crack growth rates increase significantly. It can be observed experimentally that the crack propagates during the whole period of the test. Consequently the calculation of crack growth velocities as corrosion-assisted crack advance divided by the test duration as mentioned earlier can be applied as a first estimate. Finally, for K_I values ≥ 75 MPa m^1/2 high crack growth rates up to 10⁻⁴ mm s⁻¹ can be observed. In this region the average crack growth rates are also in quite good agreement with a theoretically based crack growth model.     

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 T₀ shift value is selected for nuclear power plant applications. This shift in T₀ is compared with the temperature shift between K_Ic and K_Ia curves in ASME Boiler and Pressure Vessel Code.     

Dynamic fracture analysis of a transverse wedge-loaded compact specimen

The J-integral method cannot be applied to the elastic plastic dynamic crack propagation, because unloading and inertia force may take place. From this point of view dynamic elastic plastic scheme using J-integral is developed.Using this dynamic finite element program an MRL type specimen is analyzed. As the result, the property of path-independence of the J-integral under the existence of inertia force and unloading is confirmed. Dynamic effects are considerably small in the MRL type specimen. Also the influence of plastic zone on the crack arrest toughness is shown.Finally the present result is compared with the request of ASTM 2nd round robin test program for crack arrest toughness.     

On the determination of fracture-mechanics data using the nearfield at the tip of dynamically loaded cracks

This paper discusses some common methods of data evaluation in dynamic fracture mechanics. This is done in regard to the restriction arising from the available testing-machines as well as due to the dynamic effects in the specimen which control the crack initiation process. It is concluded that only the process at the crack tip itself determine the dynamic crack inititation at very high K-values and consequently only data from the crack tip's nearfield will yield an correct interpretation of the events in the specimen. Based on this some new experimental methods are under development at the MPA Stuttgart, Fed. Rep. Germany which are discussed in the second part of the paper.     

Constraint effects on fracture toughness of impact-loaded, precracked Charpy specimens

Impact-loaded, precracked Charpy specimens often play a crucial role in irradiation surveillance programs for nuclear power plants. However, the small specimen size B = W = 10 mm limits the maximum value of cleavage fracture toughness J_c that can be measured under elastic—plastic conditions without loss of crack tip constraint. In this investigation, plane strain impact analyses provide detailed resolution of crack tip fields for impact-loaded specimens. Crack tip stress fields are characterized in terms of J—Q trajectories and the toughness-scaling model which is applicable for a cleavage fracture mechanism. Results of the analyses suggest deformation limits at fracture in the form of b > MJ_c/σ₀, where M approaches 25–30 for a strongly rate-sensitive material at impact velocities of 3–6 m s⁻¹. Based on direct comparison of the static and dynamic J values computed using a domain integral formulation, a new proposal emerges for the transition time, the time after impact at which interial effects diminish sufficiently for simple evaluation of J using the plastic η factor approach.     

Evaluation method of corrosion lifetime of conventional stainless steel canister under oceanic air environment

Natural exposure and accelerated corrosion tests of conventional stainless steels for canisters of Types 304, 304L, and 316(LN) for concrete casks were conducted using several test specimens and 1/5 scale canister models. The welding residual stress of a full-scale model canister was also measured and the lifetime of sealability of canisters against corrosion evaluated. The maximum pitting rate and crevice corrosion rate of Type 304 were approximately 20 and 30 μm/year. Many SCC in the 4 Point Bending (4PB) test specimens were found to initiate from the bottom of the corrosion area by pitting or crevice corrosion. The SCC propagation rates in Types 304 and 304L under natural conditions were around 1.2E−12 to 1.8E−11 m/s in the K (Stress Intensity Factor) range of 0.6–9.0 MPa m^1/2, and that of the accelerated test (60 °C, 95% RHS, filled with NaCl mist) around 1.0E−10 to 3.5E−9 m/s in the K range of 0.5–30 MPa m^1/2. The SCC propagation rates under both natural and accelerated conditions were independent of K. The lifetime of sealability estimated from 1/5 scale models was longer than that from the small bending test specimens and has a safety margin as a structure.     

Fracture initiation and propagation in a cylindrical shell containing an initial surface flaw

The paper deals with the problem of fracture initiation, propagation, and arrest in a pressurized cylindrical vessel which contains an initial surface flaw. It is assumed that the flaw has the most unfavorable geometry and orientation, namely, it is a relatively long part-through axial crack.First we consider the problem of a crack which is sufficiently ‘shallow’ so that the plastic deformations are confined to the neighborhood of the crack border and part of the net section near the inner wall is still elastic. The plasticity-corrected stress intensity factor obtained from this analysis is the controlling load factor in failure considerations related to fatigue crack propagation, stress corrosion cracking, and static fracture (with the use of fracture toughness, COD, or a K_R curvetype failure criterion).The problem of relatively deep crack with fully-yielded net ligament is then considered. Plastic deformations are also assumed to spread around the crack ends through the entire wall thickness. A perfectly plastic strip model (with an eight order shell theory) is used to calculate the plastic zone size and the crack opening displacement along the crack border. Previous studies indicate that for the analysis of the type of stable and subsequent unstable crack propagation problems under consideration, the crack opening displacement δ is a more suitable load factor than the stress intensity factor K, or the crack extension force G. Thus, in this paper a ‘crack opening stretch’ type material characterization will be used.After the rupture of the net ligament under the crack, the axial crack propagation is accompanied by the depressurization of the vessel caused by leakage. From this point on the fracture problem is coupled with the related fluid mechanics or gas dynamics problem where the primary unknowns are the pressure and the crack length as functions of time. In the present study it is assumed that the volume of the vessel is finite and the crack propagation is quasi-static (this assumption, which is necessary to keep the problem within manageable proportions, is justified by the relatively low crack velocities, i.e. v_c < 0.25 c₂, c₂ being the shear wave velocity).     

Determination of the dynamic fracture toughness using a new stress pulse loading method

This paper presents a method for the determination of the dynamic fracture toughness K_Id of metallic materials at loading rates K_I of about . The method is derived from the known split Hopkinson pressure bar technique and uses a well-defined stress pulse for the loading of a fatigue precracked specimen. The interpretation of the experimental data is strictly based on a numerical analysis of the specimen under the given dynamic loading conditions. It is shown, that a conventional quasi-static approach would yield incorrect fracture toughness values. The results for some steels confirm, that the fracture toughness decreases with increasing loading rate. Therefore, in some sense the fracture toughness versus temperature curve determined with the presented stress pulse method can be regarded as lower bound curve.