Tearing–fatigue interactions in 316L(N) austenitic stainless steel

This paper presents the results from a programme of tearing, fatigue and tearing–fatigue tests performed on specimens from a 316L(N) stainless steel plate. All tests were carried out at ambient temperature. The experimental results have been compared with assessments performed using current guidance within the R6 defect assessment method. The work has shown that there is some evidence that fatigue cycling modifies the JR-curve behaviour of this material. In most cases, the data lie approximately 20–30% above the base-line JR-curve. However, whilst there may be a modest influence of fatigue crack growth on the ductile tearing characteristics, it is difficult to separate this from experimental scatter. In tearing–fatigue tests performed at a stress ratio, R=0.2, ductile tearing reduces the fatigue crack growth rates by up to 50%. This is likely to result from the presence of a residual compressive zone at the crack-tip, and increased crack closure due to the irregular and non-matching fracture surfaces generated by the ductile crack growth mechanisms. For R=0.1 tearing–fatigue tests, fatigue crack growth rates are apparently enhanced by a factor up to of 10, particularly during the latter stages of the tests when ΔK>60 MPam. This is likely to result from: (i) loading being in the elastic–plastic regime where the J-integral (rather than K) characterises the crack-tip fields, (ii) increments of ductile tearing which may occur during each fatigue cycle, and (iii) crack blunting which reduces crack closure effects. For the R=0.2 tearing–fatigue tests, the linear summation approach described in R6 provides a consistently conservative prediction of ductile, fatigue and total crack growth during the tests. However, for the R=0.1 tearing–fatigue tests, the Paris law under-predicts fatigue crack growth rates. This may be corrected by using the Kaiser equation, which acknowledges loading in the elastic–plastic regime and incorporates incremental growth due to tearing as well as fatigue. R6 provides conservative predictions of instability for the CT specimen geometry tested in the current programme, both in terms of the critical crack growth and load required for instability to occur.     

Crack growth rate in hydrogen pre-charged martensitic steels during slow strain rate tests

Crack growth rate in two high strength martensitic steels with the Mo contents of 0.43 wt.% and 1.06 wt.% was investigated by means of slow strain rate tests (SSRT) on compact tensile specimens after hydrogen pre-charging. It was found that the crack growth rate increased and the values of stress intensity factors K_IH and K_Imax decreased with the increase of pre-charged hydrogen concentration. The steel with higher Mo content showed much lower crack growth rate than the steel with lower Mo content. It could be attributed to more nano-sized precipitates that can act as the hydrogen trapping sites and mitigate hydrogen deleterious effects on crack growth rate and the K_IH and K_Imax values.     

Effects of crack depth and specimen size on ductile crack growth of SENT and SENB specimens for fracture mechanics evaluation of pipeline steels

A strong geometry dependence of ductile crack growth resistance emerges under large scale yielding. The geometry dependence is associated with different levels of crack tip constraint conditions. However, in a recent attempt to identify appropriate fracture mechanics specimens for pipeline steels, an “independent” relationship between the crack growth resistance curves and crack depths for SENT specimens has been observed experimentally. In this paper, we use the complete Gurson model to study the effects of crack depth and specimen size on ductile crack growth behavior. Crack growth resistance curves for plane strain, mode I crack growth under large scale yielding conditions have been computed. SENB and SENT specimens with three different specimen sizes, each specimen size with three different crack depths, have been selected. It has been found that crack tip constraint (Q-parameter) has a weak dependence on the crack depth for specimens in the low constraint regime.     

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.     

基于裂缝口开度的水工混凝土结构裂缝性态监控方法

针对复杂工况下带缝水工混凝土结构的裂缝扩展判据难以用于工程实际,且常规有限元法扩展法需不断进行网格重建的问题,推导了包含裂尖效应的渗流—应力耦合的扩展有限元模型,以此分析并建立了裂缝口开度与宏观裂尖开度在裂缝扩展全过程的关系,提出了基于粘着力的宏观裂尖判别法,并从数值分析角度验证了宏观裂尖开度临界值的无尺寸效应,利用其与裂缝口开度的关系建立了水工混凝土结构裂缝起裂、稳定扩展、失稳扩展全过程的监控方法,最终通过算例验证了该方法的合理性。     

Creep crack growth of seam-welded P22 and P91 pipes with artificial defects. Part I. Experimental study and post-test metallography

This paper describes the experimental work conducted on creep and fatigue crack growth behaviour in axially-notched, seam-welded pipes. The key objective of this work is to examine how the results from feature tests compare with standard laboratory specimen data using different European high temperature assessment methods, the final aim being to contribute towards the development and validation of a unified European procedure for estimating crack growth behaviour and remaining life of high temperature components. Within the context of this work, two steels P22 (2 1/4Cr1Mo) and P91 (9Cr1MoVNb) were studied at 565 and 625 °C, respectively.Two pipes were tested for each material; one pipe was subjected to a constant gas pressure (SP) and the other was tested under low cyclic pressure (CP) (10⁻⁴ Hz). Each pipe contained three axially machined notches, one in the heat affected zone (HAZ) and two in the base metal. The direct current (DC) potential drop (PD) technique was successfully applied to monitor crack growth during the tests. In both steels, the defect situated in the HAZ exhibited a higher crack growth rate, confirming that the HAZ is more vulnerable to crack initiation and propagation than the parent metal. Post-test metallography analysis showed that creep cavitation damage is the main mechanism governing the crack propagation. In the case of the cyclic tests for which the selected frequency was close to, or slightly higher than, the frequencies encountered in high temperature plants, metallographic observations showed no noticeable effect of cyclic loading in terms of transgranular crack growth.The data analysis of the experimental data obtained in this work are presented in Part II of this paper [Creep Crack Growth of Seam-welded P22 and P91 Pipes with Artificial defects—Part II. Data analysis. Second International HIDA Conference, Advances in Defects Assessment in High Temperature Plant, MPA, Stuttgart, Germany, 4–6 October, 2000].     

Investigation on behavior of trapezoidal through-wall cracks under constant pressure loading

In general, a crack found in a steam generator tube has an elliptical shape, and as soon as it penetrates through the wall, it is assumed to be rectangular for conservatism. However, the leak and crack growth behavior have not been clearly understood after the elliptical crack penetrates the tube wall. One steam generator tube failure showed the tube rupture behavior without any significant prior indication. Several experiments using steam generator tubes with a trapezoidal crack were performed under constant pressure loading by Argonne National Laboratory and exhibited crack growth behavior much faster than expected. It has been found to be caused by time-dependent fatigue crack growth. No K_I solution is available for a trapezoidal crack which can be used for fatigue crack growth analysis. The objective of this study is, therefore, to develop a new K_I solution for a trapezoidal crack using FEM.     

Time-dependent leak behavior of flawed Alloy 600 tube specimens at constant pressure

Leak rate testing has been performed using Alloy 600 tube specimens with throughwall flaws. Some specimens have shown time-dependent leak behavior at constant pressure conditions. Fractographic characterization was performed to identify the time-dependent crack growth mechanism. The fracture surface of the specimens showed the typical features of ductile fracture, as well as the distinct crystallographic facets, typical of fatigue crack growth at low ΔK level. Structural vibration appears to have been caused by the oscillation of pressure, induced by a high-pressure pump used in a test facility, and by the water jet/tube structure interaction. Analyses of the leak behaviors and crack growth indicated that both the high-pressure pump and the water jet could significantly contribute to fatigue crack growth. To determine whether the fatigue crack growth during the leak testing can occur solely by the water jet effect, leak rate tests at constant pressure without the high-pressure pump need to be performed.     

Effects of location,thermal stress and residual stress on corner cracks in nozzles with cladding

The stress intensity factors (K_I) for corner cracks in a boiling water reactor feedwater nozzle with stainless steel cladding are obtained for loading by internal pressure and a fluid quench in the nozzle. Conditions both with and without residual stress in the component are considered. The residual stress is simulated by means of a reference temperature change. The stress distribution for the uncracked structure is obtained from a three-dimensional finite element model.A three-dimensional influence function (IF) method, in conjunction with the boundary-integral equation method for structural analysis, is employed to compute K_I values from the uncracked stress distribution. For each type of loading K_I values are given for cracks at 15 nozzle locations and for six crack depths. Reasonable agreement is noted between calculated and previously published pressure-induced K_I values. Comparisons are made to determine the effect on K_I of crack location, thermal stress and residual stress, as compared with pressure stress. For the thermal transient it is shown that K_I for small crack depths is maximised early in the transient, while K_I for large cracks is maximised later under steady state conditions. Computations should, therefore, be made for several transient time points and the maximum K_I for a given crack depth should be used for design analysis. It is concluded that the effects on K_I of location, thermal stresses and residual stresses are significant and generally too complex to evaluate without advanced numerical procedures. The utilised combination of finite element analysis of the uncracked structure and three-dimensional influence function analysis of the cracked structure is demonstrated and endorsed.     

Effect of inside diameter on design fatigue life of stationary hydrogen storage vessel based on fracture mechanics

Design fatigue life of stationary hydrogen storage vessel constructed of the practical materials of low alloy steels was analyzed based on fracture mechanics in hydrogen and air of 45, 85 and 105 MPa using cylindrical model with inside diameter (Di) of 150, 250 and 350 mm. Design fatigue life of five typical model materials was also analyzed to discuss the effect of Di on the design fatigue life by hydrogen-induced crack growth of the vessel. K_IC of all the practical materials qualified the leak before burst. Design fatigue life generally increased slightly with increasing Di in air, while design fatigue life by K_IH was much shorter than that in air. Hydrogen influence on design fatigue life increased with increasing Di due to that K_I at initial crack increased with increasing Di. The design fatigue life data of the model materials under the conditions of Di, pressure, ultimate tensile strength, K_IH, fatigue crack growth rate and regulations in both hydrogen and air were proposed quantitatively for materials selection and development for stationary hydrogen storage vessel.     

The effect of microstructure on the fracture toughness of structural steels

Comprehensive research has demonstrated that fracture toughness, K_IC, may be uniquely related to the mechanical properties describing a material's crack tip behaviour and microstructure. In the work described in the present paper the behaviour of cracks in seven weldable low alloy structural steels with tensile strengths, σ_TS, in the range 455–765 MPa was investigated by measurement and observation of plane-strain fracture toughness, tensile properties and the microstructure. The information so obtained was used to establish a sophisticated K_IC calculation model. It is shown that the process zone size is the dominant microstructural factor controlling the fracture toughness and that it varies from two to six grain sizes, depending on the particular microstructure. The model proposed matches the experimental data well and can be applied to find an optimum steel microstructure for given brittle fracture design requirements.     

A review of the effect of prior inelastic deformation on high temperature mechanical response of engineering alloys

In this review article, we examine the influence of prior deformation (prestrain) on the subsequent high temperature mechanical behaviour of engineering alloys. We review the observed effects at a macroscopic level in terms of creep deformation, creep rupture times and crack growth rates from a number of sources and a range of materials. Microstructural explanations for the observed macroscopic effects are also reviewed and constitutive models which incorporate the effect of prior deformation are examined. The emphasis in the paper is on engineering steels though reference is also made to non-ferrous alloys.     

The pre-cracking of wide plate specimens

The behaviour of uniformly thick small- and large-scale fracture–toughness specimens was analysed during fatigue–crack progression. Because of the increasing crack length, the stiffness of the specimens decreases. If cyclic loading is applied in the load-control mode, the stress-intensity factor range, ΔK, and crack growth per cycle, da/dN, increase, which is not appropriate for controlling the size of the crack. The increase in ΔK for the wide plate (WP) specimens discussed is three times lower than for single edge notch bend (SENB) and compact tension (CT) specimens. The loading range decreases if cyclic loading is applied in the displacement-control mode. The values of ΔK and da/dN for SENB and CT specimens then decrease, which is helpful for controlling the crack size; however, they increase for the WP specimens in spite of lowering the loading range. This increase is one third lower than for the load-control mode, which means that it is somewhat easier to control the size of crack.     

Fatigue crack growth behavior of JIS SCM440 steel near fatigue threshold in 9-MPa hydrogen gas environment

In this study, stress intensity factor range (ΔK) decreasing tests were conducted and the in-situ observations were used to investigate the fatigue crack growth behavior of JIS SCM440 steel near the fatigue threshold in a 9-MPa hydrogen gas environment. The fatigue crack growth rate reflected the threshold behavior of the material, although the crack propagation knee point immediately before the threshold stress intensity factor range (ΔK_th) could not be distinctly identified. The fatigue crack was also observed to exhibit uneven propagation immediately before ΔK_th. In contrast, the knee points in a helium gas environment and air were very distinct. Fractographic analysis further revealed the existence of intergranular facets, which were observed immediately before ΔK_th in the hydrogen gas environment. Conversely, no facet was observed immediately before ΔK_th in the helium gas environment and air. The formation of the facets was considered to be one of the causes of the uneven crack propagation immediately before ΔK_th in the hydrogen gas environment.     

SCC growth behavior of BWR core shroud materials

Recent studies suggest that material hardening should cause enhancement in stress corrosion cracking (SCC) growth rate of stainless steels (SS). In this study, SCC growth rates of SS irradiated up to 1.2×10²⁵ n/m² and of un-irradiated SS were measured in order to obtain the reasonable estimation for SCC growth behavior in the core shrouds, using 0.5, 0.7, 1.0T-CT specimens prepared from the actual BWR components (a core shroud made of 304SS and a top guide made of 316SS) as well as 0.5T-CT specimens prepared from H3 and H4 shroud weld mock-ups made of 316L. In irradiated SS, SCC growth rate in actual core shroud of high fluence was estimated later to be 10⁻¹⁰ m/s. In un-irradiated SS, all SCC growth rates were below the K-da/dt disposition curve of the JSME NA1-2002 standard, and this fact suggests that the degree of hardening assumed in the actual shrouds’ heat-affected zone (HAZ) should bring little enhancement effect in SCC growth rates.     

Effects of hydrogen on fatigue crack growth behavior of austenitic stainless steels

The effect of hydrogen on fatigue crack growth behavior of three stainless steels has been investigated from the viewpoint of microscopic fatigue mechanisms, martensitic transformation and hydrogen content. Fatigue crack growth rates in the hydrogen-charged SUS304 and SUS316 were accelerated with respect to crack growth rates in uncharged specimens. The crack growth rate in the hydrogen-charged SUS316L was only slightly higher than that in the uncharged SUS316L. Martensitic transformation on the fatigue fracture surfaces was detected using X-ray diffraction both in the hydrogen-charged and uncharged specimens of SUS304, SUS316 and SUS316L. Materials with increased tendency for martensitic transformation also showed increased acceleration in fatigue crack growth rate due to hydrogen. It was concluded that martensitic transformation in the vicinity of the fatigue crack tip increased the local diffusion of hydrogen thus increasing crack growth rate.     

The influence of side-grooving on pre-cracked Charpy specimens in bending

Fatigue pre-cracked Charpy specimens are sometimes used to measure the fracture toughness of steels. For most steels the dimensions of the Charpy specimens are insufficient to prevent significant plastic yielding ahead of the crack during a test. A feature of the tests is that stable crack growth results in significant crack front curvature. Side-grooving compact tension specimens has been found to inhibit crack front curvature and promote straight fronted crack growth. This paper investigates the effect of side-groove depth on the fracture behaviour of pre-cracked Charpy specimens.Elastic three-dimensional finite element analyses have been performed to assess the effect of side-groove depth on the compliance, stress state and stress intensity factor.The load displacement behaviour of Charpy specimens has been measured to determine the effect of side-groove depth on the limit load and fracture resistance.Using the finite element and experimental results an expression is derived for calculating the fracture resistance of side-grooved Charpy specimens. The expression is used to determine the critical fracture resistance of 1CrMoV steel at two temperatures using Charpy specimens with and without side-grooves.     

Fatigue crack growth behavior of reactor pressure vessel steels in air and high-temperature water environments

Fatigue tests under constant amplitude load were conducted on CT specimens of A533B3 steels with four levels of sulfur content at different temperatures in air and high-temperature water environments. A modified capacitance-type COD gauge was shown to be suitable for fatigue crack length measurement at high temperatures in air. The observation that the Young's moduli measured at a strain rate of 4 × 10⁻³ s⁻¹ for the A533B3 steels at 150 °C and 300 °C did not decrease with an increase in temperature seemed to be related to the presence of dynamic strain aging. The fatigue crack growth rates at 150 °C and 300 °C in air were about two and half times slower than those tested at 400 °C, because dynamic strain aging prevailed at 150 °C and 300 °C. Fractographic examination results suggested that inclusions embedded in secondary cracks enhanced the fatigue crack initiation rather than the fatigue crack growth. The fatigue crack growth rates taken in the oxygen-saturated water environment were one order of magnitude faster than those obtained in air.     

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.