Ductile fracture properties for assessing leak-before-break issues in ferritic weldments

A Leak-Before-Break (LBB) approach is being used by Ontario Hydro's Darlington nuclear generating station as a design alternative to pipe rupture restraint hardware on the large diameter piping of the primary heat transport system. The J-resistance curves of four different ferritic weldments, fabricated by either the submerged arc weld (SAW) or shielded metal arc weld (SMAW) process, were determined as part of this program.

Results indicated that the as-welded and post-weld heat treated (PWHT) welds were susceptible to varying degrees of static or dynamic strain aging at 200 and 250°C. Dynamic strain aging effects were most significant for as-welded welds, as evidenced by sudden load drops on the load-displacement curves and ductile crack jumping. The effect of loading displacement rate and PWHT on toughness was assessed and related to the weld's tensile properties and susceptibility to dynamic strain aging. Implications of strain aging to LBB assessments are discussed.     

Compound cracks in pipes under combined bending and tension

Limit load and J-resistance curve solutions are developed for a compound crack in a pipe under combined tension and bending. The solution is based on a thick-walled cylinder assumption and the J solution can be applied with load-displacement data from one pipe test. Material resistance curves are developed for compound cracks in Type-304 stainless steel, Inconel 600 and A106 GrB base materials and a 304 stainless steel TIG weld. Tearing instability analyses are performed to assess the load-carrying capacity of pipes containing compound cracks and to evaluate nonconservatism associated with the use of C(T) specimen J-R data.     

Evaluation of fracture toughness based on results of instrumented Charpy tests

The modified Gurson model is used to determine micromechanical toughness parameters of a weld material from the irradiation surveillance of a reactor pressure vessel. These parameters were used to numerically simulate static and dynamic fracture mechanics tests for the determination of J-resistance curves. The results were confirmed by means of experiments with irradiated subsized specimens of the original material. The essential steps of the analysis—the consideration of rate-dependent stress-strain curves and the determination of the material parameters, as well as the numerical modelling of the boundary conditions at the supports of the Charpy specimen—were validated by comparing analyses and experiments with a representative unirradiated submerged are weld material. The initiation values of the J-resistance curves converted into pseudo-plane strain fracture toughness values K_Ic allowed the conservative adjustment of the ASME reference fracture toughness curve.     

Measurement of crack propagation velocity in nuclear pressure vessel steel (SA516 gr. 70)

An attempt has been made to develop a simple, reliable and cost-effective device for measuring the dynamic crack propagation velocity in a nuclear pressure vessel steel (SA516 gr. 70). The experimental method is described and a simple digital approach is proposed. The experimentally determined dynamic crack velocity has been utilized to obtain elastic dynamic stress intensity factors by INSAMCR (a two-dimensional dynamic finite element code which is a modified version of SAMCR developed by Dr Schwartz at the University of Maryland). A relationship between instantaneous crack tip velocities and dynamic stress intensity factors for pressure vessel steels is estimated using dynamic crack propagation velocities determined by a proposed measuring device. The relationship between the dynamic stress intensity factor and time history and the dynamic arrest toughness for each test are obtained using the generation mode dynamic finite element analysis. A function ƒ(å) = 1·356 − 2·672å + 6·494å² − 4·539å³ + 1·461å⁴ is suggested which may be useful to predict the relationship between the dynamic fracture toughness (K(å)) and the dynamic crack arrest toughness (K_Ia) for SA516 gr. 70 steel (say K(å) = K_Ia ƒ(å) where å is the dynamic crack propagation velocity).     

Principles of the German approach to the analysis of flawed structures

The analysis of flawed structures in the linear elastic regime (LEFM) has been treated very extensively, especially in the framework of the 4th German nuclear programme. Investigations with large specimens have demonstrated the applicability of the LEFM for a variety of materials ranging from low to high toughness levels. In the elastic-plastic (EPFM) regime, however, especially the J-integral method has been used for quantifying the safety margin, because with a required value of upper shelf Charpy energy alone this quantification is not possible.

A correlation between the crack initiation value J_i and Charpy upper shelf energy was evaluated experimentally, justifying the generally upgraded upper shelf energy level adopted in the Code. Furthermore, this correlation can be employed when fracture toughness values are not available as is mostly the case for irradiated material from surveillance programmes.

To cope with the problems resulting from repeated transients in a complex component, incipient crack intitiation and propagation under cyclic thermal load have been investigated experimentally and theoretically on a RPV nozzle corner in the HDR plant and on a thick-walled (200 mm) hollow cylinder subjected to pressurized thermal shock (PTS). Since the OCA code covers only the linear elastic range, for this loading case elastic-plastic fracture mechanics calculations have been carried out by means of the Finite Element Method. The first test performed with high toughness material has shown good agreement with the J-integral approach. Additional validation tests are under way to demonstrate the lowest tolerable toughness level to withstand PTS without catastrophic failure.

With respect to critical flaw sizes in degraded piping and vessels, the ‘leak before break’ limit curve for different loading conditions has been established and experimentally validated using piping and model vessels of different sizes, crack configurations and toughness levels.

The still existing uncertainties in the detection, sizing and interpretation of signals from nondestructive examination are the background of NDE validation programmes for both medium size and full size reactor pressure vessels. Acoustic emission trials as well as extensive ultrasonic (US) examinations will be pursued in cooperation programmes, last but not least in the framework of PISC III.     

The determination of Jq, the initiation elastic-plastic toughness parameter, using pre-cracked Charpy specimens

In order to assess the structural integrity of pressure vessel steels in the nuclear and chemical industries a need has arisen to measure fracture toughness using Charpy V-notch surveillance specimens.

This paper describes a procedure for determining the initiation fracture resistance, J_q of a material using pre-cracked Charpy specimens loaded in three-point bend. The size restrictions imposed by the geometry of a Charpy specimen on the measurement of the plane strain fracture toughness, K_IC, and the elastic-plastic toughness parameter, J_IC, are discussed for typical low- and medium-strength steels.

Consideration is given to the measurement of crack growth, side-grooving and the use of exclusion lines to interpret the fracture resistance, J, versus crack growth, Δa, curves.     

Probabilistic fracture analysis of cracked pipes with circumferential flaws

This paper describes the development of a probabilistic fracture-mechanics model for analyzing circumferential through-walled-cracked pipes subject to bending loads. It involves elastic-plastic finite element analysis for estimating energy release rates, J-tearing theory for characterizing ductile fracture, and standard structural reliability methods for conducting probabilistic analysis. The evaluation of the J-integral is based on the deformation theory of plasticity and power-law idealizations of stress-strain and fracture toughness curves. This allows J to be expressed in terms of non-dimensional influence functions (F- and h₁-functions) that depend oncrack size, pipe geometry, and material hardening constant. New equations were developed to represent these functions. Both analytical and simulation methods were formulated to determine the probabilistic characteristics of J. The same methods were used later to predict the failure probability of pipes as a function of applied load. Numerical examples are provided to illustrate the proposed methodology. The validity of the J-integral based on the proposed equations for predicting the crack driving force in a through-wall-cracked pipe was evaluated by comparison with available results in the current literature. Probability densities of the J-intergral were predicted as a function of applied loads. Failure probabilities corresponding to three different performance criteria were evaluated for stanless steel nuclear piping from a boiling water reactor plant. The results suggest that large differences may exist in the failure probability estimates produced by these performance criteria.     

Mechanical property variation within Inconel 82/182 dissimilar metal weld between low alloy steel and 316 stainless steel

In several locations of pressurized water reactors, dissimilar metal welds using Inconel welding wires are used to join the low alloy steel components to stainless-steel pipes. Because of the existence of different materials and chemistry variation within welds, mechanical properties, such as tensile and fracture properties, are expected to show spatial variation. For design and integrity assessment of the dissimilar welds, these variations should be evaluated. In this study, dissimilar metal welds composed of low alloy steel, Inconel 82/182 weld, and stainless steel were prepared by gas tungsten arc welding and shielded metal arc welding techniques. Microstructures were observed using optical and electron microscopes. Typical dendrite structures were observed in Inconel 82/182 welds. Tensile tests using standard and mini-sized specimens and micro-hardness tests were conducted to measure the variation in strength along the thickness of the weld as well as across the weld. In addition, fracture toughness specimens were taken at the bottom, middle, and top of the welds and tested to evaluate the spatial variation along the thickness. It was found that while the strength is about 50–70 MPa greater at the bottom of the weld than at the top of the weld, fracture toughness values at the top of the weld are about 70% greater than those at the bottom of the weld.     

The effect of a long post weld heat treatment on the integrity of a welded joint in a pressure vessel steel

The effect of a long post weld heat treatment on the microstructure and mechanical properties of a welded joint in a 0·2%C-1·4%Mn-0·5%Mo pressure vessel steel was studied. Multipass submerged-arc welds were made at a heat input of 1·2 and 4·3 kJ mm⁻¹. Individual microstructural regions observed in the heat-affected zone of the actual weld were simulated. These regions were brittle in the as-simulated condition. Post weld heat treatment for periods of up to 40 h at 620°C resulted in a significant improvement in the Charpy impact toughness. At the same time, a loss of the heat-affected zone and weld metal hardness and transverse weld strenghth occurred. A fracture toughness (J_Ic) of 134 kJ m⁻² was measured in the heat-affected zone of the 4·3 kJ mm⁻¹ welds after prolonged post weld heat treatment. The improvement in weldment toughness with post weld heat treatment was primarily attributed to softening of the structure.     

Fatigue life response of ASME SA 106-B steel in pressurized water reactor environments

Fatigue strain-life tests were conducted on ASME SA 106-B piping steel base metal and weld metal specimens in 288°C (550°F) pressurized water reactor (PWR) environments as a function of strain amplitude, strain ratio, notch acuity, and cyclic frequency. Notched base metal specimens tested at 0·017 Hz in 0·001 part per million (ppm) dissolved oxygen environments nearly completely used up the margins of safety of 2 on stress and 20 on cycles incorporated into the ASME Section III design curve for carbon steels. Tests conducted with smooth base metal and weld metal specimens at 1·0 Hz showed virtually no degradation in cycles to failure when compared to 288°C air test results. In all cases, however, the effect of temperature alone reduced the margin of safety offered by the design curve in the low cycle regime for the test specimens. Comparison between the fatigue life results of smooth and notched specimens suggests that fatigue crack initiation is not significantly affected by 0·001 ppm dissolved oxygen, and that most of the observed degradation may be attributed to crack growth acceleration. These results suggest that the ASME Section III methodology should be reviewed, taking into account the PWR environment variables which degrade fatigue life of pressure-retaining components.     

Tensile and fracture properties evaluation of PHT system piping material of PHWR

The aim of this paper is to report the tensile and fracture properties of SA333 Gr.6 carbon steel material which is used for the primary heat transport (PHT) system piping of the Indian pressurized heavy water reactor (PHWR). Tensile and J integral tests have been carried out on specimens machined from the base material as well as weldments of actual PHT pipes. The effects of test temperature and specimen orientation on the material properties have been discussed.     

Numerical analysis of IPIRG cracked pipe experiments subjected to dynamic and cyclic loading

This report contains results of a finite element study aiming to identify the influence of loading history and geometry for cracked pipes subjected to complex loading. The experiments have been performed within the International Piping Integrity Research Group (IPIRG) Program. The majority of the numerically analyzed experiments were conducted on straight pipes with an outside diameter of 168 mm and containing a large circumferential through-wall crack. The considered pipes were loaded in four-point bending under displacement control and at a temperature of 288°C. The types of loading were combinations of either quasi-static or dynamic and also monotonic or cyclic loading with different loading ratios R. Some analyses were also performed on surface-cracked pipes subjected to slow, monotonic loading.

In the finite element study, 20-node solid elements were used for the through-wall cracked pipes and a combination of shell and non-linear line spring elements for the surface-cracked pipes. Stable crack growth was simulated by gradual node relaxation and crack closure is accounted for by using simple contact elements. The J-integral for a remote contour is calculated and used as a characterizing fracture parameter although the cyclic loading violates the theoretical basis for this procedure. The near-tip J can not be used for growing cracks because of the weak energy singularity. The results of the numerical study confirm the trends from the experiments in that a high loading rate has a negative influence on the fracture properties of the studied carbon steel and that large cyclic loading, especially at R = −1, lowers the apparent J_R-curve for both carbon and stainless steels. To some extent geometry effects appear to be present when comparing the results from pipes containing surface cracks and through-wall cracks with results from CT specimens. These effects are more pronounced for large amounts of stable crack growth than at initiation.     

Experimental study of effects of prior overload on fracture toughness of A533B steel

This paper presents the results of an extensive study carried out to examine the effects of prior overloading over the entire fracture transition regime for 50-mm thick A533B steel. The main variables examined are temperature, crack orientation with respect to the rolling direction, level of prior overload, the initial crack length, and the statistical variation of prior overload effects. It is found that the effect of prior overload on fracture toughness at lower temperatures is dependent on orientation, so that in the L-T orientation for short and medium cracks (0·2 and 0·5 a/W) there is a benefit throughout the transition regime of 50-mm thick A533B steel. In the T-L orientation no benefit is obtained for temperatures greater than the initiation of tearing temperatures. Above these temperatures the prior overload sequence lowers the fracture toughness. For L-T orientation long cracks (a/W = 0·7) it is found for temperatures lower than −140°C that prior overload apparently increases the toughness. At higher temperatures there is a loss of toughness even though failure is cleavage dominated up to −80°C.

On the lower shelf at −170°C in the L-T orientation the fracture toughness variability after preloading is found (based on a sample of 14 specimens) to exhibit a bimodal distribution. This distribution is similar to that exhibited by non-preloaded material.     

The analytical fracture behaviour of a bar in tension containing a circumferential edge crack

This paper evaluates the analytical fracture behaviour of a bar in tension containing a circumferential edge crack. The stress intensity factor and limit load solutions available in the literature are reviewed and then used to derive the compliance and J-integral functions for a test specimen. The load at which plastic necking occurs in the specimen is also evaluated for idealized material behaviour to establish the maximum fracture toughness that can be measured as a function of specimen size. The analyses enable the optimum geometry and size of a specimen for fracture toughness measurements to be deduced.     

Fracture toughness and fatigue crack growth properties of the base metal and weld metal of a type 304 stainless steel pipeline for LNG transmission

The fatigue crack growth rate and CTOD tests on type 304 stainless steel and weld metal were studied over the temperature range −162°C to room temperature. The girth weld metal specimens were fabricated using a combination of gas-tungsten-arc-welding and shielded-metal-arc-welding. The seam weld joint was made by submerged arc welding. Fracture toughness was evaluated through CTOD tests with three point bend specimens. The fatigue crack growth rate tests were conducted using compact tension specimens in accordance with ASTM E647. The CTOD values were affected by crack orientation with respect to the rolling direction, but orientation had no influence on the fatigue crack growth rates. The fatigue crack growth rates and the CTOD values decreased with decreasing test temperature.     

Brittle fracture toughness—Experimental estimation of RPV materials and their welds containing shallow cracks

This paper presents brittle fracture resistance results for types 15X2MFA, 15X2NMFA steels and their weldments, produced by submerged arc welding as applied to nuclear reactor VVER-440 and VVER-1000 pressure vessels production and maintenance conditions. The fracture toughness/temperature dependence of these materials was determined for cracks with the depth 0·1 (small) and 0·5 (standard) from specimen thickness. The crack shape (through, semi-elliptical); material state (beginning and end of service life), and the specimen production processing (presence or absence of anticorrosive cladding) were also varied. The consideration of real crack size enables one to sufficiently increase the brittle fracture resistance of reactor materials and their welded joints. This fact should be used in the performance of control calculations.     

Application of leak-before-break to primary loop piping to eliminate pipe whip restraints in a Spanish nuclear power plant

The Spanish plant described in this study is a 982 MWe PWR plant with a three-loop primary circuit of piping made from centrifugally-cast stainless steel SA351 CF8A.

The licensee requested an exemption to GDC-4, from CSN, so as to avoid the postulation of guillotine rupture of the primary loop piping. The request was based on the generic work performed for a US PWR plant group to have such exemption to GDC-4. As the piping material in the Spanish plant is different from that in the plants included in the generic work, CSN performed a review of the applicability of the generic results to the Spanish plant. Also, aspects such as fatigue evaluation, net section collapse, crack growth and leak detection, specifically analyzed for the Spanish plant, were reviewed.

CSN found that fracture toughness test results from generic work are applicable to the Spanish plant; sufficient margin exists against unstable crack extension, and adequate leak detection capability exists with the leakage detection systems available in the plant.

Exemption to GDC-4 was approved and CSN authorized the licensee to remove protection devices against dynamic loads from guillotine breaks in the primary coolant loops.     

Fracture toughness of weld metals in steel piping for nuclear power plants

To determine toughness behaviour of dissimilar welds in steel piping and obtain data to evaluate Leak-Before-Break for these welds, an experimental study on fracture toughness was carried out. This paper provides Charpy impact properties and fracture toughness data of base and weld metals of dissimilar welds in nuclear piping.     

Fracture mechanics analysis of stable crack growth under sustained load and instability of circumferential cracks in straight water-steam pipes

In order to determine the component behaviour of pipes made from X 20 CrMoV 121 steel for the water-steam circulation system of a helium-cooled high-temperature reactor component, tests were carried out under load conditions similar to the case of emergency cooling. The aim of the tests was to determine the stable and unstable crack growth in the less tough weld filler metal of girt-welded pipes of 18·5 mm wall thickness and 400 mm inside diameter. The tests were carried out with weld filler metals of two different toughnessesSuch stable crack growth could be of particular importance in the case of residual heat removal. For this reason two tests were carried out on pipes with a circumferential crack under sustained load.In the second part of the test programme pipes with defects were loaded under sustained internal pressure, and additionally an increasing bending moment was applied. The aim of this investigation was to determine the boundary conditions for failure occurring by leak or by fracture with regard to the different toughness of the weld filler metal.The fracture mechanics analysis was carried out using a modified version of the flow-stress criterion for circumferential cracks (Batelle approach) and using also the so-called ‘effective fracture toughness’ $\text{K}{}_{\mathrm{<mtext>eff</mtext>}}$.The investigation has revealed that the component toughness is adequately high and component failure by fracture is not expected. The experimental results can be described theoretically using either the Battelle approach or the formulae of linear-elastic fracture mechanics if the data obtained in the test characterising component behaviour (flow stress or effective toughness) are used as a basis. The results show that an evaluation of component behaviour using $\text{K}{}_{\mathrm{<mtext>Jo</mtext>}}$ values determined by CT specimens according to the $\text{J}$ integral procedure is too conservative.     

Optimum material distributions for prescribed apparent fracture toughness in thick-walled FGM circular pipes

This study treats the inverse problem of evaluating optimum material distributions intending to realize prescribed apparent fracture toughness in thick-walled functionally graded material (FGM) circular pipes. The incompatible eigenstrain induced in the pipes after cooling from the sintering temperature due to the nonhomogeneous coefficient of thermal expansion is taken into consideration. An approximation method of finding stress intensity factors for a crack in the FGM pipes is introduced in which the nonhomogeneous material properties are simulated by a distribution of equivalent eigenstrain. A radial edge crack emanating from the inner surface of the homogenized pipes is considered for the case of a uniform internal pressure applied to the surfaces of the pipes and the crack. The stress intensity factors determined for the crack in the homogenized pipes represent the approximate values of the stress intensity factors for the same crack in the FGM pipes, and are used in the inverse problem of evaluating optimum material distributions intending to realize prescribed apparent fracture toughness in the FGM pipes. Numerical results obtained for a thick-walled TiC/Al₂O₃ FGM pipe reveal that the apparent fracture toughness significantly depends on the material distributions, and can be controlled within possible limits by choosing an optimum material distribution profile.