Reference fracture toughness curves for structural and turbine steels

In this paper reference fracture toughness, K_IR, curves are constructed according to the various methods developed to date. Recent treatments of reference fracture toughness curves are applied to eight steels of widely ranging strengths (455–812 MPa). Good agreement between K_IR values according to the USSR non-ductile failure design recommendations and the ASME Boiler and Pressure Vessel Code, Section III¹—and the experimental static (K_IC) and dynamic (K_Id) fracture toughness data is observed over a wide temperature range. Better agreement seems to exist with higher tensile strength steels.     

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.     

Hydrogen embrittlement of structural steels: Effect of the displacement rate on the fracture toughness of high-pressure hydrogen pre-charged samples

The aim of this paper is to study the effect of the displacement rate on the fracture toughness under internal hydrogen of two different structural steels grades used in energy applications. To this end, steel specimens were pre-charged with gaseous hydrogen at 19.5 MPa and 450 °C for 21 h and then fracture toughness tests were carried out in air at room temperature. Permeation experiments were also conducted to obtain the hydrogen diffusion coefficients of the steels. It was observed that the lower the displacement rate and the higher the steel yield strength, the stronger the reduction in fracture toughness due to the presence of internal hydrogen. A change in the fracture micromechanism was also detected. All these findings were justified in terms of hydrogen diffusion and accumulation in the crack front region in the different steel specimens.     

A quantitative description on fracture toughness of steels in hydrogen gas

Fracture toughness or critical stress intensity factor of many steels can be reduced by hydrogen gas. In this paper, a simple quantitative model to predict the fracture toughness of steels in gaseous hydrogen is proposed. This model is based on the assumption that fracture of a cracked body occurs when the maximum principal stress ahead of the crack tip reaches the critical cohesive stress for crack initiation. The critical stress is inversely proportional to the accumulated hydrogen concentration. The notion is that the crack will initiate at the elastic-plastic boundary ahead of the crack tip when hydrogen concentration reaches a maximum value after a long-term hydrogen diffusion assisted by the hydrostatic stress. The model describes the dependence of fracture toughness on hydrogen pressure, temperature and yield strength of steels. It can be used to quantitatively predict fracture toughness of steels in hydrogen gas, particularly in high pressure. Some experimental data reported in literature were used to validate the model, and a good agreement was obtained.     

An upper-shelf fracture toughness master curve for ferritic steels

When assessing the resistance of a steel structure to failure under load, fracture toughness is a key input variable. Since it is usually not possible to establish a priori if the anticipated service temperature (or temperatures) is in the fracture mode transition or on the upper shelf, it is useful to know the temperature dependence of fracture toughness in both the transition and on the upper shelf. In the transition, the master curve proposed by Wallin defines both the variation of the median value of fracture toughness with temperature and the scatter of fracture toughness about this median value. However, Wallin's master curve does not quantify fracture toughness on the upper shelf. In this paper we assemble a database of upper shelf fracture toughness data (J_Ic) for ferritic steels. These data demonstrate that the temperature dependence of upper J_Ic is consistent for all ferritic steels contained in the database and has the same form as the temperature dependence of the flow strength anticipated from dislocation mechanics considerations. This similarity between the temperature dependence of flow strength and the temperature dependence of J_Ic is physically expected because both directly depend upon the energy required for dislocation mobility in the ferrite matrix. Both the empirically derived model and the physical basis for the model are described.     

Predictive environmental hydrogen embrittlement on fracture toughness of commercial ferritic steels with hydrogen-modified fracture strain model

In this work, a practical numerical model with few parameters was proposed for the prediction of environmental hydrogen embrittlement. The proposed method adopts hydrogen enhanced plasticity-based mechanism in a fracture strain model to describe hydrogen embrittlement. Fracture toughness degradation of three commercial steels SA372J70, AISI4130 and X80 in high pressure hydrogen environment were investigated. Firstly, governing equations for hydrogen distribution and material damage evolution was established. Hydrogen enhanced localized flow softening effect was coupled within fracture strain dependency on stress triaxiality. Then, the numerical implementation and identification process of model parameters was described. Model parameters of the investigated steels were determined based on experiment results from literatures. Finally, with the calibrated model, fracture toughness reduction of the steels was predicted in a wide range of hydrogen pressure. The prediction results were compared with experimental results. Reasonable accuracy was reached. The proposed method is an attempt to reach balance between physical accurate prediction and engineering practicality. It is promising to provide a simplified numerical tool for the design and fit for service evaluation of hydrogen storage vessels.     

The effect of notch depth and orientation on the fracture toughness of multi-pass weldments

Reduction of notch depth is shown to cause dramatic increases in the crack opening displacement at unstable fracture and initiation of fibrous tearing in two high strength, multi-pass weldments tested under three point bending. The toughness range over which this geometry effect can be expected to apply is identified and the implications for structural fracture analysis are discussed. The effect of changing notch orientation on weld metal toughness is also considered.     

The effect of welding mechanical heterogeneity on fracture toughness feature of base metal

In the present paper, the effects of the mechanical heterogeneity of a weldment on the fracture toughness features of the base metal near the weld metal are studied experimentally by measuring the Crack Opening Displacement (COD) of the three point bend specimens with crack tip locating in the different distance from weld metal zone. The results show that the initiation toughness, the crack growth resistance and the tearing modulus of the base metal near the weld metal zone are greatly affected by the mechanical heterogeneity of the weldment. The nearer the crack is located to the weld metal zone, the stronger is the effect of weld metal properties. Moreover, the Strength Zone Width (SZW)_c of the crack has a good linear relation with initiation toughness and can be used as a fracture toughness parameter showing the mechanical heterogeneity of weldment.     

The influence of the stress state on fracture toughness

Conventional fracture mechanics treatments usually disregard biaxial loading modes, even though several authors have discussed the biaxiality effect of an additional load in the crack extension direction. Another biaxial effect, that related to an additional load parallel to the crack front, has not been investigated until now. This paper describes experiments and three-dimensional (3D) FE stress analyses which have been performed with plate-shaped cracked specimens under biaxial bending. In relation to stresses around the blunted crack tip, a fracture criterion based on achieving a critical stress is discussed.     

The fracture toughness behaviour of austenitic steels and weld metal including the effects of thermal ageing and irradiation

Stainless steel components used in nuclear power plant must be capable of maintaining reasonable mechanical properties after thermal ageing and irradiation damage has accumulated over the lifetime of the system. This study examines the fracture toughness behaviour of wrought and welded Type 316 material in long-term thermally aged and irradiated conditions. The results indicate that whilst some potentially detrimental microstructural changes have occurred during ageing, the degradation in mechanical properties is not large. In wrought material some comparisons are made between the toughness of Type 316 grade and recent results obtained on modified Type 316LN grade materials. The effects of welding processes on oxygen and inclusion content have been quantified in MMA and TIG welds, and the results have been used to explain the higher toughness of TIG-welded material. Comparisons of fracture toughness after irradiation have also been made between arc welds in Type 316 and other grades.     

A new engineering method for prediction of the fracture toughness temperature dependence for RPV steels

In order to predict the temperature dependence of fracture toughness for RPV steels with various degrees of embrittlement, up to extremely high levels, an engineering method is proposed which is similar to the Master Curve concept and named the Unified Curve concept. Wide verification of the Unified Curve concept is performed and comparison of the Master and Unified Curve concepts is carried out. It is shown that the Master Curve concept is a partial case of the Unified Curve concept.     

An updated correlation for crack-arrest fracture toughness for nuclear reactor pressure vessel steels

An updated and statistically-rigorous correlation is provided for crack-arrest toughness values versus normalized temperature for light-water nuclear reactor pressure vessel (RPV) steels. The database used in this effort is larger than applied heretofore and includes results from tests of laboratory-size specimens and from tests of large-scale specimens, which contain features prototypical of operating RPVs. The mathematical methodology used is based on a lognormal distribution, with its parameters calculated by orthogonal distance regression. This correlation was developed as one of several items updated for use in the US Nuclear Regulatory Commission's extensive program to evaluate and potentially revise its rule for ensuring structural integrity of operating RPVs when subjected to pressurized thermal-shock transients.     

Effects of hydrogen on the fracture toughness of CrMo and CrMoV steels quenched and tempered at different temperatures

Tempering temperatures ranging between 500 and 720 °C were applied in order to analyse the relationship between steel microstructure and the deleterious effect of hydrogen on the fracture toughness of different CrMo and CrMoV steels. The influence of hydrogen on the fracture behaviour of the steel was investigated by means of fracture toughness tests using CT specimens thermally pre-charged with hydrogen gas.First, the specimens were pre-charged with gaseous hydrogen in a pressurized reactor at 19.5 MPa and 450 °C for 21h and elasto-plastic fracture toughness tests were performed under different displacement rates. The amount of hydrogen accumulated in the steel was subsequently determined in order to justify the fracture toughness results obtained with the different steel grades. Finally, scanning electron microscopy was employed to study both the resulting steel microstructures and the fracture micromechanisms that took place during the fracture tests.According to the results, hydrogen solubility was seen to decrease with increasing tempering temperature, due to the fact that hydrogen microstructural trapping is lower in relaxed martensitic microstructures, the strong effect of the presence of vanadium carbides also being noted in this same respect. Hydrogen embrittlement was also found to be much greater in the grades tempered at the lowest temperatures (with higher yield strength). Moreover, a change in the fracture micromechanism, from ductile (microvoid coalescence, MVC), in the absence of hydrogen, to intermediate (plasticity-related hydrogen induced cracking, PRHIC) and brittle (intergranular fracture, IG), was appreciated with the increase in the embrittlement indexes.     

Prediction of fracture toughness for a pressure vessel steel with mixed bainite-ferritic microstructure

This paper presents a model for fracture toughness prediction of a 10MnNi2Mo steel having mixed bainite-ferritic microstructure as used for the pressure components of a nuclear reactor primary circuit. In addition to cleavage microcrack nucleation in carbide particles, consideration is given to a dislocation mechanism for nucleation of microcracks in ferrite as proposed by Cottrell. The presented model showed good agreement between the predicted and experimentally determined values of stress intensity factor.     

Acoustic emission characteristics from steels Part 2: Acoustic measurements from fracture toughness tests

Acoustic emission measurements have been made in conjunction with fracture toughness tests on nine steels of yield strengths from 247–1606 MNm⁻². Tests made under conditions where failure would be analysed in terms of linear elastic fracture mechanics (brittle behaviour) or general yielding fracture mechanics (ductile behaviour) have shown the emission characteristics at brittle and ductile failure to be significantly different. The applicability of relationships of the form (Acoustic Emission) = Const × (Fracture Toughness)ⁿ has been investigated and the effect of test piece size examined. The relative amounts of emission monitored during the tests have permitted an acoustic ‘rating’ of the steels to be made. The validity of the Kaiser effect has been demonstrated but a detrimental influence of possible corrosion/oxidation at the crack faces has been noted. The significance of the results has been assessed in terms of the application of acoustic emission to provide 100% coverage during the periodic inspection of pressure vessels.     

Radiation embrittlement modelling for reactor pressure vessel steels: II. Ductile fracture toughness prediction

Modelling for the irradiation effect on ductile fracture toughness of reactor pressure vessel steels (RPV) is performed on the basis of ductile fracture criterion proposed earlier by the authors. The irradiation effect on mechanisms controlling ductile fracture is considered from a physical viewpoint. Modelling of the irradiation effect is carried out on the critical strain for smooth cylindrical specimens and on the local critical strain for cracked specimens. On the basis of the performed studies a scheme that allows an evaluation of the upper shelf level of the K_IC(T) curve for irradiated RPV steels is proposed.     

Radiation embrittlement modelling for reactor pressure vessel steels: I. Brittle fracture toughness prediction

Modelling for the irradiation effect on brittle fracture toughness of reactor pressure vessel (RPV) steel is performed on the basis of the probabilistic model for fracture toughness prediction proposed by the authors earlier. The irradiation effect on parameters controlling plastic deformation and brittle fracture of RPV steels is analyzed. The physical mechanisms are considered which control the cleavage microcrack nucleation for RPV steels in the unirradiated and irradiated states and also in state after post-irradiation annealing. Prediction of the temperature dependence of brittle fracture toughness is performed as applied to irradiated 2.5Cr–Mo–V reactor pressure vessel steel. Modelling of the fluence effect and the phosphorus and copper content effect on brittle fracture toughness is carried out. It is shown that the probabilistic model based on a new formulation for brittle fracture criterion allows the adequate modelling for the irradiation effect on fracture toughness for RPV steel. Application of alternative models is discussed for fracture toughness prediction for irradiated RPV steels.     

Influence of hydrogen on the microstructure and fracture toughness of friction stir welded plates of API 5L X80 pipeline steel

In this work, the influence of hydrogen on the microstructure and fracture toughness of API 5L X80 high strength pipeline steel welded by friction stir welding was assessed. Samples were hydrogenated at room temperature for a duration of 10 h in a solution of 0.1 M H₂SO₄ + 10 mg L⁻¹ As₂O₃, with an intensity current of 20 mA cm⁻². Fracture toughness tests were performed at 0 °C in single-edged notched bending samples, using the Critical Crack Tip Opening Displacement (CTOD) parameter. Notches were positioned in different regions within the joint, such as the stir zone, hard zone, and base material. Hydrogen induces internal stress between bainite packets and ferrite plates within bainite packets. Besides, hydrogen acted as a reducer of the strain capacity of the three zones. The base metal had a moderate capacity to resist stable crack growth, displaying a ductile fracture mechanism. While the hard zone showed a brittle behavior with CTOD values below the acceptance limits for pipeline design (0.1–0.2 mm). The fracture toughness of the stir zone is higher than that of the base metal. Nevertheless, the stir zone displayed higher data dispersion due to its high inhomogeneity. Hence, it can also show a brittle behavior with critical CTOD values.     

Hydrogen effect on fracture toughness of pipeline steel welds, with in situ hydrogen charging

The API 5L X70 and X52 pipeline steel weld fracture toughness parameters are measured in a hydrogen environment and compared to the ones in air. The hydrogen environment is created by in situ hydrogen charging, using as an electrolyte a simulated soil solution, with three current densities, namely 1, 5 and 10 mA/cm². A specially designed electrolytic cell mounted onto a three-point bending arrangement is used and hydrogen charging is performed during the monotonic loading of the specimens. Ductility is measured in terms of the J₀ integral. In all cases a slight change in toughness was measured in terms of K_Q. Reduction of ductility in the base metal is observed, which increases with increasing current density. A more complex phenomenon is observed in the heat affected zone metal, where a small reduction in ductility is observed for the two current densities (1 and 5 mA/cm²) and a larger reduction for the third case (10 mA/cm²). Regarding microstructure of tested X70 and X52 base and HAZ metal, it is observed that the hydrogen degradation effect is enhanced in banded ferrite-pearlite formations. The aforementioned procedure is used for calculating the fracture toughness parameters of a through-thickness pipeline crack.