Fem prediction of the influence of warm prestressing on fracture toughness of heat-resistant steel

We present a procedure of prediction of the influence of warm prestressing combined with cycling on the brittle strength of steel 15Kh2MFA. Using a finite-element method, the effect of the combined warm prestressing on the stress-strain state at a fatigue crack tip is studied in an elastic-plastic statement. Electron microscopic observations of fracture surfaces have revealed that fracture is initiated at some distance from the fatigue crack front. Based on the pattern of influence of the plastic prestrain level on the cleavage stress of steel 15Kh2MFA and the experimental CID value, a method is put forward for finite-element modeling of the stress-strain state at a crack tip during the specimen fracture. Using the results of the finite-element modeling, the relevant curves have been plotted and an approximating formula has been proposed to represent the influence of the combined warm prestress level on the fracture toughness of steel 15Kh2MFA.     

Assessment of brittle strength of nuclear reactor pressure vessel steel upon warm prestressing

A procedure for assessing the influence of combined warm prestressing on fracture toughness of heat-resistant steel is proposed, which is based on the stress-strain state analysis by the finite-element method (in the elastic-plastic statement) and on the local fracture criterion.     

Analysis of warm prestressing effect on fracture toughness of reactor pressure vessel steels

We have performed finite element calculation of precracked Charpy-like three-point bending specimen, in order to analyze the effect of warm prestressing on fracture toughness value of reactor pressure vessel steels. Two different hardening laws were applied in the calculation. J-integral was determined in both cases and comparative analysis was made.     

Effects of void damage induced by warm prestressing (WPS) on cleavage fracture of notched steel specimens

The effects of void damage induced by warm prestressing (WPS) on cleavage fracture of notched steel specimens were studied by experiments and FEM calculations. The results show that the local stress concentration around the voids promotes the cleavage initiation and decreases the notch toughness and cleavage fracture stress. The fibrous cracks ahead of notch tips caused by the ductile tearing in the WPS obviously raise the normal stress in front of their tips and decrease fracture load and notch toughness. When the beneficial effects of WPS on improving apparent fracture toughness for specimens or structures are used, the loads in WPS need to be limited so that no obvious void damage and ductile tearing are produced in front of defects.     

THE EFFECTS OF SUB-CRITICAL CRACK GROWTH ON THE FRACTURE BEHAVIOUR OF CRACKED FERRITIC STEELS AFTER WARM PRESTRESSING

The effects of sub-critical crack extension on the fracture properties of ferritic steels after they have been subjected to warm prestressing are investigated. The crack growth is assumed to occur at the temperature of the final loading, after the warm prestressing and the subsequent unloading. Predictions of the fracture behaviour are made using continuum mechanics and a fracture criterion based on a modified J-integral. The results of these calculations are consistent with those of a micromechanistic model of cleavage fracture from a sharp crack. Both the theories predict that the beneficial elevation in fracture load produced by warm prestressing is maintained after sub-critical crack growth provided the latter is not greater than the compressive yielded zone formed on unloading.     

A model for predicting the influence of warm pre-stressing and strain ageing on the cleavage fracture toughness of ferritic steels

A micromechanistic model of warm pre-stressing is extended to predict the combined effects of warm pre-stressing and strain ageing on the cleavage fracture toughness of ferritic steels. The crack tip stress distribution after a cycle of pre-straining and strain ageing is estimated by superposition of the appropriate monotonic loading stress distributions. The Ritchie, Knott and Rice model of cleavage fracture and its associated fracture criterion are employed in conjunction with the crack tip stress distribution to predict the critical stress intensity factor after warm pre-stressing and strain ageing. Illustrative calculations are presented, based upon the published material's properties of a high nitrogen mild steel. Available experimental data for pressure vessel steels bear out the form of the predictions. At low temperatures, and after heavy pre-loads, the benefits of warm pre-stressing dominate strain ageing induced embrittlement and the toughness is apparently enhanced. At higher temperatures, or after small pre-loads, however, strain ageing dominates and the apparent toughness is reduced. Various assumptions and approximations inherent in the model are discussed. These generally tend to render the predictions conservative. Finally it is noted that the model should be equally applicable to the prediction of the combined effect of warm pre-stressing and neutron irradiation on the cleavage fracture toughness of ferritic steels.     

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.     

INVESTIGATION OF THE ROLE OF RESIDUAL STRESSES IN THE WARM PRESTRESS (WPS) EFFECT. PART I—EXPERIMENTAL

Abstract— The role of residual stresses in the warm prestress (WPS) effect has been investigated. Three types of specimen have been tested in this investigation: smooth uniaxial tensile specimens, blunt notched single edge notched bend (SENB) specimens and sharply precracked SENB specimens. Room temperature prestraining of uniaxial tensile specimens leads to a dramatic decrease in the measured nominal fracture stress at — 196°C. Such an embrittling effect may be expected to reduce the beneficial increase in subsequent fracture toughness commonly observed in WPS sequences. The blunt-notched specimens were prestressed in tension and compression. Compressive prestressing was found to lead to a decrease in subsequent fracture load whereas tensile prestressing leads to an increase. The load decrease following a compressive WPS was greater than the load increase following a tensile WPS. Various sequences of loading, unloading and cooling have been investigated and the differences in the subsequent fracture behaviour of specimens have been explained qualitatively by superposition arguments. The theories of Chell and Curry have been supported by the general trend of results.     

THE WARM PRESTRESSING EFFECT IN STEELS UNDERGOING INTERGRANULAR FRACTURE

Abstract— The warm prestressing (WPS) effect has been studied in three different steels. One of these, a Cr-Ni steel (EN36), was heat treated to cause fracture to occur either intergranularly, by cleavage, or by a mixture of cleavage and intergranular fracture. A plain carbon steel (EN3) was also used for experiments involving cleavage fracture and thermally aged A533B was used for additional experiments involving intergranular fracture. A significant WPS effect was found for each fracture mode. There were slight differences in the magnitude of the effect for different groups of specimens but, except in certain special cases, these were not considered significant. Experiments involving stress relieving treatments following warm prestressing were also carried out. They indicated that the WPS effect was not completely eliminated by stress relieving. Crack blunting during warm prestressing has been proposed to account for this and for certain other aspects of the results.     

Mechanism of effects of warm prestressing(WPS) on apparent toughness of notched steel specimens: Part I: Experimental

In this investigation, a series of experiments are carried out in blunt notched specimens to explore the various factors controlling the warm prestressing (WPS) effect on apparent toughness of a HSLA steel. A great number of specimens were tested using Cool-Fracture (CF) without WPS, Load-Cool-Fracture (LCF) and Load-Unload-Cool-Fracture (LUCF) cycles. More complex cycles have also been used to produce residual stress distributions and notch deformations different in quantities and signs. All fracture surfaces of the specimens were observed. Some specimens were unloaded after WPS and details of microscopic features in front of notch roots were investigated. Experimental results show that warm prestress cycles raising the residual compressive stress and opening the notch root improve notch toughness at low temperatures. Oppositely, WPS cycles raising the residual tensile stress and closing the notch root deteriorate notch toughness. One distinct effect of WPS involves deactivating inclusions and second phases particles. With increasing the preload of WPS, more and more particles being potential cleavage nuclei are decohered and blunted to cavities. This effect is proposed to be involved in improvement of notch toughness.     

Warm pre-stress based pre-cracking criteria for fracture toughness testing

There are presently a magnitude of different fracture toughness testing standards that have different criteria for fatigue pre-cracking specimens prior to testing. The reason for the criteria is that too high pre-fatigue load may influence the subsequently measured fracture toughness value. The criteria have to a large extent been developed specifically for each standard in question and this has lead to the considerable variability in the criteria. The basic reason for the pre-fatigue having an effect on the fracture toughness is the warm pre-stress (WPS) effect. Here, existing data relating to pre-fatigue load levels are examined with the help of a newly developed simple WPS correction and a criteria and correction procedure for too high pre-fatigue loads are proposed. The new criteria focuses on brittle fracture, but is equally applicable for ductile fracture, thus enabling a unification of pre-fatigue criteria in different fracture toughness testing standards.     

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.     

Static fracture toughness of ultra-fine grained steels processed by thermo-mechanical warm rolling

Ultra-fine grained steels were recently developed by thermo-mechanical warm rolling. Their low-temperature fracture toughness was evaluated in terms of crack tip opening displacement (CTOD) in this paper. Effect of temperature on CTOD and the correlation between CTOD and grain size were investigated, and the experimental results showed that refining ferrite grains can increase the fracture toughness of steel and lower the sensitivity of fracture toughness to temperature. The fracture toughness of the developed ultra-fine grained steels was superior to that of hot-rolled steel SM490B with similar chemical composition.     

Prediction of the Warm Prestressing Effect on an Increase in the Brittle Strength of Crack-Containing Heat-Resistant Structural Steels. Part 1. Model and Procedure of Calculating the Warm Prestressing Effect

The physicomechanical model of warm prestressing and the procedure of calculating this effect for heat-resistant steels are proposed. The procedure is based on the analysis of stress-strain state variations in the elementary volume of the material near the crack tip during warm prestressing and further loading. This procedure takes account of such factors as crack tip blunting, residual stresses, and strain hardening of the material near the crack tip, which determine the effect of warm prestressing on the brittle strength of heat-resistant steels.     

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.     

Correlation of the acoustic emission and the fracture toughness of ductile nodular cast iron

Salzbrenner has recently determined the fracture toughness of a series of ductile cast iron samples which were heat treated to produce a fully ferritic matrix. His results indicated that the fracture toughness is strongly dependent upon the average spacing between (or equally the diameter of) the spherical graphite nodules in the ferrite matrix. The acoustic emission generated during the uniaxial compressive deformation of nodular cast iron also depends strongly on the average diameter of the graphite nodules in the test sample. The present investigation was carried out to determine the correlations, if any, between the fracture toughness and the acoustic emission generated during compression of ductile cast iron. The acoustic emission generated during compression was determined using sample materials identical to those used by Salzbrenner. Excellent correlations between certain features of the measured acoustic emission and the fracture toughness were obtained. Data indicate that it should be possible to determine both the fracture toughness and the average size of the graphite nodules from the acoustic emission and load curve generated during a compression test of ductile cast iron.     

Intralaminar fracture mechanism in unidirectional CFRP composites — part II: analysis

Three kinds of representative carbon fiber reinforced unidirectional composite materials are used, and their intralaminar fracture behavior is investigated by using the double-cantilever beam (DCB) specimen with a simultaneous acoustic emission measuring. In Part I, the experimental results on the crack propagation, the bridging fibers, the intralaminar fracture toughness acoustic emission characteristics and microscope observations were obtained. Here, we use a bridging fiber model to analyze the debonding force acting on a bridging fiber and try to estimate the number of bridging fibers during the crack propagating process. At the same time, the intralaminar fracture toughness is calculated by both the adhesive force model and the finite element analysis. As a result, it is found that the intralaminar fracture toughness without the bridging fibers will have a constant value during the crack propagation, but it increases greatly when bridging fibers exist. It is clear that the bridging fibers play an important role in the intralaminar fracture toughness. The debonding forces acting on the bridging fibers and the number of bridging fibers are obtained. Furthermore, the quantitative estimation of the increment of the intralaminar fracture toughness contributed by bridging fibers is made according to the adhesive force model and it is comparable with the results obtained by the finite element analysis.     

A study concerning material fracture toughness and some small punch test data for low alloy steels

The present study has demonstrated the existence of a significant relationship between the small punch energy and the biaxial strain at the point of failure. Furthermore, using published trends relating biaxial fracture strain and fracture material toughness, relationships concerning fracture toughness and small punch energy values were generated. It was shown that the predictions from certain such relationships exhibited good commonality with published real fracture toughness data. Furthermore, one study which related fracture toughness to non-metallic inclusion distribution was promising while other models based upon microstructural details and small scale fracture roughness factors were less successful. This study must be considered provisional in nature since it represents a first attempt at relating small punch data with material fracture toughness, and as such, contains a few assumptions which are somewhat tentative.     

混凝土断裂过程区长度计算方法研究

该文基于粘聚裂缝概念,以起裂韧度作为裂缝起裂和扩展的准则,提出了混凝土断裂过程区长度的计算方法。以Ⅰ型裂缝为例,计算了不同初始缝长和起裂韧度情况下的断裂过程区长度值,结合以往大体积混凝土的试验数据对其进行了验证。进而分析了断裂过程区长度的影响因素,结果表明:断裂过程区长度随初始缝长的增大而逐渐增大,随起裂韧度的增大而逐渐减小。     

A local approach to cleavage fracture in ferritic steels following warm pre-stressing

Quantification of the enhancement in cleavage fracture toughness of ferritic steels following warm pre‐stressing has received great interest in light of its significance in the integrity assessment of such structures as pressure vessels. A Beremin type probability distribution model, i.e., a local stress‐based approach to cleavage fracture, has been developed and used for estimating cleavage fracture following prior loading (or warm pre‐stressing, WPS) in two ferritic steels with different geometry configurations. Firstly, the Weibull parameters required to match the experimental scatter in lower shelf toughness of the candidate steels are identified. These parameters are then used in two‐ and three‐dimensional finite element simulations of prior loading on the upper shelf followed by unloading and cooling to lower shelf temperatures (WPS) to determine the probability of failure. Using both isotropic hardening and kinematic hardening material models, the effect of hardening response on the predictions obtained from the suggested approach has been examined. The predictions are consistent with experimental scatter in toughness following WPS and provide a means of determining the importance of the crack tip residual stresses. We demonstrate that for our steels the crack tip residual stress is the pivotal feature in improving the fracture toughness following WPS. Predictions are compared with the available experimental data. The paper finally discusses the results in the context of the non‐uniqueness of the Weibull parameters and investigates the sensitivity of predictions to the Weibull exponent, m, and the relevance of m to the stress triaxiality factor as suggested in the literature.