Prediction of the Brittle Fracture Toughness of Neutron-Irradiated Reactor Pressure Vessel Steels. Part 2

We apply the proposed model to study the effect of irradiation on the temperature dependence of brittle fracture toughness of 15Kh2MFA steel. We analyze also the influence of irradiation and the content of phosphorus and copper on the brittle fracture toughness. It is demonstrated that the probabilistic model, which is based on a new formulation of the brittle fracture criterion, allows an adequate prediction of the irradiation effect on the fracture toughness of reactor pressure vessel steels. Alternative models of prediction of fracture toughness are discussed.     

Temperature Dependence of Brittle Fracture Toughness of Reactor Pressure-Vessel Steels upon Ductile Crack Growth

The main mechanisms of brittle fracture upon ductile crack growth are studied on the basis of the probabilistic model of brittle fracture and the deterministic model of ductile fracture, which were put forward by the authors earlier. The investigations are carried out on the reactor pressure-vessel steel 15Kh2NMFAA in the initial and embrittled states. The dependences of brittle-fracture probability on the stress intensity factor and the value of ductile crack growth are calculated for various temperatures. The temperature dependence of brittle fracture toughness in the initial and embrittled states is predicted with and without regard for ductile crack growth. The authors analyze the main factors that govern the above-mentioned relationships. The calculated results are compared to test data for CT-type compact specimens.     

Modeling of Ductile Crack Growth in Reactor Pressure-Vessel Steels and Determination of JR Curves

A method is presented for predicting JR curves for reactor pressure-vessel steels. The authors propose a procedure for determining the ductile fracture model parameters from the test results for smooth and notched cylindrical specimens. The stress and strain fields at the tip of a stationary and propagating cracks are studied by the finite-element method. The predicted JR curves are compared to the experimental data obtained for the 2T-CT-type specimens of 15Kh2NMFA-A reactor pressure-vessel steel in the initial and embrittled state.     

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

The stress-strain state at the crack tip and its relation to the crack opening displacement and the J-integral under biaxial loading have been studied by solving elastoplastic problems in a geometrically nonlinear formulation by the finite-element method. Numerical investigations have been performed for various cracks and two modes of biaxial loading (tension and bending) under conditions of both small-scale and large-scale yielding. For prediction of the influence of biaxial loading on fracture toughness (at brittle fracture) a procedure has been developed that is based on established laws of stress-strain state formation at the crack tip under biaxial loading and a criterion of brittle fracture proposed earlier. The effect of biaxial loading on fracture toughness is predicted as applied to reactor pressure-vessel steels. Calculated results are compared with avilable experimental data. Alternative approaches to prediction of the effect of biaxial loading on fracture toughness are discussed. TsNII KM “Prometei,” St. Petersburg, Russia. Translated from Problemy Prochnosti, No. 5, pp. 5–26, September–October, 1999.     

Prediction of Ductile Fracture Toughness for Neutron-Irradiated Reactor Pressure-Vessel Steels. Part 2

The influence of neutron irradiation on the critical strain in tensile testing of smooth cylindrical specimens and on the local critical strain in ductile fracture of cracked specimens is simulated for 15Kh2MFA reactor pressure-vessel steel. Based on a series of calculations, we have developed an engineering scheme for estimating the irradiation-induced decrease in the upper-shelf level of the KI _c (T ) function.     

Prediction of ductile-brittle transition for ductile crack growth in reactor pressure-vessel steels

We have investigated the local stress-strain state at the crack tip using the finite-element method in a geometrically nonlinear formulation (with account of the variations in the crack tip blunting) for both a stationary crack and a crack growing by a ductile mechanism. Combined with the criterion of brittle fracture, the derived relationships governing the generation of the stress-strain state at the tips of stationary and growing cracks allowed us to explain the ductile-brittle transition for reactor pressure-vessel steels. We propose a technique for predicting the value of the ductile crack extension up to the instant of the ductile-brittle transition depending on the test temperature, and a procedure for calculating fracture toughness taking into account ductile crack extension. The calculations for predicting the ductile-brittle transition are made as applied to reactor pressure-vessel steel 15Kh2MFA. Analysis is made of the results obtained and available experimental data. Various approaches to the interpretation of the ductile-brittle transition are discussed. TsNII KM “Prometei,” St. Petersburg, Russia. Translated from Problemy Prochnosti, No. 6, pp. 5–22, November–December, 1999.     

Prometey local approach to brittle fracture: Development and application

Approach for prediction of brittle fracture proposed by the authors over recent years and known now as Prometey approach is briefly reviewed and new results for its development and application are represented. The physical and mechanical aspects of cleavage microcrack nucleation and propagation are considered. Application of the Prometey local approach is considered for prediction of the effect of irradiation and the shallow crack effect on the fracture toughness transition curve of RPV steels. The effect of the radiation damages on the cleavage microcrack nucleation is discussed.     

Probabilistic prediction of the crack resistance of nuclear pressure-vessel steels on the basis of a local approach. Part 2

On the basis of a new local probabilistic criterion of brittle fracture, a local criterion of ductile fracture proposed by the authors earlier, and the obtained approximate solution of the problem of stress-strain state near the crack tip, we develop a probabilistic model for the prediction of the crack resistance of pressure-vessel steels. The model enables one to predict the dependence of K _Ic on temperature for any given probability of brittle fracture and the influence of the thickness of the specimen on K _Ic. Bu using this model, we can also describe the temperature range of the brittle-ductile transition. The results of numerical calculations are compared with the experimental data for 15Kh2MFA pressure-vessel steel. It is shown that the proposed model fairly well describes the spread in the experimental data on the crack resistance of this type of steel. TsNII KM “Prometei,” St. Petersburg, Russia. Translated from Problemy Prochnosti, No. 2, pp. 5–22, March–April, 1999.     

Brittle fracture local criterion and radiation embrittlement of reactor pressure vessel steels

The application of local criteria for predicting brittle fracture of reactor pressure vessel steels is discussed with an emphasis on radiation embrittlement. An association of the radiation-induced damages and the processes of initiation and propagation of cleavage microcracks is analyzed from the standpoint of the local criterion for fracture. Physical-mechanical models are put forward to describe the influence of radiation damages on the cleavage microcrack initiation. The influence of the material hardening caused by neutron irradiation and plastic deformation on the fracture toughness is studied.     

Probabilistic definition of local criterion for brittle fracture under complex thermomechanical loading

The new local criterion for brittle fracture in probabilistic definition, which was used by the authors earlier for the K_Ic(T) prediction, is modified for the case of non-isothermal non-monotonic loading. An approach is put forward which allows predicting the brittle fracture probability for cracked elements under complex thermomechanical loading typical, for example, of the case of emergency cooldown of a reactor pressure vessel. The approach has been verified by comparing the calculated and experimental data of assessment of the influence of various thermomechanical pre-loading modes on the fracture toughness of reactor pressure vessel steels.Translated from Problemy Prochnosti, No. 1, pp. 24–42, January–February, 2005.     

THE INFLUENCE OF PLASTIC PRESTRAINING ON BRITTLE FRACTURE RESISTANCE OF METALLIC MATERIALS WITH CRACKS

Abstract The purpose of the present work was to study the influence of different regimes of overloading of pressure vessel steels in different states which correspond to the steel properties at the beginning of a reactor operation and at different degrees of embrittlement (simulated by heat treatment). The experiments were performed on 25, 50 and 150 mm thick specimens with short and long cracks of various shape in the temperature range from 293 to 623 K corresponding to the service temperature range of those steels. The following factors were investigated contribution of different effects (residual stresses, strain hardening, crack tip blunting) into the enhancement of the brittle fracture resistance of steels after warm prestressing, stability of the positive warm prestressing effect during subsequent exposure of the steels to different service loading conditions; size effect on optimal regimes of thermo-mechanical prestressing and on the brittle fracture resistance characteristics of the steels studied after warm-prestressing. An approach is proposed to predict the increase in the brittle fracture resistance of steels with cracks after warm prestressing.     

On Improvement of Methods for Experimental Determination of Strength Characteristics of NPP Component Materials

The methods for experimental determination of strength characteristics as per applicable standards have been reviewed. For some structural steels used in NPP facilities, the influence of loading parameters and specimen geometry are allowed for during the assessment of static fracture toughness (K_Ic, J_Ic). Recommendations are given on the setting of cycling conditions for fracture toughness testing of standard specimens with and without crack-guiding lateral grooves. The authors substantiate the applicability of the Master Curve method to determination of fatigue strength of small specimens with subsequent use of the results for calculating brittle fracture resistance of reactor pressure vessel materials in the welded joint.     

A comparative study of ductile-brittle transition behavior and fractography of P91 and P92 steel

The modified 9Cr-1Mo (P91) and 9Cr- 0.5Mo- 1.8W (P92) steel used in fast breeder reactor is exposed to irradiation during service which severely affects the dynamic fracture resistance by increasing the ductile to brittle transition temperature (DBTT). Thus, even at room temperature, the steel can become brittle and prone to cracking. In the present investigation, to elucidate the influence of low temperature on the DBTT, Charpy toughness test was performed on creep strength enhanced ferritic P91 and P92 steel. Lower DBTT was observed for the P92 steel as compared to P91 steel. To find the mode of fracture, the fractured Charpy toughness specimens were investigated using a field electron scanning electron microscope (FESEM). The fracture surface revealed the brittle mode of fracture at a lower temperature for both the steels while the mixed mode of fracture was noticed at room temperature and above.     

Prediction of fracture toughness for heat-resistant steels considering specimen dimensions. Report 4. Fracture toughness after plastic prestraining of materials with cracks

The paper presents the results of an experimental investigation of the influence of warm prestressing (WPS) on fracture toughness characteristics of large-size specimens. The WPS has been found to be an efficient method for enhancing brittle fracture resistance of large-size bodies from the investigated materials and can be recommended for practical realization in nuclear reactors and other critical structures whose brittle fracture is impermissible both in the process of normal operation and in emergency situations. The optimum temperature-loading regime of the WPS is defined by both the properties of a given material and its thickness which governs the intensity of plastic deformation in the process of WPS. Based on the established mechanisms of the WPS effect, a physicomechanical model has been developed for the prediction of fracture toughness for pressure-vessel heat-resistant steels after WPS taking into account the influence of the stress state at the crack tip. The model makes it possible to predict fracture toughness for large-size bodies subjected to WPS with the given temperature and loading regimes from the results of testing small laboratory specimens. The most optimum regimes of the WPS can also be determined using this model and even those for several materials making up a structural component and subjected to the WPS. Translated from Problemy Prochnosti, No. 3, pp. 39–54, May–June, 1997.     

Development of the Local Approach to Fracture over the Past 25 years: Theory and Applications

This review paper is devoted to the local approach to fracture (LAF) for the prediction of the fracture toughness of structural steels. The LAF has been considerably developed over the past two decades, not only to provide a better understanding of the fracture behaviour of materials, in particular the failure micromechanisms, but also to deal with loading conditions which cannot easily be handled with the conventional linear elastic fracture mechanics and elastic–plastic fracture mechanics global approaches. The bases of this relatively newly developed methodology are first presented. Both ductile rupture and brittle cleavage fracture micromechanisms are considered. The ductile-to-brittle transition observed in ferritic steels is also briefly reviewed. Two types of LAF methods are presented: (i) those assuming that the material behaviour is not affected by damage (e.g. cleavage fracture), (ii) those using a coupling effect between damage and constitutive equations (e.g. ductile fracture). The micromechanisms of brittle and ductile fracture investigated in elementary volume elements are briefly presented. The emphasis is laid on cleavage fracture in ferritic steels. The role of second phase particles (carbides or inclusions) and grain boundaries is more thoroughly discussed. The distinction between nucleation and growth controlled fracture is made. Recent developments in the theory of cleavage fracture incorporating both the effect of stress state and that of plastic strain are presented. These theoretical results are applied to the crack tip situation to predict the fracture toughness. It is shown that the ductile-to-brittle transition curve can reasonably be well predicted using the LAF approach. Additional applications of the LAF approach methods are also shown, including: (i) the effect of loading rate and prestressing; (ii) the influence of residual stresses in welds; (iii) the mismatch effects in welds; (iv) the warm-prestressing effect. An attempt is also made to delineate research areas where large improvements should be made for a better understanding of the failure behaviour of structural materials.     

Influence of the modes of thermomechanical preloading on the resistance of heat-resistant steels to brittle fracture

We describe the methods and results of the experimental investigation of the influence of various modes of thermomechanical preloading on the resistance of nuclear pressure-vessel steels to brittle fracture. We studied specimens of different thickness (25–150 mm) made of the base and weld metals of the vessels of water-moderated, water-cooled nuclear power reactors. The materials under consideration belong to different strength classes. It is shown that the positive effect of thermomechanical preloading is preserved or even strengthened after the subcritical ductile growth of a crack in the process of preloading. We compare the available experimental data with the results of numerical calculations performed by using the Chell model. It is shown that the Chell model fails to explain some experimental data. Institute for Problems of Strength, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 2, pp. 126–138, March–April, 1999.     

Radiation embrittlement modelling in multi-scale approach to brittle fracture of RPV steels

The purpose of the present article is to develop a multi-scale brittle fracture modelling for irradiated RPV materials. For this development, applicability of local brittle fracture criteria for radiation embrittlement modelling is analysed through comparison of the predicted and test results on radiation embrittlement of RPV steels in terms of ductile-to-brittle transition temperature and fracture toughness. The influence of radiation-induced defects on the processes of cleavage microcrack nucleation and propagation is clarified. The physical-and-mechanical models of the effect of irradiation-induced defects on cleavage microcrack nucleation are developed on the basis of dislocation and brittle fracture theories. Stress-and-strain controlled fracture criterion is developed that allows the adequate prediction of radiation embrittlement by various mechanisms. The differences and commonalities are revealed in the nature of material embrittlement due to cold work and neutron irradiation. The mechanism is explained of significant recovery of fracture resistance properties with simultaneous increase of fraction of intercrystalline fracture after post-irradiation annealing. Engineering approach for prediction of the temperature dependence of fracture toughness as a function of neutron fluence is justified.     

Characterization of He embrittlement of a 9Cr–1Mo steel using local approach of brittle fracture

Ferritic-martensitic steels are prime candidate materials for future reactors. We present here the results of a study on the effects of helium implantation on the fracture behavior of 9Cr (T91) martensitic steels. Three-points static bending tests were performed at room temperature on implanted specimens and at −170 °C on un-implanted material. All these tests led to brittle fracture. Based on a mechanical analysis of the tests results using Finite Element calculations, we have proposed that the mechanism of brittle fracture is controlled by a double criterion depending on implantation temperature and helium content. Furthermore, by applying the Beremin model, the toughness of helium implanted steel has been evaluated.