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.     

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).     

Application of theoretical methods to predict overload effects on fracture toughness of A533B

In the past a number of theoretical models have been proposed in an attempt to predict the effects of prior overload sequence on the fracture performance of ferritic steel components. This paper examines two models based on plastic superposition of stresses and displacements, and also in the context of continuum modelling of crack tip behaviour for cyclic loading. An analytical solution is obtained for the stress superposition model, whereas the displacement-based model requires a numerical solution.

It is proposed that a reference stress approach can be adopted where the prior overload is automatically limited by the yield (or flow) stress.

Comparison with the experimental data for A533B steel shows that neither the stress- nor displacement-based models can predict the variation in the benefit in toughness at −170°C after prior overloading at 20°C. It is also shown that the models are unable to predict the observed differences in fracture behaviour after prior overload between the L-T and T-L orientations. An approximate equation is proposed to predict the increase in toughness or load-bearing capacity after prior overload. The approximation is found to be adequate in view of the experimental scatter.     

Instrumented drop-weight test results for normalised and tempered 9Cr1Mo steel

From instrumented drop-weight tests, the nil ductility transition temperature (T_NDT), and a conservative estimate of dynamic fracture toughness (K_Id), at T_NDT for normalised and tempered 9Cr---1Mo steel, are determined to be −25°C and 70 MPa√m, respectively. The latter value agrees well with that determined from pre-cracked Charpy tests. The K_Id/σ_Yd (σ_Yd is the dynamic yield stress) ratio at T_NDT is estimated to be 0·076 √m, in agreement with previous estimates. The uncertainties in crack profile measurement and effect of microstructural variation in the heat affected zone on fracture loads are also discussed.     

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.     

Corrosion fatigue behaviour of A508 class III steel in a simulated PWR water environment at 325°C

Fatigue crack growth data in a simulated PWR environment for A508 Class III steel at 325°C generally exhibited good agreement with crack growth data recorded for A533B steel at 288°C. All data were positioned below the ASME XI high R ‘wet’ line.

It has been shown through sulphur printing and fractography that the morphology and distribution of non-metallic sulphide inclusions play an important part in initiating environmentally assisted crack (EAC) growth which is identifiable by its fan-shaped fracture mode.

In regions of EAC crack growth a significant area of the fatigue fracture surface can still exhibit ductile striated growth. Indeed it has been shown that significant EAC growth occurs when the amount of fan-shaped growth prevalent on the fatigue surface exceeds 20%. Also, fan-shaped growth tended to occur at the centre of the test specimen.     

Influence of carbon content and tempering temperature on fatigue threshold characteristics of a low alloy steel

This paper attempts to describe the effect of carbon content on the fatigue threshold characteristics ΔK_th in various heat treated conditions. Essentially it has been shown that a tempering treatment increased ΔK_th while increasing the carbon content of steels from 0·13% to 0·8% significantly decreased the ΔK_th value by over 100%. At intermediate fatigue crack growth rates all the data show a linear relationship with ΔK level.

In terms of yield strength σ_y, the threshold stress intensity level could be given by the expression: ΔK_th = 8·74 − 3·42 × 10⁻³(σ_y).

At near threshold fatigue crack growth levels significant amounts of isolated intergranular failure were observed in the 400°C tempered condition. In the other heat treated microstructures only a flat trans-granular ductile striated failure mode was evident. A maxima in the amount of intergranular facets occurred at ΔK values approaching 15 to 20 MPa√m. It has been shown the existence of intergranular failure resulted from environmentally induced fracture (through the diffusion of hydrogen) that occurred within the crack tip enclave.     

Simplified stable crack growth analyses of welded CT specimens—comparison study of GE/EPRI, reference stress and R6 methods

This paper describes some simplified stable crack growth analyses of two kinds of inhomogeneous CT specimens. The one is machined from a submerged are welded plate of a nuclear pressure vessel A533B Class 1 steel, while the other is machined from an electron-beam welded plate of the A533B Class 1 steel and a high strength HT80 steel. In both specimens, initial cracks are placed to be normal to the fusion line. The ratio of yield stresses of the weld metal and the base metal of the A533B Class 1 steel is about 1·15, while that of the HT80 and the A533B Class 1 steels is about 1·4.

The generation phase crack growth analyses using the GE/EPRI and the reference stress methods are performed, calculating an applied load (P) and the J-value, while the application phase analyses of analyses using the R6 method are performed to calculate the maximum value of the applied load (P_max). Finally, some modification procedures of the three simplified estimation schemes are discussed in order to apply them to inhomogeneous material regimes.     

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.     

Crack growth resistance in bi-metallic weldment

Two types of welded specimens were used in an experiment relating to crack growth behavior near to and in a welding bead. One was cut from a weldment of two A533B steel plates (A533B welded specimen) and the other cut from a weldment of A533B steel with high strength steel HT80 plate (A533B-HT80 welded specimen). In the experiments, the crack growth resistance curves were measured using compact tension specimens. In all of the welded specimens an initial fatigue crack was introduced perpendicularly to the welding bead. In the A533B welded specimen the crack growth resistance curves were measured for the five cases where an initial crack tip was at a position 1·5 mm distant from the fusion line; in a heat affected zone (HAZ); at the fusion line; in the weld metal; and in the base metal. Depending on where the crack tip was located, resistance evaluated by J-integral varied in respect to the base metal, the weld metal, the HAZ, and the fusion line. The resistance values for these crack tip locations, however, spread only over a narrow range. This can be explained by the fact that the mechanical properties of the base metal, the weld metal and the HAZ are only slightly different from each other. On the other hand, in the A533B-HT80 specimens, the crack growth resistance was strongly affected by the relative positions of the steels in the specimen and the width ratio of each steel to the specimen. The experimental results are discussed in terms of the stress analysis results obtained by finite elements method (FEM).     

Thermal shock fracture experiments on large size plates of A533-B steel

The paper gives a complete report on a five-year research program concerning the fracture behaviour of the pressure vessel steel A533-B under thermal shock conditions. Five tests on 140-mm thick plates carrying a surface through-thickness crack were performed with different material conditions and thermal histories. The results confirmed the applicability of the Linear Elastic Fracture Mechanics approach and of the arrest toughness criterion. Moreover, the confidence limits achievable by a material characterization procedure based on Charpy-V tests were assessed. The prevention of crack initiation under a decreasing stress intensity factor and crack-tip temperature (‘simple’ warm pre-stressing effect) was largely confirmed, while reinitiation prevention after thermal-mechanical histories including complete unloading could not be demonstrated.     

Creep test of type 304 stainless steel tube containing a notch subjected to internal pressure

This paper summarises the results of experimental creep tests of type 304 stainless steel tube subjected to internal pressure at 650°C. The equipment used was especially developed for these tests.

The tubes without notches were tested at pressures of 9·32 and 7·36 MPa. Test results indicate that the rupture time of the tubes without notches is in good agreement with that of uniaxial specimens when the maximum stress is taken as the rupture criterion. The tubes containing axial and circumferential surface notches were tested at a pressure of 7·36 MPa. Test results indicate that the ductile fracture theory is applicable to the life prediction in the case of axial notches.

An electric potential method was very useful for monitoring the creep crack growth from the notch root. The relationship between the creep crack growth rate and the fracture mechanics parameter, σ_net or K₁, was investigated.     

Deterministic assessment of reactor pressure vessel integrity under pressurised thermal shock

Numerical investigations were carried out to assess the integrity of reactor pressure vessels under pressurised thermal shock (PTS). The 4-loop reactor pressure vessel with cladding was subjected to thermo-mechanical loading owing to loss of coolant accident. The loss of coolant accident corresponding to small break as well as hot leg breaks were considered separately, which led to axisymmetric and asymmetric thermal loading conditions respectively. Three different crack configurations, 360° circumferential part through, circumferential semi-elliptical surface and circumferential semi-elliptical under-clad cracks, were postulated in the reactor pressure vessel. Finite element method was used as a tool for transient thermo-elastic analysis. The various fracture parameters such as crack mouth opening displacement (CMOD), stress intensity factor (SIF), nil ductility transition temperature (RT_NDT) etc. were computed for each crack configuration subjected to various type of loading conditions. Finally for each crack a fracture assessment was performed concerning crack initiation based on the fracture toughness curve. The required material RT_NDT was evaluated to avoid crack initiation.     

Creep deformation of induction pressure welded 2·25Cr-1Mo steel

In this paper, experimental results involving the effect of stress and temperature on creep behaviour of induction pressure welded (IPW) 2·25Cr-1Mo steel are presented. Creep rupture tests were conducted at 550–700°C in steps of 50°C over a stress range of 112·5–180 MPa. Above 650°C failure of the specimen was enhanced due to the microstructural instability. Failure in the specimens occurred invariably in the heat affected zones (HAZ), and the fracture surfaces indicated ductile failure.     

Analysis of fracture tests on large bend beams containing an embedded flaw

As part of a collaborative project to investigate the transferability of Master Curve technology to shallow flaws in reactor pressure vessel applications, a series of fracture tests were performed on large scale bend beams, which were fabricated from a reactor pressure vessel steel and contained simulated sub-surface defects. A 2-dimensional finite element model was used to simulate the behaviour of the test pieces and to study the variation in crack tip constraint at both the near surface and deep crack tips with increasing load. Wallin's two-parameter model, which uses the Master Curve representation of the fracture toughness transition curve together with the elastic crack tip T_stress parameter to estimate the shift in the T₀ reference temperature due to constraint loss, has been applied to arrive at estimates of fracture initiation probability. These are found to be consistent with the experimental results.     

Neural network approach to estimate stable crack growth in welded specimens

This paper describes an application of the hierarchical neural network to the generation phase stable crack growth analysis of two kinds of welded CT specimens using the GE/EPRI simplified method. One of the specimens was machined from a submerged-arc-welded plate of nuclear pressure vessel A533B Class 1 steel, the other from an electron-beam-welded plate of A533B Class 1 steel and high-strength HT80 steel. A ratio of mixture of material constants was introduced to apply the GE/EPRI method to the analysis of crack growth in the welded specimens. The best ratio of mixture was identified using the neural-network-based inverse analysis approach as follows. At first, a number of generation phase crack growth analyses based on the GE/EPRI method were tested by parametrically varying the ratio of mixture. The relationship between the ratio of mixture and the calculated crack growth behavior is called here ‘learning data sets’. The neural network was then ‘trained’ using the learning data sets. In the training process, the calculated crack growth behavior is applied to the input units of the network, while the ratio of mixture is applied to its output units in the form of teaching data. Finally, the best ratio of mixture was estimated by applying measured crack growth behavior to the input units of the ‘trained network’. The effects of material inhomogeneity on crack growth behavior in the welded specimens are discussed with respect to the best ratio of mixture obtained.     

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.     

The thermal conductivity of toluene. New determinations and an appraisal of recent experimental work

New, absolute determinations of the thermal conductivity of toluene in the temperature range −20° to +112°C are reported. These results were compared with the best data published during the last 10 years, and as a result of a subsequent analysis a linear relation between the thermal conductivity and temperature is proposed, which predicts the thermal conductivity of toluene within better than 1 per cent between −20° and +112°C. In the absence of comprehensive experimental evidence from −20°C to the f.p. At −95°C, it is tentatively suggested to use this equation for extrapolations which will probably fall within 2–3 per cent of the real values.

The accuracy and consistency of the results of the various investigations from which the above relation was derived, and the favourable physical and chemical properties of toluene justify making a strong recommendation for using this substance for calibration purposes in relative determinations, or for control measurements in absolute measurements of the thermal conductivity of fluids.     

Analysis of biaxial loading effect on fracture toughness of reactor pressure vessel steels

The local stress–strain state (SSS) near the crack tip and its connection with the crack tip opening displacement and J-integral under biaxial loading have been studied by finite element methods in elastic–plastic finite strain statement. Numerical investigations have been performed for various crack lengths and two types of biaxial loading (tension and bending) under conditions of small- and large-scale yielding. To predict the biaxial loading effect on cleavage fracture toughness, the procedure has been elaborated, this being based on the revealed regularities for SSS near the crack tip under biaxial loading and brittle fracture criterion proposed earlier. Prediction of the biaxial loading effect on cleavage fracture toughness has been performed as applied to reactor pressure vessel steel. The calculated results have been compared with available experimental data. Alternative approaches for prediction of the biaxial loading effect on fracture toughness 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.