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.     

Fatigue crack growth in multi-pass butt-welded joints of mild steel

Fatigue crack growth rate properties obtained by testing multi-pass butt-welded joints in the through-the-thickness direction are presented along with a characterization of the mild steel base material. Edge-notched four-point-bending specimens are used to investigate R-ratio, specimen geometry and post-weld heat-treatment effects on fatigue crack growth rates. The pervasive influence of residual stresses on welded joint fatigue testing using the fracture mechanics approach is also discussed. For these multi-pass joints, conservative fatigue crack growth rates are obtained with post-weld heat-treated specimens.     

Effect of constraint on ductile crack growth and ductile-brittle fracture transition of a carbon steel

Ductile-brittle fracture transition was investigated using compact tension (CT) specimens from −70 to 40°C for a carbon steel. Large deformation finite element analysis was carried out to simulate the stable crack growth in the compact tension (CT, a/W = 0.6), three point-point bend [SE(B), a/W = 0.1] and centre-cracked tension [M(T), a/W = 0.5] specimens. An experimental crack tip opening displacement (CTOD) resistance curve was employed as the crack growth criterion. Ductile tearing is sensitive to constraint and tearing modulus increases with reduced constraint level. The finite element analysis shows that path-dependence of the J-integral occurs from the very beginning of crack growth and ductile crack growth elevates the opening stress on the remaining ligament. Cleavage may occur after some ductile crack growth due to the increase of opening stress. For both stationary and growing cracks, the magnitude of opening stress increases with increasing in-plane constraint. The ductile-brittle transition takes place when the opening stress ahead of the crack tip reaches the local cleavage stress as the in-plane constraint of the specimen increases.     

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.     

Ductile tearing in austenitic stainless steel weldments

The UK Technical Advisory Group on the Structural Integrity of Nuclear Plant (TAGSI) was asked to respond to a Question that was submitted by the Ministry of Defence (MoD) on the use of tearing resistance data and the interaction with fatigue crack growth, including the effects of thermal or residual stresses, in austenitic stainless steel weldments. The TAGSI response was initially prepared in 2000, but the basic response is still considered relevant and timely. Additional comments on more recent developments are included.

TAGSI concluded that for single loading conditions, material resistance curve data can be used to produce an integrity justification with up to Δa_t=2 mm of tearing. Further work would be required to justify an increase in this limit, but for applications in thick sections it would be prudent to test 50-mm thick specimens as well as standard 25-mm thick specimens. For combined tearing/fatigue conditions, TAGSI considers that the results it has reviewed are encouraging in supporting linear summation of tearing and fatigue components to assess margins. TAGSI considers that it would be prudent to include secondary loading due to welding residual stresses as well as that due to thermal loading in any prediction of stress conditions up to the crack growth limit of 2 mm for applications of the type considered.

The paper discusses a number of restrictions and limitations to the general conclusions outlined above.     

Importance of fracture criterion and crack tip material characterization in probabilistic fracture mechanics analysis of an RPV under a pressurized thermal shock

Using a probabilistic fracture mechanics code, the importance of the choice of fracture criterion and the material fracture resistance characterization at the crack tip is elucidated in the failure probability analysis of an reactor pressure vessel. The paper describes the procedure to evaluate the crack extension based on R6, where an increase in fracture resistance by ductile crack extension is considered. Two standard J–resistance curves are prepared for applying the elasto-plastic fracture criterion by R6 tearing.Case studies concerning the effect of the tearing fracture criterion were carried out using a severe pressurized thermal Shock transient. Results are discussed with respect to the fracture criterion, the material J–resistance curve, the algorithm for evaluating the crack tip fracture toughness and the initial crack geometry.The introduction of the elasto-plastic fracture criterion significantly contributes to removal of over-conservatism in applying a linear elastic fracture criterion. It was also confirmed that the algorithm of the re-evaluation method for crack tip fracture toughness has to be correctly applied.     

Material characterization by J-R curves of a 20MnMoNi55 forging

J-R curves have been determined for one specific forging of the reactor pressure vessel steel 20MnMoNi55 to characterize its fracture resistance in the upper shelf toughness regime. The multiple specimen unloading (MSU), a direct current potential drop (DCPD), and the single specimen partial unloading compliance (SSPUC) methods have been applied to test different CT-specimen geometries at temperatures between 25 and 300°C.The J–R curves are temperature dependent showing a minimum of slope and absolute values at 200°C. The critical J-values for onset of stable crack growth in the scatter of the results are independent of temperature and geometry: J_i ≈ 180 kJ/m². The values of the technical initiation toughness J_IC according to ASTM E813⁷ and according to Loss et al. (1979),⁸ the tearing modulus T, and the instability parameter J₅₀ show corresponding temperature trends as the J-R curves with a minimum at about 200°C.The material data have been used to evaluate the ductile failure initiation and tearing instability of several structural tests. The quality of agreement between calculable predictions and experiments is shown to depend on the input material J-R curve and evaluation of the loading which requires detailed knowledge about the material stress-strain behaviour and the relevant crack tip constraint.     

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.     

Some formulae for the effective stress range ratio used in crack growth of 6063-T6 aluminium alloy

The influence of stress ratio R and stress intensity range (ΔK) on fatigue crack growth experiments were determined for 6063-T6 aluminium alloy and crack growth data were analysed with different formulae for the effective stress intensity range ratio U. The data covered R values from 0 to 0·3. A good correlation was obtained from da/dN and ΔK_eff using the equation for U as a function of R.     

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.     

Stability of a crack in a cantilever beam undergoing large plastic deformation after impact

A simple model is employed to determine the dynamic response of a rigid-perfectly plastic cantilever beam with an attached tip mass and a crack, taking into account the weakening effect of the crack. The crack is assumed to be located at the base of the beam, and an initial velocity is imparted to the tip mass. The subsequent stability of the crack is considered by calculating the tearing modulus based on the J-integral associated with the deflecting beam. For the example of circumferential cracks in thin-walled piping, whose idealized geometry models some stress corrosion cracks found in service, radial propagation and instability are found to be more likely than circumferential. Once a crack penetrates the wall, however, stability in the circumferential direction is found to depend in a complex way upon loading and crack geometry.     

Constraint-based failure assessment diagrams

The loss of crack tip constraint leads to enhanced resistance to ductile tearing and cleavage. However, current defect assessment schemes use lower bound toughness obtained from highly constrained testpieces. Although this approach is safe, it is conservative, and may lead to the imposition of prohibitive repair and inspection policies when applied to defects which develop unconstrained flow fields. Recent theoretical developments indicate that geometry dependent fracture toughness effects can now be rationalised and united through a two-parameter characterisation of elastic-plastic crack tip fields. In this context a simple modification to existing failure assessment diagrams (FAD) is proposed, using a constraint matched toughness to modify one of the axes of the FAD. The procedure recovers the original failure assessment line and unifies the constraint dependent fracture toughness data.     

Fatigue growth behaviour for a corner crack at a vessel/nozzle junction

A new cruciform specimen under biaxial loading was designed for simulating the structure at vessel/nozzle junctions and for studying the fatigue growth behaviour of corner crack. The computed and experimental results show that the character of the strain distribution in the specimen is the same as that of the vessel/nozzle junction and that the fatigue growth behaviour of the corner crack is relative to the biaxial load ratio and to the shape and dimension of the initial corner crack.     

The inhibitory effect of carbon monoxide contained in hydrogen gas environment on hydrogen-accelerated fatigue crack growth and its loading frequency dependency

The effect of carbon monoxide (CO) contained in H₂ gas as an impurity on the hydrogen-accelerated fatigue crack growth of A333 pipe steel was studied in association with loading frequency dependency. The addition of CO to H₂ gas inhibited the accelerated fatigue crack growth due to the hydrogen. The inhibitory effect was affected by the CO content in the H₂ gas, loading frequency, and crack growth rate. Based on these results, it was revealed that the inhibitory effect of CO was governed by both competition between the rate of fresh surface creation by the crack growth and the rate of coverage of the surface by CO and time for hydrogen diffusion in the material to the crack tip with reduced hydrogen entry by CO.     

Effect of temperature on the threshold fatigue crack growth behaviour of spheroidal graphite cast iron

The influence of temperature at various R ratio values on the fatigue crack propagation response of a ferritic spheroidal graphitic cast iron has been studied. It has been established that the influence of R ratio on ΔK_th is strongly dependent upon the test temperature. At elevated temperatures the influence of R ratio is significantly less than that at ambient temperature. At low to intermediate R ratio values temperature initially decreased, then, with increasing temperature, increased the ΔK_th levels, causing a minimum in ΔK_th to occur at 250–300°C. At high R ratio, however, ΔK_th exhibited a small but consistent increase with temperature. The influence of temperature on the ΔK_th at various R ratio values could be adequately explained in terms of crack closure. Much intergranular failure was observed on the fatigue fracture surfaces at ambient temperature, whereas at elevated temperatures there was little evidence of this particular failure.     

Effect of weld metal properties on fatigue crack growth behaviour of gas tungsten arc welded AISI 409M grade ferritic stainless steel joints

The effect of filler metals such as austenitic stainless steel, ferritic stainless steel and duplex stainless steel on fatigue crack growth behaviour of the gas tungsten arc welded ferritic stainless steel joints was investigated. Rolled plates of 4 mm thickness were used as the base material for preparing single ‘V’ butt welded joints. Centre cracked tensile (CCT) specimens were prepared to evaluate fatigue crack growth behaviour. Servo hydraulic controlled fatigue testing machine was used to evaluate the fatigue crack growth behaviour of the welded joints. From this investigation, it was found that the joints fabricated by duplex stainless steel filler metal showed superior fatigue crack growth resistance compared to the joints fabricated by austenitic and ferritic stainless steel filler metals. Higher yield strength, hardness and relatively higher toughness may be the reasons for superior fatigue performance of the joints fabricated by duplex stainless steel filler metal.     

The development of structural integrity criteria for austenitic components

The high ductility and resistance to tearing of austenitic stainless steels, allied to the geometry and loading conditions of the European fast reactor (EFR), result in the prediction of large critical through-thickness crack sizes for EFR components. Based on this understanding of large critical crack sizes, simplified methods have been developed for the assessment of the leak-before-break behaviour of these components. The methods described avoid the need for detailed analysis of crack shape development during cyclic loading, and thus greatly reduce the calculational requirements compared with more common methods used for the assessment of known or postulated defects in high energy pressurised components. Simplifications used in the assessment of leak-tightness in EFR components are also identified.     

The effect of ductile crack growth on the temperature dependence of cleavage fracture toughness for a RPV steel with various degrees of embrittlement

The basic properties of cleavage fracture after ductile crack growth are studied on the basis of a probabilistic model for cleavage fracture and a deterministic model for ductile fracture, proposed earlier by the authors. Investigations are applied to a 2Cr–Ni–Mo–V reactor pressure vessel steel in the initial (as-received) and embrittled states. For various temperatures, the dependencies of brittle fracture probability on stress intensity factor and extent of ductile tearing are calculated. For various states of a material, the temperature dependencies of cleavage fracture toughness are predicted with and without regard for ductile crack growth. The basic factors controlling the above dependencies are analysed. The calculated results obtained are compared with test results from CT specimens.