Simulation of thermal fatigue damage in a 316L model pipe component

To contribute to the development of improved methods for assessing possible thermal fatigue damage in nuclear plant piping systems, a unique set of crack growth data has been generated for tubular test pieces in 316L(N) stainless steel subjected to cyclic thermal loads in a specially designed rig. By accurate modelling of the thermal loads and non-linear material behaviour using the finite element method, it was possible to reliably estimate the number of cycles to initiation, using standard isothermal fatigue life curves. To simulate crack growth, an engineering method was applied using published K solutions for semi-elliptical surface cracks and via 3-D elastic–plastic cracked-body analysis of selected scenarios. It was established that conservative estimates of the thermal fatigue crack growth can be obtained using the engineering model in conjunction with an upper bound fatigue crack growth law.     

海上风机支撑结构疲劳性能试验研究

  目的  针对海上风机支撑结构的疲劳问题,进行了外加强环过渡段结构疲劳性能的试验研究。  方法  采用简化的支撑结构1/5缩尺模型进行了静力与疲劳试验,获得了模型的热点应力和疲劳寿命;并根据DNV的海上风机规范,采用不同类型单元对试验模型进行了有限元分析和疲劳寿命估算,将规范方法的计算结果与试验进行了比较。  结果  结果表明:试验结果和规范方法之间的差异主要源于S-N曲线的选取。  结论  最后根据试验数据,在DNV规范的基础上给出两种改进的疲劳校核方案,可为实际应用提供指导。     

Stress intensity factors for a pipe-plate welded joint

A major component of any linear elastic fracture mechanics model for fatigue crack growth is the calculation of the crack tip stress intensity factor. This is particularly difficult for welded joints due to the complex geometry. While some data are available for cracks in welded T-plate joints, there is relatively little data available for larger cracks in more complex tubular joints. Such cracks are of significant interest since the most practical application of fracture mechanics models is the prediction of remaining life for cracks discovered in service.

A pipe-plate joint has been developed as a simplified model of tubular joint geometries for fatigue studies. Two such specimens have been tested in air, with detailed monitoring of crack growth behaviour using potential drop techniques. These data were used to obtain crack growth rate data from which estimates of stress intensity factors were made. Separately, finite element analyses for various discrete crack configurations were performed. The results of these analyses are presented and discussed, with particular emphasis on the accuracy of the results and the implications for fracture mechanics modelling.     

Preliminary investigation of PWR feedwater line cracking

Circumferential cracks were found in large diameter carbon steel piping that carries feedwater on the secondary side in PWR nuclear power plants. The cracks were confined to a narrow band in the vicinity of the counterbore region of the piping near the weld joining the piping to the feedwater nozzle on the steam generator. In most cases, the cracks were numerous, but small; a typical penetration was less than 20% through the wall. However, in some cases, the cracks were much greater in depth and, in one instance, propagated through the pipe wall, approximately 1·6 cm (0·65 in.). The environment was high temperature water containing less than 1 ppm oxygen.

A metallurgical analysis, in which both light and electron microscopy were utilized, led to the conclusion that the cracks, particularly the large ones, were caused by corrosion fatigue. This conclusion was based upon crack morphology, crack location, the presence of bench marks and striations, fracture appearance, and the presence of corrosion products. The next step in the failure analysis was to determine the source(s) of the cyclic stresses that resulted in the corrosion fatigue.

Stress and fatigue analyses were made in an effort to determine the combinations of loads and cycles necessary to produce the observed phenomena. Environmental effects were taken into account in carrying out the analyses. The results showed that plant specific mechanisms must have been responsible for the rapid fatigue growth. An analytical and experimental programme involving the investigation of thermal fluctuations was planned to determine the precise source of the required combinations of loads and cycles.     

Crack propagation rate as a function of crack tip characteristics

The dependence of crack growth rate on various crack tip parameters was studied. Experiments were performed on thin sheets of 6063-T6 Al-alloy having a central notch, to find crack tip opening displacement, total strain range, plastic strain range, crack opening stress and crack growth rate. Crack tip opening displacement and crack opening stress were measured, using a surface measurement technique, with small crack opening displacement gauges. The theoretical predictions of crack tip opening displacement compare fairly well with the experimental values. It is found that crack propagation rate vs total strain range-plastic strain range gives a straight-line fit on a log-log graph and, for positive stress ratios, the fatigue crack growth rates are found to be independent of R.

Experimental results show that the crack opening stress is not affected by the position of the gauge when it is mounted behind and near the crack tip.

The effect of mechanical properties and loading on crack growth were also studied. The specimens were fatigue cracked to a predetermined length and some specimens were annealed and again loaded cyclically. The application of cyclic loads to annealed specimens caused significant increase in crack propagation rates in comparison with the specimens having no heat-treatment. The load-displacement record was found to stabilize in about 10 cycles; the crack then extended slowly as a fatigue crack. Crack propagation rates for different values of R for annealed and work-hardened material were plotted against a crack tip parameter, ΔK*, based on notional crack lengths. Since the results of da/dN vs ΔK* for both states of material (as-received and annealed) seem to lie on the same straight line on a log-log graph, the study provides a hope that the results for a material tested in any state (annealed or work-hardened) for positive values of R (0·0≤R≤0·3) will lie on this line, thus eliminating fatigue tests on the same material under different work-hardening conditions for different values of R. Models for da/dN have been developed using various crack tip parameters.     

The near-threshold high R-ratio fatigue crack growth characteristics of SA508 cl III reactor pressure vessel steel

This paper describes the effect of frequency and environment on the near-threshold fatigue crack growth behaviour of SA508 cl III reactor pressure vessel (RPV) steel. The study has shown that in the near-threshold regime microstructure and environment markedly affect fatigue crack growth behaviour. In an aqueous environment, fatigue crack growth behaviour became even more sensitive to microstructure, and the fatigue crack growth rate increased by a factor of four in the case of the 3 Hz test, while that for the 0·3 Hz test was increased by a factor of approximately sixteen. This environmental enhancement manifested itself in the form of intergranular failure. For the 0·3 Hz test the percentage intergranular failure decreased from 18% to <1% with an increase in ΔK level. The transition from microstructure-sensitive to microstructure-insensitive occurs when the cyclic plastic zone size is of the order of the prior austenite grain size.     

Crack initiation and crack growth assessment of a high pressure steam chest

Extensive cracking had occurred in a number of high pressure steam chests. An assessment was undertaken based on the R5 British Energy methodology to assess the components for both creep–fatigue damage initiation and crack growth analysis to determine fitness for purpose. The analysis determined that the remaining base rupture endurance life of the component was greater then 1 million hours, however, due to the start-up and shutdown ramp rates, creep–fatigue damage greater then unity has occurred leading to crack initiation in a number of locations. These cracks were confirmed during internal inspection of the steam chest. A subsequent crack growth analysis determined that the component could safely be returned to service for the expected future life of the station.     

On the application of probabilistic fracture mechanics to the reliability and inspection of pressure vessels

A probabilistic fracture mechanics model was developed to analyse the failure probability of a typical high power density reactor pressure vessel. The major causes for the nuclear reactor pressure vessel failure include fatigue, corrosion fatigue and brittle failure. All these causes are greatly affected by the stress loading conditions, material properties (aged by neutron damage), and defects embedded in the structure. Both an analytical first-order second-moment approximation and a hybrid methodology were employed in this study. In addition to the static scatter of the pre-existing cracks and material properties, a random walk model based on the operating history was introduced to represent the random occurrence of the abnormal transient stresses. The failure mode is defined as the brittle failure caused by a critical crack, meaning the stress intensity factor around a critical crack exceeding the fracture toughness of the pressure vessel material. Through a sample study on a typical high power density nuclear power plant, it was found that the vessel failure probability is about 4 × 10⁻⁴ at the 40th year of operation and the failure rate is in the order of 5 × 10⁻⁶ per vessel per year, which had reasonable agreement with the value of 10⁻⁴−10⁻⁶ as reported based on real-world statistics. In addition to the failure probability caused by fatigue crack growth, the reliability of a Low Temperature Overpressure incident was also evaluated.     

Fatigue crack initiation and propagation in austenitic stainless steels for light water reactors

The determination of fatigue life of components containing defects usually takes into account crack propagation only. In a real situation, a number of cycles are often required to reach fatigue crack initiation and predictive evaluation of fatigue crack initiation phases of real defects in austenitic stainless steel welded joints are presented. Fatigue crack growth rates in wrought and cast austenitic stainless steels and associated welds are also presented. Effects of various mechanical parameters (R ratio and variable amplitude loading) of a PWR environment and of metallurgical factors (δ ferrite content and ageing in cast austenitic stainless steels) are discussed.     

A numerical simulation of fatigue growth of multiple surface initially semicircular defects under tension

The shape development of a row of five small initially semicircular surface defects through the thickness of a plate under tension is numerically predicted by the step-by-step finite element simulating technique developed by the present authors. Fatigue growth of these cracks during pre-coalescence, coalescence and post-coalescence periods is examined in detail. The fatigue crack growth of a single surface crack with the same initial size is also simulated and compared with that of the multiple crack configuration. The comparison shows that the fatigue growth of each individual defect is almost independent prior to coalescence and the interaction between them is very limited even as they approach each other very closely. The fatigue crack growth curve predicted by the numerical technique is compared with those obtained by the ASME Pressure Vessel Code and the ‘no interaction and immediate transition (NIIT)’ simplified method. The results show that the two simplified estimations are more conservative than that of the present numerical simulating method.     

Numerical simulation of tearing–fatigue interactions in 316l(N) austenitic stainless steel

The loading history of engineering components can influence the behaviour of defects in service. This paper presents, the results of a numerical study aimed at using the Gurson ductile damage model, calibrated against J R-curve data, to simulate load-history effects on ductile tearing behaviour in austenitic materials. The work has demonstrated that ductile crack growth resistance is influenced by sub-critical crack growth by an intervening mechanism such as fatigue. Fatigue crack growth under a positive R-ratio leads to increase in subsequent tearing resistance through three main mechanisms: (i) re-sharpening of the crack tip; (ii) crack extension through the fracture process zone; and (iii) cyclic loading effects on void development. The ratio of minimum to maximum stress during fatigue loading (R-ratio) has been shown to influence subsequent tearing resistance, with an R-ratio of 0.2 generally leading to a greater enhancement in tearing resistance than an R-ratio of 0.1. This behaviour is due to the influence of R-ratio on void development ahead of the fatigue crack tip. Finally, relevant experimental data compare favourably with the predicted J R-curves.     

Stress corrosion cracking of nuclear reactor pressure vessel and piping steels

This paper presents an extensive investigation of stress corrosion cracking of nuclear reactor pressure vessel and piping steels exposed to hot water. Experimental fracture mechanics results are compared with data from the literature and other laboratories. Thus a comprehensive overview of the present knowledge concerning stress corrosion crack growth rates is provided. Several sets of data confirm that ‘fast’ stress corrosion cracks with growth rates between 10⁻⁸ and 10⁻⁷ m/s and threshold stress intensities around 20 MN m^−3/2 can occur under certain conditions. However, it appears possible that specific environmental, mechanical and metallurgical conditions which may prevail in reactors can result in significantly lower stress corrosion crack growth rates. The presently known stress corrosion crack growth rate versus stress intensity curves are discussed with emphasis on their usefulness in establishing safety margins against stress corrosion cracking of components in service. Further substantial research efforts would be helpful to provide a data base which permits well founded predictions as to how stress corrosion cracking in pressure vessels and piping can be reliably excluded or tolerated. It is emphasized, however, that the nucleation of stress corrosion cracks (as opposed to their growth) is difficult and may contribute substantially to the stress corrosion free service behavior of the overwhelming majority of pressure vessels and pipes.     

Characteristics of hydrogen-related fatigue fracture in 2Mn-0.1C martensitic steel

This study investigated the hydrogen-related fatigue fracture in 2Mn-0.1C steel having a lath martensite microstructure. The presence of hydrogen significantly reduced the fatigue life. The transgranular surface was a main component in each stress range of the uncharged specimen, while the intergranular surface was frequently observed in the hydrogen-charged specimen. The crystallographic orientation analysis by electron backscattering diffraction revealed that the cracks mainly propagated along {011} planes regardless of the presence of hydrogen. Compared with the uncharged specimen, however, plastic deformation was localized near the fatigue crack in the hydrogen-charged specimen. According to the reconstructed fracture process by fracture surface topography analysis, the hydrogen-related fatigue fracture was discontinuous and composed of isolated nucleation of intergranular cracks and quasi-cleavage crack propagation initiated at the pre-existing intergranular cracks.     

Experimental investigation of fatigue behaviour for welded joint with mechanical heterogeneity

Because of material and mechanical heterogeneity in welded joints, experimental studies of low-cycle fatigue (LCF) life and fatigue crack initiation behaviour were performed on weld metal, heat-affected zone (HAZ) and base metal of 16 MnR pressure vessel steel. A fatigue damage test was carried out by introducing a new damage variable. The fatigue damage evolution laws in each zone of a welded joint were obtained. A circular-notched compact tension specimen was designed and the whole process of fatigue damage, crack initiation and crack growth in each zone was investigated. The effect of mechanical heterogeneity in welded joints on the fatigue damage behaviour and fatigue crack initiation life was also investigated. The experimental and theoretical results indicate that the fatigue behaviour is markedly different in three zones of welded joints; there is a trend toward reduction in the fatigue life of HAZ as compared with the base metal. The weld metal does not have the same fatigue behaviour as confirmed by LCF life and crack growth rate. This study leads to the conclusion that fairly accurate estimates of fatigue life for welded joints should include the whole process of fatigue damage evolution and fatigue crack initiation as well as crack growth. Furthermore, a local experimental method should be carried out for weld metal, HAZ and base metal, respectively. Finally, the local fatigue failure criterion based on the weakest chain model has been presented.     

THE INFLUENCE OF CRACK-FACE FLUID PRESSURE ON THE FATIGUE CRACK PROPAGATION DUE TO ROLLING CONTACT WITH FRICTIONAL HEAT

A three-dimensional surface planar crack problem in a half-space is considered under rolling/sliding contact with frictional heat and hydraulic pressure by the entrapped fluid within the crack. Rolling contact is simulated as a line load with both normal and shear components, moving with constant velocity over the surface of the half-space. The body force method for three-dimensional fracture mechanics is utilized to determine the three modes of stress intensity factors along the crack contour. To account for mixed-mode propagation, the modified Paris power law is used. Numerical results for the stress intensity factors and the simulations of fatigue crack propagation are given for 30-degree inclined planar surface cracks of semicircular shape. The effects of the frictional coefficient, sliding/rolling ratio, and the crack-face fluid pressure on the crack propagation life are considered for a high carbon-chromium bearing steel.     

Influence of mechanical and environmental variables on crack growth in PWR pressure vessel steels

Knowledge of the propagation rates of sub-critical cracks exposed to primary circuit coolant is essential for the use of defect assessment procedures for validation of PWR pressure vessel integrity. The results of research programmes conducted by the UKAEA to assess the conditions under which the PWR environment influences crack propagation of low alloy steels under cyclic and steady loading are described. The measured corrosion fatigue crack propagation rates for low and medium sulphur steels in good quality flowing water were well below those predicted by the ASME Section XI, Appendix A assessment curves, but higher rates were attainable for high sulphur steel or under adverse environmental conditions. No influence of material microstructure due to welding was observed in high flow water.No susceptibility to stress corrosion has been observed in bolt-loaded specimen tests on a range of parent steels and weldments. Cracking in slow strain rate tests was observed only at high potentials, such as were produced by the presence of dissolved oxygen, unless the specimen orientation was such as to maximise access of PWR coolant to sulphide inclusions in the steel.The implications of the data for operating plant and progress with the development of improved methods for assessing crack growth in operating plant are discussed.     

Comparison and evaluation of creep crack growth rate and fracture time for Alloy 800 by the Q* parameter

Creep crack growth tests were carried out under several temperature and stress conditions for Fe-based superalloy Alloy 800 and the Q* parameter was applied to an evaluation of the crack growth rate at elevated temperatures for DEN and CT specimens. Creep fracture times were evaluated with the parameter derived by integrating the creep crack growth rate formula expressed in terms of the Q* parameter for DEN, CT and smooth specimens of Alloy 800 under several temperature and stress conditions. It was found that the creep crack growth rate for CT specimens is represented as the different data bands by the Q* parameter for DEN specimens. However, it can be evaluated by parallel movement along the Q* axis in the Q* parameter-da/dt diagram for Alloy 800. The parameter {T(log₁₀t_f+20)×10⁻³} derived from the Q* parameter can compare and evaluate the difference in specimen shape and loading mechanism on creep fracture time using the identical parameter for Alloy 800.     

Creep and damage analysis of a serviced tee intersection in a boiler header: Comparison between numerical and experimental results

In the last few years modelling of damage phenomena under creep condition has been developed in order to take into account the microstructural material evolution in life predictions of high temperature components. The new analytical methods based on “Continuum Damage mechanics” and experimental creep and creep-rupture data aim at describing both stress-strain and damage field in structures in order to predict crack initiation. These models are implemented in computer finite element programs and should be subjected to rigorous experimental verification for a practical use in power plant assessment activities.

In the present paper the numerical results obtained from some creep and damagement analyses of a header component (10 CrMo 910 steel) are shown and compared with the experimental ones. The creep analyses have been performed by the computer code ABAQUS and the damage evaluation has been carried out by means of proper in-house developed user's subroutine and post processor.     

Numerical analysis of fatigue growth of external surface cracks in pressurised cylinders

Fatigue crack growth is modelled by a novel numerical technique for various external surface cracks with initially either semi-elliptical or irregular crack fronts. The technique employs a three-dimensional finite element analysis to estimate the stress intensity factors at a set of points of the crack front, then calculates local crack advances by integrating a type of Paris fatigue crack growth law at this set of points, and finally establishes a new crack front and its corresponding finite element model. The multiple degrees-of-freedom model enables the crack shape to be predicted directly during crack growth without having to make the common semi-elliptical assumption, and therefore provides more accurate predictions. Fatigue analysis results are presented and discussed, including fatigue shape developments and deviations from the semi-elliptical shape, together with aspect ratio changes, stress intensity factor variations during crack growth and fatigue life predictions. Some of these results are also compared with those obtained by two simplified predictive methods based on one and two degrees-of-freedom models together with a stress linearisation.